Timothy L. Grove · Julie M. Donnelly-Nolan Todd Housh Magmatic processes that generated the rhyolite of Glass Mountain, Medicine Lake volcano, N. California Received: 26 March 1996 Accepted: 14 November 1996 Abstract Glass Mountain consists of a 1 km 3 , compo- sitionally zoned rhyolite to dacite glass flow containing magmatic inclusions and xenoliths of underlying shallow crust. Mixing of magmas produced by fractional crys- tallization of andesite and crustal melting generated the rhyolite of Glass Mountain. Melting experiments were carried out on basaltic andesite and andesite magmatic inclusions at 100, 150 and 200 MPa, H 2 O-saturated with oxygen fugacity controlled at the nickel-nickel oxide buffer to provide evidence of the role of fractional crystallization in the origin of the rhyolite of Glass Mountain. Isotopic evidence indicates that the crustal component assimilated at Glass Mountain constitutes at least 55 to 60% of the mass of erupted rhyolite. A large volume of mafic andesite (2 to 2.5 km 3 ) periodically replenished the magma reservoir(s) beneath Glass Mountain, underwent extensive fractional crystallization and provided the heat necessary to melt the crust. The crystalline residues of fractionation as well as residual liquids expelled from the cumulate residues are pre- served as magmatic inclusions and indicate that this fractionation process occurred at two distinct depths. The presence and composition of amphibole in mag- matic inclusions preserve evidence for crystallization of the andesite at pressures of at least 200 MPa (6 km depth) under near H 2 O-saturated conditions. Mineral- ogical evidence preserved in olivine-plagioclase and oli- vine-plagioclase-high-Ca clinopyroxene-bearing mag- matic inclusions indicates that crystallization under near H 2 O-saturated conditions also occurred at pressures of 100 MPa (3 km depth) or less. Petrologic, isotopic and geochemical evidence indicate that the andesite under- went fractional crystallization to form the differentiated melts but had no chemical interaction with the melted crustal component. Heat released by the fractionation process was responsible for heating and melting the crust. Introduction Silicic magmas are the low-density, buoyant products of igneous processes and are found in a range of settings that span the distance scale of < 1 mm for the accu- mulation of melts in the late stage mesostasis in a dif- ferentiating lava flow to the scale of 1000s of km 3 for voluminous ignimbrite eruptions. The Glass Mountain eruptive center provides a spectacular example of the production and emplacement of a cubic kilometer-sized volume of silicic melt. Its young age (885 years BP, Donnelly-Nolan et al. 1990), and excellent exposure provide an opportunity for detailed examination. The Glass Mountain silicic lavas contain unusually fresh and well preserved mafic magmatic inclusions and partly melted granitic xenoliths and provide important evi- dence for understanding how silicic magmatic bodies in the km 3 volume range are formed. Lavas of Glass Mountain have continued to provide a focus for dis- cussions of magmatic processes that include the role of crustal melting (Anderson 1933, 1941) and the nature of magma mixing (Eichelberger 1975, 1980). The presence of the diverse magmatic products in Glass Mountain lavas allows quantitative modeling of the interaction of mafic magma with crust and that is the focus of this paper. In much larger silicic systems, mafic magmatic products are not commonly found (Smith 1979), but models have been developed that infer crust versus mantle inputs based only on information from silicic Contrib Mineral Petrol (1997) 127: 205–223 Springer-Verlag 1997 T. L. Grove (&) Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA J. M. Donnelly-Nolan United States Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA T. Housh Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712, USA Editorial responsibility: J. Hoefs
Transcript
Timothy L. Grove · Julie M. Donnelly-NolanTodd Housh
Magmatic processes that generated the rhyoliteof Glass Mountain, Medicine Lake volcano, N. California
Received: 26 March 1996 =Accepted: 14 November 1996
Abstract Glass Mountain consists of a 1 km3, compo-sitionally zoned rhyolite to dacite glass flow containingmagmatic inclusions and xenoliths of underlying shallowcrust. Mixing of magmas produced by fractional crys-tallization of andesite and crustal melting generated therhyolite of Glass Mountain. Melting experiments werecarried out on basaltic andesite and andesite magmaticinclusions at 100, 150 and 200 MPa, H2O-saturated withoxygen fugacity controlled at the nickel-nickel oxidebuffer to provide evidence of the role of fractionalcrystallization in the origin of the rhyolite of GlassMountain. Isotopic evidence indicates that the crustalcomponent assimilated at Glass Mountain constitutes atleast 55 to 60% of the mass of erupted rhyolite. A largevolume of mafic andesite (2 to 2.5 km3) periodicallyreplenished the magma reservoir(s) beneath GlassMountain, underwent extensive fractional crystallizationand provided the heat necessary to melt the crust. Thecrystalline residues of fractionation as well as residualliquids expelled from the cumulate residues are pre-served as magmatic inclusions and indicate that thisfractionation process occurred at two distinct depths.The presence and composition of amphibole in mag-matic inclusions preserve evidence for crystallization ofthe andesite at pressures of at least 200 MPa (6 kmdepth) under near H2O-saturated conditions. Mineral-ogical evidence preserved in olivine-plagioclase and oli-vine-plagioclase-high-Ca clinopyroxene-bearing mag-
matic inclusions indicates that crystallization under nearH2O-saturated conditions also occurred at pressures of100 MPa (3 km depth) or less. Petrologic, isotopic andgeochemical evidence indicate that the andesite under-went fractional crystallization to form the differentiatedmelts but had no chemical interaction with the meltedcrustal component. Heat released by the fractionationprocess was responsible for heating and melting thecrust.
Introduction
Silicic magmas are the low-density, buoyant products ofigneous processes and are found in a range of settingsthat span the distance scale of < 1 mm for the accu-mulation of melts in the late stage mesostasis in a dif-ferentiating lava flow to the scale of 1000s of km3 forvoluminous ignimbrite eruptions. The Glass Mountaineruptive center provides a spectacular example of theproduction and emplacement of a cubic kilometer-sizedvolume of silicic melt. Its young age (885 years BP,Donnelly-Nolan et al. 1990), and excellent exposureprovide an opportunity for detailed examination. TheGlass Mountain silicic lavas contain unusually fresh andwell preserved mafic magmatic inclusions and partlymelted granitic xenoliths and provide important evi-dence for understanding how silicic magmatic bodies inthe km3 volume range are formed. Lavas of GlassMountain have continued to provide a focus for dis-cussions of magmatic processes that include the role ofcrustal melting (Anderson 1933, 1941) and the nature ofmagma mixing (Eichelberger 1975, 1980). The presenceof the diverse magmatic products in Glass Mountainlavas allows quantitative modeling of the interaction ofmafic magma with crust and that is the focus of thispaper. In much larger silicic systems, mafic magmaticproducts are not commonly found (Smith 1979), butmodels have been developed that infer crust versusmantle inputs based only on information from silicic
Contrib Mineral Petrol (1997) 127: 205–223 Springer-Verlag 1997
T. L. Grove (&)Department of Earth, Atmospheric and Planetary Sciences,Massachusetts Institute of Technology,Cambridge, MA 02139, USA
J. M. Donnelly-NolanUnited States Geological Survey, 345 Middlefield Road,Menlo Park, CA 94025, USA
T. HoushDepartment of Geological Sciences,The University of Texas at Austin, Austin, TX 78712, USA
Editorial responsibility: J. Hoefs
eruptive products (Perry et al. 1993). This smaller vol-ume example with its diverse products may provideconstraints on plausible models for the chemical evolu-tion of these larger silicic systems.
Geologic setting
Glass Mountain represents the youngest of six eruptiveevents at Medicine Lake volcano that occurred between1250 and 850 years BP (Donnelly-Nolan et al. 1990).During this episode rhyolite to dacite lavas were volu-metrically dominant and erupted to form the MedicineDacite flow, the Hoffman flows and Little GlassMountain–Crater Glass flows. Glass Mountain eruptedon the eastern rim of the Medicine Lake caldera(Donnelly-Nolan et al. 1990, see their Fig. 1) and thelavas flowed down the steep east flank of the volcano.Ten rhyolite domes lie on a N25°W trend to the northand one dome lies to the south of the main flow (Fig. 1).A dike-like feeder (Fink and Pollard 1983) is probablyresponsible for the alignment of the vents and domes.Tephra deposits underlie the glass flows and domes(Heiken 1978). The tephras represent the earliest stagesof the eruption and are rhyolitic in composition (73.8–74.2 wt% SiO2). The early lava of the main flow variesfrom dacite to rhyolite in composition (63.8–74.2 wt%SiO2), a change caused by the mixing of rhyolite with anandesitic component that is variable in composition andthat has been blended in varying proportions prior toand during eruption. The variability in mixed compo-nents exists over a range of scales from that exhibited inthe map pattern (J. M. Donnelly-Nolan, in preparation)to that visible in hand specimens as adjacent cm-thick
bands of rhyolite, rhyodacite and dacite, to that ob-servable in thin section as mm-sized blebs of andesitesurrounded by rhyolite. The last part of the main GlassMountain flow consists of a dome of rhyolite. The do-mes to the north and south are also predominatelyrhyolitic in composition. Inclusions consist of andesiticmagma chilled by intrusion into the rhyolite, gabbrosthat are both cumulate and lithic in origin and graniticxenoliths, presumably of older subvolcanic plutonicequivalents of silicic lavas or of older shallow crust.
Petrography, petrology and geochemistry of lavasand inclusions
Analytical methods
Geochemical information on 101 samples of rhyolite of GlassMountain, rhyodacite and dacite lavas and 66 inclusions has beenobtained. A subset of inclusions and lavas was chosen from thislarger group and used as starting materials for experiments and forisotopic studies reported here. Whole-rock major element analyseswere obtained by wavelength dispersive X-ray fluorescence spec-troscopy at the US Geological Survey (USGS) laboratory inLakewood, CO. Trace elements were analyzed by energy dispersiveX-ray fluorescence at the USGS in Menlo Park and trace and rareearth elements were also obtained by neutron activation analysis atthe USGS laboratories in Lakewood, CO and Reston, VA. Isotopiccompositions of 87/86Sr, 143/144Nd and Pb were determined on theVG 54 Sector mass spectrometer at MIT and abundances of Sr andNd were obtained by isotope dilution. Housh et al. (in press) reportanalytical procedures and uncertainties. Selected data are presentedin Table 1 and sample locations are shown in Fig. 1. Compositionsof minerals in lavas and inclusions and experimental products wereobtained with the MIT five-spectrometer JEOL 733 Superprobeusing on-line data reduction and matrix correction procedures ofBence and Albee (1968) with modifications of Albee and Ray(1970). Selected mineral chemical data are presented in Table 2.
Fig. 1 Map of Glass Mountainflow and dome complex.Locations for lava andinclusion samples discussed inthe text are shown. Samplelocations are found in Table 1.See Eichelberger (1975) foradditional geologic information
206
Tab
le1
Maj
or,
trac
e,ra
reea
rth
and
isot
opic
com
posi
tion
sof
lava
san
din
clus
ions
from
Gla
ssM
ount
ain
and
1500
M+
1502
Mm
ix.
