Geology and Petrology of the Basalts of Crater Flat: Applications … · 2012. 11. 18. · The...

t^6 O_ LA-8845-MS

Geology and Petrology of the Basalts of

Crater Flat: Applications to Volcanic

Risk Assessment for the Nevada Nuclear

Waste Storage Investigations

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LOS ALAMOS SCIENTIFIC LABORATORYPost Office B0x 1663 Los Alamos. New Mexico 87545

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An Affrmative Action/Equal Opportunity Employer

This work was supported by the US Depart.ment of Energy, Nevada Nuclear WasteStorage Investigations.

Tis rort w*. Pp~jd asap i oun ofl3 wk 4q'..nmtr byv n acwy f the Vasde S's WC"aiwni. V4ahlwi ti tiiNd Sljae 4. rnagnccl Oiu n JaixfaY htvf. fla ay o th'i v nagploytws.inut. a atrafty. %pvg w ripik. .g assmons any lepal iability oir vestainslaligy Mew tha aaar..WY. V0PICUMNILa aa aa.1AaNIOCU of JnY M~ofilalkI. pg'aatu1S. plaa~d. iM g'MMc aduital. o tPccnt hat ats uaw wual n ahow prw.atly oiwned i th. Itclemawe hti t any s.stW aoam-nmer.aI pruajuat. prisu~.. air vkv y r aine. radakmik. nnua..iax . o uherwiu.. Sas nutnC6Waxusay vitstltic a nly its nurwinen. raxumaienajatiu. a favtiny y tit U~nited Sics(aiwrann oir any .Arecnay glvocuf. The views anad pnions of avihuts eresased hundj ala nt nov-amiudy a r ilea h, attho;i atedta Stalts (.avaanlr air any .apty heuI.

UNITED STATESDEPARTMENT OF ENERGYCONTRACT W7405-FENG.

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I

LA-8845-MS

UC 70Issued: June 1981

Geology and Petrology of the Basalts of

Crater Flat: Applications to Volcanic

Risk Assessment for the Nevada Nuclear

Waste Storage Investigations

D. VanimanB. Crowe

I-I ,I

-.

GEOLOGY AND PETROLOGY OF THE BASALTS OF CRATER FLAT:APPLICATIONS TO VOLCANIC RISK ASSESSMENT

FOR THE NEVADA NUCLEAR WASTE STORAGE INVESTIGATIONS

by

D. Vaniman and B. Crowe

ABSTRACT

Volcanic hazard studies of the south-centralGreat Basin, Nevada, are being conducted for theNevada Nuclear Waste Storage Investigations. Thisreport presents the results of field and petrologicstudies of the basalts of Crater Flat, a sequenceof Pliocene to Quaternary-age volcanic centerslocated near the southwestern part of the NevadaTest Site. Crater Flat is one of several basalticfields constituting a north-northeast-trendingvolcanic belt of Late Cenozoic age extending fromsouthern Death Valley, California, through theNevada Test Site region to central Nevada.

The basalts of Crater Flat are divided intothree distinct volcanic cycles (3.7, 1.1, and 0.3Myr) based upon geologic mapping, potassium-argon(K-Ar) dating, and magnetic polarity determina-tions. The cycles are characterized by eruption ofbasalt-magma of hawaiite composition that formedcinder cone clusters and associated lava flows.Total volume of erupted magma for respective cyclesis about 0.5 to 4.0 x 10 1 km'; volumes of indi-vidual cinder cone and lava flow centers are about0.3 to 1.5 km3.

The basalts of Crater Flat are sparsely tomoderately porphyritic; the major phenocryst phaseis olivine, with lesser amounts of plagioclase,clinopyroxene, and rare amphibole. Basalts of the3.7-Myr cycle contain glomeroporphyritic clots ofbytownite and augite typical of hawaiite basalts inthe southwestern United States. Major and trace-element differences between cycles, as well as thevariations within cycles (in particular the 11-Myrcycle), cannot be explained simply by crystal-liquid fractionation. However, the consistentrecurrence of evolved hawailte magmas in all threecycles points to- crystal fractionation from moreprimitive magmas at depth. A possible major

i

transition in mantle source regions through timemay be indicated by a transition from normal to Rb-depleted, Sr-enriched hawaiites in the youngerbasaltic cycles. The recurrence of small volumesof hawaiite magma at Crater Flat supports assump-tions required for probability modeling of futurevolcanic activity and provides a basis for esti-mating the effects of volcanic disruption of arepository site in the southwestern Nevada TestSite region. Preliminary data suggest that succes-sive basalt cycles at Crater Flat may be of de-creasing volume but recurring more frequently.

I. INTRO DUCTION

The Nevada Nuclear Waste Storage Investigations (NNWSI) are evaluating

the suitability of the Nevada Test Site (NTS) for location of a high-level

radioactive waste repository. Current geologic exploration studies within the

NTS are focused on Yucca Mountain (Dixon et al. 1980), a large fault block

composed of multiple sequences of ash-flow tuff erupted from the Timber

Mountain-Oasis Valley cauldron complex (Byers et al. 1976).

Yucca Mountain is located within the south-central Great Basin, a physio- )graphic subprovince of the larger Basin and Range Province, which includes

much of the western United States. The Great Basin is a tectonically active re-

gion. Its geologic history is characterized by extensional block faulting

that produced linear mountain ranges separated by broad alluvial basins (Nolan

1943; Stewart 1978; Christiansen and McKee 1978). This faulting was closely

associated in time and space with silicic volcanic activity at major cauldron

complexes. Silicic volcanic rocks as old as 40 to 45 Myr are present in the

central Great Basin. Younger volcanic rocks occur within broadly arcuate

belts that are successively younger to the south and toward the margins of theGreat Basin (Armstrong et al. 1969; Scott et al. 1971; Noble 1972; Stewart and

Carlson 1978). Silicic rocks of late Miocene age are most abundant within an

east-west-trending belt of the south-central Great Basin. This belt extends

from southeastern Nevada through the NTS region and may bend to the northwestalong the Walker Lane structural trend (Stewart and Carlson 1978). Since about

14 Myr ago, two major changes in the patterns of tectonic and volcanic activ-

ity have occurred. First, there has been a progressive concentration of tec-

tonic activity toward the margins of the Great Basin (Scholtz et al. 1971;

2

Christiansen et al. 1978). Second, silicic volcanic activity has been re-

placed by basaltic volcanism including minor amounts of bimodal basalt-

rhyolite volcanism (Christiansen and Lipman 1972; Christiansen and McKee1978). This basaltic activity occurs within distinct belts along the eastern

and western margins of the Great Basin (Stewart and Carlson 1978; Best and

Hamblin 1978) and within a less prominent northeast-trending belt in the south-

central Great Basin that extends through the NTS region (Fig. 1; Crowe and

Carr 1980).

Volcanic hazard studies, being conducted as a part of the NNWSI, are

attempting to assess the risk of disruption of a waste repository within the

NTS by future volcanic activity. Crowe and Sargent (1979) compared the geol-

ogy and geochemistry of the Silent Canyon and Black Mountain peralkaline vol-

canic centers, the latter representing the youngest major silicic volcanism

within the NTS region. They concluded that the Black Mountain cycle repre-

sents a renewed phase of silicic volcanism following the Timber Mountain-

Silent Canyon volcanic cycle. This suggests that there is a small but finite

possibility of recurrence of silicic volcanism within the NTS area. Crowe and

Carr (1980) provided a preliminary assessment of the risk of basaltic volcan-

ism within the southern Great Basin. They briefly described the Late Cenozoic

volcanic geology of the southwestern NTS region, calculated the probability of

disruption, and examined the disruption effects due to intrusion of a reposi-

tory by basaltic magma. In this report, a continuation of previous work, we

describe the detailed geology, geochronology, and petrology of the basalts of

Crater Flat. This basaltic field is located within and adjacent to Crater

Flat, an alluvial basin west and southwest of Yucca Mountain (Fig. 1). The

basalts of Crater Flat record three small volume magma pulses that are spa-

tially and temporally distinct (3.7, 1.1, and 0.3 Myr). Each pulse erupted

basalt that may be classified as hawaiite following the definition by Best and

Brimhall (1974).

II. GEOLOGY AND PETROGRAPHY OF THE BASALTS OF CRATER FLATCrater Flat contains over 15 small basaltic volcanic centers that consist

of cinder cones and associated lava flows. The distribution and tectonic set-

ting of the volcanic rocks has been described by Crowe and CUrr (1980). The

rocks are divided into three distinct cycles or magma pulses based on geologic

I I

- - - - - - ) {Min eral N~ ye * %

A;Lan oE .1,,*'*. og\ 8 '- -'- } - 3§ -

a u~~~~~~ 1~~380

a~~ ~ ~ **;. I~~~~ Mon 1X *:s:. 4 r. I

4P C~*

Inyo

\~~N .,S~~~~l -~~Clark

01aO 00 l 0

Kilometers

1180 1170 Ilse 115w 1140

Fig. 1.Distribution of Late Cenozoic basaltic volcanism in the south-central GreatBasin. Modified from Stewart and Carlson 1978, and unpublished work by W. J.Carr. Gravity symmetry axis is the line of bilateral symmetry of the observedBouguer gravity field of the Great Basin (after Eaton et al. 1978). LC, LunarCrater volcanic field; RR, Basaltic rocks of the Reveille Range; QC, Basalticrocks of the Quinn Canyon Range; BR, Basalt of Basalt Ridge; SC, Basalticrocks of the Silent Canyon cauldron; SB, Basaltic rocks of the Sleeping Buttecauldron; BB, Basalt of Buckboard Mesa; PR, Basaltic rocks of Paiute Ridge;NC, Basaltic rocks of Nye Canyon; CF, Basaltic rocks of Crater Flat; 114,Basalt of Dome Mountain; SM, Basalt of Skull Mountain and Kiwi Mesa; GM,Basaltic rocks of the Greenwater Mountains; DV, Basaltic rocks of southernDeath Valley; WL, Walker Lane; LV, Las Vegas shear zone; DV-FC, Death Valley-Furnace Creek fault.

4

N -

field relations, potassium-argon ages (Table I), and magnetic polarity deter-

minations (Fig. 2).

* 3.7-Myr cycle (Tb, Tbp) Rocks of the older cycle-consist

of deeply dissected cones and flows with locally exposed

feeder dikes. ' They crop out in the central and

southeastern part of Crater Flat (Fig. 2).

* 1.1-Myr cycle (Qb1, Qbp1): Basaltic rocks of this cycle

consist of cinder cones and lava flows located along a

northeast-trending structural arc near the center of

TABLE I

POTASSIUM-ARGON WHOLE-ROCK AGES OF BASALTS FROM CRATER FLATa

Group and Center

0.3-Myr basalt cycleLathrop Wells Cone

1.1-Myr basalt cycle

Sample No.

T.SY -1

TSV-128

.K2 0

1.818, 1.8241.820, 1.810

1.723,,1.7121.723, 1.727

Age (Myr)

0.29+0.2

1.14±0.3

Black Cone TSV-2".

TSV-2A

1.806, 1.7971.797, 1.799

1.632, 1.6341.637, 1.635

1.680, 1.6831.678, 1.680

-1.571.60

1.09±0.3

1.07±0.4

1.11±0.3

3.84+0.2

Little Cones

3.7-Myr basalt cycle

TSV-3

CF-72-24-8

. CF-79-26-1 1.771.76

3.64±0.1

aSamples from Lathrop Wells and western centers determined by R. J. Fleck,U.S. Geological Survey, Menlo Park, California; southeastern Crater Flat byR. F. Marvin, U.S. Geological Survef, Denver, Colorado.

Uncertainties represent the larger of either the value calculated usingthe expression of Cox and Dalrymple (1967) or the calculated standarddeviation of the reolicate analyses.

D

I .

Crater Flat. From northeast to southwest, the major

cones in this cycle include a northernmost cone (un- )named), Black, Red, and Little Cones.

* 0.3-Myr cycle (Qb2, Qbp2, Qbs): The youngest cycle is

marked by cones and flows of the Lathrop Wells center.

This center is located in the southeast corner of Fig. 2,

outside of Crater Flat.

III. 3.7-MYR BASALT CYCLE

The older basalts of Crater Flat crop out as deeply dissected cinder

cones with minor lava flows (central Crater Flat) and as moderately extensive

lava flows with no associated cone deposits (southeastern Crater Flat). The

cinder cone and flow deposits are significantly eroded. The cones retain no

evidence of original forms or slopes; cone scoria is preserved only where on-

lapped by lava flows or where dikes have ncreased the erosional resistance of

the deposits. Dikes exposed within the dissected cones trend north-south to

north-northeast parallel to basin-range faults within Yucca Mountain (Fig. 2).

This suggests the older basalts were erupted along preexisting basin-range

faults (Crowe and Carr 1980). The dikes are of variable but generally small )width (0.3 to 2 m) and are laterally discontinuous (Figs. 3 and 4). They

pinch and swell and branch and coalesce, with dips ranging from vertical to

less than 20° (Fig. 4). Locally the dikes are arcuate with inward dips (Fig.

4). The dikes are feeder intrusives that fed former surface eruptions and

underlie and intrude cinder cones. Lava flows of the older basalts (central

Crater Flat) thicken adjacent to the cone scoria indicating the cones were the

source of the flows. The flows are highly modified by erosion. They lack

flow fronts and primary flow topography. Lava surfaces are pediments with

areas of desert pavement and local thin soils. Two separate and more exten-

sive lava flow sequences crop out in southeastern Crater Flat (Fig. 2) and are

equally modified by erosion. They are locally offset by north-northeast-trend-

ing normal faults with displacements down to the northwest. There are no vent

or cone scoria deposits associated with either lava sequence. Aeromagnetic

data suggest the flows are not continuous in the subsurface. The flows were

probably erupted from separate vents that are now buried by alluvium.

The basalts of the older cycle are moderately porphyritic; total pheno-

cryst content ranges from 12 to greater than 20 modal percent and averages 13 )

I

Z

116g35' 116*30'

.:,A, ' * rjA -/ /-#t-,, ,-

CRATER FLAT / 3

BAKCONE

prE CONE IToo,g ' @"@i . %eb.. @1

'':1~ ~~ ~~~ U.'~' Z1

A a | g t rTL tI ' ..

CONES

" ' 2 ;) 36045'

1.SI.6444 *4mS 4

ge"em.. £e# Ii Bf*e. p

.04 Ut4.' *4sea .sI1~ 4 .. .'^.I..':";"P'' 444a .,ME. . , ,SIH. f

NEV AD it . A| '

* LArNRO WELLS2 ; i 4'QM(?(R5 NE

36*40'

Fig. 2.Generalized geologic map of the southern Crater Flat area (from Crowe et al.19'sC'.

I ,

EXPL ANA 71ONCORRELATION OF MAP UNITS

)

BASESURGE

I OUArERNARYQTo

I PLIOCENEBASALT: LAVA FLOWS PYROCLASTIC

DEPOSITSALLUVIUM

7;SLIDE BLOCK OF PALEOZOIC ROCKS

rUFF, LAVA, AND SEDIMENTARY ROCKS

TERTIARY

[ MIOCENE

SEDIMENTARY ROCKSI PALEOZOIC

)

- ~ CONTACT

.* .

