Rhyolite Thermobarometry and the Shallowing of the Magma Reservoir, Coso Volcanic Field, California

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1 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 1 PAGES Rhyolite Thermobarometry and the Shallowing of the Magma Reservoir, Coso Volcanic Field, California CURTIS R. MANLEY 1 AND CHARLES R. BACON 2 1 DEPARTMENT OF GEOLOGY, UNIVERSITY OF NORTH CAROLINA, CHAPEL HILL, NC , USA 2 US GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD MS-910, MENLO PARK, CA , USA RECEIVED JUNE 17, 1998; REVISED TYPESCRIPT ACCEPTED JULY 5, 1999 The compositionally bimodal Pleistocene Coso volcanic field is histories of their magmatic systems (e.g. Long Valley located at the western margin of the Basin and Range province Glass Mountain, Metz & Mahood, 1991), and they >60 km north of the Garlock fault. Thirty-nine nearly aphyric potentially provide insight into magma chamber growth high-silica rhyolite domes were emplaced in the past million years: processes before caldera collapse. The Coso volcanic field one at 1 Ma from a transient magma reservoir, one at >0 6 Ma, of eastern California is such a system in which phenocryst and the rest since >0 3 Ma. Over the past 0 6 My, the depth thermobarometry of highly differentiated rhyolite pro- from which the rhyolites erupted has decreased and their temperatures vides the opportunity to determine the depth and thermal have become slightly higher. Pre-eruptive conditions of the rhyolite history of the silicic magma reservoir(s) over a span of magmas, calculated from phenocryst compositions using the two- nearly 0 6 My. Although we cannot know whether the oxide thermometer and the Al-in-hornblende barometer, ranged Coso volcanic field will eventually produce a large-volume from 740 C and 270 MPa (2 7 kbar; >10 km depth) for the eruption, the processes of accumulation of rhyolitic >0 6 Ma magma, to 770 C and 140 MPa (1 4 kbar; magma and its apparent migration to shallow depth are >5 5 km) for the youngest (>0 04 Ma) magma. Results are believed to be similar to the early stages of development consistent with either a single rhyolitic reservoir moving upward of a Long Valley-like system. through the crust, or a series of successively shallower reservoirs. As The Pliocene Pleistocene Coso volcanic field ( Fig. 1) the reservoir has become closer to the surface, eruptions have become is located at the western margin of the Basin and Range both more frequent and more voluminous. province just east of the Sierra Nevada and >60 km north of the Garlock fault. About 39 high-silica rhyolite domes and lava flows (hereafter simply termed domes), between 70 and 170 m thick and totaling 1 6 km 3, were KEY WORDS: Al-in-hornblende; caldera; eruption; geothermal; rhyolite emplaced in the western part of the field in the past million years, all but two since >0 3 Ma (Duffield et al., 1980). Explosive eruptions that preceded extrusion of INTRODUCTION many of the domes produced >0 3 km 3 of pyroclastic deposits that form partial to complete rings around some Many large-volume, shallow crustal magma reservoirs domes (Duffield et al., 1980; Bacon et al., 1981). These have extended histories that may culminate in calderasome deposits blanket the pre-eruptive topography and mantle forming pyroclastic eruptions. Changes over geologic of the older domes. The dome field is flanked on time in the depth, volume, and magmatic temperature three sides by monogenetic basaltic volcanoes, and several of these systems are poorly known and are not amenable rhyolite domes carry enclaves of hybrid basaltic andesite to study by geophysical methods. Silicic lava domes fed (Bacon & Metz, 1984). An active geothermal system from such reservoirs can yield partial records of the early centered within the rhyolite field sustains at present four Corresponding author. Present address: NE 11th Street, Bellevue, WA , USA. Telephone: crmanley@mindspring.com Oxford University Press 2000

