American Mineralogist, Volume 68, pages 1146-1159, 1983 Allogenic layer silicate minerals in boreholeElmore #1, Salton SeaGeothermalField. California S. Douclns McDowell Departmentof Geologyand Geological Engineering Michigan Technological University, Houghton, Michigan 4993 I eNo Wrlpnpo A. Elpens Institute of Geophysics and Planetary Physics University of California, Riverside, California 92521 Abstract Reaction of coarsegrained allogeniclayer silicate mineralswith the hot, hypersaline brine of the Salton Sea Geothermal System has resulted in the formation of a series of metastable intermediate mineralphases that were created within the system, have a finite temperature range over which they exist, and react with the system in a regular but incomplete manner.Intensecalcite and dolomite/ankerite cementation allowed a suite of allogenicbiotite, chlorite, and muscovite grains to be preserved as unstablemineral phases to temperatures near 200'C. At this stage removal of significantportions of the cement and access of the fluid phaseto theseminerals initiateda series of complexreactions. Muscovite reacted to very fine grained interlayered illite/smectite through a phengitic muscovite intermediate phase that persists in the geothermal system for less than 50'C. The overall reactionof muscovite occurredin two steps, the first involving change within the 2:l layer via Mg additionand Al lossbut little change in the interlayer sheet, and the second involving significant K loss in the interlayer site as expandable smectite layers with exchangable Mg formed. Allogenic Ti-bearing biotite reacted completely at temperatures near 200oC to a metastableoptically anomalous titaniferous chlorite phase by a reaction mechanism that apparentlyallowed the 2:l octahedral sheetofthe reactantbiotite to be preserved intact in the product chlorite. The metastable chlorite persists throughoutthe entirechlorite zone, showssome systematic compositional variation with temperature, but was steadily reduced in amount by reaction to fine-grained Ti-free equilibrated authigenic chlorite. At the biotite isograd at 325C, the remainingmetastable titaniferouschlorite reacted completely and rapidly to a metastable titaniferous biotite in a reaction that involvedall structural sites in the minerals. This biotite reacted, within 10" above the biotite isograd, to Ti-free equilibrium authigenic biotite. A significant degree of the compositional scatter observed in low grade metamorphic layer silicate minerals may be due to the existence of metastable mineral phases which have preserved structural elements of the mineral they originally replaced. The data suggests that some sites within mineralscan remain inert while others continueto react with the fluid phase, creating partially equilibrated mineral phases and mineral reactions that must be dealt with on a site-bv-site basis. Introduction Investigations of prograde hydrothermal metamor- phism in the Salton Sea (Niland) Geothermal System have revealed a large number of systematic variations with increasing temperature in the geothermal system (Muffierand White, 1969; Helgeson, 1968; McDowell and Elders, 1980). In particular, systematic changes in authi- genic layer silicate mineralogy (McDowell and Elders, 0003-004x/E3/1I l2-l 146$02.00 fi46 1980) with increasing temperature in borehole Elmore I suggest that in fine grained sandstone a significantdegree of equilibration has occurred between the authigenic minerals and fluid phase in the geothermal system. It is possible to distinguish these authigenic layer silicates from a group of layer silicates that exhibit properties rangingfrom clearly allogenic to intermediateand ambig- uous. Within this group there are numerousindications of
Transcript
American Mineralogist, Volume 68, pages 1146-1159, 1983
Allogenic layer silicate minerals in borehole Elmore #1,Salton Sea Geothermal Field. California
S. Douclns McDowell
Department of Geology and Geological EngineeringMichigan Technological University, Houghton, Michigan 4993 I
eNo Wrlpnpo A. Elpens
Institute of Geophysics and Planetary PhysicsUniversity of California, Riverside, California 92521
Abstract
Reaction of coarse grained allogenic layer silicate minerals with the hot, hypersalinebrine of the Salton Sea Geothermal System has resulted in the formation of a series ofmetastable intermediate mineral phases that were created within the system, have a finitetemperature range over which they exist, and react with the system in a regular butincomplete manner. Intense calcite and dolomite/ankerite cementation allowed a suite ofallogenic biotite, chlorite, and muscovite grains to be preserved as unstable mineral phasesto temperatures near 200'C. At this stage removal of significant portions of the cement andaccess of the fluid phase to these minerals initiated a series of complex reactions.
Muscovite reacted to very fine grained interlayered illite/smectite through a phengiticmuscovite intermediate phase that persists in the geothermal system for less than 50'C. Theoverall reaction of muscovite occurred in two steps, the first involving change within the2:l layer via Mg addition and Al loss but little change in the interlayer sheet, and the secondinvolving significant K loss in the interlayer site as expandable smectite layers withexchangable Mg formed. Allogenic Ti-bearing biotite reacted completely at temperaturesnear 200oC to a metastable optically anomalous titaniferous chlorite phase by a reactionmechanism that apparently allowed the 2:l octahedral sheet ofthe reactant biotite to bepreserved intact in the product chlorite. The metastable chlorite persists throughout theentire chlorite zone, shows some systematic compositional variation with temperature, butwas steadily reduced in amount by reaction to fine-grained Ti-free equilibrated authigenicchlorite. At the biotite isograd at 325C, the remaining metastable titaniferous chloritereacted completely and rapidly to a metastable titaniferous biotite in a reaction thatinvolved all structural sites in the minerals. This biotite reacted, within 10" above the biotiteisograd, to Ti-free equilibrium authigenic biotite.
A significant degree of the compositional scatter observed in low grade metamorphiclayer silicate minerals may be due to the existence of metastable mineral phases which havepreserved structural elements of the mineral they originally replaced. The data suggeststhat some sites within minerals can remain inert while others continue to react with thefluid phase, creating partially equilibrated mineral phases and mineral reactions that mustbe dealt with on a site-bv-site basis.
IntroductionInvestigations of prograde hydrothermal metamor-
phism in the Salton Sea (Niland) Geothermal Systemhave revealed a large number of systematic variationswith increasing temperature in the geothermal system(Muffier and White, 1969; Helgeson, 1968; McDowell andElders, 1980). In particular, systematic changes in authi-genic layer silicate mineralogy (McDowell and Elders,
0003-004x/E3/1I l2-l 146$02.00 fi46
1980) with increasing temperature in borehole Elmore Isuggest that in fine grained sandstone a significant degreeof equilibration has occurred between the authigenicminerals and fluid phase in the geothermal system. It ispossible to distinguish these authigenic layer silicatesfrom a group of layer silicates that exhibit propertiesranging from clearly allogenic to intermediate and ambig-uous. Within this group there are numerous indications of
partial reaction with the fluid phase to form layer silicatephases that appear to be metastable with respect to thesystem. The purpose of this paper is to briefly discussseveral examples of such metastable layer silicates andthe reactions that might control their formation.
Geologic background
There exists in Elmore I a regular prograde zonation insandstone from clay carbonate metamorphic facies (Zen,1959) through chlorite and biotite zone greenschist faciesto, at the base ofthe borehole, an andradite-garnet zone.The mineral assemblages and temperatures of these vari-ous zones are summarized in Table l.