FAULT, dotted where concealed;bar and ball on downthrownSide

AhL&A&&. Bse of slide block, dotted whereconcealed

Direction of loro flow

* Buried volcanic Center, locatedfrom aeromagnetic anomaly

Fig. 2. (Cont.)Generalized geologic map of the southern Crater Flat area (from Crowe et al.1980). )

a 'i

.w - *- I'll

9 PC[ C1I OI -1I doI

r- Pd-,,

~~~~'I

Pb

z v ,> K-Ar 20

/ CS- 7I~~

I t Pd |

1~~~~~0

/ /30P

Pe r '7

|s~~~~~'A :/I

'4 ' ~ ~Pc f N

I

5041

' Pb

Pb

'I

1

K-ArA

k Pb

Pcf

Geologic contact

Inferred cinder cone boundary

Flow foliation

Basalt dike

Reversed magnetic polarity

Dating locality

Pliocene basalt flow

Pliocene cone facies

0

Scale 1:12000I

.0.5-. Kilometeris1.0

Fig. 3.Geologic map of the northern outcrop area of the 3.7-Myr basalt of CraterFlat. Map compiled by direct transfer from uncorrected aerial photographs.

9

)

EM.j tN rv l - ' Geologic contact

"°/ -as '-off ho ,,,% Inferred cinder cone

t-Z>^s ' '° Aft boundary

5SI I'm - * s Basalt dike with dip60,>1;.> \ direction

j70 (ten t # 20 Strike and dip of cinderc %0c8] ) { cone bedding

3 o few Pb J Lava flow foliation

f' Magnetic Polarity stationasi1[ \> 2 \n R-reversed

~~,I~~~f20i ~~N-normal)

Pb Pliocene olivine basalt flow

7Pc Pliocene cone facies

CT PO

IS 'I

Scale 1:12 000Pb

a 0.5 1.0Kilometers

Fig. 4.Geologic map of the north-central part of the outcrop area of the 3.7-Myrbasalt of Crater Flat. Compiled by direct transfer from uncorrected aerial )Dhotographs.

to 14% (Table II). The older basalts can be distinguished from the basalts ofyounger cycles by their greater total phenocryst abundance and by the presence

of clinopyroxene, plagioclase, and olivine as coexisting phenocryst phases in

some samples. Olivine (Fo80 75) is the major phenocryst phase in all samples

of older basalt. Crystals are subhedral to euhedral, commonly embayed due to

resorption and partly to completely altered to iddingsite. Phenocrystic

plagioclase (An82 - 68 ), euhedral with moderate normal zoning, is present in the

majority of samples. Groundmass feldspars may be zoned from labradorite to

alkali feldspar. Rare isolated plagioclase phenocrysts are strongly resorbed

and cloudy due to the presence of abundant glass inclusions--these crystals

are probably xenocrystic. Clinopyroxene phenocrysts are present in about one-

half of the basalt samples studied. Glomeroporphyritic clots are abundant in

many samples and have a varied assemblage including olivine, olivine + plagio-

clase, olivine + plagioclase + clinopyroxene, and plagioclase + clinopyroxene,

the latter being most abundant. These clots do not occur in the younger ba-

salts of Crater Flat. As discussed elsewhere, glomeroporphyritic clots ap-

parently formed under hi-gher temperature or higher pH20 than the latter stage

phenocryst and groundmass assemblages. Except for rare samples showing evi-

dence of resorption, the phenocryst zonation trends and euhedral shapes argue

against a xenocrystic origin.

Groundmass phases within the older basalts include olivine (largely al-

tered to iddingsite), pyroxene, feldspar, iron-titanium oxides, and rare apa-

tite. Phlogopite, which is pleochroic from clear to pale red, occurs in the

groundmass of coarse-grained basalts and as a vein fill in dike rocks. These

occurrences suggest that phlogopite formed as a late-stage deuteric phase.

Textures of the older basalts are generally intergranular with interstitial

spaces between plagioclase laths filled by pyroxene, olivine; iron-titanium

oxides, and rare glass. Textures of basalt samples collected from the inter-

ior of thick lava flows (southeastern Crater Flat) are diabasic. Vesicle and

vein-fill phases in the older basalt are primarily calcite with minor amounts

of a pale brown, fibrous to amorphous zeolite(?).

K-Ar determinations of two samples of the older basalts yield an average

age of about 3.7 Myr (Table I). These dates are consistent with the reversed

magnetic polarity of the basalts and indicate a correspondence to the Gilbert

Reversed Magnetic Epoch (Mankinen and Dalrymple 1979).

. 9

I *k

)TABLE II

MODAL AND PETROGRAPHIC DATA, CRATER FLAT BASALTSa

1st cycle: 3.7 Myr

Sample

Phenocrystso lvelne cilnopyroxenefeldsparamphibole

F878-14 CF12-6-12 CF12-6-10 CF12-7-6 CFl2-7-1 F378-15 av

7.4 7.7- 3.26.7 9.3

S.84.42.7

3.3 6.1 8.27.5 - 1.55.1 1.5

6.42.84.2

Groundmassol IV nepyroxenefeldsparamphibolebiotiteilmenitemagnetitehematiteapatite

9.19.5

63.9

3.0

2.812. 062. 3

2.6

6.516.760.4

0.4

2.9

0.2

10.662.9

1.0

5.70.4

5.716.568.0-btr.

3.30.4

9.013.460. 1

0.12.5

5.513.162.9

0.20.13.3°-lbtr.

Glass orMesostasis

CYesicles)a

0.4 3.9 3.5 1.3

(5.2) (3.6))

(7.6) (1.4) (4.0) (1.4) (1.8)

PhenocrystSizes () 0.2-1.0 0.2-1.0 0.2-1.2 0.2-1.5 0.2-1.0 0.2-2.0

Grounduassplagloclasedimensions 0.lx0.01 0.lx0.03 0.2x0.05(average in mm)

0.lxO.01 0.lSXO.015 0.i5XO.015

Sampl e dike dike flow dike flow flow

-

aNote: Modalabundance.sample.

data for the first 14 entries inPercentages listed as vesicles-

each coluin are normalized to vesicle-freerepresent the volume of vesicles in each

bSymbol tr.- ndicates that mineral is present in trace amounts (less than O.1S); a dashindicates that mineral or phase is not present.

)

,'

I KL

TABLE 11 (Cont.)

MODAL AND PETROGRAPIIIC DATA, CRATER FLAT BASALTSa

2ndCycle: 1.1 yr

NorthernC~r2 -i t B 8- 10o

Bl ackf8?8-1 f078-4 FD78-5

Cf_______ Red Little .4.01 Little N.E. -CffI2-4- CI24-6 C12-141 FI2-4-12A FB78-9- CF12-4-13A CF12-4-13 av

Phenocrysts

cl inopyroxenefeldsparamphibole

Groundmassoilv Tnepyroxenefeldsparamph1boleb1otiteIlmenitemagnetitehematiteapatite

Glass orHesostasis

(Yesicles a

0.8 1.1 2.0 1.1 2.6 1.8 1.5 2.6 1.6 1.4 2.4 3.4 1.9

0.9 0.1_ 0.5

14.75.7

67.7

3.41.3

8.74.8

66.2

10.1 3.7 6.34.5 11.4 8.1

62.8 77.8 70.7

6.07.8

75.4

10.1 9.911.3 7.768.1 62.2

3.1 3.5

4.3 3.8 9.37.7 13.4 7.8

66.8 74.7 61.6

5.69.5

62.36t; .3

-r btr.8.6

7.78.3

68.0 btr.

tr.b

4.40.1

0.1 - 0.63.0 2.0 2.6 2.6 3.8 6.9 6.0 6.9

6.5 16.3 18.6 2.8 9.8 5.3 5.9 14.1

(4.6) (9.0)

12.8 0.7 11.7 9.8 9.5

(4.6) (5.4) (10.8) (29.8) (14.0) (20.2) (24.8) (16.2) (33.2) (44.1) (18.1)

PhenocrystSi zes (om) 0.2-1.0 0.2-1.2 0.2-0.5 0.2-1.0 0.2-0.75 0.2-2.0 0.2-1.0 0.2-1.0 0.2-0.5 0.2-0.5 0.2-0.5 0.2-0.5

Groundnassplagloclasedimensions 0.2x0.025 0.2x0.02 0.2x0.02 0.18x0.02 0.02 .02 0.2x0.03(average in n)

0.2xO.025 .2xO.02 0.08X0.00 0,O.005 0XO.01 0O.01

Sample flow flow flow dike flow flow flow bomb bomb flow bomb bomb

aNote: Modal data for the first 14 entries in each column are normalized to vesicle-freeabundance. Percentages listed as 'vesicles represent the volume of vesicles in eachsample.

bSymbol tr.' indicates that mineral is present in trace amounts (less than 0.11; a dashindicates that mineral or phase is not present.

7

TABLE II (Cont.) )

MODAL AND PETROGRAPHIC DATA, CRATER FLAT BASALTS

3rd Cycle: 0.3

Lathrop Wells

CF11-7-1 CF11-7-2 F378-7 av

Phenocrsts

cli nopyroxenefeldsparamphi bole

Groundmassol1vin epyroxenefel dsoaramphibolebiotiteilmenitemagneti tehematiteapati te

Glass orMesostasi s

(Vesicles)a

PhenocrystSizes (mm)

Groundnasspl agioclasedimensions 0

Samp e

2.6

6.94.1

68.8

2.2

15.5

3.0 2.9 .2.8

6.24.6

55.7

9.3 7.52.6 3.8

63.2 62.6

2.4 1.2 1.9

)28.1 20.8 21.5

C ) i: ) C )

0.2-1.5 0.2-0.5 0.2-1.25

0.15x0.01

fl ow

0.12x0.02 0.lxO;01

bomb flow

aNote: Modalnomal i zed toas vesicles'sample.

data for the first 14 entries in each column arevesicle-free abundance. Percentages listedrepresent the volume of vesicles in each

)

1 1

I ..

Magma volume calculations were determined for the older basalts based on

outcrop area, area of inferred subsurface outcrop from aeromagnetic data, and

measured thicknesses. Lava volumes were converted to magma volumes assuming a

magma density of 2.7 g/cm 3; pyroclastic volumes were calculated using a cone

porosity of 25% and assuming that 400 of the tephra was deposited over one

cone diameter from the vent. Bomb density values were taken from McGetchin et

al. (1974). The total volume of magma erupted during the older basalt cycle

is about 4.0 x 1O-1 km3.

IV. 1.1-MYR BASALT CYCLE

Cinder cones and associated lava flows dated at 1.1 Myr define a

northeast-trending arc within the central part of Crater Flat (Figs. 2 and 5).

This basalt cycle may follow one of a system of faults of northeast trend with-

in the Walker Lane fault system in the southern Great Basin (Carr 1974; Crowe

and Carr 1980).

Little Cones, the southwesternmost center of the arc, consists of two

separate cinder cones. The southwestern cone (base diam 0.3 km) is deeply

rilled with approximately 200 of the cone removed by erosion. The cone is

breached on the south side by the vent of a small lava flow. This flow is

largely concealed by colluvium with local outcrops; original flow margins can

be inferred from slope changes in the colluvial surface. Aeromagnetic data

suggest the presence of an older flow, now buried. This flow appears to have

extended in the same direction as the younger flow, but about 1 km farther.

The second cone of the Little Cones (Fig. 2) has an inferred base diameter of

0.2 km and is equally modified by erosion. Based upon outcrops and aeromag-

netic data, this cone appears to lack associated lava flows.

Red Cone and Black Cone, the middle cones in the northeast-trending arc,

are very similar in field occurrence, mineralogy, and petrology; Red Cone will

be described in some detail. The oldest deposits of Red Cone consist of small

coalesced cinder cones (base diameter <0.2 km) that occur southeast and south

of the main cone (Fig. 6). These cones are deeply eroded and onlapped by aa

flows from Red Cone. Red Cone itself, the largest cone of the center, is a

typical Strombolian cinder cone. It has a base diameter of 0.5 km and origin-

ally rose about 80 m above the alluvial surface. Approximately 20% of the

cone has been removed by erosion. Two small dikes, which are probable off-

shoots of the main conduit, are exposed in the western cone wall. The summit

IA

I

)

Fig. 5.1.1-Myr basalt cycle viewed from the southwest. Volcanic cones include fromright to left, Little Cone, Red cone, and Black Cone. The northernmost coneis not visible in the photograph.

crater of Red Cone was infilled by inward-dipping spatter (Fig. 6) with bombs

exceeding 2 m in length. This spatter draped the vent of Red Cone during the

waning stages of activity when the ejection velocity of bombs was insufficient

to crest the walls of the summit crater. During growth of Red Cone, aa flows

extruded from southeastern and possibly southwestern flank vents. These stub-

by flows partly surrounded and onlapped the older cinder cones and extended

slightly more than 1 km from the vent. Steep lava flow fronts are preserved,

althouch primary surface flow toooaraphy is completely modified by erosion.

16

a---

a- -O

I- I

Aid i 05e

C. \~~~~~~~~~~)O

- Geologic Contact

_ Inferred cinder coneboundary

- Basalt dike

20 Strike and dip ofcinder cone bedding

isp Lava flow foliation

Magnetic polarity stationR-ReversedN-Normal

20

\4

"I.

1 /

Lava flow source andflow direction

Oc Quaternary scoria colluvit

Ocf Quaternary cone facies

ats Quaternary vent spatter

t Quaternary basalt

- __.

.I I~~~~~~I -#I .

Scale 1:12000

I. II6 -

-' Kilometefs1.00.5

Fig. 6.Geologic map of the Red Cone volcanic center. Compiled by direct transfer from uncorrected aerial photo-graphs.

P

- 7- - -

Fig. 7.Black Cone volcanic center. Note the dissection of the cinder cone, cappinglava fill sequence, preservation of original lava flow fronts, as well as modi-fication of the original lava flow surface.

The Black Cone center includes several coalesced cinder cones located

directly south of Black Cone (Fig. 7). Aa flows vented from the north-

northeast and southeast sides of Black Cone (Fig. 2). Black Cone is capped by

inward-dipping flows that ponded within the summit vent of the cinder cone.The northernmost center of the 1.1-Myr basalt cycle is more deeply in-

cised than other centers of the arc. Margins and flow tops of lava outcrops

are completely modified by erosion. Local scoria deposits in the northern

part of the center are probably remnants of the original cinder cone. All

primary cone features have been destroyed and the deposits are lower

i0

I