2 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 1 JANUARY 2000 Fig. 1. Simplified geologic map of the Coso volcanic field, showing Pleistocene rhyolite domes and generalized outcrops of the Pliocene Pleistocene volcanic (basaltic, andesitic and rhyodacitic) rocks, and older granitoid and metamorphic basement. Modified after Duffield et al. (1980) and Duffield & Bacon (1981). significant crystal fractionation from the parent basalt or more-evolved intermediate magmas, with concomitant but relatively minor assimilation of upper crust (Miller et al., 1996; J. S. Miller, in preparation). The volcanic field is the product of a small magmatic system that has neither produced ignimbrites nor undergone caldera collapse; the roof of its magma reservoir has thus remained intact throughout its active history. Dome extrusions over the past 1 My provide information through bulk compositions, mineral as- semblages and mineral compositions, and magmatic enclaves about the internal state of the magmatic system at the times of the eruptions. Phase assemblages necessary for thermometry (two Fe Ti oxides; two feldspars; hornblende and plagioclase) and Al-in-hornblende barometry (hornblende and at least seven other minerals, plus free water vapor) are present in domes of ages that span that of the system s lifetime, allowing the thermal and depth history of the top of the Coso rhyolitic magma reservoir to be estimated. electrical generating stations built during the 1980s and 1990s. These stations produce a total of >260 MW of electricity (Duffield et al., 1994). The Coso magmatic system appears to be maintained by intrusion of basaltic magmas facilitated, in part, by continuing deformation of the region, which lies in a transition region between the Basin and Range province (undergoing approximately WNW ESE extension) and the San Andreas right-slip province. The tectonic situation has led to block faulting and intense seismicity (Weaver & Hill, 1979; Roquemore, 1980). Extension favors intrusion of basalt into the crust, providing the heat and mass flux to sustain a long-lived (>4 My) locus of magmatism (Bacon et al., 1981; Bacon, 1982; Novak & Bacon, 1986). New Sr and Nd isotopic analyses (Miller et al., 1996) indicate that much of the mass in the Coso rhyolites is derived from mantle basalts, either by partial melting of underplated basalts or by differentiation of mantle melts. The highly evolved character of the major and trace element signatures of the rhyolites reflects 150

3 MANLEY AND BACON COSO RHYOLITE THERMOBAROMETRY Fig. 2. Distribution of the rhyolite domes (some reconstructed) and basalt vents of the Pleistocene portion of the Coso volcanic field. Maps 1 7 show roughly coeval rhyolite dome groups, based largely on similarities in trace element contents, and approximately coeval basalt vents. Domes (and samples of the domes) are designated by numbers 1 39 (Duffield et al., 1980; Bacon et al., 1981); the numbering order implies no chronological or geographical sequence. Domes labeled with dome numbers on maps 1 7 are those for which the most complete thermobarometry data are available (see Fig. 8). Dates for each group are weighted mean ages (Bacon et al., 1981). Map at lower right shows all domes and basalt vents. The Devils Kitchen dome is dome 28; Sugarloaf Mtn is dome 26. Figure modified after Bacon et al. (1981). Mineralogy Except for three domes, including the two oldest ones, the Coso rhyolites are nearly aphyric. Typical crystal contents range from 1 wt % to <0 001 wt % (Bacon et al., 1981). Thin sections are almost useless to observe and study the minerals in such rocks. Grain mounts must be used, but at the expense of knowing the detailed petrographic relationships among grains. This is espe- cially significant in thermobarometry; without contact relations, equilibrium among grains must be assessed compositionally and thermodynamically. The mineral assemblage table ( Fig. 3) shows the observed mineral phases in each dome and indicates whether each is thought to be present as phenocrysts or xenocrysts or both. COMPOSITION AND MINERALOGY Lava compositions and dome groups All the Pleistocene Coso domes consist of high-silica rhyolite (Bacon et al., 1981; Macdonald et al., 1992) with a rather uniform major element composition, but with differing minor and trace element contents. Bacon et al. (1981) used trace element contents and K Ar ages to divide the 39 domes into seven groups ( Fig. 2) they thought reflected the eruption of discrete batches of magma. All domes fed by an individual magma batch did not necessarily erupt simultaneously but may have been tapped over some longer period of time. Bacon et al. (1981) assigned weighted mean ages to each of the dome groups (Fig. 2) based on the dates of individual domes ( Fig. 3). 151