The gross stratigraphy ofthe geothermal system (Ran-dall, 1974) consists of an upper unconsolidated to poorlyconsolidated clay silt-evaporite lacustrine sequence atdepths shallower than approximately 350 m, and a lowermoderately to well-consolidated thinly interbedded sand-stone-siltstone-mudstone sequence of fluvial-deltaic ori-gin originating from the Colorado River System (Muffierand Doe, 1968; Van de Camp, 1973). The active geother-mal system is operating entirely within the deeper fluvial-deltaic sequence, while the overlying lacustrine sequenceacts as a very effective stratigraphic permeability cap onthat system. Within the deeper fluvial-deltaic sequence,fluid/rock interaction has resulted in a series of physicalchanges in the sandstones that correlate directly with themetamorphic zones. The clay-carbonate metamorphicfacies consists of well-cemented low porosity sandstone
Table l. Summary of assemblages in sandstone, Elmore I , withinfluvial-deltaic sequence. Lacustrine sequence occurs at depths
of less than approximately 427 m.
r185'C depth - ^"427n
l t47
SALTON SEA GEOTHERMAL FIELD
Fig. l. Location of boreholes in Salton Sea Geothermal Field.Solid lines = temperature ("C) at 914 m (3,0(X) ft) depth, modifiedafter Palmer (1975) and Randall (1974). Dashed lines = relativeheat flux in l0-3 watts/m2 from Lee and Cohen (1979). Rhyoliteextrusives: OB = Obsidian Butte; RH : Rock Hill; RI : Redlsland: MI = Mullet Island.
which has formed a relatively impermeable dome-shapedcarbonate cap on the geothermal system entirely withinthe fluvial-deltaic sequence. Sandstone from the deeperchlorite zone, while still having some patchy carbonatecement, is porous and makes up the main reservoir rockin the geothermal system. The contact between the claycarbonate facies carbonate cap and the porous chloritezone is relatively sharp and well defined in the interiorportions of the geothermal field where borehole Elmore Iis located (Figure l). This contact, at a depth of -439 m(190"C) in Elmore l, represents a distinct porosity/perme-ability discontinuity in the geothermal system as well asdefining an efective chlorite isograd. With increasingdepth, sandstone porosity gradually decreases from val-ues of l5-20Vo in the shallowest, lowest temperatureportions of the chlorite zone to 9Vo inthe highest tempera-ture portions of the biotite zone where andradit€ garnetappears. Within the biotite zone recrystallization of bothquartz and feldspar overgrowths is sufrciently intensethat overgrowths are no longer commonly visible, and thesandstone has evolved into relatively low porosity horn-fels.
MCDOWELL AND ELDERS: ALLOGENIC LAYER SILICATE MINERALS
340'C 1325nSee a6 ebover but ecthollteappeers aDd @acovlte oo Longeraeen
350'C CARNET 2I20nElotite + qErtz + epldote +gahet + albl te + sctLDol l te+ pyrlte + 6phoe
a
oD
7RHa
I l4E
In the shallowest, least metamorphosed sandstonesthat could be examined in borehole Elmore 1, a widevariety of layer silicate grains is observed ranging frompore-filling clay-sized aggregates to single grains up to0.10 mm (rarely 0.34 mm) in length. At these depths, insandstones completely cemented by microcrystalline topoikiloblastic carbonate cement, the distinction betweenthe very fine-gained highly reactive authigenic clay-sizedlayer silicates and the coarser grained allogenic grains isclear. The allogenic layer silicates show no texturalevidence of reaction to their finer grained counterparts,and have compositions consistant with a higher tempera-ture igneous/metamorphic origin. These clearly detritalgrains have apparently not been altered by the geothermalsystem, and have been preserved intact to temperaturesup to 190'C or more in highly cemented, low porositysandstone where no access to the fluid phase was avail-able. X-ray diffraction investigations of the fine grainedfraction of sandstones from nearby wells, which havebetter sample coverage within the carbonate cap, indi-cates that the clay fraction is continuously reacting withchanging temperature in highly cemented sandstone.
Removal of the bulk of the carbonate cement at thechlorite isograd with increasing temperature and develop-ment ofhighly porous calcite-chlorite zone alteration hasresulted in a series of reactions that affect the allogeniclayer silicates. Some react directly but at variable rates tofine grained authigenic phases such as illite or chloritewhich are in apparent equilibrium with the geothermalfluids (McDowell and Elders, 1980). Others react tocoarse grained layer silicate minerals which are createdby replacement of the allogenic layer silicates, exist overa finite range of temperatures, but react continuously totheir fine grained apparently equilibrated equivalentsthroughout that temperature range. Such coarse grainedphases are authigenic in the sense that they were createdat the observed depth/temperature/fluid composition,etc., but are clearly not equilibrated with the system. Forwant of a better term, these will be referred to asmetastable phases which may represent kinetic interme-diate stages in the overall reaction of coarse grainedunequilibrated allogenic layer silicates to fine grainedequilibrated authigenic layer silicates.
The fine grained equilibrated authigenic layer silicatesprogressed through a series of regular changes withincreasing temperature (McDowell and Elders, 1980) thatincluded a gradual coarsening of the originally clay-sizedmaterial to eventually produce coarser grained (up to 0. l0mm) idioblastic single crystals. The general sequenceobserved for the transformation of dioctahedral mica withincreasing temperature starts with <0.005 mm regularmixed layer illite/smectite with about llVo expandablelayers at temperatures near l90oC and progresses through<0.01 mm illite that gradually coalesces into aggregatesof subparallel grains with overall dimensions approaching0.10 mm at temperatures near 280'C. These grains appearto recrystallize into 0.10 mm clear idioblastic muscovite
MCDOWELL AND ELDERS: ALLOGENIC LAYER SILICATE MINERALS
grains at temperatures above 290'C. Unlike dioctahedralmica, however, both fine grained xenoblastic and coarsegrained idioblastic chlorite persist to the biotite isograd attemperatures above 325'C. The textural and composition-al variability of chlorite is far greater than that of whitemica, reflecting the diversity of reactions that producechlorite in sandstone. At higher temperatures, authigenicbiotite also undergoes a gradual coarsening with increas-ing temperature.
Method of investigation
The data and observations for this paper are basedmainly on petrographic examination and microprobe(mainly energy dispersive, some wavelength dispersive)analyses of layer silicate minerals in sandstone cuttingsfrom Elmore 1. The microprobe techniques used areidentical to those reported by McDowell and Elders(1980), and the reader is referred to that paper for furtheranalytical details. The dfficulties encountered due toepoxy-contamination of microprobe analyses of the finegrained authigenic layer silicate aggregates were muchless prevelent on analyzing the coarser grained layersilicate minerals that are the main subject of this report.
Allogenic layer silicates
The coarse grained allogenic layer silicates in thecarbonate cap sandstone consist ofunreacted muscovite,chlorite and biotite which have been preserved as unsta-ble phases to temperatures as high as 200'C. Examples ofthe compositions of these minerals are given in Table 2.This entire suite of minerals makes up less than 2 percentby volume of sandstone, but their relatively coarse sizeallows the progressive reaction of these phases to beinvestigated in some detail. The criteria for their identifi-cation includes various combinations of the following: (l)coarser grained, clear crystals with evidence of accumu-lated internal strain; (2) well developed internal cleavagereflecting a long pre-geothermal history; (3) bent andbroken forms, often with splayed ends of internal separa-tion along cleavage surfaces; (4) alteration to a variety ofauthigenic layer silicates; (5) compositions distinctly dif-ferent from their authigenic counterpart, and characteris-tic of a higher temperature origin; and (6) lack of anysystematic compositional change with depth/temperaturein the geothermal system.
Allogenic muscovite occurs as clear, often bent orbroken plates up to 0.03 mm thick and 0. l0 mm long withwell developed cleavage and often shadowy extinction. Itis especially prevalent and is preserved to greater depthsin siltstone, relative to sandstone. Its composition (Table2,a) is distinctly different from the regular mixed layerillite/smectite (Table 3,a) stable at the boundary betweencarbonate cap and chlorite zone. Relative to the authi-genic idioblastic coarse grained muscovite (Table 3,b)stable in the geothermal system at temperatures above290'C, the allogenic muscovite is more sodic and containsmore Fe and Mg. These characteristics suggest that the
Table 2. Allogenic layer silicates at 439 m (190'C) in completelycalcite-cemented sandstone. A) Calculated assuming )cations -
Na,K,Ca = 6.00(a) or 7.ffi(b) or 10.00(c). B) Calculatedassuming ! + charge = 22.N andE6.5Vo of Fe1o,4 = Fe3*(a) or
l$EoFes*(b), or ) + charge = 28.00 and l$Vo Fe3*(c).
fi49
rocks and contrasts sharply with the small variability andregular compositional variation of authigenic biotite withchanging temperature.
In addition allogenic biotite is distinctly titaniferous,with 0.19-0.25 Til3.00 octahedral cations, again contrast-ing sharply with the almost Ti-free authigenic biotite. Thehigh Ti-content of allogenic biotite also supports a highertemperature origin for these biotites. The range andintensity of pleochroism in allogenic biotite increases asthe Ti-content increases. The correlation ofoptical prop-erties and Ti-content also holds true for reaction productsof allogenic biotite deeper in the geothermal system, afact which allows these reaction products to be tracedback to their biotite progenitor.