topographicaliy than~ the lavas. The greater dissection o the northernmost

canter probably is due to both higher elevation and a steeper drainage gradi-

ent within this part of the Crater Flat basin.

The basalts of the intermediate age cycle are aphyric to sparsely porphy-

ritic (less than 34 total phenocrysts, Table II). Olivine is the only major

phenocryst phase and occurs as subhedral to euhedral crystals that are fresh

or exhibit minor alteration to iddingsite at grain margins and along frac-

tures. Olivine compositions range from Fo77 to F 62 and are more iron rich

than olivines from either the older or younger cycles. Basaltic hornblende

(high-Ti amphibole) occurs as phenocrysts in one cone from the Little Cone

center and in the groundmass of basalt samples from Red Cone. It is markedly

pleochroic (shades of red-brown) and is fringed by reaction rims composed of

granular intergrowths of plagioclase, pyroxene, and iron-titanium oxides.

Plagioclase (from An71 to more alkaline compositions) is the major groundmassphase, present as microlites and as larger crystals that approach micropheno-

dryst size. Additional groundmass phases are olivine, pyroxene, ron-titanium

oxides, and variable amounts of deep brown glass. A two-pyroxene groundmass

assemblage (high-Ca and low-Ca pyroxene) occurs in Black Cone and the northern-

most cone. Textures of the lavas are mostly intergranular, though some have

pilotaxitic textures. Samples containing appreciable amounts of groundmass

glass show intersertal or hyalopilitic textures. The basalts are largely un-

altered; clays are present as vesicle filling along with small amounts of cal-

cite.

K-Ar ages for Little Cones, Black Cone, and northernmost cone centers are

all about 1.1 Myr (Table I). Magnetic polarity determinations for all the

centers are reversed, in agreement with the K-Ar ages; the basalts of the 1.1-

Myr cycle thus belong to the Matuyama Reversed Magnetic Epoch. Calculated

magma volume for the basalt cycle is 3.0 x 10 1 km3.

V. 0.3-MYR BASALT CYCLE

The 0.3-Myr basalt cycle of Crater Flat includes the Lathrop Wells center

located about 5 km southeast of the southeastern edge of Crater Flat (Fig. 2).

Here, a large cinder cone with two small satellitic cones overlie and are

flanked to the east by aa flows (Fig. 8). The satellitic cones are overlapped

by deposits of the main cone. The large cone, referred to as the Lathrop

Wells cone, has a height/width ratio of 0.23. The summit crater and the cone

19

I I

)

4A '� .�.A .�

1% �

o 0

3

a.25I-

I

I.li i

j 1%r I

'I

,- N'"- %

f Lj

toI. f

I

.U

8

I-

'U

4.'

a,

u

0u

E0

L

4-

L.

a,

'4-

u

U,

'U

4.)

-

CL

.5-

-

a,4C

u

u

U

0

-

-C

_4.'

CJs

- C

)

i * '':

e -I- % ac ,., - - '_ I, , f../ 0

, , .'-, _ v

AE-l

)

I ;I

- - - --- - tWw-

- ~~ f * es:

Fig. 9.Well-bedded base-surge deposits exposed in the -northwestern part of theLathrop Wells Cone.

are elongate to the northwest, probably due to prevailing winds from the south-

east. The cone -appears unmodified by erosion except for minor slumping of

steep cone slopes. The probable oldest deposits of the Lathrop Wells cone are

well-bedded base-surge deposits (Fig. 9) that are exposed only on the north-

west side of the cone where they overlap a topographic ridge upheld by tuff.

They probably underlie the scoria deposits of the cone and thus record an epi-

sode of phreatomagmatic activity during the early eruptive stages of the cen-

ter. Two aa flows vented at several sites along the east flank of the Lathrop

Wells cone (Fig. 8). Flow vents are marked by arcuate spatter ridges

I .i

extending east and southeast of the cone. The lavas have unmodified flow mar-

gins and rubbly flow surfaces consistent with their young age. They are local- )ly covered by aeolian sands.

The basalts of the Lathrop Wells center are sparsely porphyritic with

olivine as the major phenocryst phase (3 modal percent). In thin section the

basalts can be distinguished from the intermediate cycle basalts by slightly

greater olivine contents and a greater abundance of deep brown interstitial

glass. Moreover the olivine phenocrysts have slightly more magnesium-rich

cores (Fo80_77) than olivines of the 1.1-Myr cycle (Fo77-76). Groundmass

phases in the basalts include plagioclase (zoned from An68 to more alkaline

compositions) and minor amounts of olivine, pyroxene, and iron-titanium oxides

plus interstitial glass. Basalt textures are hyalopilitic to pilotaxitic and

reflect the high content of groundmass glass.

Lavas of the Lathrop Wells center have been dated at about 0.3 Myr, con-

sistent with the lack of erosional modification of both cones and flows. The

basalts are normally magnetized and thus assigned to the Brunhes Normal Mag-

netic Epoch. Calculated magma volume is about 0.5 x 10 km3

VI. MINERAL CHEMISTRY )Mineral compositions of basalts from Crater Flat were determined by elec-

tron microprobe. An automated Cameca electron microprobe was used, with ac-

celerating potential fixed at 15 kv and sample current at 0.015 A on thorium.

Counts were collected for a maximum of either 20 s or 30 000 counts for each

element. Complete tables of representative mineral compositions are included

in the Appendix (Tables A-I to A-V). Brief descriptions of the mineral data

and applications to petrology follow.

A. OlivineData on olivine for the basalts of Crater Flat are summarized in Fig. 10.

This figure indicates continuous zonation of most olivine phenocrysts; the

zoning is normal, without notable reversals. Phenocryst olivine is more Mg-

rich than groundmass olivine; the apparent overlap of Red Cone phenocryst and

groundmass compositions in Fig. 10 is not real because two samples with dif-

ferent zonation ranges have been superimposed in one diagram. Olivine pheno-

crysts in fact may have formed with rims as Fe-rich as the groundmass oli-

vines; if so, the Fe-rich rims have been totally altered.to iddingsite.

)

22

I ,

N

The distribution of re and M between olivine and basalt c melt may pro-

vide evidence for or against equilibrium crystallization; i equilibrium

crystallization has occurred and the composition of early crystallized oli-

vines can be determined, the olivine-melt system can be used to estimate the

highest (or first) temperature of olivine appearance. The Mg:Fe composition

of the basalt may be assumed to approximate the initial liquid composition if

no other Fe-, Mg-rich phases preceded olivine in the crystallization sequence.

Experimental studies (Knutson and Green 1975) indicate that hawaiites similar

to the basalts of Crater Flat are cosaturated with olivine and clinopyroxene ±

plagioclase ± amphibole. Because olivine forms early, the natural

olivine/rock compositions can be compared using experimentally calibrated

olivine/liquid compositions (Shibata et al. 1979).

Experimental studies (Roedder and Emslie 1970; Longhi et al. 1978) docu-ment an exchange distribution coefficient (KD) that is constant at 0.30 to0.33 for basalts similar in composition to the basalts of Crater Flat. Theappropriate KD curves are drawn in Fig. 11, with plotted points representing

3.7-Myr Basalts

I I * .-m I I I -Fa P .

I I . , , . _

Fig. 10.Ca-Mg-Fe compositions of pyroxene and amphibole, andlivine and biotite in basalts of Crater Flat.

Mg-Fe compsition of

7.,

I *'

I

)

Mg

F0.

-

Rfld0miL I . .

ru-- U-- I I

)

Mg

FI i | & x n X i l i i i Fa

Fig.Ca-Mg-Fe compositions of pyroxene andvine and biotite in asalts of Crater

lO.(Cont.)amphibole, and Mg-Fe composition of oli-Flat .

)

2

CdMg

Ma V

I\ A AX A\ /E CFS1.1-Myr Basalts

go Lttl Cone S.W.

* Groundmass pyroxent

0 Phenocryst gnvine

V V V V V V V ^Fe

Fal I fI l;

CaM

Mg V V

iII I I I's

, ~ _. ,*At . . . r.rol I I .- .[ - I _ III ra

Fig. 1O.(Cont.)Ca-Mg-Fe compositions of pyroxene and amphibole, and Mg-Fe composition of oli-vine and biotite in basalts of Crater Flat.

£5

I 'I

/ y

1.1-Nlyr 3asaltu/ \ ~~~~~~~~~~Red Cne

/* Groundmass Pyroxen.

* &oundmassi mphlbole

Pnocryst* Groundmoss

v V V V V V V V _ Ft

!

mg .

Rim ffi I & 11ri 9 a ., i i i i - iFo

)

4dli4Fo, - I I I IF

Fig. 10.(Cont.)Ca-Ilg-Fe compositions of pyroxene and amphibole, and Mg-Fe composition of oli-vine and biotite in basalts of Crater Flat. )

I I

-0.8

C_

5 0.4-

0 NoK c- 0.2 N

u. .

0~~~~~~~~~

0.2 0.4 0.6 0.8 1.0

(\x Fe \~) liquidFXe+ M8

Fig. 11.Comparisons of cation fraction Fel (Fe 2 + Mg) in olivines and liquids(inferred from rock compositions) of the basalts at Crater Flat. Symbols rep-resent (1) the 3.7-Myr basalts, (2) the 1.1-Myr basalts, both nepheline andhypersthene normative, and (3) the 0.3-Myr basalts. Arcuate lines enclose thecompositional range where olivine and liquid may be in equilibrium; olivineresorption or accumulation will result in points plotting above the arcuatelines, whereas olivine loss will result in points plotting below the arcuatelines. -The composition of Fe' in the original liquid is assumed to be 0.9 xtotal Fe, following the arguments of Shibata et al. (1979) for maintaining aliquid composition near fayalite-magnetite-quartz stability.

,

I 01

I

olivine/rock pairs from Crater Flat samples. Considerable effort was under-

taken to find the most Mg-rich olivine phenocryst cores in the basalt samples

in order to obtain an analysis approaching the composition of the first oliv-

ine that crystallized from the basalt.

If the first-formed olivine reacts with the melt, it will become more Fe

rich and project upward above the KD = 0.30 to 0.33 envelope in Fig. 11. Adisplacement to the left of the envelope will occur if olivine has accumulated

in the sample, resulting in an increase of the apparent Mg content of the host

liquid. From Fig. 11, it is apparent that olivine phenocryst cores in several

of the 3.7-Myr basalts are more Fe rich than permitted by an equilibrium

model; plotted points are displaced upward, suggesting re-equilibration of the

initial Mg-olivine cores to a more Fe-rich olivine. This interpretation is

supported by the coarse grain size and the scarcity of glass or mesostasis in

the 3.7-Myr basalts, features in accord with slow cooling that would permit re-

equilibration of olivine with subliquidus Fe-enriched liquids.

It is possible to estimate the liquidus temperature (first olivine preci-

pitation) using relations developed by Roedder and Emslie (1970) and expanded

by Longhi et al. (1978) and Leeman (1978) using samples that maintain the equi-librium olivine/melt relation (Fig. 11). These temperature estimates are less

accurate in samples with high Na and K contents, and although the hawaiites of

Crater Flat are not alkali rich, they contain enough Na and K to yield anoma-

lously low temperature estimates. Leeman (1978) has provided some guidelines

for evaluating temperature estimates in alkaline basalts; using his Fig. 5 and

K, relationships, we estimate the temperature of olivine appearance at 12000C

in the basalts of Crater Flat. This estimate is crude because of the moderate

alkali content of Crater Flat basalts and carries a large uncertainty of ±750C.

B. Feldspar

With the exception of feldspar phenocrysts in the 3.7-Myr basalts, the

most Ca-rich plagioclase cores are high-Ca labradorite (An70) in the basalts

of Crater Flat. Plagioclase zonation is limited in some samples; in other

samples disequilibrium zonation may extend in shallowly bowed paths across the

feldspar solvus to a Ca-free alkali feldspar composition (Fig. 12). Note that

the cores of plagioclase phenocrysts in the 3.7-Myr basalts of Crater Flat are

significantly Ca enriched (An80; Fig. 12 and Table A-II). The Ca-enriched

phenocrysts of the 3.7-Myr basalts have important implications for the petro-

genesis of basalts of this cycle. )

28

Analogous to the formation of the most Mg-rich olivine in the initial

stages of a crystallizing melt, Ca-rich plagioclase cores represent the first

plagioclase to form. Drake (1976) described several empirical relations that

relate plagioclase/melt compositions to crystallization temperature. These

relations are reliable for a broad range of basalt types, provided the vapor

pressure of water (pH20) is not high during crystallization. Experimental

data (Knutson et al. 1975) indicate that plagioclase is a near-liquidus phase

for ow-H 20 hawaiite compositions, and the coupled Al-Ca substitution in plag-