Maj
orel
emen
tan
alys
esw
ere
done
atU
.S.
Geo
logi
calS
urve
y(U
SGS)
,Lak
ewoo
d,C
O,b
yJ.
Bak
er,A
.J.B
arte
l,R
.V.M
ende
s,D
.Sie
ms,
K.S
tew
art
and
J.T
agga
rt.T
race
elem
ent
(ppm
)an
alys
esw
ere
perf
orm
edat
the
USG
S,M
enlo
Par
k,by
P.E
.Bru
ggm
an,T
.Fro
st,B
.Kin
g,J.
R.L
inds
ayan
dD
.Viv
it.R
are
eart
hel
emen
ts(p
pm)o
btai
ned
byin
stru
men
taln
eutr
onac
tiva
tion
anal
ysis
atth
eU
SGS,
Lak
ewoo
d,C
O,
byJ.
B.
Mill
ard
Jr.
and
Res
ton,
VA
,by
J.S.
Mee
and
G.A
.W
andl
ess.
See
text
for
anal
ytic
alm
etho
ds.
Isot
opic
anal
yses
perf
orm
edat
Mas
sach
uset
tsIn
stit
ute
ofT
echn
olog
yby
T.
Hou
sh,
S.B
owri
ngan
dD
.C
olem
an.
Typ
ical
stan
dard
erro
rsfo
ris
otop
era
tios
are
(2si
gma)
:Sr
,0.
003
%,
Nd
0.00
18%
and
0.01
%fo
ral
lP
bra
tios
Rhy
olit
esD
acit
esIn
clus
ions
404M
1406
M14
29M
1413
M13
63M
1368
M65
0Ma
FG
I11
39M
aA
CI
1140
Mf
FG
IA11
41M
CI
1149
Mb
FG
IA15
43M
GI
1543
Ma
FG
I15
44M
CI
1544
Ma
FG
I16
79M
FG
IA16
90M
GI
1691
MA
CI
1500
M+
1502
M
SiO
273
.073
.969
.763
.365
.571
.653
.853
.956
.351
.854
.176
.062
.652
.357
.359
.669
.854
.267
.15
Al 2
O3
13.9
13.5
14.7
16.1
15.8
14.2
18.4
17.8
17.4
20.0
18.9
12.6
16.0
19.4
16.5
16.5
14.9
18.2
16.0
5F
eO*a
1.68
1.57
2.78
4.62
4.11
2.16
6.71
7.69
7.17
6.13
6.54
0.82
6.39
6.60
7.14
6.55
1.98
6.89
4.20
MgO
0.35
0.30
1.24
2.93
2.15
0.54
5.62
5.80
4.42
7.87
5.73
0.14
2.57
7.66
3.47
2.36
0.71
5.40
1.15
CaO
1.25
1.14
2.64
5.13
4.18
1.72
8.41
9.06
7.88
9.53
9.49
0.62
4.38
9.34
7.40
5.39
2.16
8.62
2.75
Na 2
O4.
013.
964.
023.
804.
084.
153.
313.
333.
542.
683.
173.
594.
252.
913.
804.
424.
563.
244.
95K
2O4.
334.
443.
652.
502.
824.
010.
980.
881.
260.
750.
894.
901.
830.
751.
371.
793.
361.
022.
65T
iO2
0.27
0.24
0.41
0.63
0.60
0.35
0.76
0.95
0.95
0.46
0.71
0.14
1.17
0.61
1.09
1.19
0.48
0.86
0.80
P2O
50.
050.
000.
090.
140.
130.
070.
160.
170.
180.
100.
140.
000.
270.
130.
240.
290.
080.
170.
20M
nO0.
030.
020.
050.
080.
070.
030.
110.
130.
120.
090.
110.
000.
160.
110.
130.
120.
070.
120.
10T
otal
98.8
799
.07
99.2
899
.23
99.4
498
.83
98.2
699
.71
99.2
299
.41
99.7
898
.81
99.6
299
.81
98.4
498
.21
98.1
98.7
210
0.00
LO
I0.
160.
200.
100.
040.
150.
500.
290.
120.
290.
110.
180.
40<
0.01
<0.
010.
150.
030.
350.
19L
a23
.021
.817
.619
.523
.310
.910
.013
.07.
239.
521
.212
.58.
1114
.818
.227
.310
.4C
e44
.839
.733
.638
.343
.025
.321
.027
.014
.719
.239
.630
.416
.731
.235
.949
.722
.1N
d19
.818
.216
.417
.917
.614
.013
.014
.07.
9–
16.0
18.4
9.2
18.4
17.0
20.5
11.9
Sm4.
144.
103.
834.
194.
283.
343.
213.
741.
992.
773.
335.
302.
354.
335.
003.
903.
43E
u0.
630.
720.
850.
890.
700.
980.
961.
100.
640.
830.
361.
050.
751.
201.
410.
910.
99T
b0.
630.
590.
530.
690.
670.
580.
560.
650.
320.
450.
500.
910.
350.
670.
810.
470.
52Y
b2.
612.
522.
262.
462.
701.
751.
902.
201.
021.
592.
483.
061.
242.
552.
971.
962.
00L
u0.
400.
360.
320.
370.
380.
250.
300.
300.
170.
230.
360.
410.
180.
340.
410.
290.
28R
b14
212
686
8613
334
2641
2222
155
4030
4457
9433
Sr11
019
831
825
715
146
644
344
249
748
341
218
484
416
390
200
467
Y34
3331
3132
1920
2114
1523
3316
2633
2423
Zr
217
226
185
190
230
115
110
132
8510
014
415
491
149
184
185
121
Nb
89
109
93
34
35
814
76
106
4B
a82
877
760
260
081
930
825
836
923
526
666
122
322
842
148
781
431
1N
i9
2455
309
8445
2513
055
37
168
115
353
Th
15.0
12.0
7.8
9.1
13.0
2.8
2.0
3.4
2.0
2.4
18.0
3.6
5.3
7.1
2.8
87/8
6 Sr0.
7041
60.
7041
70.
7040
00.
7038
80.
7041
90.
7038
60.
7041
20.
7038
40.
7038
40.
7037
814
3/14
4 Nd
0.51
2805
0.51
2816
0.51
2806
0.51
2813
0.51
2810
0.51
2827
0.51
2891
0.51
2815
0.51
2845
206/
204 P
b19
.000
18.9
9918
.987
19.0
1918
.994
18.9
9718
.933
18.9
6418
.998
18.9
1420
7/20
4 Pb
15.6
0815
.605
15.5
9215
.635
15.6
0215
.605
15.5
9015
.605
15.6
0215
.585
208/
204 P
b38
.675
38.6
5938
.615
38.7
6938
.647
38.6
4838
.559
38.6
3338
.648
38.5
27N
41°b
37.2
36.6
35.2
37.1
36.3
35.2
37.1
37.6
35.1
3737
.237
.237
.237
.25
37.2
537
.135
.135
.1W
121°
28.8
30.9
27.5
26.4
28.7
30.2
26.4
31.3
29.7
3131
.231
.231
.231
.15
31.1
531
.229
.829
.8
a FeO
*is
Fe
calc
ulat
edas
FeO
;L
OI
islo
sson
igni
tion
bSa
mpl
elo
cati
ons
inL
atit
ude
and
Lon
gitu
dem
easu
red
inm
inut
esus
ing
adi
giti
zing
tabl
et
207
Lavas
The rhyolite of Glass Mountain is a glass that contains flow alignedmicrolites of plagioclase and rare phenocrysts (~ 1 mm long andoften less than one grain per thin section) of dominantly plagioclaseand orthopyroxene (see Anderson 1941; Eichelberger 1975; Groveand Donnelly-Nolan 1986). The dacite and rhyodacite consist of agroundmass of glass that contains blebs of andesite. These blebsrange in size from single 100–200 micron glomerocrysts of oli-vine + plagioclase surrounded by a felty dark groundmass of pla-gioclase, pyroxene and oxide to cm-sized intergranular olivine+ plagioclase + high-Ca pyroxene inclusions to the quenchedmagmatic inclusions described below. Also found are phenocrysts ofNa-rich plagioclase 0.5 to 2 mm in dimension, often intergrown withorthopyroxene and spinel. Amphibole is also rarely encountered inthin sections of the silicic lavas. Samples 1363 M and 1413 M (Ta-bles 1 and 2) are dacites that were chosen for detailed study.
Inclusions
Several varieties of inclusions are represented in the 66 sampledfrom Glass Mountain lavas. Of the sample set > 90% are magmaticinclusions ranging in SiO2 content from 50.0 to 62.8 wt%. The sizeof the analyzed inclusions ranges from ~ 10 cm to > 100 cm inlongest dimension. Smaller inclusions are abundant in the mixed
dacite, but were not sampled for chemical analysis. Magmatic in-clusions typically exhibit both a coarse sugary texture (~ 1–2 mmeuhedral to subhedral interlocking grains) and a fine-graineddarker patchy matrix (0.1–0.4 mm laths of pyroxene in plagioclase)in hand sample. The coarser facies consists of an intergranular tosubophitic intergrowth of olivine + plagioclase ± high-Ca clino-pyroxene ± oxide. The fine-grained groundmass consists of silicatelaths and oxide granules set in a brown glass matrix. These patchycoarser- and finer- textured inclusions are designated fine-grainedinclusions (FGI). In ten inclusions hornblende is also present aspart of this assemblage (fine-grained + amphibole, FGIA) andorthopyroxene is found in the coarse facies of about half of theinclusions. In amphibole-bearing inclusions high-Ca pyroxene issurrounded by amphibole reaction rims and orthopyroxene oftenmantles olivine. Amphibole microlites are also present in the fine-grained facies in 19 of the inclusions. Textures of the coarse andfine regions of the inclusions are similar to those illustrated inGrove and Donnelly-Nolan (1986) in Figs. 2A, B (coarse) andFig. 3C (fine). Samples 1544M and 1140Mf are representativeof FGIs containing olivine + plagioclase (1544M) and olivine+ plagioclase + high-Ca pyroxene (1140 Mf). Sample 1544M, a100 × 50 × 20 cm tombstone-shaped inclusion is unusual in that arind of the finer-grained facies had concentrated on the outermargin in sufficient quantity to allow analysis (1544Ma). Sample1139Ma contains amphibole in its coarse and fine facies.
Only four inclusions (1045Ma, 1145Ma, 1139Ma, 1691M) re-semble the coarser-grained (~ 5–10 mm) cumulate hornblende
Table 2 Compositions of minerals in Glass Mountain inclusions and lavas. Analyses are of cores of minerals that appear to havecrystallized simultaneously (ilm ilmenite). Other abbreviations defined in Table 4
a ol + pl in 1544M, ol + pl + cpx in 1149Mb, ol + amp + pl in 1139Ma and amp + pl in 1679M and 1691Mb are intergrowths ofeuhedral crystalsb ol in this intergrowth contains an Mg-rich overgrowth rimc opx in this intergrowth is overgrown by an Mg-rich rim and pl is rimmed by an An-rich overgrowth; sp + ilm occur in an intergrowthwith opx + pl
208
gabbros that Grove and Donnelly-Nolan (1986) found to be thedominant inclusion type in the rhyolites of Little Glass Mountainand Crater Glass flow (amphibole cumulate inclusions, ACI). Mostof the magmatic inclusions (39) contain < 5% phenocrysts. Eigh-teen contain from 30 to 75% large crystals, lack amphibole and areclassified based on this textural characteristic as cumulates (cu-mulate inclusions, CI). Two inclusions (1543Ma at 62.8 and1679 M at 60.7 wt% SiO2) are fine grained and aphanitic and re-semble the quenched andesite inclusion type described by Groveand Donnelly-Nolan (1986). Analytical data for these inclusions ispresented in Tables 1 and 2, and these textural distinctions will beused to differentiate inclusion types in the figures.