Allogenic chlorite occurs as poorly formed grains withlength:width ratios of 1.5:l to 3:l and typical maximumdimensions near 0. I mm, although very rare grains up to0.35 mm were noted. It exhibits all the textural propertiesof allogenic biotite in the completely cemented sand-stones, and like biotite it shows no evidence of reaction toany other phase. It is, however, distinctly more roundedthan biotite. In these rocks only pale green or gray-green,very weakly pleochroic chlorite grains are observed, andthere is a clear optical distinction between allogenicchlorite and the more highly colored, more stronglypleochroic allogenic biotite. The allogenic biotite:chloriteratio is approximately 2: l, while the total volume percent-age of the two is seldom over 3Vo of any given carbonate-cemented sandstone.
The composition of allogenic chlorite (Table 2,c) fromcarbonate cap sandstones is grossly similar to identicalappearing pale green weakly pleochroic coarse grainedchlorite from the chlorite zone at higher temperatures. Itis also similar to fine grained authigenic chlorite (Table3,d), although the authigenic chlorite is texturally quitedissimilar and usually is a brighter green color veryreminiscent of chrome diopside. Allogenic chlorite fromthe carbonate cap is Ti-poor, unlike many coarse grainedchlorite grains observed in the deeper chlorite zone. Thecompositional overlap of coarse-grained allogenic chlo-rite with other chlorite types is illustrated in Figure 3.
Allogenic rnuscovite reaction
Unreacted allogenic muscovite is preserved only incompletely calcite-cemented sandstones of the calcitecap. Alteration of muscovite starts immediately on expo-sure to the fluid phase as the sandstone cement isremoved at temperatures > 190'C, and the last identifiableallogenic muscovite is observed at2&"C (675 m depth).The only unreacted muscovite that is observed in theshallow portions of the chlorit e zone occurs in patches ofcalcite cement preserved in sandstone. Muscovite reactsto whatever dioctahedral phase is stable at a giventemperature. The lack of identifiable allogenic muscoviteat higher temperatures indicates that the relatively slug-gish alteration reaction was complete at these tempera-
McDOWELL AND ELDERS: ALLOGENIC LAYER SILICATE MINERALS
c
Chlor l te
#analyeee/{grahe 7 | 3
As l 3 . 0 7 ! 0 . 0 3
rv l r 0 .93 ! o .o3Vr l l 1 .84 ! o .o3
uS 0 .02 ! 0 .02
F" . 0 .09 ! 0 .04
T l 0 .04 ! 0 .02
Itr <0.01
N a 0 . 1 3 ! 0 . 0 3
K 0 .84 s 0 .02
Ca 0 .01 ! 0 .01
Bs r 3 . 0 6
rvel 0.94Vrl, l
L.B2
uS O.O2
F.2{ o.o1
" "$ o .o8
T1 0 .04
Itr <0.01
tv r r .98
Na 0 .13
K 0 . 8 4
ca 0 .01
L7 ILO
2 , 9 3 ! O . L z
1 . 0 7 ! 0 . 1 2
0 . 4 9 ! 0 . 1 8
L . 2 6 ! O . 4 4
1 . 0 5 t 0 . 3 1
0 . r 8 ! 0 . 0 4
0 . 0 2 ! 0 . 0 1
Trace, <0.01
0 . 8 8 ! 0 . 0 6
0
2 . 8 3
t . r 7
0 . 3 4
r . 2 2o.920 . 1 0
0 . 1 7
0 . 0 2
2 , 7 7
o
0 . 8 5
0
3 lL
2 . 9 7 ! 0 . 0 3
1 . 0 3 r 0 . 0 3
1 . 2 3 i 0 . 0 2
2 . 3 3 ! 0 . 0 2
2 , 3 3 ! 0 . 0 0
0 . 0 1 ! 0 . 0 1
0 . 0 3 ! 0 . 0 3
o . o 0
0 . 0 1 ! 0 . 0 1
0 . 0 6 ! 0 . 0 2
2 . 9 4
1.06
l . 1 8
2 . 3 0
2 . 0 8
o . 2 3
0 . 0 1
0 . 0 3
5 . 8 3
0 . 0 0
0 . 0 r
0 .06
allogenic muscovite originated at a significantly highertemperature than did the authigenic muscovite from thegeothermal system (Guidotti and Sassi, 1976).
Allogenic biotite takes the form of ragged, often bent,variably pleochroic grains up to 0.20 mm in length. Colorsvary from weakly pleochroic dull green through variousshades of green-brown and brown to very strongly pleo-chroic light brown to dark brown to red-brown. Thebiotite grains often have been physically disrupted bypenetration of the calcite cement along cleavage planes,but there is no evidence of chemical alteration of thesegrains. Most biotite grains show little evidence of round-ing, either by transport or alteration.
The composition of allogenic biotite (Table 2,b) is mostsimilar to biotite from medium grade pelitic metamorphicrocks (Fig. 2) or from acid-intermediate plutonic rocks(Dodge et aI., 1969). The allogenic biotite at 190'C isdistinctly more aluminous than the authigenic biotite(Table 3,c) that exists at >325"C in the geothermalsystem, and unlike the authigenic biotite contains a fullcomplement of interlayer cations. The large scatter ob-served in allogenic biotite composition reflects the ran-dom mixing of grains originating from a variety of source
a
Ifuacovlte
bBlot l te
I t50 MCDOWELL AND ELDERS: ALLOGENIC LAYER SILICATE MINERALS
Table 3. Authigenic, equilibrated layer silicates. A) Calculated assuming )cations - Na,K,Ca = 6.00(a,b), 7.00(c), or 10.00(d). B)Calculated assuming E + charge : 22.00 and E6.5Vo Fe3*(a,b) or lSVo Fe3*(c), or ) + charge = 2E.00 and lOVo Fe3+(d). Analyses
from McDowell and Elders (1980).
a bI11lte/102 Srectite ldtoblasttc ltuacovlte
439 n (190 'c ) 844.s n (295"c)
c dBlot i te Chlor i te
1143 n (330"c) 675 n (264"c)
#aoalyees
As l
ruatvrAl
s1rv^ .vret
Mg- 2 +! e
F"3+
TL
lftr
tvI
Na
K
Ca
l 9
3 . 5 3 r 0 . 0 7
0 . 4 7 ! 0 . 0 7
1 , 4 0 ! 0 . 0 7
0 . 3 0 r 0 . 0 4
o . 2 7 ! O . 0 4
0 . 0 2 t 0 . 0 2
0 , 0 0
0 . 0 6 ! 0 . 0 2
0 . 5 6 ! 0 . 0 4
0 . 0 3 ! 0 . 0 2
5
3 , 0 9 t 0 . 0 6
0 . 9 1 ! 0 . 0 6
1 . 7 3 I 0 . 0 8
0 . 0 8 t 0 . 0 5
0 . 1 6 ! 0 . 0 3
0 . 0 2 I 0 . 0 1
0 . 0 0
0 . 0 8 t 0 . 0 4
0 . 8 3 r 0 . 0 7
0 . 0 1 r 0 . 0 1
3 . 0 9
0 . 9 1
0 . 0 8
0 . o 2
0 , 1 4
0 . 0 2
0
2 . 0 1
0 . 0 8
0 . 8 3
0 . 0 r
4
3 , 2 5 r 0 . 0 5
0 . 7 5 I 0 . 0 5
0 . 0 3 ! 0 . 0 2
2 . 3 8 t 0 . 0 9
0 . 5 6 ! 0 . 0 8
0 . 0 1 t 0 . 0 1
0 . 0 2 ! 0 . 0 1
0 . 0 0
0 . 6 3 ! 0 . 0 3
0 . 0 2 ! 0 . 0 1
3 . 2 4
o . 7 5
o . 0 2
2 . 3 7
0 . 1 0
0 . 0 1
0 . 0 2
2 . 9 8
0 . 0 0
0 . 6 3
o . o 2
6
3 . 1 0 ! 0 . 0 9
0 . 9 0 ! 0 . 0 9
1 . 5 8 I 0 . 1 5
2 . 2 3 ! O . O 5
2 . 1 3 ! 0 . 0 8
0 . 0 0
0 . 0 2 r 0 . 0 2
0 . 0 0
0 . 0 2 ! o . 0 1
0 , 0 2 I 0 . 0 2
3 . 0 1
0 . 9 9
t . 4 2
2 . t 7
1 . 8 6
o . 2 L
0
o . o 25 .68
0
o .o2
o .o2
MS
Ftt
ltr
Na
K
Ca
3 . 5 s
o :45
r . 4 2
0. 30
0 .04
o . 2 4
o .o2
0
2 . 0 3
0 .06
0 . 56
0 . 0 3
tures and for the length of time necessary to reach thesetemperatures.