4 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 1 JANUARY 2000 Fig. 3. Observed apparent phenocryst and xenocryst mineral assemblages of individual Coso rhyolite domes, showing sample numbers, crystal contents in weight percent (Bacon et al., 1981), and ages (see text) of dated domes. Phases assigned to phenocryst or xenocryst based on relative composition, morphology, and identity (see text); some phases may have been stirred into the magma at the time of eruption (e.g. Nakada et al., 1994; Nakamura, 1995). K Ar dates on sanidine and obsidian were determined for 18 rhyolite domes (Lanphere et al., 1975; Duffield et al., 1980; G. B. Dalrymple & C. R. Bacon, unpublished data, 1980). Friedman & Obradovich (1981) measured obsidian hydration rind thicknesses and calculated dates for 13 domes. Only those dates accepted by Bacon et al. (1981) are shown. 152

5 MANLEY AND BACON COSO RHYOLITE THERMOBAROMETRY Salic phases Quartz occurs in 16 of the 39 domes. The oldest dome (group 1) and the group 5 domes appear to contain no quartz, but quartz has been found in one or more domes in each of the other dome groups. Unzoned sanidine phenocrysts ( Table 1) are present in 29 of the 39 domes, and in all the dome groups. Compositions of sanidine phenocrysts from domes of groups 2 7 fall in the narrow range Or 55 to Or 59. Sanidine found in the oldest dome (group 1) is Or 67. Xenocrystic potassium feldspar grains with compositions that range from Or 91 to Or 93 are found in three domes from groups 3, 6, and 7. Unzoned plagioclase (Table 1) is found in all but one of the 39 domes; that dome has wt % crystals. Plagioclase grains that are clearly phenocrysts have compositions from An 7 7 to An 10, but an absolute cut-off between phenocryst and xenocryst compositions is not apparent. Most xenocrysts have compositions An 26 to An 79, but grains of An 13, An 14, An 15, An 18, An 19, and An 22 are present, and these are probably xenocrysts also. Although the Coso rhyolites are nearly aphyric, they contain a great number of mineral phases that appear to be phenocrysts, plus up to seven that probably are xenocrysts. At Coso, sources of xenocrysts include wall rock around the reservoir and the conduit walls, as well as andesitic enclaves found in several of the domes. The enclaves are thought to have formed by hybridization between silicic and underlying basaltic magmas; these hybrids were subsequently dispersed into the rhyolitic magma (Bacon & Metz, 1984). Multiple populations of ferromagnesian phases are common, and it is not always obvious which were in equilibrium and which were not. Even for those phases not thought to be xenocrysts, recent studies have suggested how little we may know about the nucleation, growth, and subsequent history of the crystals commonly called phenocrysts in nearly aphy- ric high-silica rhyolites. Evidence from laboratory ex- periments and from other volcanic systems indicates that crystallization may preferentially occur at the walls of the reservoir, where heat loss to the country rock enhances the thermal gradient (Chen & Turner, 1980; Turner & Gustafson, 1981). Crystals may grow partly attached to the reservoir s wall; the liquid not incorporated into the crystals becomes enriched in H 2 O and other incompatible species, and its lower density allows it to rise along the boundary layer (Turner & Gustafson, 1981) and accumulate at the top of the reservoir. As the magma continues to cool in this new location, additional crystals may nucleate and grow (Manley, 1996). An ideal eruption would bring to the surface a rhyolite liquid with a small population of euhedral crystals. Complications arise when eruptive stirring mixes in some phenocrysts that grew near the reservoir margins (Nakada et al., 1994; Wolff et al., 1999); although such crystals may not have actually grown within the liquid enclosing them in the erupted rock, they grew from it when it was in the boundary layer near the reservoir wall. The Coso rhyolites represent highly fractionated yet crystal-poor magmas that must have separated from a more crystal-rich environment. The magma s content of dissolved water, and the number of phases with which it is saturated, can thus be unrelated to the magma s actual crystal content. Mafic phases Ilmenite and/or magnetite (Table 2) were found in 35 domes from all dome groups. Hornblende (Table 3) was found in 32 domes from all dome groups. Apparent phenocrysts have mg-numbers [100 Mg/( Mg + Fe)] that range from five to 17 or possibly 39. Probable xenocrysts have mg-numbers from 45 to 77. Pyroxene (Table 4) was found in 29 domes from all dome groups, although the oldest dome appears to have only xenocrystic clinopyroxene. Pyroxene mg-numbers increase with time, but this is not true of hornblende or biotite. Or- thopyroxene phenocrysts have mg-numbers that range from 19 to 32; probable xenocrysts have mg-numbers from 29 to 51. Clinopyroxene phenocrysts have mg- numbers from 30 to 48; probable xenocrysts have mg- numbers from 64 to 77. Fourteen domes appear to have pyroxene phenocrysts, and 24 domes display pyroxene xenocrysts. Biotite (Table 5) is found in 29 domes of all dome groups. The biotite is probably phenocrystic; biotite mg-numbers range from 24 to 56. Olivine (Table 6) is found in 20 domes, of all dome groups except group 1. Apparent olivine phenocrysts have mg-numbers that range from four to 20; probable xenocrysts have mg-numbers from 27 to 44. Accessory minerals Allanite is present in grain mounts of 15 of the 39 domes, only from dome groups 3, 6, and 7. Discrete apatite crystals have been identified with certainty in grain mounts from six of the domes and tentatively in 20 other domes, from all dome groups except groups 1 and 2. Apatite needles are common inclusions in phenocrysts from nearly all dome groups. Titanite occurs in grain mounts of four domes, of groups 1, 3, and 7. Zircon crystals have been identified with certainty from 14 of the domes (groups 1, 3, 4, 6, and 7) and tentatively in 11 other domes, including those of group 5. Zircon inclusions are common in phenocrysts in nearly all dome groups. Xenocrystic Cr-spinel has been found in only two of the domes, both from dome group