The three granitic inclusions (GI) collected from the GlassMountain flow are texturally and geochemically similar to the plu-tonic samples described in Grove et al. (1988), Grove and Donnelly-Nolan (1986) and Mertzman and Williams (1981). These inclusionscontain quartz, microcline, plagioclase and biotite. Sample 1543M isremarkable in that it is surrounded by a rind of fine-grained and-esitic quenched magma (1543Ma), which was, in turn, chilled in thehost rhyolite. The samples have grain size (up to 1 cm) and texturescharacteristic of hypabyssal igneous rocks. Thus, textures are nottypical of coarse-grained batholithic igneous rocks, but they do notexclude mid-crustal crystallization conditions.
Experiments
Experimental procedures
Synthesis experiments were performed using powdered samples ofGlass Mountain inclusions (1544M and 1140Mf) and a mixture of
two andesite tuff pumices (1500M and 1502M). Major elementcompositions of the starting materials are found in Table 1. Ex-perimental procedures followed those described in Sisson andGrove (1993a, b) with exceptions noted below. Experiments wereconducted at 100, 150 and 200 MPa under H2O-saturated condi-tions in TZM (titanium-zirconium-molybdenum) and ZHM (zir-conium-hafnium carbide-molybdenum) cold seal pressure vessels.The experiments used Au inner and outer capsules and oxygen fu-gacity ( fO2 ) was buffered at the nickel-nickel oxide (NNO) buffer.The buffer assemblage was isolated from contact with the Au byplacing it in two or three unsealed Pt or Ag70Pd30 capsules. Theupper temperature limit of the experiments (1050 °C) was imposedby the melting point of the Au sample container. Temperaturemeasurement, calibration, pressure application, and gas mixturesare as described in Sisson and Grove (1993a, b). The pressure vesselwas positioned vertically in a Deltech DT31VT furnace and held atpressure and temperature of the experiment for its duration. Ex-periments were terminated by removing the vessel from the furnace,inverting it and rapping on the hot portion of the vessel with awrench. The capsule dropped to the water-cooled pressure seal andquenched rapidly with no growth of quench crystals. Followingquenching samples were weighed to measure volatile loss or gain,checked for the presence of H2O and both phases of the bufferassemblage. Approximately 25% of the experiments successfullypassed these tests and were subjected to further analysis. Analyticalconditions and precision are described in Sisson and Grove (1993a).
Results
Experimental conditions, duration, phase assemblagesand phase proportions are summarized in Table 3.
Table 3 Experimental conditions and products. All experiments buffered at NNO, PH2O � Ptotal. See Table 4 for phase compositions
a Phase proportions calculated by mass balance, ignoring MnO and P2O5. Abbreviations for phases are in Table 4b Apparent loss or gain of FeO estimated as 100*(FeOcalc ) FeOstarting material)/FeOstarting material
209
Tab
le4
Ele
ctro
nm
icro
prob
ean
alys
esof
expe
rim
enta
lph
ases
(gl
glas
s,ol
oliv
ine,
cpx
clin
opyr
oxen
e,pl
plag
iocl
ase,
spsp
inel
,am
pam
phib
ole,
apap
atit
e)
Run
no.
Pha
seSi
O2
TiO
2A
l 2O
3C
r 2O
3F
eOM
nOM
gOC
aON
a 2O
K2O
P2O
5N
iOT
otal
1140
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#18
gla (8
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20.2
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40.2
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0.29
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0.09
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100.
511
40m
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57.6
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0.95
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6.80
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99.4
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8.19
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94.8
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23.9
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1.68
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0.13
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100.
9cp
x(4)
51.3
(11)
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2.98
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0.30
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42(5
)98
.5pl
(6)
51.3
(3)
30.5
(6)
0.75
(9)
0.13
(3)
14.2
(3)
3.44
(15)
0.15
(3)
100.
4sp
(4)
0.20
(6)
9.54
(6)
4.33
(29)
2.03
(47)
76.1
(3)
0.36
(1)
2.25
(18)
0.24
(2)
0.07
(2)
95.2
1140
mf
#48
gl(6
)68
.3(6
)0.
30(1
)16
.2(1
)3.
73(9
)0.
11(5
)0.
86(3
)3.
34(1
4)4.
59(1
3)2.
35(4
)0.
22(3
)93
.7am
p(4)
41.6
(9)
3.62
(35)
11.5
(9)
0.07
(6)
17.0
(8)
0.36
(7)
11.8
(5)
10.0
(4)
2.49
(9)
0.40
(4)
98.8
pl(7
)53
.5(9
)29
.6(7
)0.
74(8
)0.
07(2
)12
.2(8
)4.
29(4
2)0.
14(2
)10
0.5
sp(4
)0.
27(2
)11
.5(1
)3.
67(4
)1.
22(6
)76
.1(2
)0.
39(3
)1.
62(8
)0.
24(2
)95
.015
00m
–150
2m#
2gl
(8)
68.0
(4)
0.61
(4)
15.9
(1)
3.70
(10)
0.11
(2)
0.91
(3)
2.53
(6)
5.31
(19)
2.77
(4)
0.15
(3)
94.8
1500
m–1
502m
#3
gl(9
)71
.3(5
)0.
40(9
)15
.1(1
)2.
44(1
4)0.
12(4
)0.
44(6
)1.
65(1
2)5.
05(2
0)3.
30(8
)0.
09(4
)95
.6op
x(5)
52.8
(4)
0.27
(7)
1.46
(63)
0.10
(3)
22.9
(9)
1.11
(18)
20.5
(11)
1.61
(3)
0.14
(13)
100.
9pl
(8)
58.9
(3)
25.8
(4)
0.51
(11)
0.06
(4)
8.03
(24)
6.91
(30)
0.40
(6)
100.
6sp
(6)
0.01
(1)
15.7
(1)
1.77
(3)
0.13
(3)
75.1
(6)
0.80
(4)
1.69
(3)
0.10
(2)
0.16
(1)
95.5
For
foot
note
sse
ene
xtpa
ge(C
onti
nued
)
211
Average compositions of minerals and glasses from theexperiments are presented in Table 4. A materials bal-ance technique (Bryan et al. 1969) was used to estimatethe phase proportions from the phase compositions inTable 3. Iron loss from the silicate material to the Aucontainer was estimated using the results of the massbalance calculations (Table 3) and is considered to beacceptably low. All of the experiments in this study aredirect synthesis from the glass and crystals present in thestarting materials and the phase appearance sequenceand mineral compositions have not been reversed. Weconclude that the experiments presented here show asufficiently close approach to equilibrium to allow themto be used to understand the petrogenesis of the GlassMountain lava and inclusion suite. As discussed inSisson and Grove (1993a) the evidence taken to indicatean approach to equilibrium includes: maintenance ofconstant sample bulk composition, especially with ref-erence to total iron; achievement of consistent mineral-melt partitioning for Fe-Mg for olivine, high-Ca py-roxene, orthopyroxene, amphibole and Ca-Na for pla-gioclase as expected from other experimental studies andfrom natural samples and generation of regular andconsistent partitioning of elements in minerals grownfrom the melt independent of their presence or absencein the initial starting materials.
Mineral phase appearance and liquid lines of descent
The phase appearance sequence for sample 1140Mf at100, 150 and 200 MPa is summarized in Fig. 2. At
Tab
le4
(con
tinu
ed)
Run
no.
Pha
seSi
O2
TiO
2A
l 2O
3C
r 2O
3F
eOM
nOM
gOC
aON
a 2O
K2O
P2O
5N
iOT
otal
1500
m–1
502m
#7
gl(8
)71
.9(7
)0.
36(7
)15
.0(4
)2.
30(1
2)0.
08(4
)0.
34(3
)1.
51(1
6)5.
06(1
5)3.
38(1
2)0.
11(3
)94
.815
44m
#10
gl(9
)57
.3(3
)0.
96(3
)18
.2(4
)6.
35(1
6)0.
10(2
)4.
06(1
3)7.
80(2
4)4.
03(1
3)1.
19(5
)95
.9ol
(7)
39.2
(2)
0.04
(2)
17.5
(2)
0.24
(3)
43.2
(2)
0.26
(1)
0.12
(2)
100.
6pl
(8)
48.4
(6)
33.2
(4)
0.70
(8)
0.12
(2)
16.5
(5)
2.22
(25)
0.06
(1)
101.
1sp
(4)
0.37
(3)
2.36
(25)
16.1
(1.0
)30
.5(7
)38
.6(1
.3)
0.29
(2)
9.53
(39)
0.34
(10)
0.12
(2)
98.2
1544
m#
7gl
(8)
58.1
(7)
1.03
(3)
18.1
(2)
6.14
(19)
0.07
(3)
3.45
(38)
7.21
(42)
4.33
(27)
1.35
(6)
0.24
(3)
94.3
ol(6
)38
.5(2
)20
.6(5
)0.
29(3
)40
.5(4
)0.
26(2
)0.
02(1
)10
0.5
cpx(
8)52
.3(4
)0.
62(7
)2.
52(3
8)0.
38(8
)6.
48(1
7)0.
16(4
)16
.3(3
)21
.3(2
)0.
27(4
)10
0.5
pl(7
)49
.4(7
)32
.3(4
)0.
70(6
)0.
13(2
)15
.5(6
)2.
75(3
4)0.
08(2
)10
0.9
sp(4
)0.
12(2
)3.
04(7
7)14
.6(1
.6)
30.1
(1.2
)42
.3(1
.6)
0.33
(2)
7.7(
3)0.
21(6
)0.
13(3
)98
.515
44m
#2
gl(1
1)58
.9(3
)1.
02(3
)17
.8(3
)0.
005.
99(1
4)0.
14(6
)3.
39(9
)6.
75(1
6)4.
28(1
8)1.
42(4
)0.
24(3
)95
.4ol
(4)
39.6
(2)
0.01
(1)
0.03
(1)
19.2
(12)
0.22
(1)
41.8
(12)
0.23
(6)
0.17
(4)
101.
3cp
x(10
)52
.3(4
)0.
62(4
)2.
75(4
1)0.
40(5
)6.
37(2
4)0.
18(2
)16
.4(2
)21
.3(3
)0.
27(4
)10
0.6
pl(7
)49
.5(7
)32
.2(7
)0.
61(8
)0.
14(8
)15
.6(6
)2.
64(2
7)0.
09(4
)10
0.7
aG
lass
anal
yses
are
norm
aliz
edto
100%
anhy
drou
s,w
ith
all
Fe
asF
eO.