Within the lower temperature portions of the chloritezone, allogenic muscovite exhibits a range of alterationtextures on reaction to the fine grained alteration prod-ucts. Where the ends of allogenic grains project into openpores, expansion and splaying of muscovite occurs. Thetextures so produced are almost identical to the kaolini-tized white mica illusrrated in Millot (1970, Plate l, Figs.l, 2, 5 and 6, p.240-241). The splayed ends have oftenbeen completely replaced by aggregates of sericite-likeillite grains in subparallel orientation. Partial to completereaction rims of illite are common. In a few cases, theoriginal muscovite has been completely replaced by amass of illite grains in subparallel orientation with theoriginal shape of the muscovite generally preserved.
The compositional changes resulting from alteration ofmuscovite to the stable dioctahedral layer silicate phaseare summarized in Table 4 and Figures 4, 5, and 6. Mostexamples are taken from sandstone at a depth of 439 m(190"C), where an illite/smectite (I/S) with approximatelyl$Vo expandable layers is the stable authigenic phase
(McDowell and Elders, 1980). The composition of I/Swhich formed by complete replacement of preexistingallogenic muscovite (squares, Fig. 4 and Table 4,a) isdistinctly different than the average composition of finegrained authigenic I/S (Table 3,a, and star on Fig. 4), mostof which appears to have reacted continuously withincreasing temperature from the original smectite-richclay matrix. In particular I/S after muscovite containsmore tetrahedral Al substitution and less octahedral Fe *Mg substitution, but similar interlayer occupancies, rela-tive to authigenic I/S from the sandstone matrix. Thedistribution of authigenic matrix I/S in Figure 4 is suchthat most analyses cluster in an elliptical area (dashedline) around the average value, while a few analyses plotnear the compositions ofI/S that replaced coarse grainedallogenic muscovite. It is possible that these few authi-genic matrix I/S also formed by replacement of smallerallogenic muscovite grains, a reaction for which convinc-ing textural evidence is lacking or ambiguous.
A clue to the actual reaction path followed duringalteration of allogenic muscovite to authigenic I/S or illiteis given in a series of analyses across a partially reacted
Fig. 2. Allogenic (large dots) vs. Authigenic (small dots)biotites in Al-total Fe-Mg system (molecular proportions).Arrows indicate change in authigenic biotite composition onincreasing temperature. Area within solid line-biotites frompelitic rocks (Brown, 1967; Mather, 1970; Lambert, 1959;Chinner, 1960; Butler, 1967); within dashed line=frommetabasic rocks (Cooper, 1972).
allogenic muscovite grain that shows three distinct con-centric zones: (1) a clear muscovite core (Table 4,b) withuniform extinction and no evidence ofalteration (x, Figs.5 and 6); (2) a wide irregular zone (Table 4,c) which showsmottled extinction to various degrees (closed circles,Figs. 5 and 6); and (3) a thin, discontinuous outer zone(Table 4,d) consisting of an aggregate of minute, physical-ly distinct I/S grains in subparallel orientation (opencircles, Figs. 5 and 6). These zones appear to represent aprogressive, stepwise replacement of muscovite by illite/smectite. The single analysis of the outermost zoneapproaches the composition of I/S that formed by com-plete replacement of another muscovite (Table 4,a).
The zonation suggests that the reaction occurs in twodiscrete steps. Starting with slightly phengitic allogenicmuscovite, the first step in the reaction occurs entirelywithin the 2:l layer of the original muscovite, withessentially no change in the interlayer position. Celadon-itic substitution by Mg in the octahedral site, coupledwith Si substitution in the tetrahedral site and resultingredistribution of Al, mark the major compositionalchanges in this step. The Fe-content remains approxi-mately constant, resulting in a large increase in the Mg/(Mg+Fe.) ratio. Charge balance between layer and inter-Iayer units, calculated assuming a fixed Fe3*/Fe total =
0.865', is excellent for all analyses, and yields octahedraloccupancies of 2.fi)-r0.01 in all cases. Note that anabsolute loss of Al is required by this reaction.
Examination of the mottled zone at high magnificationsin the petrographic microscope reveals a thin optically
rAverage Fe'* percentage of illite from analyses tabulated byWeaver and Pollard (1973).
I t 5 l
gradational zone separating slightly larger regions ordomains of variable size with relatively uniform extinc-tion. No sharp break in optical properties is visible. Theoptically gradational boundary zones between domainsmight represent zones of lattice defect concentration that,on continuing reaction, produce true, discontinuous grainboundaries during the second reaction step when physi-cally separable white mica grains are produced.
The formation of the optically variable defect-richboundary zones is probably the result oflattice distortionand defect pileup during the diffusion of Si, Al, and Mg inthe 2:l layer. The resulting structurally more homoge-neous domains and adjacent high energy defect zonesmay represent a metastable minimum energy situationwhich would greatly enhance the ability to move small,highly charged cations such as Al and Si within thesystem. True lattice diffusion would be required only tothe nearest defect zone, and all further movement couldtake place much more rapidly through the interconnecteddomain boundaries and out of the original grain entirely.Note that the analyses of the mottled zone (Table 4,c)were made with an electron beam spot size (30-50microns) considerably larger than the domains, and thusrepresent an average value for cations contained bothwithin the domains and the defect-rich boundary zones.The calculated formulae in Table 4 have assumed allcations to be located within the mica structure, not in thedefects, and this may not be the case in reality.
The second step in the reaction sequence involves thecomplete disruption of the original muscovite lattice andformation ofseparate I/S grains. It is during this step thatmost of the interlayer K is lost, while little changeappears to occur within the tetrahedral site during thisreaction step. The major change in interlayer K corre-sponds with a major change in the long range order of thecrystal structure, in going from a distorted but still intactphengite to the fine grained retrograde I/S aggregate. Thisis analogous to the sharp prograde increase in interlayerK content observed with increasing temperature in theequilibrated authigenic phases at temperatures near 290'Cin going from the fine grained illite to coarse grainedidioblastic phengite (McDowell and Elders, 1980). In both
Fig. 3. Allogenic chlorite (triangles), low-Ti fine-grainedchlorite (open circles), and all coarse grained high-Ti chlorite(filled circles) in Al-total Fe-Mg system (molecular proportions).
MCDOWELL AND ELDERS: ALLOGENIC LAYER SILICATE MINERALS
M9Fe
rt52
Fig. 4. Authigenic (open circles) illite/smectite, partiallyaltered allogenic muscovite (solid circles), and allogenicmuscovite completely replaced by illite/smectite from 439 mdepth (190'C) (squares) plotted in Muscovite (Mu)-Celadonote(Ce)-Pyrophyllite (Py) system using method of Hower andMowatt (1966). Unaltered muscovite plots in Mu corner (notshown). Average authigenic I/S indicated by star, andcomposition of most I/S outlined by dashed line. Contours ofpercent expandable layers based on data of Hower and Mowatt(1966) but drawn parallel to Mu-Ce join.
cases the major change in K content is associated with amajor change in the structural integrity of the dioctahe-dral phase.