ioclase resists re-equilibration at subliquidus temperatures even in slowly

cooled basalt samples. Using plagioclase/rock relations fitted to the plagio-

clase/melt relations of Drake (1976), temperature estimates for the basalts at

Crater Flat are listed in Table III.

The average plagioclase/rock temperature estimate for the basalts of

Crater Flat is 1208 ± 160C. Within the error for this method (80'C), this

3.7-Myr Bash

* Phamoyg. & _oumdm

Fig. 12.Or-Ab-An compositions of feldspar in basalts of Crater Flat.

29

I I

1.14Myr BsafltsOrclrwo nas Fewsa;u

of

)NOrU Con*

Ab

Rod Con*

a ae O VM, L\~~~~~a ~A

)

A

Lb

L"U Ccn. N. L

Fig. 12.(Cont.)Or-Ab-An compositions of feldspar in basalts of Crater Flat. )

30

I 1

03-Myr BasaftsGroundmass Foldspen.

Lathrop won$ Cn*

Fig. 12. (Cont.)Or-Ab-An compositions of feldspar in basalts of Crater Flat.

TABLE III

TEMPERATURE ESTIMATESa FRO4 PLAGCLASE/ROCK CPOSITIONS IN THEBASALTS CF CRATER FLAT

3.7-Myr Basalts

CF12-6-1

GroundmassPlagioclase

1218"C

Relic orPhenocrystPlagloclase

1374 C

1.1-Myr Basalts

FB78-10 (Northern Cone)FB78-5 (Black Cone)CF12-4-11 (Red Cone)CF12-4-12A (Little Cone S.W.)CF12-4-13A (Little Cone N.E.)

0.3-Myr asalts

CF11-7-1 (Lathrop Wells)

1218-C1187 C1201 C11859C1208%C

1225C

1285 C

1Uncertainties in lagloclase/rock temperature estimates are +80C.

31

I TI

estimate is in close accord with the olivine/rock temperature estimate of

1200"C. The similarity in calculated temperatures is expected from experi-

mental and petrographic evidence for multiple phase cosaturation in hawaiites.

The 3.7-Myr basalts of Crater Flat yield phenocryst plagioclase/rock

temperature estimates that are significantly higher than the plagioclase/rock

temperature estimates obtained from groundmass plagioclase. These high temper-

ature estimates for the older basalts are based on phenocryst plagioclase

grains with very Ca-rich cores (An75-83, Fig. 12). Drake (1976) noted that

basalts that crystallized under appreciable pH20 generated anomalously Ca-rich

plagioclase, although this effect was reversed at pH20 > 10Kb. Although the

pH20 effect has not been quantified for anomalously Ca-rich plagioclase, the

data compiled by Drake (1976) indicate that Ca-rich plagioclase phenocrysts,

as found in the 3.7-Myr basalts of Crater Flat, may occur by crystallization

at pressure (that is, at moderate depth) before eruption. The same effect may

also lead to the anomalously high temperature estimates from very rare relic

plagioclase in the basalts of Red Cone (Table III).

C. Pyroxene

Pyroxene end-member compositions in the basalts of Crater Flat are sum-

marized in Fig. 10 for the components CaSiO3 - MgSiO3 - FeSiO3. Representa-tive pyroxene analyses are listed in Table A-III. The basaltic pyroxenes are

diopsidic augites with a narrow range of compositions. Pyroxene minor-element

contents are very low; Al+Ti+Na+Mn+Cr contents are less than 0.35 and commonly

below 0.2 on a six-oxygen mineral formula basis. The groundmass pyroxenes of

all basaltic cycles are broadly similar and less Mg rich than the clinopy-

roxene phenocrysts that occur in basalts of the 3.7-Myr cycle (Fig. 10). The

Mg-rich pyroxene phenocrysts in the older basalts represent early crystalliza-

tion, coinciding with the development of olivine and feldspar phenocrysts at

depth before eruption. Coprecipitation of clinopyroxene, olivine, and plagio-

clase phenocrysts in the 3.7-Myr basalts of Crater Flat is indicated by the

occurrence of all three phases in some glomeroporphyritic clots. Other compo-

sitional features also point to the earlier growth of Mg-rich clinopyroxene

phenocrysts: the phenocrysts have a distinctly higher Al/Ti ratio (6.5 vs

3.5, Table A-III), and the phenocrysts commonly contain Cr203 in amounts above

microprobe detection limits (0-0.1 wt.%), whereas groundmass pyroxenes seldom

contain detectable Cr.

32

I .

The groundmass- minerals of dike and lava flow samples from the two

northernmost cones of the 1.1-Myr cycle (Fig. 2) include both low-Ca and high-

Ca pyroxenes (Fig. 10). These cones consist of hypersthene-normative basalts

in which low-Ca pyroxene and olivine both formed during late crystallization.

If temperatures of groundmass crystallization are inferred using the two-

pyroxene geothemometer of Wells (1977), the results are anomalously high

(2100C). These unreasonably high temperature estimates indicate that the

two-pyroxene association at the northernmost cone and at Black Cone is not an

equilibrium pyroxene intergrowth.

D. Oxide Mineralogy

The common primary oxide mineral in Crater Flat basalts is magnetite,

with much smaller amounts of primary Ilmenite (Table A-IV). Scarcity of pri-

mary ilmenite can be related to the alkaline nature of Crater Flat basalts, in

which Ti is incorporated into aluminous pyroxene (R2+TiAl206) rather than

ilmenite (FeTiO3) to liberate the silicon required by formation of alkaline

phases. Magnetite and ilmenite both occur in the older and intermediate age

basalts, although the oxidation and exsolution of magnetite to ilmenite, hema-

tite, and pseudobrookite obscure the primary magnetite compositions and pre-

vents useful application of magnetite-ilmenite geothermometry.

Microprobe studies show that the groundmass Fe-Ti oxides in the basalts

of Crater Flat are low in minor elements such as Cr,. Al, and low in Mg/(Mg

Fe). The spinels enclosed in and therefore coprecipitating with olivine

phenocrysts are considerably more Mg-, Al-, and Cr-rich than spinels found in

the basalt groundmasses (Table A-IV). Early coprecipitatfon of olivine and

Mg-, Cr-, Al-rich spinel is consistent with the evidence from clinopyroxene-

olivine-feldspar glomeroporphyritic clots suggesting that saturation in most

major phases began early in the crystallization histories of the basalts at

Crater Flat.

E. Amphiboles and Biotites

Biotite grains occur in the groundmass of some samples of the older

basalt cycle. These grains are Mg-rich phlogopites (Fig. 10), which formed

late in the crystallization history of the host basalts. The occurrence of

biotite in the 3.7-Myr basalts can be attributed to protracted late-stage

crystallization under relatively hydrous conditions.

Amphibole grains occur both as phenocrysts and as groundmass minerals in

basalts of the 1.1-Myr cycle (Fig. 10). There is little difference in

33

I .I

i .

composition between phenocryst or groundmass amphibole; both are high-Ti ba-

saltic hornblendes. The occurrence of amphibole is restricted to basalts of

Red Cone and Little Cone that are located at the southwest end of the 1.1-Myr

basaltic arc. The occurrence of amphibole as phenocrysts indicates that the

host magmas were not dry and may have been relatively water rich at the time

of eruption. The inference of high volatile (water) content in these samples

is supported by the very high vesicle content (33 to 44%; Table II) of basalts

with amphibole phenocrysts. Representative amphibole analyses are included in

Table A-V. The importance of amphibole in the basalt fractionation histories

is discussed in the following section.

VII. MAJOR-ELEMENT CHEMISTRY OF THE BASALTS AT CRATER FLATWhole-rock major-element analyses (Si, T, Al, Fe, Mg, Ca, Na, K, and P)

were obtained by electron microprobe analysis of glass beads fused from whole-

rock powders on an Ir-strip resistance furnace. Details of this technique and

analytical uncertainty are given by Baldridge (1979).

The basalts of Crater Flat all fall within the classification of hawaiite

as used by Best and Brimhall (1974) for this abundant basalt type in the

western Colorado Plateau. The principal features of this basalt type are (1)

normative plagioclase An content between 40% and 52% and (2) transitional alka-

line affinities, with compositions generally near the normative nepheline/

hypersthene divide. The major-element chemistry and calculated cation norms

for basalts from Crater Flat are listed in Table IV. The textural and mineral-

ogical descriptions of Colorado Plateau hawaiites by Best and Brimhall (1974)

are very similar to the descriptions of the basalts of Crater Flat outlined

above, including the common occurrence of diopsidic augite + bytownite glomero-

porphyritic clots as in the 3.7-Myr basalt cycle at Crater Flat.

An important characteristic of hawaiite basalts is their transitional

alkaline composition. MacDonald and Katsura (1964) defined a generally ac-

cepted division between tholeiitic and alkaline basalts based on a plot of

total alkalis versus silica content (Fig. 13); hawaiites such as the basalts

of Crater Flat project slightly above the MacDonald-Katsura line. A recent

revision of alkaline and tholeiitic lineages by Chayes (1979) shifted the

dividing line upwards, leaving an undefined zone within which most hawaiites

plot. This transitional alkaline characteristic is an important feature of

Miyashiro's (1978) classification for Straddle-A type alkaline basalts. The

34

I It

TABLE V

BASALTS OF CRATER FLAT: MAJOR-ELEMENT CHEMISTRY AND CALCULATED CATION NORMS

3.7-Hbr Cycle 1.1-Ntr Cycle

Calc.Parent

''1°2 48.41102 1.55A) 03 13.3

FtO 11.0Milo 0.21ill° 10.4

(:dO 10.911a 0 2.42K20 1.23

P205 0.55100.0

CF12-6-10

49.4

1.60

15.5

10.8

0.23

7.1

9.8

3.00

1.58

0.62

99.6

CF CF12-6-12 12-7-8

48.5

1.62

15.2

11.5

0.21

8.69.22.93

1.37

0.68

99.8

49.3

1.64

15.7

11.3

0.23

7.0

9.1

3.05

1.49

0.71

99.5

Fe78-15

48.11.77

:15.211.8

0.217.8

10.2

2.711.50

0.71100.0

NorthernCone

Fa FB78-17 78-10

48.8

1.70

15.6

11.2

0.24

7.0

9.2

3.08

1.53

0.74

99.1

49.9

1.41

17.0

9.9

0.21

5.0

9.0

3.39

1.57

1.13

98.5

81 ackConeFB

78-5

51.0

1.45

17.2

9.9

0.19

5.2

8.7

3.42

1.60

1.09

99.8

RedConeCF

12-4-11

50.9

1.19

17.2

9.8

0.23

5.2

8.8

3.36

1.69

1.20

99.6

LittleCone S.W.

CF12-4-12A

47.8

2.05

16.0

10.8

0.21

4.8

10.7

3.82

1.91

1.47

99.6

LittleCone N.E.

CF12-4-13A

48.7

2.3116.311.50.145.18.63.792.071.21

99.7

LittleCone N.E.

CF12-4-138

47.4

2.24

15.8

11.5

0.17

5.1

9.6

3.71

2.03

1.24

98.8

48.5

1.82

16.6

10.8

0.20

5.9

8.8

3.501.76

1.14

99.0

48.41.76

16.410.7

0.19

5.89.13.701.771.40

99.2

0.3-Myr Cycle

Lathrop el1sCF CF

11-7-1 11-7-2

Qza - -

Or

P1

(An)

tie

1(I

'it

7.2 9.4

42.8 51.3

(50.9) (47.2)

0.0 -

23.1 16.4

0.0- 1.1

22.2 17.0

1.3 1.3

8.1

50.4

(47.8)

13.6

1.3

21.7

1.3

2.2

1.4

8.9

52.4

(47.4)

12.8

4.9

16.0

1.3

2.3

1.5

8.9

48.6

(51.3)

0.4

16.8

20.0

1.4

2.5

1.5

9.1

52.5

(46.8)

13.2

2.1

17.8

1.3

2.4

1.6

9.4

58.1(46.6)

8.5

9.3

9.1

1.2

2.0

9.5 '57.8

(46.7) (1^

7.413.0 16.9

1.22.0

10.0 11.4

57.4 44.517.0) (47.5)- 6.7

7.2 18.112.9 -

7.2 12.0

1.2 1.31.7 2.92.5 3.1

12.351.2

(41.8)

2.710.8

15.81.43.22.6

12.245.0

(46.2)'5.8

15.7

14.21.43.22.6

10.5 10.5:55.2 53.1

(43.5) (43.5)

0.7 2.1

9.5 10.4

17.8 17.3

11. 2.2 2.2

Ap 1.2 1.3

1.3

2.6

2.4

1.2

2.5

3.02.4 2.3

aNote: cation norms are calculated assuming atomic ratio Fe2 /Fe 3 + * 9/1. Row (An) represents the cation percent anorthtte in plagioclase (1't.).All analyses obtained by electron microprobe analysis of fused rock powder. The derivation of the calculated parent composition (Mg - 0.65) sd Iscussed on the following page.

primary requirements of this classification are that the less evolved members

of a volcanic sequence straddle the dividing line between normative nepheline

and hypersthene fields, whereas the more evolved members of the sequence in-

clude two distinct fractionation trends, one into the nepheline field and

another into the hypersthene field. The atomic ratio of Mg/(Mg+Fe), or Mg',

provides a reliable measure of how evolved a sample is. Figure 14 is a plot

of Mg' vs nepheline (Ne) or hypersthene (Hy) normative composition for thebasalts of Crater Flat. The diverging trends in Fig. 14 are characteristic

of Miyashiro's Straddle-A classification. A low Mg' ratio near 0.5 (Fig. 14)

is also characteristic of hawaiite basalts (Knutson and Green 1975; Green et

al. 1974). This feature is perhaps the most important of all hawaiite charac-

teristics, for the low Mg' ratio requires that all hawaiites be derived from

more primitive basalts. Where hawaiites are erupted along with their probable

parental precursors, the parental magmas are distinctly alkaline (Green et al.