6 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 1 JANUARY 2000 Table 1: Averaged analyses of representative feldspars from the Coso rhyolite domes Dome: Phase: san plag san plag san plag san plag san plag san plag san plag san plag san plag san plag Age (Ma): Dome group: wt % oxides SiO Al 2 O CaO Na 2 O K 2 O Total per 32 O Si Al Ca Na K Z X molar An Ab Or F&L 2-Fsp T ( 2 kbar

7 MANLEY AND BACON COSO RHYOLITE THERMOBAROMETRY Dome: Phase: san plag san plag plag san san plag plag san plag san plag san plag san plag Age (Ma): <0 1 <0 1 Dome group: wt % oxides SiO Al 2 O CaO Na 2 O K 2 O Total per 32 O Si Al Ca Na K Z X molar An Ab Or F&L 2-Fsp T ( 2 kbar san, sanidine; plag, plagioclase; xenocryst compositions not shown; all temperatures shown are referenced to a pressure of 2 kbar. 155

8 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 1 JANUARY 2000 Table 2: Representative analyses of Fe Ti oxides from the Coso rhyolite domes Dome: Phase: mag ilm mag ilm mag ilm mag ilm mag ilm mag ilm mag ilm mag ilm mag ilm Age (Ma): Dome group: Grain: Mt-9 Il-7 Mt-10 Il-8 Mt-11 Il-9 Mt Il-3 Mt-1 Il-10 Mt-2 Il-6 Mt-1 Il-2 Mt-1 Il-1 Mt-2 Il-2 Association: on on in in on in w/il-10 w/mt-1 on opx in opx, in opx, Opx-3, Opx-3, Opx-4 Opx-4 Opx-5, Opx-5, w/il-2 w/mt-1 near near near near Il-7 Mt-9 Il-9 Mt-11 wt % oxides SiO TiO Al 2 O FeO MnO MgO CaO V 2 O Cr 2 O ZnO Original total Fe 2 O FeO New total Oxide projection mag ilm Usp Ilm Mt Hem QUILF projection mag ilm NTi XHem NMg XGk NMn XPy Log(Mg/Mn) Partitioning test: w/il-7 w/mt-11 w/il-8 w/il-9 w/mt w/il-10 w/mt-2 w/il-6 w/mt-1 w/mt-1 w/il-1 w/il-2 Passes test? close yes very yes no yes yes yes yes yes no yes close T ( C) QUILF 2-Ox log f O 2 kbar kbar Uncertainty / In very yes very yes yes yes close yes yes close equilibrium? close close 156