Note that the charge balance of the single outer illitezone analysis (Table 4,d) and the illite/smectite aftermuscovite analyses (Table 4,a) is very poor as calculated. If analysis 4a is recalculated assuming Fe+3 : 0.865
MCDOWELL AND ELDERS: ALLOGENIC LAYER SILICATE MINERALS
o.5
x
x2 . 5
AI
2 . O
3 . 5
3.o Si 3.5
Fig. 6. Compositional changes during alteration of singleconcentrically zoned muscovite (formula proportions). Symbolssame as in Fig. 5, except that composition of muscovitecompletely replaced by illite/smectite (squares in Fig. 5)indicated by area within solid line. Arrows emphasize two-stepalteration sequence. Al and Fe indicate total Al and Fe,respectively. See text for details.
total Fe, and )+charge = 22.00, the formula be-comes (Kq.60Nao.osMgo. ro)ll(Tio.orFdjrFe62.[aMg6.21Al,.5o;vllAlo.*Sir.3o)Ivoro(oH)2. The charge balance is quitegood (-0.89, +0.85) assuming all Mg in excess of theideal 2.00 octahedral occupancy occurs as exchangeableinterlayer Mg in expandable smectite layers. Thus thesecond reaction steps between mottled zone and outeraggregate zone is complicated by the creation ofexpand-able layers in this temperature range.
An overall reaction from allogenic muscovite (Table2,a) to fine grained illite/smectite (Table 4,a) has beenwritten below, assuming one mole of illite/smectite isproduced by one mole of muscovite. Note that significantAl is lost during this reaction, and is probably consumedafter moving very short distances in the fluid phase bygrowing authigenic phases such as chlorite, alkali feld-spar, albite, or aluminous sphene. Minute chlorite grainsaxe often observed immediately adjacent to the outeredges of allogenic muscovite grains in various stages ofalteration, so that chlorite is the most plausible Al sink. Ifthe concentrically zones grains discussed above are rep-resentative of the actual reactions, then a more realisticmethod of writing the overall reaction might be in theform of two steps, the first balanced assuming constantinterlayer occupancy, and the second balanced assumingconstant tetrahedral Si occupancy.
Fig. 5. Compositional changes during alteration of allogenicmuscovite (plus) to authigenic illite/smectite in Muscovite (Mu)_Celadonite (Ce)-Pyrophyllite (py) system after Hower andMowatt (1966). Sequence ofanalyses from single concentricallyzoned muscovite: clear muscovite core (X), mottled zone (filledcircles), illitic rim (open circles). Muscovite completely replacedby illite/smectite shown by squares. Arrows indicate reactionpath. 0.5 = 50Vo Mtt. See text for details.
Texturally and compositionally distinct coarse grainedtitaniferous biotite and optically normal low-Ti chloritemake up the mafic detrital suite in the highly cementedsandstone of the carbonate cap. Chlorite is the onlycoarse grained mafic layer silicate observed in the cal-cite-chlorite zone from 622 m to the biotite isograd nearll00 m and 325'C. No coarse grained biotite is found inthis interval.
Examination of sandstone cuttings at the boundary ofthe carbonate cap and chlorite zones at 439 m depth(190'C) indicates that all coarse grained allogenic biotitereacts very rapidly to some form of chlorite. In a fewcases, alteration of highly pleochroic titaniferous biotitedirectly to fine grained pale green low-Ti authigenicchlorite is observed. In most cases, however, singlegrains of coarse grained allogenic biotite were completelyreplaced by single grains of chlorite (Table 5,a) throughthe progressive layer by layer pattern so commonlyobserved during retrograde alteration of biotite in meta-morphic or igneous rocks. The shape, distribution andoptical properties of the product chlorite grains mimicthose of the reactant biotite, producing a series of optical-ly anomalous coarse grained chlorites at the chloriteisograd. These chlorites are strongly pleochroic, withcolors ranging from light yellow-brown to medium or darkbrown or red brown, and birefringence as high as 0.021.As in the case of the reactant biotite, the range andintensity ofpleochroic colors and the birefringence oftheproduct chlorite increases as the Ti-content increases to a
Table 4. Alteration of allogenic muscovite at 439 m (190"C). Allanalyses calculated assuming )cations - Na,K,Ca : 6.00. A)Muscovite completely replaced by illite/smectite. B), C), D)sequence of alteration of a single allogenic muscovite-see text
for details.
I t53
maximum of 0.23 Ti/formula unit. The titaniferous chlo-rite persists through the entire calcite-chlorite zone indiminishing amounts, and everywhere within its -135"C
temperature range of existence shows evidence of reac-tion to fine grained low-Ti equilibrated authigenic chlo-rite. Thus, while authigenic, it is clearly not equilibratedbut appears to be metastable with respect to the geother-mal system.
The result, in the lowest temperature portions of thecalcite-chlorite zone, is a series of authigenic coarsegrained chlorites with highly variable optical propertiesranging from optically normal pale-green weakly pleo-chroic grains with low birefringence to the high-Ti opti-cally anomalous grainsjust described. The percentage ofcoarse grained authigenic chlorite is approximately equalto the combined percentage of allogenic chlorite andbiotite in the adjacent carbonate cap sandstones. Sincethe ratio of allogenic biotite to chlorite was approximately2:1 in the highly cemented sandstones, most of the coarsegrained single chlorite crystals in the shallowest calcite-chlorite zone must have been the product of the biotite tochlorite reaction. The optically anomalous titaniferouschlorites were formed from the most titaniferous biotites,while lower titanium biotite produced more normal ap-pearing chlorite. Much of the original allogenic chloritefrom the carbonate cap persists into the chlorite zonewith little recognizable reaction at the facies boundary.
Many coarse grained chlorites preserve features of adetrital origin such as bent or broken forms with splayedends well into the chlorite zone. Shadowy extinctionindicative ofstored lattice strain is not observed at depthsgreater than 622 m (250'C), indicating some degree ofinternal reorganization and possible chemical exchange inthese grains. The gradual coarsening of the fine grainedchlorite, as well as pore filling by epidote and by quartzand feldspar overgrowths, cause features indicative of adetrital origin to gradually disappear. The titaniferousmetastable chlorite remains distinctly identifiable andpersists to the biotite isograd.
Relative to fine grained chlorite, coarse grained chloritehas a higher Mg/(Mg+Fe1) ratio (usually >0.5), is similarin Al-content, and may be richer in Ti if formed byreaction from allogenic biotite (Fig. 3). The compositionalsimilarity of all chlorite types is apparent from Figure 3.The compositional scatter of chlorite appears to bestrongly controlled by the specific reactant that wasaltered to form the authigenic chlorite, and not to varia-tions in bulk composition or even fluid composition.Unpublished bulk chemical analyses of sandstone andshale in adjacent wells indicate relatively little composi-tional variation through the calcite-chlorite zone. Thesandstones in this zone are very porous, and both oxygenisotope analyses (Kendall, 1976) and authigenic mineralcompositions support significant fluid-rock interchangein this system. In addition, those fine grained authigenicchlorites that have replaced different allogenic phasessuch as alkali feldspar, plagioclase, biotite, white mica, or
McDOWELL AND ELDERS: ALLOGENIC LAYER SILICATE MINERALS
I1 l tEe /Snect iEe
after ltuscovite
4 3 9 n ( 1 9 0 " c )
#analyses 4
s l 3 . 2 9 ! O . O 2r v A l
0 . 7 1 i o . o 2v r l t
1 . 4 1 ! o . o 4
l , t g 0 . 3 1 t 0 . 0 1
F e . 0 . 2 6 ! 0 . 0 3
T i 0 . 0 2 t 0 , 0 L
lftr Trece
N a 0 . 0 5 ! 0 . 0 2
K 0 . 5 9 ! 0 . 0 2
C a 0 . 0 2 i 0 . 0 I
bClear Core
2
3 . 1 4 r 0 . 0 4
0 , 8 6 ! 0 . 0 4
1 , 6 9 r 0 . 0 3
0 . 1 0 t 0 . 0 4
0 . 1 9 t 0 . 0 1
0 . 0 2 ! 0 . 0 0
Trace
0 . 0 5 t 0 . 0 2
0 . 9 2 I 0 . 0 r
0 . 0 1 ! 0 . 0 1
c d
Mottled Zone Outer
I I l 1 E E
Zone
3 r
3 , 3 5 t 0 . 0 6 3 . 3 1
0 . 5 4 r 0 . 0 6 0 . 6 9
1 . 5 9 ! 0 . 0 3 r . 4 2
0 . 2 4 ! 0 . 0 4 0 , 2 7
0 . 1 6 t 0 . 0 5 0 . 3 1
0 . 0 1 t 0 . 0 1 0 . 0 I
0 . 0 2 i 0 . 0 1 0 . 0 9
0 . 8 7 I 0 . 0 4 0 . 6 9
0.01 I 0 .00 0 .01
I 154
Table 5. Coarse grained chlorite and biotite. A) Calculatedassuming )cations - Na,K,Ca = 10.00(a) or 7.00(b,c). B)Calculated assuming ) + charge : 28.00 and lOVo Fe*3(a), or )f charge = 22.ffi and lSVo Fe(b,c). Compare (c) with Table 3-c
authigenic biotite.