Unn
orm
aliz
edto
tal
isre
port
edb
Num
ber
ofm
icro
prob
ean
alys
escO
nest
anda
rdde
viat
ion
ofre
plic
ate
anal
yses
inte
rms
ofle
ast
unit
cite
d;th
us0.
15(6
)sh
ould
bere
adas
0.15
±0.
06
Fig. 2 Summary of phase appearance sequence produced in H2O-saturated experiments on andesite 1140Mf. See Tables 3 and 4 forexperimental details and for experiments performed on 1544M and1500M + 1502M mixture. Note that the 1500M + 1502 M mixture isused to extend the 100 MPa results on 1140Mf to temperatures below895 °C
212
100 MPa olivine is the liquidus phase and is present inthe highest temperature experiment (1050 °C). Olivinecrystallizes until high-Ca pyroxene (hi-Ca pyx) andplagioclase (plag) appear at about 1005 °C. Spinel (sp)joins the crystallizing assemblage at 995 °C, and olivine,hi-Ca pyx, plag and spinel cocrystallize until 945 °C,when orthopyroxene (opx) appears and olivine disap-pears through a reaction relation. From 945 to 910 °Copx, hi-Ca pyx, plag and sp cocrystallize. To investigatefurther crystallization at 100 MPa a mix that approxi-mated the composition of experimentally produced glassin experiment 1140Mf #29 was created by mixing twosamples of a tuff erupted from the volcano in latePleistocene time (1500M and 1502M). Three experi-ments were carried out from 895 to 865 °C using thismixture. These produced glass, opx, plagioclase, spineland apatite as crystallizing phases. In experiment #2 and#7 the crystalline products could be identified, but weretoo small to analyze. The hi-Ca pyx was not present inthese experiments. The small difference in bulk compo-sition between the 1500M + 1502M mix and the residualliquid in 1140mf #29 may lead to the absence of hi-Capyx in these experiments. Alternatively, the boundaryopx + hi-Ca pyx + plag + sp boundary may be curvedso that a reaction relation develops, but this curvaturemust be small. At 150 MPa olivine and hi-Ca pyx areboth present in the highest temperature experiment(1000 °C). Plagioclase appears at 975 °C. Olivine, hi-Capyx and plag cocrystallize until opx appears in a reactionrelation that removes olivine from coexistence with liq-uid. Spinel also appears late in the crystallizing assem-blage. At 200 MPa olivine and hi-Ca pyx are againpresent in the highest temperature experiment at1000 °C. Plagioclase and amphibole join the crystalliz-ing assemblage by 980 °C. The four-phase assemblageolivine (oliv) + amphibole (amph) + hi-Ca pyx + plagcrystallizes over a small temperature interval under areaction relation where hi-Ca pyx and liquid react toproduce amphibole. The hi-Ca pyx disappears from thecrystallizing assemblage at ~ 970 °C and the crystallizingassemblage becomes olivine + amphibole + plagioclase.Another reaction involving olivine + liquid to produceamphibole removes olivine as a phase stable with liquid,and the lowest temperature experiment at 200 MPa(920 °C) contains amphibole + plagioclase + spinel.
The 100 MPa H2O-saturated phase appearance se-quence of sample 1544M showed an interval of olivine+ plag + sp crystallization at the highest temperature(1050 °C). Saturation with hi-Ca pyx occurred at 1030 °Cand the liquids follow the 100 MPa olivine + plag + hi-Ca pyx saturation boundary defined by the experimentson 1140Mf.
Mineral-melt chemical systematics in experiments
Table 3 illustrates the variation in composition of min-erals in the H2O-saturated melting using Fe-Mg andCa-Na distribution coefficients. Olivine-melt Fe-Mg ex-
change distribution coefficients �KDFe-Mg
��XFeOlXMg
Liq�=
�XMgOlXFe
Liq�� have an average value of 0.30(� 0:05 2r)
for 15 mineral-melt experimental pairs. The high-Capyroxene KD
Fe-Mg is 0:25�� 0:09 2r� for 13 experimentalassemblages and the amphibole-melt KD
Fe-Mg� 0:35
�� 0:02 2r� for the four amphibole-bearing experiments.The orthopyroxene- melt KD
Fe-Mg is 0:29�� 0:16 2r�for five experiments. Plagioclase-melt Ca-Na exchangedistribution coefficients (KD
Ca-Na� �XCa
PlXNaLiq�=
�XNaPlXCa
Liq�) display a systematic variation with in-
creasing H2O. The KDCa-Na
� 3:76�� 0:2 2r� for theeleven 100 MPa experimental pairs. In the 150 MPaexperiments average KD
Ca-Na is 4:6�� 0:5 2r� and theaverage KD
Ca-Na value in the 200 MPa experiments is4:4�� 0:2 2r�. The values for the exchange KDs andtheir variations provide an indication of the extent towhich a mineral has established Fe-Mg or Ca-Na ex-change equilibrium with its enclosing melt. Olivine-meltpairs show a regular and nearly constant value for KD.High-Ca pyroxene shows a larger variability in KD
Fe-Mg
and this mineral often showed patchy overgrowth rims.Plagioclase-melt KD
Ca-Na at 200 MPa is similar to thevalues found by Gaetani et al. (1994) for Lau Basinbasalt and lower than the value of 5.5 determined bySisson and Grove (1993a) for basaltic melts. This sys-tematic difference may be the result of a melt composi-tional effect attributed to the higher SiO2 and Na2Ocontents of the 1140Mf amphibole-saturated liquids at200 MPa.
Discussion
Compositional variations in Glass Mountain lavasand inclusions
Part of the motivation for this study was to understandthe large variation in chemical composition representedin the Glass Mountain inclusion and lava suite, whichspans a range from 50 to 76 wt% SiO2 and Mg# [Mg/(Mg + Fe2+)] from 0.7 to 0.18. Figures 3 and 4 illustratethis compositional diversity in variation diagrams thatwere chosen because they separate the data and allow itto be compared to the experimentally produced liquidlines of descent. Inclusions that contain olivine + low-Capyx + plag phenocrysts define the low-SiO2 and high-Mg# end of the inclusion suite and hornblende- andopx-bearing inclusions define the high-SiO2 and low-Mg# extreme. The three high-SiO2 inclusions are theplutonic-textured granites. Dacite, rhyodacite andrhyolite lavas range from 62 to 74 wt% SiO2. Glassy,nearly phenocryst free rhyodacite and rhyolite define arestricted range of composition in Na2O versus SiO2(Fig. 4) from 4.5% Na2O and 70% SiO2 to 4% Na2O and74% SiO2. Inclusion-bearing mixed dacite and rhyoda-cite define a field that extends to lower Na2O and SiO2that reflects a range of mafic end members.
In contrast to the inclusions from Little GlassMountain (LGM, Grove and Donnelly-Nolan 1986) the
213
fine-grained, equigranular Glass Mountain inclusionsuite (FGI and FGIA) shows a limited range of com-position. Most are andesitic with about 56% SiO2, andone of these (1140Mf) was used as starting material forthe experimental study. Two samples (1543Ma and1679M) contain > 60% SiO2 and define high-SiO2extremes.
Mineral chemical variations in inclusions and lavas
Mineral compositions in magmatic inclusions (Table 2and Fig. 5) are reported from cores of intergrown min-erals. In the olivine + plag + hi-Ca pyx-bearing inclu-sions olivine + plag (1544 M, Table 2) or olivine +plag + hi-Ca pyx (650Ma, Table 2) glomerocrystic in-tergrowths have textural appearances that indicate thecoexisting phases grew simultaneously. In the cores ofmany hornblende phenocrysts, hi-Ca pyx is preserved asreacted/resorbed remnants and hornblende is intergrownwith plag, opx and spinel (1139Ma and 1679M, Table 2).Mineral assemblages in mixed dacites (1363M and1413M, Table 2 and Fig. 5) consist of fine-grained oli-vine + plag intergrowths in mm-sized blobs of andesite,and coarser mm-long intergrowths of opx + plag +
oxides in rhyodacite (see Eichelberger, 1975). In severalinstances coexisting spinel + ilmenite are associated withopx + plag intergrowths and provide T -fO2 for the latter
Fig. 3 Plot of Mg# versus Al2O3 content of inclusions and lavas fromGlass Mountain and experimentally produced liquids. Numbers referto samples discussed in the text. For Mg# > 0.44 the oliv + plag+ hi-Ca pyx + sp 200 MPa boundaries of Sisson and Grove (1993a,b)have been used to extend the Mg# versus Al2O3 saturation boundaryfor liquids multiply saturated with oliv + plag + cpx. The low Mg#versus Al2O3 dacite lava array extends from Mg# 0.3 to 0.45 andAl2O3 from 14 to 15 wt%. The high-Al2O3 array extends over thesame Mg# range at ~ 16 wt% Al2O3. (Abbreviations for inclusionsbased on textural criteria: FGI fine-grained inclusions, FGIA fine-grained inclusions + amphibole, CI cumulate inclusions, ACI amphi-bole-bearing cumulate inclusions, GI (granitic inclusions)
Fig. 4 Variation of Na2O versus SiO2 in lavas, inclusions andexperiments.A fan of mixing trends in this diagram illustrates therange of mafic differentiates that were mixed with crustal melt. Oneextreme of the mixing array extends from high Na2O (4.7 %), lowSiO2 (67 %) to the rhyolite extreme (4.0 % Na2O, 74.5 % SiO2). Theother extreme of the array extends from low Na2O (3.8 %), low SiO2(62 %) to the rhyolite extreme (4.0 % Na2O, 74.5 % SiO2). See Fig. 3caption for abbreviations of inclusion types based on textural criteria
Fig. 5 Olivine-plagioclase and orthopyroxene-plagioclase covariationin Glass Mountain lavas and minerals produced in meltingexperiments. Chemical analyses are found in Tables 2 and 4
214
stages of crystallization of the Glass Mountain system.The assemblage reported from 1363M in Table 2 recordsa temperature of 855 °C and fO2 of 0.8 log units abovethe quartz-fayalite magnetite buffer (Anderson et al.1988; Ghiorso and Sack 1990), which is only slightlymore oxidizing than NNO. Other assemblages give sim-ilar values for fO2 and extend to temperatures of ~950 °C.