9 MANLEY AND BACON COSO RHYOLITE THERMOBAROMETRY Dome: Phase: mag mag ilm ilm ilm mag ilm mag ilm mag mag mag mag mag ilm ilm ilm ilm Age (Ma): Dome group: Grain: Mt-1 Mt-2 Il-5 Il-6 Il-8 Mt-1 Il-1 Mt-2 Il-2 Mt-2 Mt-6 Mt-7 Mt-8 Mt-9 Il-1 Il-2 Il-3 Il-5 Association: w/il-1 w/mt-1 w/il-2 w/mt-2 inclusion inclusion inclusion on opx in in in silicate silicate silicate wt % oxides SiO TiO Al 2 O FeO MnO MgO CaO V 2 O Cr 2 O ZnO Original total Fe 2 O FeO New total Oxide projection mag ilm Usp Ilm Mt Hem QUILF projection mag ilm NTi XHem NMg XGk NMn XPy Log(Mg/Mn) Partitioning w/il-6 w/il-5 w/mt-1 w/mt-2 w/mt-2 w/il-1 w/il-2 w/mt-1 w/il-2 w/il-2 w/il-1 w/il-2 w/il-3 w/mt-9 w/mt-8 w/mt-6 test: Passes test? yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes T ( C) QUILF 2-Ox log f O 2 kbar kbar Uncertainty 18/16 21/0 18/14 4/2 26/ /13 10/8 14/ /9 22/3 23/ In equilibrium? yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes mag, magnetite; ilm, ilmenite; all f O 2 values shown are referenced to a pressure of 2 kbar; uncertainty values are mathematical uncertainties in the temperature values shown; approach to equilibrium based on change of projected components and temperatures when compositions are allowed to vary. Fe expressed as FeO. 157

10 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 1 JANUARY 2000 Table 3: Averaged analyses of hornblendes from the Coso rhyolite domes Probable phenocrysts Dome: Average: B A B B Age (Ma): <0 1 Dome group: No. grains averaged: No. points averaged: Position on grain: r c, r c, r inc. in fsp c, r c, r r c, r c, r r wt % oxides SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cl Total per 13 cations (Si + Ti + Al + Fe + Mn + Mg) Si Ti Al Fe Mn Mg Total Fe/(Fe + Mg) mg-no. (atomic) Al(tot) T ( C) est T ( C) QUILF 2- Ox P (kbar) (A&S,95) Depth (km) THERMOBAROMETRY Analytical methods Pieces of obsidian or pumice weighing several kilograms were crushed, and minerals separated from the glassy matrix by magnetic and heavy liquid techniques. The resultant mineral separates were weighed to estimate crystal content of the domes ( Fig. 3). Compositions of phenocrysts and xenocrysts were determined by wavelength-dispersive electron microprobe analysis of polished grain mounts. Hornblende analyses were performed by C.R.M. on a four-spectrometer Cameca MBX instrument at Duke University using a Kakanui kaersutite standard and 15 na and 15 kv analytical conditions. 158

11 MANLEY AND BACON COSO RHYOLITE THERMOBAROMETRY Probable xenocrysts Dome: Average: A B A A Age (Ma): Dome group: No. grains averaged: No. points averaged: Position on grain: c, r c, r c, r c, r c, r c, r c, r c, r c, r c, r c c, r c, r c, r wt % oxides SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O Cl Total per 13 cations (Si + Ti + Al + Fe + Mn + Mg) Si Ti Al Fe Mn Mg Total Fe/(Fe + Mg) mg-no. (atomic) Al(tot) T ( C) est T ( C) QUILF 2-Ox P (kbar) (A&S, 95) Depth (km) na, element not analyzed for; position on grain (where recorded): c, core; r, rim. mg-number (atomic) = 100 Mg/(Mg + Fe). All Fe expressed as FeO. Other phases were analyzed by C.R.B. at the US Geological Survey, Menlo Park, CA, with an ARL EMX instrument under analytical conditions described by Bacon & Duffield (1981). Reported mineral compositions are averages of two or more points per grain, or are averages of two or more grains, with two or more analyzed points on each. In cases of zoned grains, rim and core compositions are averaged separately. 159