MCDOWELL AND ELDERS: ALLOGENIC LAYER SILICATE MINERALS
#eelyseB/,gratns
other chlorite have differing compositions, reflecting thestrong control of the chlorite precursor in the finalchlorite chemistry. Much of the large compositional scat-ter observed in most low grade chlorite from regional andburial metamorphic terrains may be due to similar varia-tions in the origin of individual chlorite grains.
The compositional variation of fine and coarse grainedauthigenic chlorite from calcite-chlorite zone sandstonesis shown in Figure 7 as a function of temperature.Compared to other authigenic minerals, the composition-al variation of chlorite with temperature is very weak.However, all authigenic chlorite shows the same slightdecrease in octahedral Al, increase in total Mg * Fe1, andincrease in octahedral occupancy, with increasing tem-perature. Thus authigenic chlorite, whether the coarse orfine grained optically normal varieties, or the coarsegrained metastable titaniferous variety, appears to beresponding in a systematic way to the change in depth ortemperature through the calcite-chlorite zone. In particu-lar, the metastable titaniferous chlorite, formed by directreplacement of allogenic biotite at the carbonate-cap/chlorite zone boundary, also appears to react at least inpart with the fluid phase, at the same time it reacts to theapparently stable equilibrated fine grained low-Ti authi-genic chlorite. Thus the metastable chlorite that wascreated within the geothermal system appears to respondto temperature changes within the system, yet continu-
ously reacts with that system to produce an apparentlystable, equilibrated chlorite variety.
Biotite/chlorite reaction near the carbonate-cap/chlorite zone boundary
The majority of the titaniferous allogenic biotite ap-pears to react to fine grained equilibrated chlorite in twodistinct steps: (l) rapid reaction at the zonal boundary toa metastable, optically anomalous coarse grained titanif-erous chlorite; and (2) sluggish reaction of the metastablechlorite to fine grained equilibrated authigenic chloritethroughout the calcite-chlorite zone.
There is a strong correlation between the developmentof secondary porosity and the progress of the first reac-tion. Sandstones from which most of the calcite has beenremoved contain only traces of the original biotite, whichitself was preserved as an unstable phase to temperaturesnear 190'C by lack offluid access in the highly cementedrocks. By using the average allogenic biotite (Table 2) andtitaniferous metastable chlorite (Table 5a) analyses, it ispossible to write an aluminum conservative version of thefirst reaction as follows:
The critical characteristic of the metastable chlorite is itshigh content of the small, highly charged titanium ion, anelement that is immobile in most fluid systems. Thecreation of a metastable phase whose Ti-content appearsto reflect that of the reactant biotite implies that specificstructural elements present in the biotite are passed on tothe product chlorite during the layer-by-layer replace-
ta
Metastable Coar€eGiatued fl-Chlorlte
675 i (254'cl
6 1 3
3.06 t 0.o5
0 . 9 4 r 0 . 0 5
1 . 3 r ! 0 . 1 4
2 . 3 3 t 0 . 0 8
2 . L 2 t 0 . L 3
0 . 1 7 ! 0 . 0 5
o . 0 4 r 0 . 0 1
0 . 0 0
0 . 0 3 ! 0 , 0 1
0.00
bhleatable coarreCrshed ll-Blotlte
u43 r (330"c)
5 1 2
2 . 9 1 ! 0 . 0 5
1 . 0 9 r 0 . 0 5
0 . 4 7 ! 0 . 0 8
1 . 3 6 ! 0 . I 3
0 . 9 9 ! 0 . 0 6
0 . 1 5 ! 0 . 0 r
0 , 0 3 ! 0 , 0 1
0 . 0 0
0 . 8 0 r 0 . 0 2
0 . 0 0
Steble Coara€ GrahedTl-rree Blottte
1143 ! (330'c)
5 1 4
3 . 1 9 ! 0 . 0 5
0 - 8 1 ! 0 . 0 5
0 . 0 4 ! 0 . 0 2
2 . 3 3 ! 0 . 0 9
0 . 5 1 r 0 . 0 8
0 . 0 1 ! 0 . 0 1
0 . 0 1 r 0 . 0 1
0 . 0 0
0 . 5 6 ! 0 . 0 3
o . 0 0
tv^.
vr^,
uS
F"a
Tt
h
Na
K
3 . 1 9
0 . 8 r
0 . 0 4
0 . 5 0
0 . 1 1
0 . 0 r
0 . 0 r
3 . 0 0
0 , 0 0
o . 6 4
0 . 0 0
2 . 8 3
r . 1 7
0 . 3 6
I . 3 I
o . 7 7
o . 1 7
0 . 1 5
0 . 0 3
2 . 7 9
o
o . 7 8
0
sr 2.97rvrr r.o3vrf l
1.15
lf8 2.25
F"2+ 1.85
F e ' 0 . 2 1
T 1 O . I ?
h 0 . 0 4
w r 5 . 5 8
N a 0
K O . 0 3
C a 0
t.5A14
350T
250i 'c L ,
t 50 +3.!
M g + F e =vI Alr
Fig. 7. Composition variation of average fine grained (solidcircles) and coarse grained (X) authigenic chlorite from chloritezone as a function of borehole temperatures ('C) (formulaproportions). Bar indicates average composition of allogenicchlorite. Triangle represents average authigenic chlorite fromdolomite-hematite-bearing sandstone. VI = total octahedraloccupancy assuming ) + charge = 28.(X) and lOVo ferric iron.
ment process. An alternative to the above reaction is toassume that the titanium stabilizes the octahedral sheet ofthe biotite 2:l layer, passing on this sheet intact to theproduct metastable chlorite and thus allowing that meta-stable phase to be created and persist in the geothermalsystem. A hypothetical alternative reaction written usingmineral formulae calculated assuming that l0Vo of thetotal Fe is ferric, and combining Fe2+ + Mg * Mn as R2+and Fe3* * octahedral Al as R3* is as follows:
The gain in R3+ (mainly Al) is easily accounted for due tothe local Al mobility in the geothermally altered sand-stone required to form newly nucleated albite, orthoclase,epidote and other authigenic phases. In particular, amajor Al source is represented by the absolute decreasein the authigenic illite and muscovite content on increas-ing temperature in these rocks. The product chlorite has abrucitelike interlayer sheet with approximately one thirdof the filled sites occupied by Al, a value close to themaximum observed in chlorite (Deer et al., 1962).Changes in the interlayer site are balanced by changes inthe tetrahedral sheet of the 2:l layer packet, while theoctahedral sheet remains constant. The R2*/R3* ratios ininterlayer and octahedral sheets are distinctly different,and this along with the Ti-stabilized 2:l octahedral sheet,may cause the slow but gradual breakdown of the meta-stable chlorite to equilibrated chlorite.
The continuous reaction of the metastable Ti-chloriteto Ti-free equilibrated chlorite implies complete exchangeof the Ti-stabilized 2:l octahedral sheet and recrystalliza-tion of a large single grain'into an aggregate of smallgrains. Since the 2:1 octahedral sheet does not maintainits integrity, and thus there is no clear criteria for parti-tioning cations among the two octahedral sheets in theproduct chlorite, the reaction has been written by com-bining octahedral sheets as follows:
The reaction as written requires essentially no changein the tetrahedral site and little variation in the number ofprobable octahedral vacancies. Again the needed Alcould be provided by the on-going continuous reaction ofdioctahedral mica, while the immediate sink for the Tiproduced is authigenic sphene.