Comparison with experimental liquids
The experimentally determined liquid lines of descentillustrate paths that approximate the ones followedduring fractional crystallization. In practice there wouldbe a divergence of the compositions of melts derived byequilibrium versus fractional crystallization; especiallyat high F (> 0.90). For the purposes of this discussionthe differences at lower F (< 0.7) are small enough toallow a meaningful approximation of the series of iso-thermal experiments to a fractional crystallization se-quence. A comparison of Al2O3 versus Mg# variation inlavas, inclusions and experiments serves as a usefulstarting point (Fig. 3). The liquids from the 100 and150 MPa experiments on 1140Mf define oliv + pla-g + hi-Ca pyx + sp and opx + plag + hi-Ca pyx + spmultiple saturation boundaries; the 200 MPa experi-ments on 1140Mf define oliv + plag ± cpx ± amph andamph + plag + sp boundaries. For Mg# > 0.44 theoliv + plag + hi-Ca pyx + sp boundaries of Sisson andGrove (1993a, b) have been used to illustrate the ex-pected Mg# versus Al2O3 variation in Fig. 3. Compar-ison of inclusions with these multiple saturationboundaries is appropriate, because the most commonmineral assemblages in the inclusions consist of inter-growths of the phases present in the multiply saturatedliquids. Six of the fine-grained inclusions plot nearest themultiple saturation boundary defined by the 100 MPaH2O-saturated experiments in Fig. 3. Most clusteraround the 1140Mf composition and lie at lower Al2O3abundance for their Mg# than the 100 MPa boundary.Cumulate inclusions with Mg# > 0.65 represent samplesthat have lost interstitial melt or accumulated earlyformed crystals. Cumulate-textured sample 1544M andthe aphanitic outer rind 1544Ma that surrounds thislarge inclusion show that this inclusion expelled an SiO2-rich, evolved melt after it was emplaced in its hostrhyolite during eruption and/or cooling. Presumably,this melt was formed after a meter-sized magmatic"blob" was injected into rhyolite, cooled to the temper-ature of the host rhyolite and then subsequently expe-rienced slow cooling prior to eruption. During this slowcooling, liquid was lost to the surrounding rhyolite host,possibly by a process that involved vesiculation and gasfilter pressing (Bacon 1986). It appears that many of theinclusions have experienced some exchange of elementswith their enclosing rhyolite host, and a plot of MgOversus K2O (Fig. 6) illustrates this interaction. The in-clusions plot above the experimental trend between 9
and 3 wt% MgO and define a trend consistent with theincorporation of a small amount of K2O-rich rhyolitemelt that occurred as the partly molten inclusions ex-changed elements with the host lava during cooling priorto or during eruption. The composition of phenocrysticolivine provides an important additional line of evidencethat some of the inclusions are cumulate. The Mg# of1544M is ~ 0.70, so it should contain a liquidus olivineof Fo88 (Fig. 3). The most Mg-rich olivine in 1544M isFo82.7, and this corresponds to a liquidus olivine for thefine-grained andesitic compositions that constitute thedominant set of magmatic inclusions. Amphibole-bear-ing, cumulate-textured inclusion 1691M contains Fo72olivine, as opposed to Fo84, the expected liquidus olivinefor a composition with this Mg#. Furthermore, theAl2O3 content of 1691M (Fig. 3) is significantly lowerthan that expected for an amphibole + olivine + plag +hi-Ca pyx-saturated melt.
Several inclusions have compositional and mineral-ogical characteristics that are similar to 100 MPa H2O-saturated multiply saturated liquid compositions. Sam-ples 1149Mb and 650Ma (Tables 1, 2) contain olivinewith Fo content and plagioclase with An content similarto that expected for liquidus phases at 100 MPa H2O-saturated. The hi-Ca pyx that is intergrown with olivineand plagioclase is slightly more Fe-rich than that ex-pected to coprecipitate with the olivine, and indicatesthat hi-Ca pyx crystallized soon after the olivine andplag.
The process of residual melt expulsion and incorpo-ration of enclosing rhyolite magma that is evident bet-ween cumulate sample 1544M and its aphanitic rind
Fig. 6 Variation of MgO versus K2O in inclusions, lavas andexperimentally produced liquids. See Fig. 3 caption for abbreviationsof inclusion types based on textural criteria
215
(1544Ma) may have been experienced by many of theinclusions. Sample 1140Mf was chosen for experimentalwork because it is a fine-grained, aphanitic amphibole-bearing inclusion and appeared to represent a preservedmelt composition that might be multiply saturated at itsliquidus with olivine + plag + hi-Ca pyx (100 MPa) oroliv + plag + hi-Ca pyx + amph (200 MPa). However,1140Mf, although it is close, is not multiply saturated,and crystallizes < 5% olivine or olivine + hi-Ca pyxbefore it reaches multiple saturation. Thus, 1140Mf andother inclusions that plot below the multiple saturationboundaries in Fig. 3 have lost a small (< 1%) amount ofresidual liquid that formed after the inclusion was in-corporated and cooled in the rhyolite host.
Another characteristic of the inclusion suite is theabsence of a systematic relationship among the fine-grained inclusions, fine-grained amphibole-bearing in-clusions, the cumulate inclusions and the amphibole-bearing cumulate inclusions. There is no clear compo-sitional separation between fine-grained inclusions thatcontain the dominant assemblage (oliv + plag + hi-Capyx) and those that contain amphibole. Both types arefound at either end of the compositional spectrum.Seven of the cumulate-textured inclusion samples showclear compositional characteristics of cumulates (dis-tinctly high MgO, Fig. 7), but of the other 11 cumulate-textured inclusions, several have compositions that aresimilar to 100 MPa H2O-saturated liquids.
The lavas of Glass Mountain show the influence ofmagma mixing (Anderson 1933, 1941). In the Mg#versus Al2O3 plot (Fig. 3) the mixed dacites andrhyodacites define a trend that follows the low end of theMg# versus Al2O3 variation of the lavas. This trendrepresents one extreme of mixing lines between silicicand mafic end members. A number of lavas (Fig. 3) plotat higher Al2O3 values. The lavas with highest Al2O3values define the other extreme of the mixing field. Avariation in the compositions of the mafic end membersof mixing is also distinguishable in plots of MgO versusTiO2 and Na2O versus SiO2 (Figs. 7, 4) and fan-shapedarrays of lavas are evident. The range is caused bymixing of compositionally variable mafic componentswith a single silicic component and variations in theproportions of the components in the mixture. Lavasthat lie at the high-MgO, low-TiO2 extreme are mixturesdominated by an MgO-rich, SiO2-poor mafic compo-nent. Lavas that lie at the low-MgO, high-TiO2 extremeare mixtures dominated by the MgO-poor, SiO2-richmafic differentiate. Samples 1363M and 1413M, two ofthe deviant low-SiO2, high-MgO dacites, were chosenfor detailed petrologic analysis. These dacites containglomerocrystic clots of Fo83–84 olivine intergrown withAn85–78 plagioclase that reside in mafic patches of in-completely mixed andesite melt (not reported in Table 2,see Eichelberger, 1975), isolated coarse-grained Fo71.8and An81.7 glomerocrysts (Table 2) similar to thosefound in amphibole-bearing inclusions (e.g., 1139Ma,1691M, Table 2) and iron-rich opx (Mg# = 64.5) andAn51.7 plagioclase that represent evolved SiO2-richmelts. Thus, the mixed dacites and rhyodacites aremulti-component mixtures of several mafic componentsthat represent various stages of differentiation and ofevolved SiO2-rich component(s).
Comparison of mineral chemical variationsin experiments, inclusions and lavas
The compositional variation of coexisting mineralsprovides another mechanism for comparing the mag-matic products of the Glass Mountain system with theexperimental results. Variations in the composition ofcoexisting olivine + plag or opx + plag from experi-ments, inclusions and lavas are summarized in Fig. 5.For the experiments, the plotted points represent co-crystallizing phases. In lavas and inclusions, mineralpairs are analyses of cores of minerals in glomerocryststhat appear to have nucleated and grown at the sametime. The 100 MPa experiments on 1140Mf have olivineas a liquidus phase (not shown on Fig. 5). When plagfirst appears it crystallizes with hi-Ca pyx and after ashort interval sp joins the assemblage. The sharp bend inthe 100 MPa curve at Mg# = 0.68 and An = 0.68 occurswhere olivine + liquid react to opx. Subsequent crys-tallization involves opx + plag + sp ± hi-Ca pyx. Thevariation in Mg# versus An, reflects the abundance of spin the crystallizing assemblage. The sp buffers the Mg#
Fig. 7 Variation of MgO versus TiO2 in inclusions, lavas andexperimentally produced liquids. A fan of mixing trends in thisdiagram illustrates the range of mafic differentiates that were mixedwith crustal melt. One extreme of the mixing array extends from TiO2of 0.75 %, low MgO (1.5 %) to the rhyolite extreme (0.25 % TiO2,0.3 % MgO). The other extreme of the array extends from TiO2 of0.75 %, high MgO (3.5 %) to the rhyolite extreme (0.25 % TiO2, 0.3 %MgO). See Fig. 3 caption for abbreviations of inclusion types basedon textural criteria
216
of the melt, and significant differentiation occurs withoutmuch change in the Mg# of the evolving liquid. Thecrystallization of an An-rich plag and hi-Ca pyx depletesthe An content of the melt and leads to crystallization ofAn-poor plag. At 200 MPa, pargasitic amphibole isstabilized early in the crystallization assemblage. TheNa2O and TiO2 content of the amphibole and the de-crease in plag abundance keep the An content of themelt at a higher value. The result is that the An contentof plag that crystallizes at 200 MPa follows a less pre-cipitous decline. The amphibole also contains high-TiO2and higher-FeO abundances, which lead to a lowerabundance of these elements in the melt and delaysspinel precipitation. The 200 MPa trend is one of con-tinuously decreasing Mg# with An displaced to highervalues because the increased magmatic H2O increasesthe plag KD
Ca-Na.The inclusions divide into two groups in Fig. 5. The
high-Mg#, high-An group (5 samples) represents theolivine + plag + hi-Ca pyx group of inclusions. Themineral compositions in this group provide evidencethat the parental liquids of the inclusion suite are bestrepresented by 650Ma and 1149Mb, which contain oli-vine and plagioclase that are in equilibrium with thebulk MgO-FeO and CaO-Na2O of these two inclusionsat 100 and 200 MPa H2O-saturated, respectively. Thelow-Mg#, high-An group (3 samples) represents theolivine + plagioclase intergrowths in the amphibole-bearing inclusions. The compositional variations shownin these minerals are most similar to those produced inthe 200 MPa H2O-saturated experiments.
Figure 5 shows the compositional variation in min-erals found in the lavas. Olivine + plag glomerocrystsare similar to those produced in the 100 MPa, H2O-saturated experiments or consist of an assemblage ofolivine with plag, where the plag is lower in An than thatproduced in the H2O-saturated experiments. The lowerAn content records crystallization of olivine + plag atlower pressures (shallower depths) (Wagner et al. 1995).Also present are olivine + plagioclase assemblages sim-ilar to those produced in amphibole-bearing inclusions(1363M, Table 2). A third trend that has been foundonly in the mixed dacites is represented as opx + plagand parallels the 100 MPa opx + plag + sp ± hi-Ca pyxtrend produced in the 100 MPa experiments.