1974). Where, as at Crater Flat, the parental magmas do not occur at the sur-

face, an alkaline parentage can only be inferred.A parental composition, with Mg' = 0.65, has been calculated from the

least evolved basalts at Crater Flat: those of the 3.7-Myr cycle that plot

near the straddling nepheline-hypersthene divide in Fig. 14, with Mg' compo-sitions of 0.57. Inspection of bivariate oxide plots has shown that olivine,

amphibole, and clinopyroxene are phases that might be removed from an Mg' =

0.65 basalt to arrive at the Mg' = 0.57 Crater Flat composition. By trial-

and-error modeling of olivine, clinopyroxene, and amphibole fractionation, we

have found that clinopyroxene and/or olivine addition to the Mg' = 0.57 compo-

sition lead t calculated parental compositions that also plot near the strad-

dling position. These calculated parental magmas are represented by the

question mark at Mg' = 0.65 in Fig. 14. Amphibole addition results in paren-

tal compositions that plot well within the nepheline field, which is not

expected for the least-fractionated members of a Straddle-A association. It

would be highly unlikely for the least evolved magmas to arise far from the

straddling position, fractionate towards that position, and then diverge from

the straddling position in two opposed directions. These calculations suggest

that olivine or clinopyroxene may be involved in the evolution of the 3.7-Myr

Crater Flat basalts from parental magma. The parental composition that is

listed in Table IV requires fractionation of 5 olivine plus 12% clinopyroxene

to lead to the Mg' = 0.57 basalt of the 3.7-Myr cycle. With this

4Ca.

S10,

Fig. 13.of Na2O + K20 vs SiO2 (wt%) in the basalts of Crater Flat. Symbols(1) the 3.7-Myr basalts, (2) the 1.1-Myr basalts,,and (3) the 0.3-Myr

Variationrepresentbasalts.

H4 E

12 -I

Ny

&UU4

Noi4

Fig. 14.A plot of cation normative hyersthene or nepheline content vs atomic ratio ofMg to Mg + Fe2+ in the basalts of Crater Flat. Fe2+, symbolized by "Fe*, isstandardized as 90" of total atomic Fe analyzed as FeO. Weight ratios of La/Smand wt% TiO 2 are shown for various normative fields. Divergence into bothhypersthene- and nepheline-normative fields from less-evolved "straddling"compositions Is characteristic of Miyashiro's (1978) straddle-A-type alkalinebasalt clan. The arrow labelled "kaersutite removal" is discussed in the textand reproduced in Fig. 15.

37

I :,

fractionation scheme, the calculated parental magma has an A1203/CaO ratio of1.2, a ratio that we have chosen as realistic for the parental magma (for sup-port of this argument, see Frey et al. 1978).

No attempt has been made to calculate parental compositions for theyounger basalts of Crater Flat, those of the 1.1- and 0.3-Myr cycles, becausethose basalts were certainly derived by more complex fractionation histories.To a certain extent these complexities can be seen in Fig. 14, particularly in

the origin of the hypersthene-normative 1.1-Myr basalts. The solid arrow inFig. 14 shows a trend of amphibole fractionation; the evidence for amphibolefractionation includes the steady increase of La/Sm along this trend (a pointdiscussed below) and a steady decrease of TiO2 along this trend. No otherearly-crystallizing phases except amphibole could account for these TiO 2 andLa/Sm trends in a hawaiite composition. The evidence of Ti variation is par-ticularly telling, for no other phase but amphibole is likely to remove ap-preciable amounts of Ti from the fractionating melt.

The evolved Ne-normative basalts are almost certainly products of complexolivine, clinopyroxene, and amphibole fractionation. The irregular variationof La/Sm and Ti with Mg' (Fig. 14) suggests that amphibole fractionation wasprominent in some samples (for example, La/Sm >10) but less important inothers, particularly within the nepheline field. Parental compositions have

not been calculated for the nepheline-normative basalts, because the many pos-sible fractionation histories involved introduce a large degree of ambiguity.

A projection of normative mineral compositions employs chemical distinc-tions based on most of the major elements. Six of the major elements or majorelement groups (SiO2. A1203 , CaO, FeO-MgO, Na2 -K 20, TiO2) play a significantrole in plotting the position of a basalt on the projection of Fig. 15. Inthis multielement projection, the three Crater Flat basaltic cycles can bedistinguished. As in Fig. 14, there is a lobe of the 3.7-Myr basalts

extending towards the hypersthene-normative 1.1-Myr basalts. The arrow inFig. 15 is consistent with a single model of amphibole fractionation, a modelmore fully developed in Fig. 14. However, because of the approximate 2.6-Myr

age difference between these two basalt cycles, they cannot be the products of

a single fractionation event. This point is important, for it proves that aspecific pattern of amphibole fractionation has been repeated at least twiceamong the basalts of Crater Flat.

33

I *

OLH Hy

Fig. 15.Cation-basis normative diopside-olivine-nepheline-hypersthene plot for thebasalts of Crater Flat. Symbols are defined in Fig. 13; the question marksignifies a proposed parental composition (Table IV), and the arrow reproducesthe kaersutite removal trend of Fig. 14.

A comparison of Figs. 13, 14, and 15 leads to the following conclusions

concerning the origins of the basalts at Crater Flat. First, the three cycles

of hawaiite volcanism at Crater Flat are compositionally distinctive. Second,

where pronounced compositional variation does occur, as within the 1.1-Myr

cycle, the variation cannot be modeled by fractionation of one erupted variant

from another; all variants of the 1.1-Myr hawaiites arose either from one par-

ent magma by varied modes of fractionation or from two or more cosanguineous

parent magmas. Third, the various volcanic cycles are distinctive but each

cycle reproduces hawaiite-clan volcanism. This third conclusion is very im-

portant, for it strongly suggests that the mechanics of mantle melting and

parental magma evolution have remained fundamentally unchanged beneath Crater

Flat for the past 3.7 Myr. This last conclusion is strengthened by the re-

peated occurrence of a specific amphibole fractionation trend among the 3.7-

and 1.1-Myr basalts of Crater Flat.

I 'I

VIII. GENERAL TRACE-ELEMENT CHEMISTRY PD Rb-Sr SYSTEMATICS OF THE BASALTS AT

CRATER FLAT

Trace-element abundances were obtained by instrumental neutron activation

analysis (INAA). Both thermal and epithermal neutron irradiations were used.Several 250-mg aliquots of each whole-rock powder were encapsulated in poly-ethylene vials and irradiated in the following neutron fluxes at the Los

Alamos Omega West Reactor: (+Thermal = 1 x 1013 and OEpithermal = 5 x

10' 0 n/cm2/s). Different irradiation lengths, decay intervals, and counting

times were employed to determine Sc, V, Cr, Mn, Co, Ga, As, Rb, Sr, Cs, Ba,

La, Ce, Sm, Eu, Tb, Yb, Hf, Ta, and Th by direct counting of gamma radiation

on large Ge(Li) crystals coupled to 4096-channel pulse-height analyzers. All

gamma-ray spectra were stored either directly on Digital Equipment Corporation

(DEC) RL02 disks or on magnetic tape for subsequent transfer to disk. Data

reduction was done off-line on a DEC PP 11/34 minicomputer under the RSX-11M

operating system. Uranium was determined by delayed neutron assay (DNA). All

procedures are described in Gladney et al. (1980a,b,c).

As with major elements, trace-element concentrations discriminate between

the three basal tic cycles at Crater Flat. The incompatible trace elements are

distinctively enriched or depleted in the various basaltic cycles of Crater )Flat (Table V). The 3.7-Myr cycle is relatively low in most incompatible ele-

ments including the light rare-earth elements (La and Sm, Fig. 16), the high-

valency actinide elements (U and Th, Fig. 17), and large cations such as Sr

(Fig. 18). Despite a broad range in composition, the 1.1-Myr basalts are re-

markably enriched in all incompatible trace elements except Rb (Fig. 18); the

northeastern cinder cone of the Little Cone center is an exception to the

1.1-Myr enrichment in most incompatible elements, although it is comparable to

other 1.1-Myr basalts in Sr enrichment. The final basalt cycle at Crater Flat

(0.3 Myr) is generally intermediate in trace-element composition, between the

two preceding basaltic cycles. The enrichment of incompatible trace elements

In the two younger cycles of Crater Flat basalts is much greater than in other

comparable hawaiite basalts (Price and Taylor 1980; Frey et al. 1978; Fitton

and Hughes 1977). The implications of this enrichment are discussed below.

The origins of the basalts at Crater Flat are partially obscured by their

evolved nature. Basalts are known to have high Mg' values (>0.65) at their

source regions in the upper mantle, and basalts with lower Mg' values have

evolved from their parental compositions. The basalts at Crater Flat, with )

I I

TABLE V

TRACE ELEMENTa COMPOSITIONS OF CRATER FLAT BASALTS (ppm)

3.7-Myr Cycle

FB78-14

Cs 0.92.

Rb 66

Ba 1040

Sr 920

La 104

Ce 188

Sm 11.5

Eu 3.3

Yb 2.4

Th 7.5

U 1.5

Hf 7.5

Ta 1.54

V 173

Sc 22

Ga 18

As 1.5

CF12-6-12

0.5136

1260

800

66

126

8.5

2.5

2..6

6.2

-1.1

6.6

1.27

220

27

18

0.6

CF12-6- 10

0.72

65

780

840

63

128

8.1

2.5

2.9

6.2

1.2

6.2

1.24

187

29

21

1.4

CF12-7-6

0.57

22

950

770

72

136

9.0

2.6

2.8

6.0

1.2

6.5

1.38

217

27

16

0.8

CF12-7-1

0.41

18

1020

800

58

119

9.3

2.9

3.2

4.6

1.0

6.6

1.50

259

30

15

2.4

FB CF78-15 12-7-8

0.75 0.83

28 30

890 710

770 750

- 73

- 140

- 9.1

- . 2.8

- 2.7

5.4 5.6

0.8 1.2

5.9 7.9

1.10 1.40

248 243

27 29

19 18

1.2 1.0

FB78-17

0.68

39

930

780

6.4

1.1

6.4

1.26

189

25

18

0.5

-

aAll analyses reported in ppm.errors are 10: for Sr, La, Eu,202 for b, n, and Yb.

Data obtained by nstrumental neutron activation. PelativeTb. Th. U, Hf. Ta, V, and Sc; 15: for Cs, Ba, Ce, Ga, and As;

41

I I

I .TABLE V (Cont.)

TRACE ELEMENTa COMPOSITIONS OF CRATER FLAT BASALTS (ppm)

1.1-Myr Cycle 0.3-Myr Cycle

Lathro e! llsNorthern Cone Black Cone Red Cone Little Cone S.W. Little Cone .E.-

Cf FB FU FB FB CF CF12-6-3 7-10 711-L 78-4 8-5 12-4-4 12-4-6

CF CF FO12-4-11 _j?.A.1jA 78-9

CF CF CF12-4-13A 12-4-138 11-7-1

Cs

14,

Oa

Sr

La

Ce

s

Eu

Yb

Th

UlitllfTa

V

Sc

Ga

As

131

liV

1.1 2.0

19 45

10 1410

10 1170

- 122

- 217

- 12.3

- 3.7

- 3.0

10 9.6

3.0 3.0

8.2 8.7

1.64 1.77

75 190

18 19

18 16

1.6 1.3

2.5

20

1420

1040

2.6

36

1010

1100

2.0

20

1140

1200

116

206

11.6' 3.4

2.6

10

3.4

8.5

1.76

151

22

21

1.2

2.6

36

1340

1600

2.0 2.6

33 31

1350 1500

1340 1230

- 121

- 202

- 11.4

0.87

14

1430

1750

111

207

13.1

3.7

2.6

7.6

3.3

0.72

24

1390

1900

10

1.9

1.4

14

1280

1320

93

1.1

32

1170

1180

1.5

19

1330

1380

94

1

11

3.4

9.1

1.69

145

22

17

1.9

- - ~~3.2- 2.5

15 14 12

4.4 3.6 3.6

9.9

3.2

8.8

1.63

160

20

19

1.6

186 - 181

12.9 - 12.0

3.7 - 3.6

2.7 - 2.5

5.0 4.9 6.70.9 1.5 2.28.2 8.0 8.01.83 1.56 1.56

224 209 20719 18 1920 16 181.3 5.0 1.3

Cf11-7-2

1.1

18

1310

1450

88

184

11.9

3.5

2.7

6.4

2.0

8.2

1.62

152

19

19

1.2

f B

2. 3

1 J!j0I

1290

7.52. 4

1.b3

.0

178

2.

18

1.5

8.1

1.65

166

19

20

2.3

8.8

1.78

181

19

17

2.1

8.9 9.4 8.6

1.59148

20

17

1.8

1.88

179

19

19

0.8

2.1200

1922

0.8

aAll analyses reported In ppm. Data obtained by instrumental neutronand Sc; 15S for Cs, Ba, Ce, Ga. and As; 201 for R, S, and b.

activation. Relative errors are 10 for Sr, La. Eu, T. Th, U, lf, Id V,

15

10.

0 so 100 150 200La (ppml

Fig. 16.Plot of Sm vs La for the basalts of Crater Flat; symbols represent (1) the 3.7-Myr basalts, (2) the 1.1-Myr basalts, and (3) the 0.3-Myr basalts. The stip-pled field represents the common range of compositions for tholeiitic to alka-line basalts, including most hawaiites. Olivine (0L), clinopyroxene (CPX),and amphibole (AMPH) compositions are shown as calculated for minerals inequilibrium with the range of basalt compositions at Crater Flat. The youngerbasalts at Crater Flat have very high La/Sm ratios (discussion in text).

:.

4,

1-

1*0 __

U l b ( p p m )lb Ispm

Fig. 17.Plot of Th vs U for the basalts of Crater Flat; symbols as in Fig. 16. Notethe constant ratio but increased content of U and Th in the youngest basalts!2 *''