Metastable chlorite/biotite reactions near the biotiteisograd
Despite the continuous progressive alteration of meta-stable titaniferous chlorite, small amounts of it are pre-served to the biotite isograd. The morphology of suchchlorite is still distinct in that it maintains its raggedappearance and irregular form, in contrast to the muchmore idiomorphic, low-Ti chlorite which by these tem-peratures has coarsened considerably. The metastablechlorite is internally much more irregular, showing vari-able optical extinction and mottling of pleochroic colors,while the idioblastic chlorite is clear, and shows light,uniform colors, minor pleochroism, and uniform extinc-tion.
At the biotite isograd, almost all chlorite reacts directlyto biotite. The light green low-Ti idioblastic chloritereacts to a morphologically identical light green Mg-richbiotite. This biotite is compositionally identical to theblocky <0.02 mm pale green weakly pleochroic authi-genic biotite that nucleates in pore spaces at the biotiteisograd (Table 3,c). The compositional uniformity of bothnewly nucleated biotite and biotite after idioblastic low-Tichlorite (Table 5,c) strongly indicates complete equilibra-tion with the fluid phase. These biotites show no evidenceof reaction to any other phase at a given temperature.With increasing temperature, however, these biotitesundergo a series of very regular compositional changeswith temperature (McDowell and Elders, 1980) as theygradually make the transition from a siliceous, interlayerdeficient biotite at the biotite isograd (325'C) to a compo-sitionally typical greenschist facies metamorphic biotiteat the garnet isograd (360"C).
The metastable Ti-chlorite does not react directly toequilibrated low-Ti biotite. Instead it reacts to an idio-blastic to sub-idioblastic titaniferous, strongly pleochroicbiotite with colors ranging to deep green-brown, brown-green, or brown and birefringence up to 0.040. Thistitaniferous biotite is also apparently metastable, as it isimmediately replaced by fine grained pale green low-Tiequilibrated biotite. In the 1140-1146 m depth intervalwhere authigenic biotite first appears, almost all of themetastable titaniferous biotite shows some evidence ofreaction to equilibrated biotite, and in a few instancesgrains almost completely replaced by equilibrated biotiteaggregates were noted. In the next deeper sampled inter-val at 1213-1219 m depth (-335'C) coarse grained titanif-erous biotite is very rare, and the few grains that areobserved are almost completely replaced by equilibrated
MCDOWELL AND ELDERS: ALLOGENIC LAYER SILICATE MINERALS
Equilibrated Chlorite
I t56
authigenic biotite. No evidence of the metastable Ti-biotite is observed at greater depth. Thus metastableauthigenic titaniferous chlorite, which originally formedby the rapid replacement of unstable allogenic biotite atthe chlorite isograd near l90oC, is itself replaced at thebiotite isograd at 325'C by a metastable authigenic titanif-erous biotite that is in turn completely altered to stableauthigenic biotite within about 10" of the biotite isograd.
Average analyses of metastable Ti-rich and equilibrat-ed Ti-free biotite have been listed in Table 5. Unlike theoriginal allogenic biotite or metastable chlorite, thecoarse grained biotite from the biotite isograd shows veryregular cation substitutions which span the gap betweenthe broad compositional field of the original allogenicbiotite and the very restricted range ofequilibrated authi-genic biotite compositions from the same depth (Fig. 8).The contrast between the compositional scatter of allo-genic biotite, and the very restricted compositional rangeof the Ti-rich metastable biotite, is especially marked.
There is a regular increase in Ti-content in the coarsegrained biotite from K- and Al-poor, Mg-rich grains(Table 5,c) identical in composition to fine grained equili-brated biotite (Table 3,c) ro K- and Al-rich, Mg-poorgrains that are very similar to the average composition ofthe original allogenic biotite. As Ti increases, Si, Mg+Fe,and Mg/(Mg+Fe,) decreases while IvAl, vIAl, K, and Al/Al * Fe, + Mg increase in a very regular way. The mostTi-rich metastable grains, however, still have low inter-layer occupancies more similar to authigenic than allo-genic biotite. The Ti substitution scheme within thecoarse grained sub-idioblastic biotite is very complex,and can be generalized as follows:
Fig. 8. Comparison of composition of allogenic biotite (filledcircles), Low-Ti authigenic biotite (open circles), and metastable(triangles) Ti-biotite that exists just above 325oC only, in Al-totalFe-Mg system (molecular proportions). Compositional trend ofauthigenic biotite on increasing temperature indicated by arrow.
MCDOWELL AND ELDERS: ALLOGENIC LAYER SILICATE MINERALS
M g
The data strongly indicates that those titaniferouscoarse grained metastable chlorite grains that had not yetreacted to fine grained Ti-free equilibrated chlorite in thechlorite zone by the time temperatures reached 325"C,reacted very rapidly at that temperature to produce ametastable coarse grained sub-idioblastic titaniferous bio-tite. The most titaniferous chlorite produced a homoge-neous titaniferous biotite very similar to the averagecomposition of allogenic biotite that had ceased to exist attemperatures some 125"C lower than the biotite isograd.Other chlorite grains of varying Ti-contents in turn werereplaced by a suite of metastable biotites with veryregular intergrain compositional variations. All coarsegrained xenoblastic chlorite grains that originated byreaction from original allogenic biotite were completelyreplaced at the biotite isograd by metastable coarsegrained biotite. Those few idioblastic coarse grainedchlorite grains that formed by reerystallization of finegrained equilibrated chlorite in the deeper parts of thechlorite zone did not react at the biotite isograd, butpersist in trace amounts well into the biotite zone.
The reaction of metastable chlorite to metastable bio-tite at the biotite isograd show some critical differencesrelative to the reaction which originally formed the chlo-rite. In particular, the metastable biotite is distinctly moreidioblastic than the chlorite it replaces, and is internallyoptically homogeneous. It shows none of the intracrystal-line color and extinction variations so common in themore xenoblastic metastable chlorite, even though theintense pleochroic colors and high birefringence of thechlorite are mimiced in the product biotite. In addition,the compositional range of the metastable biotite is veryrestricted compared with that of the reactant chlorite. Itthus appears that, unlike the chlorite isograd reactionwhere specific structural elements of the biotite couldhave been passed onto the product chlorite, at the biotiteisograd the reaction of metastable chlorite to metastablebiotite involves the complete recrystallization and struc-tural reorganization of the product biotite. The titanifer-ous 2:l octahedral sheet that is postulated to have al-Iowed the metastable chlorite to persist through thecalcite-chlorite zone probably was finally disrupted at thebiotite isograd. Once this "stabilizing" 2:l sheet wasremoved, the metastable biotite reacted relatively rapidlyto equilibrated biotite.
The reaction of metastable titaniferous chlorite tometastable titaniferous biotite at the biotite isograd, andof that biotite to Ti-free biotite at temperatures just abovethe biotite isograd are given below, using the same cationcombination procedures as in previous reactions. Notethat the biotites have been calculated assuming that 18percent of total Fe is ferric after McDowell and Elders(1980).
IIG *l*o s tR35sRej51313d'5 [Ab srsi3 rs]-o ilo,.(oH),
0.t5Ti4* 0)+ 0.74Rr+
0.16K*
Stable Ti-Free Biotite
The first reaction is almost the reverse of the reactionthat produced metastable chlorite from allogenic biotite atthe chlorite isograd, although the chemical variability ofthe products of the two reactions is distinctly different.The 2:l octahedral sheet of the metastable Ti-chlorite,which was carried through from the same sheet of theallogenic biotite, is not preserved in the reaction at thebiotite isograd. As noted above, this is consistent with thehigh degree ofchemical exchange and structural reorgani-zation of this reaction. In common with titaniferousallogenic biotite, the metastable Ti-biotite has a signifi-cant number of apparent octahedral vacancies. The Alproduced during this reaction could be consumed by thenucleation of fine grained authigenic biotite in the sand-stone pore spaces.
The reaction of metastable Ti-biotite to a coarsegrained Ti-free biotite that is identical in composition withfine grained authigenic biotite occurs at the same tem-perature. This reaction, which removes the last of themetastable titaniferous mafic layer silicate phases ob-served in the geothermal system, produces a biotite withno vacancies, low interlayer occupancy, and a low octa-hedral Al content.