Conditions of fractional crystallizationin Glass Mountain magmatic system
The H2O-saturated experiments provide a starting pointfor interpreting the physical conditions that led to theevolution of the Glass Mountain magmatic system, butthey do not allow exploration of all the possible differ-entiation conditions in PÿT ÿpH2OÿfO2 space. Themineral assemblages may have been produced at H2O-undersaturated conditions at Ptotal > PH2O. Rutherford(1985) and Rutherford and Devine (1988) have exploredthe influence of variable fO2 and pH2O on the phase
appearance sequence of Mt. St. Helens dacite. Theyshow that the effect of variations in fO2 is reflected in theMg# of crystallizing amphibole; the Mg# increasing asfO2 increases and generates Fe3+, thus diminishing Fe2+
available for incorporation into the silicates. The Mg#of crystallizing silicates in the experiments closelymatches that of the Mg# range found in the GlassMountain lavas and inclusions and the fO2 of the ex-periments is similar to the fO2 deduced from coexistingoxides found in Glass Mountain dacite 1363M (Table 2)and in the coexisting oxide pair analyzed by Carmichael(1967, 1991) in rhyolite of Glass Mountain. Therefore,fO2 in experiments and in the Glass Mountain system arecomparable. The crystallizing assemblage in the Mt. St.Helens dacite is amph + opx + hi-Ca pyx + oxides, andthe Rutherford and Devine (1988) experimental resultsare relevant to the latest stages of crystallization of theGlass Mountain system, after olivine has reacted out as acrystallizing phase. Rutherford and Devine (1988) showthat amphibole will only remain stable with pyrox-ene + plag when XH2O > 0:8 under H2O-undersaturatedconditions. At XH2O > 0:8, Ptotal � 220 MPa, Rutherfordand Devine (1988) find a crystallizing assemblage that issimilar to that found in the amphibole-bearing inclu-sions and in the 200 MPa, H2O-saturated experiments.At XH2O � 0:5, Ptotal � 220 MPa, the crystallizing as-semblage resembles that produced in the 100 MPa GlassMountain H2O-saturated experiments. The H2O-un-dersaturated experiments also demonstrate that the ef-fect of variable H2O content is the controlling factor onAn content. As pH2O decreases, An content of the pla-gioclase drops. Thus, a possible interpretation of themineral compositional variations observed in the GlassMountain system is that crystallization commenced at apressure sufficient to stabilize amphibole ~ 200 MPa,but at pH2O � 0:5 Ptotal. As the Glass Mountain systemcrystallized, H2O was concentrated as an incompatiblecomponent in the residual liquid and XH2O increaseduntil amphibole became stabilized.
Several lines of evidence indicate that a single crys-tallization event in which H2O content of the residualmelt increased as differentiation proceeded is not theprocess that led to the generation of rhyolite of GlassMountain and that the situation is more complex. Themost primitive inclusions (650Ma and 1149Mb) containmineral assemblages that indicate variable H2O withinthe system. The An content of the plagioclase in 650Mais consistent with crystallization under 100 MPa H2O-saturated conditions, and 1149Mb contains a plagioclasethat would have cocrystallized with olivine at 200 MPaH2O-saturated conditions. Furthermore, there are twodistinct and separate differentiation paths preserved ininclusions from the Glass Mountain magmatic system:one involving amphibole + higher-An plag + olivinecrystallization and one involving opx + lower-An plag +spinel. These two trends were produced by spatial/tem-poral variations in H2O content (e.g. Ptotal � 200 MPaand pH2O � variable) or by variations in pressure atH2O-saturated conditions (Ptotal�pH2O; Ptotal� variable).
217
The conclusions drawn from this variation are that therewas either a heterogeneous distribution of H2O in themagmatic system, or a pressure gradient.
The role of crustal assimilation in the developmentof rhyolite of Glass Mountain
To establish the role played by assimilation of crustduring crystallization processes inclusions and lavaswere chosen for isotopic analysis and Fig. 8 shows thevariation in the Sr and Nd isotope systems. The moststriking characteristic of these data is that the magmaticinclusions are isotopically similar with 87/86Sr of~ 0.7038 and the rhyolite is more radiogenic at 0.70404.When the abundance levels of Sr in the andesitic inclu-sions (~ 450 ppm) and rhyolite (~ 125 ppm) are com-pared, it is clear that the rhyolite has seen a significantcontribution from another source. Grove et al. (1988)and Baker et al. (1991) discussed the role of crustalcontamination in mafic Medicine Lake lavas and pro-posed that a granitic component is the most likelycrustal source incorporated during a process that in-volves crystallization of the mafic magma, melting ofshallow crust and replenishment by undifferentiatedmafic magma. The granitic component that underlies thevolcano is present as inclusions in Pleistocene andHolocene lavas. The range of isotopic compositions ofthese potential crustal assimilants is shown on Fig. 8.The data include granitic inclusions from Glass Moun-tain and other Holocene and Pleistocene Medicine Lakelavas (Grove et al. 1988).
Baker et al. (1991) propose a process involving frac-tional crystallization, assimilation, replenishment andmixing (FARM) to generate basaltic andesite andandesite at the Holocene Burnt Lava and Giant Craterflows. Evidence preserved in the chemical compositionof minerals in the Glass Mountain mixed magmas and inthe diversity of inclusions found in Glass Mountain la-vas is also consistent with the FARM model. Both in-clusions and minerals in the lavas show evidence of aparental magma of andesite composition similar to1140Mf, 650Ma and 1149Ma (Table 1). Several high-SiO2, low-MgO andesite inclusions (1679M and1543Ma) represent products of fractional crystallization.Amphibole-bearing cumulates (1139Ma and 1691M)represent residues of fractional crystallization. Graniticinclusions represent candidate plutonic material thatwas heated and melted during the FARM process. TheMg-rich olivine and An-rich plagioclase in the mixeddacite imply reinjection of parent andesite into the
Fig. 8a–c Variations in Sr and Nd isotopic abundances in GlassMountain inclusions and lavas. Symbol referred to as other granites isfor analyses from Grove et al. (1988) which are granitic inclusionsfrom other Holocene and Pleistocene lavas on Medicine Lake vol-cano. a 87/86Sr versus SiO2 content; b 87/86Sr versus Sr abundance;c 143/144Nd versus Nd abundance
c
218
magma reservoir, triggering mixing of several magmaticcomponents. In the FARM model fractional crystalli-zation of parental mafic magma occurs in the absence ofinteraction with surrounding crustal material. Thecrystallization event provides heat to melt surroundingwall rocks, but these crustal melts do not interact withthe fractionating magma. Less dense crustal melts collectat the top of the reservoir, denser cumulates from frac-tionation develop at the base, and differentiated meltsexpelled from these cumulates pond above the crystal-rich residue and beneath the low density SiO2-richcrustal melts. Two lines of evidence in the GlassMountain system support this scenario: (1) The closeisotopic similarity of the magmatic inclusions; parental,evolved and cumulate inclusions are similar and do notappear to have been significantly affected by crustalcontamination. (2) The association preserved whereby aquenched andesite rind (1543Ma) surrounds a highlymelted granitic inclusion (1543 M). The granite andandesite retain their isotopic differences and the andesiteis isotopically identical to the parental andesites. Ande-site inclusion 1543Ma most closely resembles a 100 MPadifferentiate of an andesite parent (Fig. 3) and lacks thecompositional characteristics of a disequilibrium melt ofthe granite host (high Al2O3 and high Na2O + K2O;Grove et al. 1988; Knesel and Davidson 1996).
A model (Table 5) tests the plausibility of the FARMprocess. It uses the most primitive parent andesite(1140Mf and 650Ma), an estimated differentiate, and anassimilant represented by the spectrum of granitic crustalinclusions that have been found at Medicine Lake todevelop a geochemical model of the trace element andisotopic characteristics of the rhyolite and mixed dacitelavas of Glass Mountain. The parent andesite is repre-sented by inclusions 650Ma and 1140Mf. The differen-tiate is not directly represented as a sampled inclusion,and we devised several model compositions. Mineralchemical variations in mixed lavas indicate the existenceof evolved and differentiated residual liquids that aresimilar to the lowest temperature residual liquids pro-duced in the 100 and 200 MPa experiments. The mixingtrends displayed by the rhyolite and dacite lavas alsoindicate a spectrum of evolved melts with SiO2 contents< 73 wt% and > 68% (Fig. 5). The mineral compositionsin experimentally produced liquids with this range ofSiO2 abundance are similar to the chemical compositionsof the Mg-poor and An-poor assemblages (Fig. 6). Ac-cordingly, experimental liquids at 100 MPa(1502m1503m #3) and 200 MPa (1140mf #48) werechosen as representative of the differentiated component.These melts represent ~ 68 and ~ 61 wt% crystallizationof the parent andesite. The phase proportions producedexperimentally were used to develop a model for frac-tional crystallization at 100 MPa and 200 MPa. Becausethe crystallizing assemblage changes over the solidifica-tion interval, a two-stage model of Rayleigh fractiona-tion was applied. Trace element abundances measured inthe andesite parent and experimentally determined dis-tribution coefficients for a set of elements were used to
calculate the abundances in an evolved melt (Table 5).The assimilant composition was assumed to be repre-sented within the spectrum of compositions spanned bythe average granite inclusion from Medicine Lake(Grove et al. 1988) and the granitic crustal materialfound in the Glass Mountain lavas (Tables 1 and 5).
The major element characteristics of rhyolites andmixed dacites were estimated by a least squares materialbalance using the major element compositions of ande-site parent, differentiated melt and granitic crust. Arange of models was considered and the ones that pro-vide the best approximation to rhyolite of GlassMountain are presented in Table 5. These models usedthe 100 MPa differentiated melt and a granitic GlassMountain crustal inclusion (1543M). Similar modelingresults were obtained when a 200 MPa amphibole-bearing liquid was substituted as the differentiated melt.If a crustal inclusion similar to the average granitic in-clusion (Grove et al. 1988) is used, the proportion ofandesite parent that is required to generate rhyolitedrops to zero. This situation is represented by the secondrhyolite model in Table 5 that incorporates only differ-entiate of andesite and granite crust. Models that gen-erated the mixed dacite required all three components(Table 5).
To further test the mixing models we used the majorelement-based estimates of mixture proportions to cal-culate the abundance of several trace and rare earth el-ements as well as the isotopic compositions of Sr and Ndin rhyolite and dacite. The mass proportions of themagmatic components estimated by least squares wereused as input and the results are presented in Table 5. Arange of estimates is included that reflects the variabilityin trace and rare earth element partition coefficients usedto model the element abundances in the differentiatedmelt. This range could be broadened further if we in-corporated the variability in element abundance in thegranitic inclusions that have been found in MedicineLake lavas (Grove et al. 1988). The match between theestimated abundance of trace and rare earth elements isclose enough to indicate that the mixing models providea plausible mechanism for generating the rhyolite andmixed dacites. A plot of two incompatible elements(Fig. 9) shows the constraints provided by trace ele-ments on the mixing process. At least three componentsare necessary to generate the observed abundances inrhyolite and dacite lavas. Trace element abundancesrequire nearly equal proportions of parent andesite,differentiated liquid and granite to produce the mixeddacites. In the rhyolites differentiated melt and granitedominate the mixture and some rhyolite lavas containonly these two components. The proportions estimatedfrom the major element model, the trace element abun-dances and isotopic compositions in the mix compo-nents were used to calculate Sr and Nd isotopiccompositions of dacite and rhyolite lavas. The calculatedSr and Nd isotopic compositions of dacite and rhyoliteare very similar to those measured in the lavas, and in-dicate a significant melted crust component.