100- oft

cc~~~~'

2 233

Sf tppml

Fig. 18.Plot of Rb vs Sr for the basalts of Crater Flat; symbols as in Fig. 16. Theyoungest basalts (2, 3) have high Sr contents but very low Rb/Sr ratios; highradiogenic Sr content in the 0.3-Myr basalts (3), Sr'7/Sr8" = 0.7075, stronglysuggests Rb depletion. Arrows indicate the paths of fractionation from thefield of common tholeiitic to alkaline basalt compositions; this path is in-variably one of Rb-enrichment. The line RbiSr = 0.032 represents a whole-earth model ratio (Carter et al. 1978).

Mg' values of 0.58 to 0.46 (Fig. 14), are so evolved. The Mg' value of a par-

ental magma is reduced by removal of Mg-rich silicate phases, of which oli-

vine, clinopyroxene, and kaersutite (or another Ti-rich amphibole) are pos-

sible candidates that may lead to the hawaiites of Crater Flat. Plagioclase

removal does not affect the Mg' value of the evolving magma and can be ruled

out for the Crater Flat basalts on the basis of smooth chondrite-normalized

patterns for all of the rare earth elements, including Eu, in even the most

lanthanide-enriched basalts of the 1.1-Myr cycle.

Without samples of the parental magma, the development of detailed crys-

tal fractionation schemes for Crater Flat basalts is highly speculative.However, the incompatible trace-element compositions of the 1.1- and 0.3-Myrbasaltic cycles provide some indication of which minerals were removed from

parental magma to generate Crater Flat basalts. Minerals that tend to reject

all lanthanide elements, but disfavor light lanthanide elements (for example,

La) more than other lanthanide elements (for example, Sm), will increase the

concentrations of La and Sm and raise the La/Sm ratio in the evolved magma.

44

.Fractionation of kaersutite or clinopyroxene leads. to this result. Figure 16

shows that the La/Sm ratios in the 1.1- and 0.3-Myr Crater Flat basalts are

very high (10 to 14), much higher than in other hawaiites evolved to similar

Mg' values (La/Sm <6, Price 1980; Frey et al. 1978). High La/Sm ratios may

become common as more trace-element data are obtained for basalts in the NTS

area, but the immediate inference from this data is that large amounts ofclinopyroxene or kaersutite,- or both, were fractionated from the magmas that

were parental to the two younger basaltic cycles at Crater Flat. The impor-

tance of amphibole fractionation has been discussed above in conjunction withFigs. 14 and 15. Clinopyroxene and kaersutite fractionation probably included

the crystallization and removal of olivine, to account for the combined de-

crease in Mg' and pronounced increase in incompatible elements. The less

evolved basalts of the 3.7-Myr Crater Flat cycle are not greatly enriched in

La/Sm and could be derived from parental magma(s) through crystal fractiona-

tion dominated by olivine, though the calculation of parental magma types sug-

gests that clinopyroxene was also involved. Whatever their parentage, the

hawaiites of Crater Flat, particularly the younger basalts, did not rise

abruptly from their mantle sources, but were derived from parental magmas thatwere held at depth and partially crystallized before eruption.

Processes that enrich a magma in one incompatible element will generally

result in enrichment in other incompatible elements, though the final ratios

between incompatible elements may vary. The 1.1-Myr basalts of Crater Flat

are enriched in almost all incompatible trace elements, with the notable ex-

ception of Rb, relative to the 3.7-Myr basalts. Figure 18 shows the low Rb

content and high Sr content in Crater Flat basalts relative to other common

basalt types. The high Sr content of the younger basalts of Crater Flat may

.be attributed to the extensive crystal fractionation that is required to ex-

plain the other incompatible-element enrichments. However, all realistic frac-tionation models should increase Rb as much as Sr or more. This low-Rb anom-

aly in the younger Crater Flat basalts strongly suggests an b depletion of

the mantle source region, before the melting event that generated the 1.1- and

0.3-Myr basalts of Crater Flat.

The scenario of Rb depletion is complicated by isotopic data. Analyses

published by Leeman (1970) show that the 0.3-Myr Lathrop Wells basalt is en-

riched in Sr87, the radiogenic daughter of Rb, with an Sr87186 ratio of

0.7075. The low b content of this and other Crater Flat samples rules out

I T,

.he ossibiliy of crustl contani naticn. Thus the mantle source -or the

0.3-Myr basalt (and probably the other basalts of Crater Flat as well, thoughisotopic data have not been collected for these samples) was enriched in Rb

through some event in the distant past. The requirements for past Rb enrich-

ment are illustrated by the line Rb/Sr 0.032 in Fig. 18. This is a model

whole-earth ratio (Carter et al. 1978), which would lead to a present-day Sr87/86 of 0.705, if left undisturbed throughout the earth's history. Some dis-

turbance involving Rb enrichment must have occurred in the ancient mantle that

was to become the Late Cenozoic source region for the basalts of Crater Flat.

A single-stage Rb enrichment model based on the highest Rb/Sr content of the

3.7-Myr basalts would place this enrichment event at about 900-Myr ago. TheRb-enriched mantle remained static until the Late Cenozoic and generated alarge amount of Sr87 from the high b concentration. Finally, this mantle

source region lost much of its Rb. The loss of R could not have been coupled

to a loss of other trace elements, or the great enrichment of lanthanides and

other incompatible elements would not be seen in the younger basalts of Crater

Flat (Figs. 16 and 17). A selective depletion of Rb is possible through flux-

ing by aqueous volatile-rich fluids; data of Shaw (1978) show that in such

fluids the solubility of b is greater than the solubility of other incompati-

ble elements. Alternatively, Rb might be selectively lost by destabilization

of an Rb-rich mantle phase (for example, phlogopite) in an upwelling mantleenvironment.

The isotopic data suggest that Rb depletion must have occurred in the

near past. Could this event have been the magma genesis of the 3.7-Myr Crater

Flat basalts? .Or was it a much larger event, associated with one or more of

the silicic caldera-forming eruptions of the Timber Mountain-Oasis Valley caul-

dron complex? A proof of this second possibility would provide documentation

of an important mechanism for generating mantle nhomogeneities. Another pos-

sibility for selective Rb loss would be the general crustal thinning and

mantle upwelling associated with late Cenozoic Basin-Range tectonism. Further

petrographic, chemical, and isotopic studies will address these questions.

IX. VOLCANIC RISK ASSESSMENT

Recurrence of basaltic volcanism within the Crater Flat area is of con-

cern to siting a waste repository at Yucca Mountain: the Quaternary-age

Lathrop Wells center is located less than 20 km from the southern edge of the

-36

Yucca Mountain-exploration block. Crowe and. Carr (1980) defined maximum prob-ability limits (10-8 to 109 /year) for the likelihood of volcanic disruption

of a repository at Yucca Mountain. They briefly considered the direct disrup-tion effects of volcanism and examined the regional volcanic setting of basal-

tic volcanism within the south-central Great Basin. Several conclusions from

this work add data with respect to the earlier volcanic risk assessment.

First, field, geochronologic, and geochemistry studies all support therecognition of cycles of basaltic activity within the Crater Flat area. Each

cycle is distinct in space and time and can be discriminated through major- ortrace-element abundances. Absolute volumes of erupted lava for each cycle are

relatively small (<1 km 3), and the actual number of eruptive vents for each

cycle is variable but generally small (less than 10 vents per cycle). Thus,

if this pattern of past basaltic activity can be assumed to continue into the

future, it is likely that future volcanism in the Crater Flat area will be of

relatively small volume with a limited number of volcanic vents.

Second, there is no clear evidence of an increase in rates of volcanic

activity or volumes of erupted magma within the last 3.7 Myr. This is illus-

trated by Fig. 19, a plot of calculated magma volume vs time. Two interpreta-

tions are suggested by this figure although the interpretations are sharply

limited by the small number of data points. There is a near-linear decline in

volume of magma for successively younger volcanic cycles. This suggests a

possible waning in basaltic activity within the last 3.7 Myr. This trend con-

trasts with a decrease in the intervals between eruptions with time that could

indicate an acceleration of basaltic activity. Both of these interpretations

need to be tested through examination of the history of basaltic volcanism

(postsilicic volcanism) for the entire NTS region. Such studies are in

progress.

Third, compositional studies indicate that each of the three volcanic

cycles at Crater Flat produced similar hawaiite magmas. There is a strong sug-

gestion of source region variation with time from the fact that the oldest

basaltic cycle includes samples that are not Rb-depleted, unlike the younger

basaltic cycles. On the other hand, compositionally similar basalt types were

erupted repeatedly within the Crater Flat area, reflecting relatively constant

conditions of magma generation through time.

17

I ,

1.0_.

0.5.

I>.2 0.1

2 2g2 0.05f

0.01 I' - 2.64 32I0

TIME(million years)

Fig. 19.Plot of volume vs age for the three basaltic cycles at Crater Flat.

The above data argue that, within the Crater Flat area for the last 3.7

!4yr, basalt types have remained relatively similar and volumes have been

small. The general assumptions of continuity in magmatic processes for the

Crater Flat area (last 3.7 Myr) required for probability calculations thus far

are broadly supported by the continuing field, dating, and petrologic work.

Two additional areas of investigation are required. (1) The history of

basaltic volcanism for a larger area of the NTS region needs to be studied.

Young basalts are present at two additional localities. Two cinder cone and

lava flow centers dated at about 0.3 Myr are present north of Crater Flat (SB

of Fig. 1); the basalts of Buckboard Mesa (BM of Fig. ) have been dated at

about 2.8 Myr (W. J. Carr, personal communication 1980). Scattered basalts

younger than 11 yr are also present within the NTS area. These basalts will

be compared petrologically with the basalts of Crater Flat. The concept of

discrete cycles or pulses of basaltic activity will be tested through regional

studies and the volume/time plot completed for Crater Flat basalts will be

expanded to include the entire NTS region. (2) The Lunar Crater volcanic

field of central Nevada (probably Pliocene and Quaternary age) is the northern-

most basalt field of the volcanic belt. Volumes of basalt in this field-

I .,

exceed several tens of cubic kilometers. Cone density of uaternary-age

cinder cones within the Lunar Crater field is about 0.1/km2, in contrast to

the Quaternary cone density for the NTS region of about 10-3 to 10-4/km2. It

is important to determine why the contemporary rates and volumes of basaltic

activity for the Lunar Crater volcanic field are so much greater than for the

Crater Flat field. Studies under way indicate the compositional range of ba-

salt types is much greater in the Lunar Crater field than the Crater Flat

field. These fields need to be contrasted petrologically and geochemically in

order to further understand basaltic volcanism in the southern Great Basin.

ACKNOWLEDGMENTS

W. S. Carr, U.S. Geological Survey, participated in many aspects of the

geologic studies of the basalts of Crater Flat. We benefited from his know-

ledge of the tectonic and volcanic history of the Great Basin. We are grate-

ful to E. S. Gladney of the Los Alamos Health Sciences Division for his excel-

lent work in INAA analysis of our samples. We also gratefully acknowledge the

assistance of R. J. Fleck and R. F. Marvin of the U.S. Geological Survey who

determined the K-Ar whole rock ages for the basalts. The manuscript was

reviewed by F. M. Byers, W. J. Carr, and A. C. Waters. Editorial review was