12 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 1 JANUARY 2000 Table 4: Averaged analyses of representative pyroxenes from the Coso rhyolite domes Dome: Phase: opx cpx opx cpx opx opx cpx opx cpx cpx opx cpx opx cpx opx cpx opx cpx Average mg-no.: low low high high low low high low low high high low low high high Age (Ma): Age group: No. grains averaged: Grain no.: 3,5,7 2,3,4,5 1,2,4,6 1 1,2 1,6 3 1,2 1,3 3 1,2,5,6 1,2,3 1,2,6 2,3,4 1,2,3 2,3,4 2 Position on grain: c, r c, r c, r c, r? c, r c, r c c r r wt % oxides SiO TiO Al 2 O Cr 2 O FeO MnO MgO CaO Na 2 O Original total per6o Si Al(iv) Al(vi) Ti Cr Fe Mn Mg Ca Na Total

13 MANLEY AND BACON COSO RHYOLITE THERMOBAROMETRY Dome: Phase: opx cpx opx cpx opx opx cpx opx cpx cpx opx cpx opx cpx opx cpx opx cpx Average mg-no.: low low high high low low high low low high high low low high high Age (Ma): Age group: No. grains averaged: Grain no.: 3,5,7 2,3,4,5 1,2,4,6 1 1,2 1,6 3 1,2 1,3 3 1,2,5,6 1,2,3 1,2,6 2,3,4 1,2,3 2,3,4 2 Position on grain: c, r c, r c, r c, r? c, r c, r c c r r wt % oxides Fe/(Fe + Mg) mg-no. (atomic) Pyx quad Ca Mg Fe Phase opx cpx opx cpx opx opx cpx opx cpx cpx opx cpx opx cpx opx cpx opx cpx QUILF 2-Pyx T ( 2 kbar Uncertainty Varying components Cpx XEn: T ( 2 kbar Uncertainty Opx XEn: T ( 2 kbar Uncertainty opx, orthopyroxene; cpx, clinopyroxene; position on grain (where recorded): c, core; r, rim. mg-number (atomic) = 100 Mg/(Mg + Fe); all temperatures shown are referenced to a pressure of 2 kbar. Fe expressed as FeO. 161

14 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 1 JANUARY 2000 Table 5: Averaged analyses of biotites from the Coso rhyolite domes Dome: Age (Ma): Dome group: No. grains averaged: wt % oxides SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O F Estimated H 2 O Original total Less O for F Total per 24 O,OH,F Si Al(iv) Al(vi) Ti Fe Mn Mg Ca Na K F Estimated OH Original total Z Y X OH, F Fe/(Fe + Mg) mg-no. (atomic) mg-number (atomic) = 100 Mg/(Mg + Fe). Fe expressed as FeO. Two-oxide temperatures and oxygen temperatures and estimate the oxygen fugacity of the fugacities rhyolitic magmas ( Fig. 4). QUILF assesses equilibria Compositions of apparently equilibrated, coexisting among oxide minerals, pyroxenes, olivine, and quartz, Fe Ti oxide minerals were used in the QUILF 4 1 and uses improvements to previous solution models by software of Andersen et al. (1993) to determine equilibrium Lindsley and coworkers (Lindsey & Andersen, 1983; 162