219
Tab
le5
FA
RM
mod
els
for
gene
rati
onof
Gla
ssM
ount
ain
rhyo
lite
and
daci
tela
vas
Com
pone
nts
ofm
ixin
gm
odel
SiO
2T
iO2
Al 2
O3
FeO
MgO
CaO
Na 2
OK
2OP
2O5
La
Ce
SmE
uY
bL
uT
hB
aR
bZ
rSr
87/8
6N
d14
3/14
4ΣR
2
Ass
imila
ntco
mpo
siti
onA
GI
(Gro
veet
al.,
1988
,T
able
s8,
9,10
)73
.88
0.26
14.2
51.
650.
451.
483.
874.
070.
0823
464.
20.
392.
40.
3513
815
144
189
157
0.70
460
190.
5127
7015
43M
76.9
0.14
12.8
0.83
0.14
0.63
3.63
4.96
0.00
2140
3.3
0.36
2.5
0.36
1866
115
514
441
0.70
412
160.
5128
91D
iffer
enti
ated
liqui
dco
mpo
siti
on–c
alcu
late
dtr
ace
elem
ent
abun
danc
es15
02m
1503
m#
3aF
=0.
3271
.46
0.40
15.1
32.
450.
441.
655.
063.
310.
0932
567
1.0
3.4
0.55
7.9
858
112
338
131
3136
7912
3.2
6.4
0.70
8.4
910
125
388
137
41F
ract
iona
tion
mod
elb
Ds
used
toes
tim
ate
trac
eel
emen
tab
unda
nces
liste
dab
ove.
Par
t1
=0.
14ol
+0.
27cp
x+
0.58
pl,
0.11
0.05
0.04
0.06
0.03
0.27
0.01
0.12
0.02
0.06
1.29
0.04
F=
1.0
to0.
650.
190.
110.
501.
10.
580.
450.
160.
120.
101.
330.
25P
art
2=
0.17
opx
+0.
19cp
x+
0.58
pl+
0.06
sp0.
120.
050.
050.
070.
070.
260.
020.
260.
020.
052.
470.
05F
=0.
65to
0.32
0.24
0.38
0.54
1.1
0.63
0.47
0.12
0.32
0.12
0.22
2.51
0.34
Thr
eeco
mpo
nent
mix
ing
mod
elsc
Rhy
olit
e40
4Man
d14
06M
.M
odel
1=
0.55
assi
mila
nt+
0.40
diffe
rent
iate
+0.
05pa
rent
ande
site
.M
odel
2=
0.66
assi
mila
nt+
0.34
Diff
eren
tiat
e73
.80
0.27
14.1
01.
890.
351.
264.
064.
380.
0523
454.
10.
632.
60.
4015
828
142
217
110
0.70
416
18.2
0.51
2805
73.8
60.
2613
.96
1.64
0.25
1.11
4.40
4.07
0.06
2649
5.2
0.67
2.8
0.43
1181
012
624
516
10.
7042
924
0.51
2801
0.15
2858
7.2
1.5
4.0
0.48
1183
013
126
616
629
73.8
50.
1813
.93
1.77
0.31
1.48
3.98
4.04
0.8
2649
5.1
0.6
2.7
0.42
1182
913
324
014
80.
7044
240.
5127
950.
2428
586.
81.
33.
80.
4711
847
138
260
150
28D
acit
e14
13M
Mod
el1
=0.
21as
sim
ilant
+0.
62di
ffere
ntia
te+
0.17
pare
ntan
desi
te63
.80
0.63
16.2
05.
172.
955.
173.
832.
520.
1418
343.
80.
852.
30.
327.
860
286
185
318
0.70
388
160.
5128
1363
.82
0.65
16.1
35.
062.
815.
383.
752.
390.
1418
364
0.93
2.4
0.35
854
675
179
329
0.70
393
180.
5128
140.
1219
405
1.3
3.0
0.38
855
577
188
330
20
aT
race
elem
ent
and
rare
eart
hel
emen
tab
unda
nces
calc
ulat
edus
ing
atw
ost
epfr
acti
onal
crys
talli
zati
onm
odel
desc
ribe
dbe
low
inth
ista
ble
bF
ract
iona
tion
asse
mbl
age
used
for
two
step
trac
eel
emen
tm
odel
ing.
Pha
sepr
opor
tion
sfr
omT
able
3,10
0M
Pa
expe
rim
ents
.P
arti
tion
coeffi
cien
tso
urce
san
dm
odel
assu
mpt
ions
:(1
)ta
bula
tion
ofG
rove
and
Don
nelly
-Nol
an(1
986)
;(2
)B
lund
yan
dW
ood
(199
1);
(3)
Siss
on(1
991)
;(4
)G
aeta
nian
dG
rove
(199
5);
(6)W
atso
nan
dH
arri
son
(198
3).
The
rang
ein
abun
danc
ere
sult
sfr
omth
era
nge
inpo
tent
ial
Dva
lues
.cT
hem
odel
sas
sum
ea
FA
RM
proc
ess
disc
usse
dby
Bak
eret
al.
(199
1).
Par
ent
mag
ma
isan
desi
te(1
140M
fan
d65
0Ma)
and
diffe
rent
iate
dm
agm
ais
mod
eled
usin
gex
peri
men
tally
prod
uced
resi
dual
liqui
d15
02m
1503
m#
3th
atre
pres
ents
68w
t%cr
ysta
lliza
tion
ofth
ean
desi
tepa
rent
.M
iner
alpr
opor
tion
sar
eus
edto
calc
ulat
etr
ace
elem
ent
abun
danc
esof
the
frac
tion
ated
mel
tus
ing
mea
sure
dab
unda
nces
in65
0Ma
asa
star
ting
poin
tan
dth
em
odel
pres
ente
din
this
tabl
e.M
odel
1m
ixes
allt
hree
com
pone
nts
and
mod
el2
mix
eson
lym
elte
dgr
anit
ean
ddi
ffere
ntia
ted
liqui
d.F
irst
row
show
s40
4Mco
mpo
siti
onal
data
.Nex
tis
mod
el1
wit
hra
nge
oftr
ace
elm
enta
bund
ance
s,fo
llow
edby
mod
el2.
Com
posi
tion
of14
13M
daci
teis
next
,fo
llow
edby
leas
tsq
uare
sm
odel
and
calc
ulat
edtr
ace
elem
ents
220
Implications for magmatic processes
The mass balance models developed in the precedingsection imply that the ~ 1 km3 volume of rhyolite ofGlass Mountain consists of from 55 to 66 wt% of meltedcrust. To produce a shallow crustal melt that is 0.55 to0.66 the mass fraction of the 1 km3 total of rhyolite ofGlass Mountain requires a substantial input of heatfrom an external source; presumably a hot mafic pa-rental magma intruded to shallow levels. The minimummass of melted crust is 1.4 to 1:7 � 1015 gm. Geophysi-cal models of crustal structure beneath Medicine Lakevolcano identify low velocity, low density silicic crust ata depth of > 6 km (Zucca et al. 1986). To melt graniticcrust at this shallow depth, its temperature must first beraised from ambient levels to the solidus (200 °C to900 °C) and then melted. The thermal energy required tocarry out this process includes specific heat(Cp � 0:3 cal gmÿ1 �Cÿ1) of � 210 cal gmÿ1 and a la-tent heat of fusion of � 50 cal gmÿ1 (Weill and Hon1980). The total heat required is atleast 3.6 to4:9 � 1017 cal. The model developed in Table 5 indicates68% crystallization of a mafic andesite parent. The heatnecessary would be supplied by 68% crystallization of5.3 to 6:3 � 1015 gm of magma or ~ 2 to 2.5 km3 volumeof parent andesite.This 68% crystallization of 2–2.5 km3
of mafic andesite would produce the heat necessary toraise the temperature of the shallow crust and melt amass that would constitute the 55 to 60% that makes upthe 1 km3 erupted volume. Another way to estimate theoriginal mass of andesite is to use the mass fraction ofderivative liquid that is present in the rhyolite of GlassMountain and was produced by fractional crystalliza-tion of mafic andesite. The amount of andesite parentresidual from 68% fractional crystallization (40–34 wt%,Table 5) implies the presence of an original mass ofandesite of 3 � 1015 gm or approximately 1–1.3 km3 ofmafic andesite parent. Thus, the constraint provided bythe thermal energy required to produce the 55% of therhyolite that is a crustal melt predicts a significantlygreater mass of andesite than the amount represented bythe fractionated component. The remaining andesite ispresumably stored as mafic cumulates at depth. Sparksand Marshall (1986) show that mixing of melts withdiffering liquidus temperatures and physical propertiesdoes not generally lead to the production of hybridmagmas. Mixing can only occur after thermal equili-bration takes place. The consequence is that large vol-umes of mafic cumulates develop at the base of silicicmagma reservoirs, and relatively smaller volumes ofdifferentiate of the parent magma are added to crustallyderived melts.
Model for the Glass Mountain magmatic system
A model of Glass Mountain magmatic processes can beconstructed by integrating the petrologic and geochemi-cal evidence described in this work with the results ofgravity and seismic studies of the volcano. The geo-physical studies provide information on subsurface ge-ology and structure (Finn and Williams 1982; Zucca et al.1986) and image the magma reservoir beneath GlassMountain (Evans and Zucca 1988). An E–W cross sec-tion of the volcano (Fig. 10) summarizes the results ofthese geophysical studies and provides a context for in-terpreting the petrologic evidence. Evans and Zucca(1988) locate a silicic magma reservoir at a depth of 1 to3 km beneath the volcanic edifice. The maximum hori-zontal dimensions of the body are 2 to 3 km allowing forstorage of a mass of material equivalent of up to 3 km3.This magma body is underlain by low velocity graniticcrust to a depth of ~ 10 km, and a feeder system of maficdikes that supplied parental magma inferred to lie be-neath the volcano. To be consistent with the geochemicaland petrologic data a model for the evolution of theGlass Mountain magmatic system must have the fol-lowing properties: (1) crystallization processes over arange of pressures from 200 to 100 MPa (6–3 km depth);(2) assimilation of granitic crustal material to provide themelted crustal component represented in rhyolite ofGlass Mountain. Figure 10 illustrates such a model. Thepresence of amphibole-bearing cumulates is consistentwith crystallization of an H2O-rich magma in the crustthat underlies the volcano. This crystallization process
Fig. 9 Abundance variation of Th and Zr in Glass Mountaininclusions and lavas. Triangle shows proposed three componentmixing that leads to the production of dacite and rhyolite lavas. AGI isthe average granitic inclusion of Grove et al. (1988), and the solid barextending to 1543M indicates variability in the granitic inclusionssampled from beneath Medicine Lake volcano. The range in Zr-Thabundance in the fractionated liquids is a result of variability in the Dsused in the fractionation calculation found in Table 5. Temperaturerange and compositional variations of liquids do not reach zirconsaturation (Watson and Harrison 1983). See Fig. 3 caption forabbreviations of inclusion types based on textural criteria
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supplied heat to melt the crust. In Fig. 10 the process isenvisioned as one in which dikes of mafic andesite areemplaced into the granite, where they cool, crystallizeand transfer heat to melt surrounding wall rock. As thesedikes cool and crystallize the less dense residual liquidsand melted crust ascend to the shallow magma reservoirand collect there. The process of emplacement andcrystallization occurs episodically but the repeatedheating eventually leads to melting of the granitic crust.At the same time differentiated magma and crustal par-tial melts ascend and collect in a shallow reservoir. Theselow density melts mix to form rhyolite that consistsdominantly of melted crust and differentiated andesite.Dynamics of magma emplacement are such that andesiteis intruded into the developing shallow silicic magmareservoir where it cools and crystallizes in the rhyolitemagma. This process is illustrated in Fig. 10 as the em-placement of sill-like bodies of andesite at the base of thesilicic chamber, where the higher density andesite istrapped beneath the less dense rhyolite. These shallowerandesites also differentiate and expel their residual melts,adding to the mass of evolved silicic magma. The cul-mination of this process was the eruption of the GlassMountain system. Presumably, an influx of andesitemagma overpressured the system and triggered theeruption. As the andesite ascended toward the surface, itpicked up samples of the deeper magmatic cumulates andcrustal material and shallower cumulates. The andesiteand its inclusion samples were injected into overlying
rhyolite, and the mixture erupted. The mixed dacite andrhyodacite magmas of Glass Mountain contain partlydisaggregated magmatic inclusions from both the deepand shallow cumulates, along with granitic crust andchilled magma that was injected to shallow levels in thereservoir.