Many coarse grained chlorite analyses in the 106l-1067m depth interval at temperatures near 3l5oC containsignificant K, and optical examination of these grainsindicates that these chlorites contain thin bands of biotiteparallel to cleavage. When these analyses are plotted on avariation diagram the trends produced indicate that thesethin bands consist of equilibrated low K, low Al and highMg biotite, not allogenic biotite. Thus, at temperatures ofapproximately l0'C lower than the biotite isograd, traceamounts of authigenic biotite form in the micro-composi-tional environment of the coarse grained chlorite where aprecursor micalike lattice exists, but do not form awayfrom chlorite where independent nucleation of biotitewould be required.
Discussion
The conversion of a suite of allogenic layer silicateminerals to their authigenic equivalents in porous sand-
n57
stone within the currently active Salton Sea Geothermalsystem was a complex process involving a number ofapparently metastable intermediate phases. These meta-stable phases were produced within the geothermal sys-tem and at least partially equilibrated with that system,but reacted at varying rates to eventually produce authi-genic minerals that were as equilibrated as possible withinthe time-temperature constraints of the evolving geother-mal system. The apparent reaction rates of the allogeniclayer silicates at temperatures near 190'C reflect, ininverse order, the stability ranges of their authigeniccounterparts in the geothermal system. Biotite, which isnot stable within the geothermal system for anotherl25oC, reacts very rapidly to either equilibrated chloriteor, for the most part, to a metastable titaniferous chlorite.Muscovite, which is not created by the prograde se-quence from dioctahedral clay for another 90oC, reacts ata moderate rate such that it persists for some 65'C intothe chlorite zone before it is completely replaced. Chlo-rite, which is stable in the geothermal system at 190'C,apparently reacts very little, as evidence of reaction ofcoarse grained allogenic chlorite, with normal opticalproperties, to fine grained authigenic chlorite, while ob-served, is very rare. The compositional overlap of allo-genic and equilibrated chlorite is suffcient that littlereaction is to be expected.
Investigation of the actual reactions strongly indicatesthat specific structural elements may be preserved intactin the product layer silicates, and that different latticesites react at diferent rates and to different degrees. Attemperatures near 190"C, the reaction of allogenic musco-vite to mixed layer illite/smectite apparently occurs intwo steps, producing a shortJived metastable phengiticphase with a domain structure as an intermediate reactionphase. In the first reaction step, little exchange occurs inthe interlayer site while significant Si for Al exchange andMg addition occur in the 2:1 layer. In the second step, inwhich the original muscovite structure is completelydisrupted to form an aggregate of <0.fi)5 mm grains, asharp decrease in interlayer K occupancy to valuestypical of slightly expandable illite/smectite occurs. Ex-pandable layers with exchangeable Mg are formed at thistime. Illite/smectite after muscovite is similar to matrixillite/smectite except for a lower phengite content. Atslightly higher temperatures, muscovite reacts directly tonon-expandable illite.
The metastable titaniferous chlorite produced by reac-tion of allogenic biotite has the greatest range of persis-tence in the geothermal system of any of the metastablelayer silicate phases. Chlorite replaced biotite directly ona layer-by-layer basis, and during this reaction the 2:loctahedral sheet of the reactant biotite was apparentlytransferred intact to the product metastable chlorite.Stabilization of the 2:l octahedral sheet was probablycaused by the small, highly charged Tia+ cation. Themetastable chlorite thus took on anomalous biotite-likeoptical properties directly related to the Ti-content ofthe
McDOWELL AND ELDERS: ALLOGENIC LAYER SILICATE MINERALS
l l5E
2:l octahedral sheet. The higher Mg/(Fe+Mg) ratio ofmetastable chlorite relative to all other chlorites suggeststhat the newly created interlayer octahedral sheet wasMg-rich. Since other sites in the metastable chlorite wereapparently free to react, the activation energy barrierresponsible for persistence of the metastable chlorite iseffectively equivalent to the energy necessary to removeTia+ from the specific sites it occupies in the 2:l octahe-dral sheet. This energy barrier was sufficiently large thatcomplete disruption of the 2:1 octahedral sheet did notoccur until at interlayer deficient, siliceous biotite wasitself stable within the geothermal system. Despite thecomplete exchange of the 2:l octahedral sheet at thebiotite isograd at 325oC, the product of reaction of themetastable chlorite was not the stable form of biotite, butwas in part a completely recrystallized metastable titanif-erous biotite. This metastable biotite reacted within lO"Cof the biotite isograd to stable Ti-free authigenic biotite,implying that once the Ti-rich 2: I octahedral sheet of themetastable chlorite was disrupted, all sites within themetastable biotite were reactive and reaction to the stablephase was relatively rapid.
The importance of structural factors is supported bythe observation that the first authigenic biotite observedin the system is formed entirely within pre-existing grainsof idioblastic, coarse grained equilibrated chlorite, attemperatures some l0 degrees lower than compositional-ly identical biotite is able to nucleate from the fluid phase.This emphasizes the sluggishness of many of these reac-tions as well as the importance of a preexisting lattice onwhich to form a presumably equilibrium mineral phase.Much of the compositional variability observed in lowgrade layer silicates appears due to the specific reactionthat produced each grain. Those grains produced byreplacement reactions appear to inherit at least some ofthe compositional characteristics of the reactant phase,and in some cases whole structural elements may bepassed on intact. Thus in the geothermal system chloritesafter biotite, chlorite, feldspar or dioctahedral layer sili-cate each may be compositionally distinct, and all differ-ent from chlorite nucleated directly from the fluid phase.The effects of structural control and replacement reactionvariability appear to be more important in these veryporous sandstones than effects due to variations in fluidchemistry in various micro environments. Once thesevarious phases are formed, the intergrain compositionalvariability gradually decreases as reaction with the perva-sive fluid phase continues with increasing temperature.This effect is reflected in the geothermal system by thelarge compositional variability of mixed layered illite/smectite at 190"C, the moderate variability of illite near250"C, and the very restricted composition of authigenicmuscovite at temperatures over 290"C. Authigenic chlo-rite shows considerable compositional variabil itythroughout the calcite-chlorite zone, reflecting the appar-ent slow equilibration rate of chlorite. The variability ofchlorite contrasts sharply with that of authigenic biotite,
MCDOWELL AND ELDERS: ALLOGENIC LAYER SILICATE MINERALS
most of which is the reaction product of chlorite break-down and which shows very restricted compositionalvariability.
It is clear that the reaction among the various coarsegrained layer silicates in the geothermal system is strong-Iy dependent on kinetic factors, and must be consideredin a site-by-site basis. The number of metastable phasesproduced and the variable rates of reaction observedindicate that great care must be taken in low temperaturesystems such as these in applying equilibrium thermo-chemical concepts to reactions in these systems. It is alsoclear that, in geothermal systems such as this in whichtime, temperature, and fluid compositions can be deter-mined to reasonable degrees of reliability, it will bepossible to eventually understand these complex reac-tions and to gain insight into the energetics of thesereactions.
Acknowledgments
This research has been supported by NSF grants EAR78-22-755 and 8l-20-821 (to S. D. McDowell), and United StatesGeological Survey Grants l4-08-0001-G2,f4 (to W. A. Elders)and l4-08-001-G430 (to W. A. Elders and S. D. McDowell).Technical assistance by Peter Collier and Paul Johnson of UCRis greatly appreciated. Thanks are given to Art Chodos and theCal Tech microprobe laboratory for access to, and advice inoperation of, that first-class facility. In addition, the staff of theInstitute of Mineral Research at MTU and especially W. A.Hockings and N. Scofield are thanked for access to theirmicroprobe facility and for help in a number of aspects of thisresearch. The manuscript was typed by Julene Erickson and PatCline (MTU), and drafting done by Deborah McDowell. Com-ments by reviewer M. A. Bodine were greatly appreciated andhave resulted in a much improved manuscript.
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MCDOWELL AND ELDERS: ALLOGENIC LAYER SILICATE MINERALS