contributed by M. G. Wilson.

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I I

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APPENDIX

TABLES OF MINERAL ANALYSES

Table A-I: Olivine analyses normalized to 4 oxygens.

Table A-II: Feldspar analyses normalized to 8 oxygens.

Table A-III: Pyroxene analyses normalized to 6 oxygens.

Table A-IV: Oxide analyses normalized to 2 cations (rhombic) or 3 cations

(isometric).

Table A-V: Amphibole analyses normalized to 23 oxygens.

[Note: (n.a.) not analyzed; (-) = below microprobe detection limits.]

Z s

I *.

t)-I-

TABLE A-I

OLIVINE ANALYSES, CRATER FLAT ASALTS

3.744yr Cycle

fU78-14 CF 12-7-6A CF12-7-1

Si02

Al 03

7°2

FeO

MnOMg0

CaO

Cr 203

z

Si

Al

I1

Fe

Hn

Hg

Ca

Cr

lcatlons

Fo

I'd

Phenoc rys ts

38.2 37.9

n.a. n.a.

21.9 23.1

0.17 0.24

40.9 39.4

0.14 0.20

101.3 100.0

Groundindss Phenocrysts Phenocrysts Groundmass

36.6 36.7 38.4 38.9 39.7 39.5 39.0 35.5 35.7

- 0.06 n.a. n.a. 0.06 - - 0.07 0.20

0.12 0.13 - - - - - 0.15 0.16

34.5 34.0 18.8 19.3 19.3 19.4 21.9 39.9 38.9

0.91 0.83 0.21 0.22 0.25 0.24 0.23 0.78 0.79

29.8 29.4 42.4 41.2 42.5 41.9 39.7 25.4 24.0

0.15 0.15 0.14 0.12 0.20 0.21 0.21 0.43 0.69

0.09 0.10 - - - - - -

102.2 101.4 100.0 99.7 102.0 101.2 101.0 102.2 101.2

0.980

0.469

0.003

1.564

0.003

3.019

0.983 0.990 0.999 0. 984 0.9U7 0.995 0.999 0.998 0.981 0.996

n.a. - 0.001 0.1 - - 0.001 0.006

- 0.002. 0. 2 - - - - 0. 002 0.002

0.500 0.779 0.773 0.402 0.420 0.403 0.410 0.468 0.927 0.905

0.005 0.021 0.018 0.005 0.005 0.005 0.004 0.004 0.018 0.018

1.524 1.201 1.194 1.620 1.598 1.587 1.577 1.518 1.054 1.030

0.005 0.004 0.004 O.003 0.003 0.005 0.005 0.007 0.012 0.020

- 0.001 0.001 _ _ _

3.017 2.998 2.992 3.014 3.013 2.996 2.995 2.995 3.001 2.977

0.77 0.75 0.61 0.61 0.80 0.79 0.U0

0.23 0.25 0.39 0.39 0.20 0.21 0.20

0. 79 0.76 0.53 0.53

0.21 0.24 0.47 0.47

vf-

TABLE A-I (Cont.)

OLIVINE ANALYSES, CRATER FLAT ASALTS

1.1-Myr CycleNorthern Cone 81ack Cone

CF12-6-3 F878-1 F878-4

Phenocrysts gohs Phenocrysts Grounmhass Phenocrysts Groundeass

SiO2 38.6 38.1 36.2 38.5 38.5 37.3 36.8 39.1 37.1 36.3 35.8Al203 - - - - - - - - - -TiO 2 - - 0.09 _ - 0.10 0.11.. - - 0.13 0.11FeO 21.5 27.5 33.5 22.5 26.3 30.8 33.3 22.4 31.3 35.8 37.5MnO 0.24 0.41 0.74 0.38 0.38 0.63 0.64 0.32 0.59 0.76 0.75

MgO 40.5 35.6 30.4 40.0. 36.7 32.6 30.3 40.7 .32.0 28.2 26.5CaO 0.14 0.17 0.32 0.16 0.17 0.33 0.38 0.16 0.27 0.30 0.52Cr 2 03 - - - - - - - - - - -

z 101.0 101.8 101.2 101.5 102.0 101.8 101.5 102.7 101.3 101.5 101.2

Si 0.991 0.997 0.985 0.985 0.997 0.993 0.996 0.987 0.996 0.994 0.995Al - - - - .. - - - - - -

TI - - 0.001 - - 0.001 0.001 - - 0.002 0.001Fe 0.460 0.601 0.760 0.482 0.571 0.687 0.753 0.473 0.701 0.820 0.873Mn 0.004 0.010 0.017 0.007 0.007 0.014 0.014 0.007 0.013 0.017 0.017Mg 1.547 1.388 1.233 1.527 1.419 1.295 1.222 1.533 1.278 1.151 1.097Ca 0.003 0.004 0.008 0.003 0.005 0.009 0.010 0.003 0.007 0.008 0.015Cr - - - - - - - - - -rcations 3.006 3.000 3.004 3.004 2.999 2.999 2.996 3.003 2.99S 2.992 2.998

Fo 0.77 0.70 0.62 0.76 0.71 0.65 0.62 0.76 0.65 0.58 0.56Fa 0.23 0.30 0.38 0.24 0.29 0.35 0.38 0.24 0.35 0.42 0.44

(3mMS

TABLE A-I (Cont.).,.

OLIVINE ANALYSES, CRATER FLAT BASALTS

1.1-Hyr Cycle

Red Cone Little Cone. S.U.

.... . CEi2A-.h____ . __ _ _ 12=__ 2Ph!enocr-sts Groundimass Phenocrysts Groundmass Phenocrysts

S102 39.0 38.2 36.8 36.1 39.1 37.2 36.2 35.6 38.4 38.5 38.5A1203 - - 0.06 - - 0.05 0.08 0.07 - -

1102 - - - 0.11 - - 0.11 0.12 0.09 - _

FeO 21.7 26.6 32.1 34.0 23.0 33.4 36.7 40.8 22.3 24.3 25.1

MnO 0.32 0.50 0.74 0.90 0.27 0.68 0.83 0.88 0.45 0.63 0.66HgO 39.6 36.5 31.2 28.8 39.4 30.6 27.4 24.4 38.6 37.9 37.3

CadO 0.16 0.20 0.20 0.46 0.13 0.25 0.33 0.29 0.23 0.28 0.32Cr203 - - - - - - - - - -

z 100.8 102.0 101.0 100.4 101.9 102.1 101.6 102.2 100.1 101.6 101.9

Si 1.001 0.993 0.993 0.994 0.997 0.999 0.997 0.994 0.999 0.995 0. 96Al - - - 0.001 - - 0.001 0.002 0.001 - -

TI - - - O.Ol - 0.001 0.002 0.001 - -

Fe 0.465 0.577 0.727 0.704 0.491 0.750 0.845 0.951 0.484 0.524 0.S43Mn 0.006 0.010 0.017 0.021 0.005 0.015 0.019 0.020 0.009 0.013 0.014

Hg 1.516 1.414 1.255 1.114 1.502 1.225 1.125 1.016 1.496 1.459 1.437Ca 0.003 0.005 O.OU5 0.013 0.003 0.007 0.009 0.008 0.006 0.007 0.008Cr - - - _ _ _

Icatlons 2.991 2.999 2.997 2.998 2.998 2.996 2.997 2.Y93 2.996 2.998 2.998

Fo 0.77 0.71 0.63 0.60 0.75 0.62 0.57 0.52 0.76 0.74 0.73Fa 0.23 0.29 0.37 0.40 0.25 0.38 0.43 0.48 0.24 0.26 0.27

TABLE A-I (Cont.)

OLIVINE ANALYSES, CRATER FLAT BASALTS

1.1-yr Cycle Cont'd.)

Little Cone, .E.CF12-4-13H

0.3-Myr Cycle

La'throp Wells ConeFB78-7

Phenocrysts Groundmass Phenocrysts

Si02

A1203TI 02

FeO

HgO"goCaO

Cr203

t

38.1 37.5 36.1

n.a. n.a. 0.05

- 0.08 0.17

22.8 24.9 31.5

0.26 0.36 1.02

39.1 37.8 30.5

0.15 0.25 0.43

- - 0.09

100.4 100.9 99.9

36.1

0.04

0.14

31.7

0.98

30.6

0.45

0.01

100.1

39.0

19.9

0.26

41.2

0.13

38.8

21.8

0.27

40.3

0.16

Groundmass

37.3 36.2

0.11 0.29

- 0.14

27.6 31.5

2 0.48 0.54

34.8 29.9

0 0.48 0.95

100.5 101.3 101.8 100.8 99.5

Si

Al

TI

Fe

Mn

Mg

Ca

Cr

1cations

0.989

n.a.

0.495

0.005

1.515

0.004

3.008

0.981

n.a.

0.001.0.5410.0081.4710.007

3.009

0.989

0.001

0.003

0.722

0.023

1.247

0.012

0.001

2.998

0.988

0.001

0.002

0.725

0.023

1.249

0.013

('.001

3.002

0. 7 0.992

0.426 0.466-

0.005 0.O05

1.571 1.535

0.003 0.003

3.002 3.001

0.989 0. 988

- 0.002

O. 569

0.009

1.431

0.005

0.6120.010

1.3750.013

0.995

0.009

0.002

0.722

0.012

1.223

0.027

2.9903.003 3.000

Fo

Fa

0.76 0.73 0.63 0.63

0.24 0.27 0.37 0.37

0.79 0.77 0.72 0.69 0.63

0.21 0.23 0.28 0.31 0.37

)I

(II TABLE A-l

FELDSPAR ANALYSES, CRATER FLAT ASLTS

3.l-Hyr Cycle

IS /81-4

lSoJ

ft0

"-p

tdo

stoado

NA 20

KIo

9

49.3

IJ.2

o. o

0. )9

.

A.S.

N.A.

2.64

0.16

100.S

Phenocrysts Groundeass

0.5 60.2 S?.! 55.2 6Z.4 65.8

3l.S 30.1 29.2 26.8 22.3 31.s

0.81 0.99 3.36 0.98 0.60 0.41

0.09 - 0.01 0.09 - 0.61

14.1 14.4 £2.9 30.8 3.9, 0.48

A.A. A.A. n.A. ... '".. N.A.

N.A. . .. n.h. A.A. R.A. h.A.

2.61 3.36 3.9i S.11 S.0O 3.23

0.20 0.09 0.22 0.Jo 6.09 11.4

100.7 99.1 300.0 99.3 100.0 99.4

CF 12-6-12.

Pheocrysts

41.6 60.6 51.1

32.4 30.9 33.4

0.61 0.71 0.49

".4. A.A. NI.

16.1 14.2 13.93.4. A.S. N.G.

B.S. A.A. 4.4.

2.21 3.11 3.3

0.24 0.34 0.3s

99.4 59.9 100.6

Si

Al

lSt.Al

Fc

H9

Es

sr

a

Ua

K

ix-.cti033

ctlos

Or

Ab

2.24S 2.290 2.316

1.129 1.682 1.636

J.914 3.912 3.962

0.025 0.030 0.038

0.00s 0.00s -

0.171 0.116 0.712

N.A. N.. A.4.

n.s. N.. G.A.

0.223 0.2S3 0.303

0.008 0.010 0.004

3.038 1.012 1.oss

4.992 4.984 6.001

0.01 0.03 0.m3

0.22 0.26 0.30

0.11 0.13 0.70

2.392 2.634 2.191 L.aos

3.661 1.440 1.181 0.942

. sss 3.964 3.984 3.947

0.042 O.0J6 0.422 0.016

0.004 0.006 - 0.031

0.628 0.624 0.390 0.022

A.A. ".S. S.d. A.A.

n.s. B.S. N.d. n.a.

0.34S 0.43 0.431 0.286

0.012 0.017 0.g4 0.66s

1.033 1.033 0.998 3.025

4.990 4.987 4.982 4.912

0.03 0.02 0.36 0.68

0.3s 0.45 0.46 0.29

0.64 U.sJ 0.19 0.03

2.208 2.316 2.314

1.164 1.664 3.616

3.912 3. 980 3.99

0.023 0.026 0.0£1

A.A. A.S. 3.8.

0.198 0.696 0.675

U... A.S. I e" .

B.S. A.S. B.S.

0.39 0.21 0.296

0.013 0.039 0.020

1.033 1.016 3.001

S.0S 4.996 4.991

0.0£ 0.02 0.02

0.20 0.28 0.30

0.19 0.70 0.68

.to I

TABLE A-II (Cont.)

FELDSPAR ANALYSES, CRATER FLAT BASALTS

J.J-Hyr Cycle

S102

tao

1490

Lao

'-0

840

2°

50.0 521

Jo.7 30. S

O.SI 0.96

0.12 0.11

14.8 13.4

O.J0 0.18

J.OS 3.58

0.?I 0.2J

99.9 JOD.5

1S 2.298 2.340

Al 3.880 1.512

I St-Al 3.95 3.912

F. 0.01S 0.0)6

iI 0.00 0.O6

C 0. 19 0.so

St 0.006 0.004

as -

Ns 0.211 0.314

R 0.01? 0.013

a.-ctlons I.OSI 1.02J

I catlons 5.W0 4.99S

S4.1

28.8

0.42

0.04

11.1

0.74

4.91

0.29

300.6

2.530

3.961

0.014

0.001

0.1US

0.001

0.429

0.0281

.LON

S.WI

Sl.S *.

101.1 1.1

21.2 20.1

0.81 0.11

0.11 0.20

0.31 0.09

0.7s3 O.WS

0. s 0. W8

0.486 9.314

103.0 100.1

2.161 2.1113

2.4?SI 1.102

0. 02 0.021

0.00 0.010

0.1JWI O.0D89

O. 01)4 O. 005

0.028 0.142

I3.0Ot4 1.01 9

*.4? S9. S.a

64.4

19.7

0. 4

0.01

0.84

0.25

4.21

10.0

2.*50

3.Ujs| .Is$

U. 01 I

O.O JII0. 03

O.WOt0.0J140.114

0. 5,6

5.005

CFl,--10 CE2-t1.6phe1ocr ilts __e__or_ _ts '

41.1 48.4 41.S 49.1 10.4 10.1 54.1 59.J

32.6 11.6 331 32.1 31.6 31.3 28.0 24.6

0.18 0.19 0.12 0.62 O.S9 O.S6 0.6 0.44

A.S. O.&. R.*. .. 4. N.S. A. L.a. L.S.

12.9 14.1 1.2 IS.$ I4.S 13.8 10.9 S.8,

A.. .4. *.* ".a. .S. A. L.S. N.A. -

L.4. M.S. L.a. P.A. L.a. L.A. L.A. n.a.

1.S6 2.19 2.09 2.66 2.89 3.41 .S9 k.11

0.09 O.2O 0.09 0.21 0.20 0.24 0.40 1.31

99.2 98.S 100.1 200.3 100.2 9S.4 ".2 98.6

2.18 2.212 2.19 .241 t. 4 2.299 2.414 '7.689

2.11 132 1.814 I.t3O 3. G 1.912 I.SOJ. 1.311

J.969 3.464 3.993 3.911 3.991 3.991 j.981 4.000

0.07 o 0.0 2Z0 0.01 0.022 0.021 0.020 0.0?4 0.0w8

N.A. A.S. B.S. A.S. L.i. A.S. L.*. f.a.

0.838 0.132 0.19" 0.12 O.yob 0.611 0.31 0.281

0.. L.a. S .a. NL.a. L.a. ... 8.4.

N.A. L.a. L.S. L.A. L.a. M... l.a. I.e.

0.218 O.2S1 0.186 0.238 0.255 0.30 0.406 0.621

O.0U4 0.033 o.004 0.OU8 Q.010 0.013 0.051 0.01

1.0JS 1.024 1.011 I.028 0.n2 ? 1.01 .04 2.000

S.Ou s.aw. 5.004 S.0S 4. "S S.004 4.945 S.OO

or 0.01 0.01 0.07 0.Oj 0.5s

Ab 0.?1 O.12 0.44 0.58 0.16

An 0.12 0.61 O.SS 0.41 0.09

o.S

U.3J

u.u4

0.02 0.03 0.00 0.02 0.0l 0.03 0. us 0.0

0.11 0.7 0.1 2 0.23 0.28 0A 8.42 0.u

1.87 0.14 0.81 0.18 0.13 0.68 0.S4 0.29

(IAL

(X.TABLE A-II (Cont.)

FELDSPAR AALYSES, CRATER FLAT BASALTS

3.144yr Cycle

Northern Cone(f12-6-3firawam

Sb?2A1203Fe0

14go

C&O

Sro

80

~2°z20£z

49.9 Sl.0

29.8 30.3

0.98 0.82

i.e. i.A.

14. 14.0

P.e. M.S.

N.e. i.e.

3.39 .8

0.24 0.22

go.8 99.1

50.9

30.3

I.01

N.A.

13.8

N.A.

N.A.

3.41

0.26

99.1

813ck Conef`1-1 1118-4

Groundss Crouadmass

51.4 51.2 14.9 1.8 63.3 13.8 88.129.9 29.4 21.0 28.1 26.8 28.0 18.3

0.81 1.03 0.85 0.88 0.98 0.98 0.63

N.A. i.e. N.A. 0.12 0.11 0.10

13.9 13.8 Il.S 13.8 12.4 11.8 0.29

M.S. N.A. i.A. i.e. i.e. i.e. N.A.

i.C. i.e. i.A. i.e. i.e. N.e. i.e.

3.80 3.61 4.80 3.41 4.23 4.68 3.15

0.24 0.22 0.41 0.24 0.38 0.45 10.8

99.8 99.3 59. 98.2 98.2 .8 99.9

Red Con.Cf 12-4-4Grounmaess

30.9 30.2

0.91 3.OSN.A. N.A.

13.1 13.3

N.e. M.e.

i.A. i.e.

3. " 3.90

0.22 0.21

101.0 100.8

Red Cne Cont)CF 12.4-6

Groundeass

13.4 51.3

30.4 30.6

0.89 1.04

0.09 0.11

33.7 13.8

i.e. i.e.

N.e. i.e.

3.i3 3.36

0.26 0.28

1300.3 300.S

13.8

29.3

I .6d

0.12

4.340.41

101.3

S 2.320 2.37

Al 1. 24 1.636

St-Al 3.944 3.913

Fe 0.036 o.031

Mg R.. i.e.

Ca 0.129 0.885

Sr i.&. i.e.

ad i.e. i.4.

Ns 0.281 0.282

K 0.013 0.012

la-catlons l.09 1.010

X catbons 5.009 4.983

2.333

I.841

3.974

0.038

".e.

0. 69

i.e.

M.e.

0.303

0.01

1.025

4. 999

2.349 2.31 2.502 2.396 2.481 2.441 3.001

1.811 1.S94 1.450 I.S 1.462 1.499 0.911

3.980 3.9SI 3.9S52 3.933 3.929 3.948 3.918

0.031 0.039 0.032 0.034 0.233 0.016 0.022

i.&. i.e. M.S. 0.006 0.00 0.0006 -

.680 0.83 0.562 0.685 0.S17 0.1SS 0.013

i.e. i.e. N.e. N.A. i.e. N.A. i.e.

N.e. N.e. i.e. N.e. N.e. i.e. i.e.

0.319 0.321 0.424 0.306 0.319 0.412 0.329

0.013 0.012 0.022 0.011 0.021 0.025 0.821

1.043 .o55 1.040 1.044 I.00 1.054 0.949

S. WI S.008 4.99z 4.911 4.989 S.O00 4.961

2.332

1.641

3.9Y9

0.033

i.e.

0.62

N.e.

N.e.

0.322

0.012

1.029

5.008

2.318I.6133.9690. 039

i.e.0.4i.S.i.e.0.3420.012

1.040

S.009

2.341

3.912

0.033

0.005

0. 68

0.292

1.013

4. S8

2.329

I.16

3.961

0.019

0.0060.812

0.294

0.026

1.021

4.992

2.413

1.56o

3.963

0.062

0.006

0.562

n.e.

i.e.

0.111

0.022

1.029

4.992

Or 0.01 0.01

Ab 0.28 U. 29

An 0.11 0.10

0.03 0.03 0.02

0.31 0.32 0.32

0.88 0.87 0.61

0.02 0.01

0.42 0.31

0.56 0.8

0.02 0.02 0.85

0.31 0.41 0.34

0.81 0.51 0.01

0.01 0.01

3.32 0.34

0.81 O.8s

0.02

0.30

0.68

0.02

0. 3

0.8

0.02

0.40

0.58

(l I

TABLE A-TI (Cont.)

FELDSPAR ANALYSES, CRATER FLAT BASALTS

1.1 4yr Cycle

tlittle Cone 5.11.r11O-1

LiI~l Co. .1.lE*tt V

.3.eyr Cycle

Latbrop Wells Cap*a010.1

"waf8es.

F tO

N0

(*O

io0

K20

51

Il

Fe

C.

5.

"aI i it

K

la-catlees

I cations

12.1 St. 5).3 1.1 51.6 SI.1 I 1.6

20.3 25.5 20.1 Zl.7 29.6 20.9 25.0

0.U0 1.01 1.08 0.15 1.20 0.89 1.18

0.09 0.13 0.09 R. .e. a#.

12.9 12.1 11.0 4.96 1Z. 12. 32.0

a... a^a. A.S. nRe. MAs.. A.. n.

A... A.S. a... A.S. P.&. 4.e. , .4.

I.9S 4.1 4.44 S.10 3.ro J.

0. 26 0. 2 0. 3 S. 0. 4 0.39 0.38

9.6 9.9 99.I 99.2 'S5 99.1 9.

2.403 2.318 ?.435 2.199 2.J66 2.407 2.360