This model describes the evolution of a small silicicmagma chamber, although, several of its petrologicfeatures are shared with larger magmatic systems. In theclimactic Crater Lake eruption of ~ 50 km3 of rhyoda-cite magma, there is good evidence that the silicic mag-ma body grew by expulsion of differentiated melt fromcumulates crystallized from andesitic parents and thatassimilation of shallow (3–5 km deep) granodiorite crustwas important (Bacon and Druitt 1988; Bacon 1992).Cumulates with similar mineralogy and inferred crys-tallization conditions underlie the Sierra Nevadabatholith and similar parent melts were important inproducing the voluminous granodiorite magmas of theSierra Nevada (Sisson et al. 1997). Thus, the types ofmagmatic processes preserved in the lavas of GlassMountain may be significant in much larger systems.
Acknowledgments The authors thank R. Powers and G. Gaetanifor assistance with the experiments, and S. Bowring and D. Cole-man for providing isotopic data. N. Chatterjee and M. Jercinovicexpertly maintained the MIT electron microprobe during thecourse of this study. Careful and thoughtful reviews were providedby C. Bacon, M. Clynne, R. Lange and M. Rutherford. This re-search was supported by NSF Grant EAR9204661 andEAR9406177.
References
Albee AL, Ray L (1970) Correction factors for electron probemicroanalysis of silicates, oxides, carbonates, phosphates andsulfates. Anal Chem 42: 1408–1414
Andersen DJ, Lindsley DH (1988) Internally consistent solutionmodel for Fe-Ti-Mn-Ti oxides: Fe-Ti oxides. Am Mineral 73:714–726
Anderson CA (1933) Volcanic history of Glass Mountain, northernCalifornia. Am J Sci 26: 485–506
Anderson CA (1941) Volcanoes of the Medicine Lake Highland,California. Univ Calif Depp. Geol Sci Bull 25: 347–442
Bacon CR (1986) Magmatic inclusions in silicic and intermediatevolcanic rocks. J Geophys Res 91: 6091–6112
Bacon CR (1992) Partially melted granodiorite and related rocksejected from Crater Lake caldera, Oregon. Trans R Soc Edin-burgh 83: 27–47
Bacon CR, Druitt TH (1988) Compositional evolution of the zonedcalcalkaline magma chamber of Mt. Mazama, Crater Lake,Oregon. Contrib Mineral Petrol 98: 224–256
Baker MB, Grove TL, Kinzler RJ, Donnelly-Nolan JM,Wandless GA (1991) Origin of compositional zonation (high-alumina basalt to basaltic andesite) in the Giant Crater lavafield: Medicine Lake Volcano, northern California. J GeophysRes 96: 21819–21842
Bence AE, Albee AL (1968) Empirical correction factors for theelectron microanalysis of silicates and oxides. J Geol 76: 382–403
Blundy JD, Wood BJ (1991) Crystal- chemical controls on thepartitioning of Sr and Ba between plagioclase feldspar, silicatemelts, and hydrothermal solutions. Geochim Cosmochim Acta55: 193–208
Fig. 10 Cross section of the Glass Mountain magma chamber basedon inferences from geochemistry, petrology and geophysics. Depthscale is based on petrologic and geophysical constraints
222
Bryan WB, Finger LW, Chayes F (1969) Estimating proportions inpetrographic mixing equations by least squares approximation.Science 163: 926–927
Carmichael ISE (1967) The iron titanium oxides of salic volcanicrocks and their associated ferromagnesian silicates. ContribMineral Petrol 14: 36–64
Carmichael ISE (1991) The redox states of basic and silicic mag-mas: a reflection of their source regions? Contrib Mineral Petrol106: 129–141
Donnelly-Nolan JM (1988) A magmatic model of Medicine Lakevolcano, California. J Geophys Res 93: 4412–4420
Donnelly-Nolan JM, Champion DE, Miller CD, Grove TL,Trimble DA (1990) Post-11,000-year volcanism at MedicineLake volcano, Cascade Range, northern California. J GeophysRes: 95: 19693–19704
Donnelly-Nolan JM, Champion DE, Grove TL, Baker MB, Tag-gart JEJr, Bruggman PE (1991) The Giant Crater lava field:geology and geochemistry of a compositionally zoned, high-alumina basalt to basaltic andesite eruption at Medicine LakeHighland, California. J Geophys Res 96: 21843–21863
Eichelberger JC (1975) Origin of andesite and dacite: evidence ofmixing at Glass Mountain in California and at other circum-Pacific volcanoes. Geol Soc Am Bull 86: 1381–1391
Eichelberger JC (1980) Vesiculation of mafic magma during re-plenishment of silicic magma reservoirs. Nature 288: 446–450
Evans JR, Zucca JJ (1988) Active high-resolution seismic tomo-graphy of compressional wave velocity and attenuation struc-ture at Medicine Lake volcano, northern California CascadeRange. J Geophys Res 93: 15016–15036
Fink JH, Pollard DD (1983) Structural evidence for dikes beneathsilicic domes, Medicine Lake Highland Volcano, California.Geology 11: 458–461
Finn C, Williams DL (1982) Gravity evidence for a shallow intrusionunder Medicine Lake volcano, California. Geology 10: 503–507
Gaetani GA, Grove TL (1995) Partitioning of rare earth elementsbetween clinopyroxene and silicate melt: crystal-chemical con-trols. Geochim Cosmochim Acta 59: 1951–1962
Gaetani GA, Grove TL, Bryan WB (1994) Experimental phaserelations of basaltic andesite from Hole 839B under hydrousand anhydrous conditions. In: Hawkins J, et al. (eds) Proc.Ocean Drilling Program (Sci Results, vol 135) pp 557–563
Ghiorso M, Sack RO (1991) Fe-Ti oxide geothermometry: ther-modynamic formulation and the estimation of intensive vari-ables in silicic magmas. Contrib Mineral Petrol 108: 485–510
Grove TL, Donnelly-Nolan JM (1986) The evolution of youngsilicic lavas at Medicine Lake volcano, California: implicationsfor the origin of compositional gaps in calc-alkaline series lavas.Contrib Mineral Petrol 92: 281–302
Grove TL, Gerlach DC, Sando TW (1982) Origin of calc-alkalineseries lavas at Medicine Lake volcano by fractionation, assim-ilation and mixing. Contrib Mineral Petrol 80: 162–180
Grove TL, Kinzler RJ, Baker MB, Donnelly-Nolan JM, Lesher CE(1988) Assimilation of granitic crust by basaltic magma atBurnt Lava Flow, Medicine Lake volcano, northern California.Contrib Mineral Petrol 99: 320–343
Heiken G (1978) Plinian-type eruptions in the Medicine LakeHighland, California and the nature of the underlying magma. JVolcanol Geotherm Res 2: 1–16
Hon R, Weill DF (1982) Heat balance of basaltic intrusion versusgranitic fusion in the lower crust (abstract). EOS Trans AmGeophys Union 63: 470
Housh T, Bowring SA, Dungan MA (1997) Isotopic constraints onpetrogenesis and lithospheric development within the Ocate andRaton-Clayton volcanic fields, New Mexico. Contrib MineralPetrol (in press)
Knesel KM, Davidson JP (1996) Isotopic disequilibrium duringmelting of granite and implications for crustal contamination ofmagmas. Geology 24: 243–246
Mertzman SA, Williams RJ (1981) Genesis of recent silicic mag-matism in the Medicine Lake Highland, California: evidencefrom cognate inclusions found at Little Glass Mountain. Ge-ochim Cosmochim Acta 45: 1463–1478
Perry FV, DePaolo DJ, Baldridge WS (1993) Neodymium isotopicevidence for decreasing crustal contributions to Cenozoicignimbrites of the western United States: implications for thethermal evolution of the Cordilleran crust. Geol Soc Am Bull105: 872–882
Rutherford MJ, Devine JD (1988)) The May 18, 1980 eruption ofMt. St. Helens 3. Stability and chemistry of amphibole in themagma chamber. J Geophys Res 93: 11949–11959
Rutherford MJ, Sigurdsson H, Carey S, Davis A (1985) TheMay 18, 1980 eruption of Mt. St. Helens 1. Melt compositionand experimental phase relations. J Geophys Res 90: 2929–2947
Sisson TW (1991) Pyroxene-high silica rhyolite trace element par-tition coefficients measured by ion microprobe. Geochim Cos-mochim Acta 55: 1575–1585
Sisson TW, Grove TL (1993a) Experimental investigations of therole of water in calc-alkaline differentiation and subductionzone magmatism. Contrib Mineral Petrol 113: 143–166
Sisson TW, Grove TL (1993b) Temperature and H2O contents oflow-MgO high-alumina basalts. Contrib Mineral Petrol 113:167–184
Sisson TW, Grove TL, Coleman DS (1997) Hornblende gabbro sillcomplex at Onion Valley, California, and a mixing origin forthe Sierra Nevada batholith. Contrib Mineral Petrol 126: 81–108
Smith RL (1979) Ash-flow magmatism. In: Chapin CE, Elston WE(eds) Ash-flow tuffs. Geol Soc Am Spec Pap 180: 5–27
Sparks RSJ, Marshall LA (1986) Thermal and mechanical con-straints on mixing between mafic and silicic magmas. J VolcanolGeotherm Res 29: 99–124
Taggart JEJr, Lindsay JR, Scott BA, Vivit DV, Bartel AJ, Stew-art KC (1987) Analysis of geologic materials by wavelength-dispersive X-ray fluorescence spectrometry, US Geol Surv Bull1170: E1–E19
Wagner TP, Donnelly-Nolan JM, Grove TL (1995) Evidence ofhydrous differentiation and crystal accumulation in the low-MgO, high-Al2O3 Lake Basalt from Medicine Lake volcano,California. Contrib Mineral Petrol 121: 201–216
Watson EB, Harrison TM (1983) Zircon saturation revisited:temperature and composition effects in a variety of crustalmagma types. Earth Planet Sci Lett 64: 295–304