1.S11 1.549 1.514 1.159 I.b09 1. 54 1.89

J. 0 J."? I. 2 3.956 3.911 3.9 41 3. 96

0.036 0.040 0.040 0.079 0.045 0.03) 0.44

0.DOS 0.005 0.001 - n.j. n.e. ..0. G)s 0.611 0. Sa0 0. 20 0.629 0.602 0. 02

n.e. as... n"e. .&. R.&. II.&. A.S.

a.*. * .. a.. .. n ".e. * ^ as. M... A.A.

0.352 0.3?1 O.)9 0.09 0.33* U.JS? 0.348O.Ols 0.011 0.019 0.90 0.019 0.0?2 O.021

1.043 1.051 1.038 1.030 1.029 1.014 I.075

4.9d3 s.000 4.990 4. "a S. M 4.99 ou.0

49. S2.3 S).I SS.I

11.1 30.0 26. 26.?

0. 1. 03 O.9 1.01

0.10 o.S 0.09 0.09

13.2 12.) II.z 10.00.43 0.44 0.31 0.38

- 0.24 0.11 O.Z4

3.22 3. 1 4.45 4.09

0.21 0.39 o.S0 0.94

99.0 100.6 100.2 100.0

2. 1 2.311 t.436 2.529

1.700 1. m 1.542 1.4?9

3.991 ).1 3.930 3.VS8

0.021 0.039 0.031 0.040

0.06 0.009 0.00 O.0S

O.6S O.59" 0.545 0.44

0.011 0.010 0. 0 0.

0.004 0.001 O.040. 5t1 0. 25 0.391 0.431

0.01 0.022 0.028 O.OS4

e.m 3.004 .ots l.0W9

4.996 4.985 4.993 4.961

Or

Pb

An

0.01 0.02 0.02 - 0.9 0.02 0.02 0.0?

0.35 0.31 0. 40 U.St . 4 0.31 0.30

0.64 0.61 0.s0 0.20 0.64 0.03 0.62

0.02 0.02

0. 30 0.3

." - 0.63

0.03 0.

0.41 0.44

.S4 0.50

o

TABLE A-111

PYROXENE ANALYSES

I1l.Nvr CYcIe-- zS::

s102

A1203

1102

N"o

Ca"

04r203

1'°

Cr2')3£

Ma rt mrn lom Uk 9..nR ed ConeCf_12-6_3. fU/81 i 47- 6

51.4 1.3 2.4 5.3 1.1 50.6 S 0.6 63.8 52 1.4 Croumnd sza2.22 2.09 1.68 0.50 2.14 2.96 3.22 3.29 0.65 2.28 3.25 2.62 2.S0 1.921.30 0.98 0.90 0.39 1.06 1.30 I.1S 1.42 0.46 0.68 1.23 1.09 1.06 0.1930.7 10.4 10.2 23.3 11.4 10.4 12.3 33.5 22.5 1.2 9.5 30.5 30.6 11.40.30 0.32 0.30 0.65 0.30 8.25 0.39 0.42 0.7s 0.21 0.21 0.25 0.25 0.34

14.8 14.9 lb.3 20.S 16.1 14.3 s.5 13.1 19.1 16.4 14.1 14.0 14.1 IS.81I1.1 19.2 18.8 1.6 18.3 19.9 17.8 18.1 2.80 I8.8 19.6 19.8 19.9 11.810.49 0.43 0.47 0.06 0.36 U.41 0.39 0.59 0.20 0.27 U.36 0.43 0.33 U.32

98.9 99.6 100.0 98.3 99.8 100.1 9.8 101.4 99.3 9.1 91.0 100.9 100.8 300.4

SI1

VAI

stet

TIt

FeNA

mg

Cahe

Cr

zoct and

Ications

1.931 I.S2S

0.065 0.01S

2. 0 2. OU

0.033 0.017

0.036 0.021

0.331 U.325

0.009 0.009

0.829 0.832

0.133 0.167

0.03S 0.032

3.92 2.OL%93.SY2 4.009

1.949

0.01

2. O

0.021

0.0260.315

0.0080.846

0.747

0.034

1.9/0 3.931

0.021 0.08

".91 2.W- U.010

0.009 0.02S0.161 0.38

0.020 0. 91.172 0.043

0.063 0.7340.003 0.025

1.092 3. o 1.91.1 I.9S3 1.914 1.ow0 1.916 1.912 1.9320.108 0.10 0.09 0.007 0.086 0.120 0.082 0.06 U.U8b2. iO0 2.000 2.0 2.00 2. 00 2.000O 2.000 2.00 2.O0.021 0.042 0.04 0.021 0.013 0.026 0.033 0.020 0.01

0.035 0.044 0.039 0.012 0.038 0.033 0.029 0.029 0.0210.324 0.389 0.354 0.715 0.288 0.300 0.326 0.324 0.3540.0WY 0.012 0.013 0.024 0.037. 0.001 O.OD) 0.001 0.0100.197 0./59 0.715S .093 0.914 0.825 0.761 0.311 0.8?30.19 0.16 0.131 0.113 0.71 0. 790 0.192 0.791 0.1010.021 0.028 0.041 0.035 0.018 0.025 0.030 0.022 0.022

2.006 1.990 1. 953 1.953 2.009 2.006 3.99 20 04 2. O44.006 3.990 3.993 3.913 4.009 4.006 3.911 4.04 4.004

1.997 2.014 2.0083.59/ 4.00S 4.008

Io

En

Fs

AMIl

0.38

0.44

0.t3

2.1

0.40

0.43

0.17

3.4

0. 39

0.44

0.11

2.8

0.03 0.38 0.41 0.38 U.40 0.06 0.380.59 0.44 0.42 0.41 0.41 0.7 1 0.410.38 0.18 U.A1 U.21 0.19 0.31 O.1S2.] 3.2 3.6 .I 3.1 2.3 5.5

0.41

0.43

0.16

4.4

0.42

0.41

0.21

4.0

0.41

0.42

U.2

3.I

0.31

0.4S

0.28

4.0

TABLE A-111 (Cont.)

PYROXENE ANALYSES

3.l.Hyr Cycle

Soo 2

Al 201

FeoflO

"tno

"go

c&O

No20

Cr 203

C

M-14_

50.4 50. 5

3.6. 3.11

1.62 1.41

10.4 9.9

0.34 0.2s

14.3 14.6

20.2 20.3

0.32 0.34

303.0 100.4

CF 12-14A6

-- tmulQfts-S0. 6 s0. 3.0

3.44 3.S0 3.69

1.01 0." 0.9

1.6 7.S 1.2

0.39 0.33 0.3

3s.0 14.7 14.

21.3 12.0 22.6

0.2s 0.20 0.2s

99.4 99.7 100.6

Cf12-4- 1678-35

S3.6 49.1 50.3 S3.6 50.6 48.6 49.9 s.4 50.6

1.73 4.06 2.7 . 3.71 3.6s 6.1 4.38 2.94 2.95

1.31 1.14 1.29 1.04 0.f6 1.40 1.34 3.30 L.06

9.6 10.3 9.3 10.7 6.5 7.2 1.0 6.0 6.1

0.22 0.23 0.20 0.32 0.39 0.22 0.19 0.34 o.32

3.s 13.4 33.6 13.8 IS.3 13.? 14.6 14.1 3s.0

20.9 20.5 21.6 21.1 22.9 23.0 22.1 21.6 20.8

O.3S 0.35 0.3 0.47 0.27 0.30 0.24 0.36 o.29

0.04 - - . 0.22 0.3? O.09 0.20 0.36

99.s 99.? 99.6 300.6 100.6 100.9 100.3 300.6 99.9

so 1.669 I.Mo I.686 3.8 3.683 3."4 3.654 3.62 1.926 1.669 1.79? 3.65 3.697 1.8

IVAl 0.333 0.120 0.112 0.334 0.119 0.056 0.344 0.108 O.07s 0.133 0.203 0.350 0.303 0.333

tel 2.000 2.000 2.000 2.0uo 2.000 2.00 2.000 2.000 2.0O 2.000 2.00o 2.000 2.000 2.000

IAl 0.027 o.ols 0.o 0.038 0.040 0.020 0.034 0.03 - O.OS 0.060 0.040 0.024 0.018

1 0.044 0.0.J9 0.027 0.021 0.026 0.033 0.048 0.036 0.028 0.026 0.037 0.030 0.030 0.025

F. 0.322 0.309 0.242 0.233 0.223 0.306 0.322 0.296 0.332 0.197 0.21 0.213 0.246 0.266

" 0.030 0.0My 0. 003 0.003 0.003 0.006 0.006 0.eos 0.030 0.0O 0.006 0.e05 0.006 0.006

mg 0.163 0.833 0.036 0.6s 0.609 0.1s4 0.749 . 0.71 0. 2s 6.7Sl 0.602 0.60s 0.30

c 0. ml 0.608 0.64 0.816 0. 94 0.839 0.626 6.661 0.640 0.900 0.901 0.6O" 0.856 0.630

Na 0.021 0.024 0.017 0.014 0.037 0.025 0.024 0.026 0.034 0.017 0.020 0.037 0.024 0.020

Cr . . . . 0.002 0.003 - - - 0.oos 6.030 0.00 0.005 0.004

tct find

Vill 2.006 2.01s 2.000 2.006 2.012 1.96 2.009 2.009 2.033 2.012 2.0o9 2.006 2.000 2.001

Ccatloes 4.008 4.01S 4.008 4.006 4.012 3.586 4.009 4.009 4.011 4.309 4.009 4.0ON 4.000 4.001

we 0.42 0.42

to 0.41 0.42

Fs 0.37 0.16

Alm71 3.6 3.1

0.44

0.43

0.3

S.6

0.46

0.42

0.12

S.6

0.46

0.42

0.32

6.2

0.440.400.16

2.3

0.44

0.39

0.37

3.0

0.45 0.43 0.41

0.40 0.40 6.43

O.AS 0.1 0.10

3.4 2.7 6.4

0.46 0.47 .4S

0.40 0.42 0.42

0.12 0.33 0.3

7.0 6.2 4.2

0.43

0.43

0.14

4.4

01.,.

0) TABLE A-Ill (Cont.)

PYROXENE ANALYSES

I 1441LS-cdtcIC 8C± ... _ __°_- Q.l3Itr CjcleU11 Cone .14...... -WH..11l (f.w fl. _______

F27H-S Cf12.4-lie 157.F1-

IUO.7 4 d. crF _ 4 I b Cr4 8 4d8s.4- W0.b 49. s0.0 St.lSiot

A1203

1302Fell

hooU

hgO

Cau

Cr2oCrO 3

4.85 6.2 4.92

3. 0 1.92 1. S6.6 9.U 6.6

0.20 0.21 0.23

13.6 31.5 14.1

20.9 2.9 20.9

0.56 0.38 0.43

100.8 300.6 100.1

3.89

1.93

10.0

0.36

14.1

19.9

0.34

100.2

6.9

2.95

30.S

0.32

11.'

19.0

0.56

99.8

5.4 2.54

2.23 1.32

9.5 11.1

0.St 0.2?

13.1 14.6

19.8 18.8

0.60 0.42

100.2 100.0

3.63

1.10

10.2

0.20

13.4

20.0

0.68

99.6

2.98 2.23

I.59 1.13

9. 9.2

0.21 0.23

13.8 IS.0

20.4 20.9

0.48 0.34

99.4 100.1

SoI W^ -

IttVI Al111

Fe

No

Ca

wie

Cr

tact and

ll

ticat leas

ho

[aC

Fs

AIi

lctcn

1.89 3.1002 1.634

0.141 0.398 0.166

2. o0 2.000 2.000

0.069 0.011 0.049

0.046 0.053 0.041

0.265 0.218 0.26)

O.0S 0.00S 0.006

0.147 0.741 0.71

0.826 0.828 0.613

0.041 0.021 0.030

2.0W0 2.009 2.008

4.WI3 4.009 4.008

1. hS6 1. 19S

U. 144 0.205

2. W0 2.000

0.026 0. U0

0.0SJ 0.0O4

0. J13 0.3130

0.011 0.00

0. 716 0.659.

0.7ff 0.166

0.024 0.041

2.0W I .990

.00 3.994

3.605 1.S02

0. IS 0.098

2.0 00 2. 000.041 0.013

0.061 0.036

0.29) 0.341

0.036 0.001

0.162 0.82S

0. 190 0. 15

0.042 0.029

2. Ws 2. N

4.009 4.0Ws

1.611

0.123

2.0000.0J6

0.04)

0.321

0.0050.7500.600

0.00

I. 5 1. S

0.IIS 0.095

2.000 2.000

0.0350.044 0.029

0.310 0.265

U.O06 0.006

0.115 0.32

0.82S 0.631

0.034 0.024

2.009 2.0W9 2.001

4.009 4.009 4.001

0.41

0.34

4.6

U.4SS0.45

0.40

U.IS

S.l

U.44

0.42

U. 14

4.5

0.42

0.42

U.36

j.2

0.44

. d

0.3 8

J.6

0.43

0.41

0.16

3.9

0.39

0.41

0.18

3.1

0.43

0.40

0.11

j.4

0.43

0.41

U.6

3.0

0.43

0.42

0.Is

TABLE A-IV

OXIDE MINERAL ANALYSES, BASALTS OF CRATER FLAT

3.7-Myr Cycle 1.1-lyr CycleSlac Cone

Northern ConeCF12-6-3CF12-7-6A Bo 8_-

0to?

Al203

FeO

Fe203M90

MnOCr203

V2 03

lmenite

47.00.14

37.611.1

1.031.03

.A.

98. '

Magnet i te

8.65.00

37.746.80.360.36

0.12n.a.

99.8

0.239

0.2171.1631.3010.0650.011

0.004

IlmenI te

46.80.11

37.011.9

0.600.60

n.a.98.9

0. 8850.0030.778O. Z260.094

.0.013

Magnet I te

13.733.61

40.038.6*0.52

* 0.521.37o.a.

100.5

0.375

0.1551.216

1.0560.1440.0160.039

n.a.3. 000

Magneti tcin

01 ivine

3.4113.431.029.60.441 0.44

18.9n.a.

100.3

0. 0880.5420.8930.768

0.182

0.013O.514

.a.3. 0

Magneti te

15.9

2.7940.334.50.580.58

1.540.62

99.8

0.435

0.1201.2250.947

0.193

0.018

0.0440.0183.UUo

Magneti te

20.21.33

44.628.70.660.660.160.71

99.4

0. 9

0.0581.3720. 98

0.1660.0210.0050.0213.000

Magneti teIn

01 tvIne

13.04.81

36.736.60.380.38j.23

0.5999.8

0.350

0.2031.100

0.989

0.2380.012

0.092

0.0173.000

8lackt Cone

Magneti teIn

Magnet ite

15.01.46

40.037.90.580.580.11

0.5398.3

0.424

0.0641.252

1.0680.1540.0180.003

0.0163.000

01 IvIne

7.98.2

31.140.H0.430.435.3(). 52

99.6

0.20

0.3410.9161.0810.219

0.0130.147

0.015

3.000

TI

AlFe2+

Fe3

Mg

Mn

CrVI

0.892

0.0040.7940.2110.077

0.022

n.h.2.000

n.a. n.a.3.000 2.000

magnetiteUlvospinel1imenite

a; 40 1 hematite

0.740.26

0.s5

0.45

0.89

0.11

0.45

0.55

0.310.69

0.530.47

0.490.51

0.71

0.290.89

0.11

0.890.11

O.

TABLE A-IV (Cont.)

OXIDE IIHERAL AIJALYSES, BASALTS OF CRATER FLAT

1.1-Hyr Cycle

* Red Cone LittleCF12-4-4 CFl2

MagnetiteIn

Magnetite 01Ivine Flagnetite

e Cone M.E.2-4-130-

T10

Al203

FeO

Fe2 O3

HgO

HnO

Cr2 03

9203

T IAlFe2 '

Fe3

Hg

Hn

Cr

W

Magnetite

Ulvospinel

Ilmenite

hematite

19.7

1.40

43.4

30.5

3.58

0.79

0.13

0.71

100.2

0.540

0.060

1.321

0.836

0.194

0.024

0. 004

0.021

3.000

0.31

0.69

7.57.1

31.0

45.8

5.00.38

2.43

0.34

99.6

0.I99

0.297

0.9Z2

1.226

0.266

0.011

0.068

0.010

3.000

0.72

0.28

17.8

2.79

41.8

32.8

3.5?

0.63

n.a.

99.4

0.489

0.120

1.276

0.901

0.194

0.019

n.j.

3.000

0.38

0.62

.Hnetite

14.74. 70

39.5

37.23.700.480.31

n.A.I00.6

0.3950.19U1.13

1.0020.1970.015

0.009

n.a.3.000

0.500.50

0.3-Hyr Cycle

Lathrop WellsFU78-7

MagnetiteIn

Wagnetlte 1oivne

14.3 9.5

3.46 7.3

37.5 31.2

37.1 41.8

4.62 6.3

0.50 0.36

2.43 3.04

0.56 0.54

100.5 100.0

0. 850.146

1.123

0.999

0.246

0.015

0.069

0.016

3.000

0.48

0.52

0.249

0.302

0.911

1.100

0.327

0.011

0.084

0.016

3.000

0.62

0.38

TABLE A-V

N4PHIBOLE ANALYSES, BASALTS OfRED CONE AND LITTLE CONE N.E.

1.1-Myr Cycle

Little Cone N.E.CF1 2-4-l 3BPhenocrys ts

Red ConeCF12-4-4Groundmass

Sio2

Al 203

TiO2

FeO

MnO

M9O

CaO

Na2 0

K20

Cr2 03

(H 20)

I

SiIV Al

YIAl

Ti

Fe

Mn

Mg

Ca

Na

K

Cr

zcations

39.4

14.3

3.88

11.4

13.6

11.2

2.53

1.20

n .a.

2.5

100.0

5.851

2.1490.345

0.431

1.410

3.000

1.785

0.727

0.222

n.a.

15.920

40.0

12.9

3.94

11.2

0.05

13.9

11.8

2.45

1.00

n .a.

2.8

100.0

5.950

2.050

0.205

0.441

1.390

0.004

3.082

1.879

0.704

0.200

n.a.

15. 905

40.8

13.8

2.99

11.0

0.09

14.4

11.5

2.55

0.80

0.082.0

100.0

5.992

2.0080.384

* 0.328

1.351

0.006

3.151

1.806

0.719

0.144

0.006

15.895

40.9

13.4

3.17

10.6

0.11

14.2

11.6

2.59

0.800.14

2.5

100.0

6.026

1.974

0.349

0.351

1.305

0.010

3.117

1.829

0.730

0.144

0.011

15.846

Mg/(Fe+Mg) 0.68 0.69 0.70 0 .70

67*U.S. GOVERNMENT PRsNTiNG OFFICE: 19.O.-t777-022167

S a %

I IN

TABLE A-V

A4PHIBOLE ANALYSES, BASALTS OFRED CONE AND LITTLE CONE N.E.

1.1-Myr Cycle

Little Cone N.E.CFl2-4-13BPhenocrysts

Red ConeCF12-4-4Groundmass

SiO2

Al203

TiO2

FeO

MnO

MGO

CaO

Na20

K2 0

Cr2 03

(H20)

SiI AI

Vi AlYIA

Fe

Mn

Mg

Ca

Na

K

Cr

zcations

39.4

14.3

3.88

11.4

13.6

11.2

2.53

1.20

n .a.

2.5

100.0

5.851

2.149

0.345

0.431

1.410

3.000

1.785

0.727

0.222

n.a.

15.920

40.0

12.9

3.94

11.2

0.05

13.9

11.8

2.45

1. 00

n.a.

2.8

100.0

5.950

2.050

0.205

0.441

1.390

0.004

3.082

1.879

0. 704

0.200

n.a.

15.905

40.8

13.8

2.99

11.0

0.09

14.4

11.5

2.55

0.80

0.08

2.0

100.0

5.992

2.008

0.384

0.328

1.351

* 0.006

3.151

1.806

0.719

0.144

0.006

15.895

40.9

13.4

3.17

10.6

0.11

14.2

11.6

2.59

0.80

0.14

2.5

100.0

6.026

1.974

0.349

0.351

1.305

0.010

3.117

1.829

0.730

0.144

0.011

15.846

Mg/(Fe+Mg) 0.68 0.69 0.70 0.70

67*U.S. GOVERNMENT PRINTING OFFICE; 1941-0-777 022187

* bv

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Microfiche 3.0 (AOl)

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001025 S 5.00 A02 1175 311.00 AO 301-32 31700 AI4 451475 S23.00 A20026450 4.00 A03 174-200 12.00 A09 326-350 IS.00 AUS 474-500 24.00 A2105 475 7.00 £04 201.225 13.00 AIO 35 1-37 19.00 A16 501525 25.00 A22076-100 3.00 AS 226-250 14.00 All 376400 20.00 A17 S26-SSO 2600 £23101-125 9.00 £06 251.275 15.00 A12 401425 21.00 Al 551-575 27.00 £24126-150 50.00 o 6? N1O 16.00 A13 426450 22.00 A19 576400 26JO0 A25

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