15 MANLEY AND BACON COSO RHYOLITE THERMOBAROMETRY Table 6: Averaged analyses of olivines from the Coso rhyolite domes Dome: Age (Ma): Age group: Sample: CD-24 CD-23 CD-30 CD-33 CD-12 No. grains averaged: wt % oxides SiO TiO Al 2 O Cr 2 O Fig. 4. Temperature vs f O 2 for all Coso rhyolite Fe Ti oxide pairs FeO that pass the Mg/Mn partitioning test of Bacon & Hirschmann (1988). MnO In this and subsequent figures, g2 indicates dome group 2, etc. Temperatures MgO and f O 2 values [shown at 200 MPa (2 kbar)] were determined CaO by the QUILF 4 1 software of Andersen et al. (1993). Error bars show combined solution model and analytical uncertainties (Andersen et al., Na 2 O ). The position of the FMQ buffer (Frost, 1991) is consistent with Original total the equilibria used by the QUILF program; the NNO buffer (Huebner per4o Si & Sato, 1970) is shown for comparison. Fe oxide grains may be xenocrysts that were stirred into the Mn magma so soon before the eruption that re-equilibration Mg was not possible (Nakamura, 1995). Ca QUILF assesses how close the input mineral com- Na positions are to equilibrium, and we used the software Ti in the following manner. Two-oxide temperatures were Original total calculated without assuming any other phases (i.e. pyroxene or olivine) were in equilibrium with the oxides. Si Compositions of apparently equilibrated, coexisting Original total Fe Ti oxide minerals were input as electron microprobe Fe/(Fe + Mg) major oxide analyses. The software recasts these analyses mg-no. (atomic) to four compositional parameters for each phase and XFa calculates temperatures based on the solution model of Andersen et al. (1991). Temperatures and oxygen fugacities mg-number (atomic) = 100 Mg/(Mg + Fe); XFa = Fe/(Fe were first calculated from the input compositions, + Mn + Mg + Ca). then magnetite NMg (number of Mg atoms per formula Fe expressed as FeO. unit) was allowed to vary. In almost every case, varying NMg lowered the mathematical uncertainties in the fit. In some cases, the adjustment in NMg was excessive, Andersen & Lindsey, 1988; Davidson & Lindsley, 1989; and the reported temperature was calculated from the Andersen et al., 1991). All oxide mineral pairs used for original composition. In most cases, temperatures caltemperature calculation pass the Mg/Mn partitioning culated with and without adjusting NMg vary less than test of Bacon & Hirschmann (1988). Because Fe Ti the stated uncertainty of the model (about ±25 C; oxides are thought to re-equilibrate more rapidly than Andersen et al., 1993). do silicates after a change in P T X conditions (Gardner et al., 1995), they seem the most likely phenocryst phases to record pressure and temperature conditions before Two-feldspar temperatures eruption (Buddington & Lindsley, 1964; Frost et al., 1988). Sanidine and plagioclase phenocrysts apparently co- Nonetheless, only seven Coso domes contained Fe Ti existed in equilibrium in many of the Coso rhyolite oxide pairs that pass the partitioning test (Fig. 4); many magmas. Equilibration temperatures were determined 163

16 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 1 JANUARY 2000 with the SOLVCALC 1 0 software of Wen & Nekvasil Zen, 1986) was formulated for calc-alkalic plutons with (1994) using the feldspar site mixing model of Fuhrman near-solidus phase assemblages of seven minerals. John- & Lindsley (1988), which has stated uncertainties of son & Rutherford (1989) revised the barometer on the ±30 C. Like the MTHERM3 software of Fuhrman & basis of laboratory experiments and extended it to include Lindsley (1988), SOLVCALC varies the input com- silicic volcanic rocks as well as plutonic ones. Application positions of the feldspars to achieve better fits. To cal- to high-silica rhyolites is often complicated by the fact culate two-feldspar temperatures, one must assume a that although a near-solidus mineral assemblage may value for pressure. Because pressure and temperature exist, the rock is so nearly aphyric that some rarer are directly proportional, calculated temperatures should components of the assemblage will probably not be increase smoothly with pressure (although the rate of observed even if a larger than normal sample is crushed temperature increase with pressure is model dependent). and separated for mineral grains. In practice, however, the software s adjustments of com- The equilibrium phase assemblage quartz + sanidine position lead to jumps (to both higher and lower values) + plagioclase + hornblende + biotite + magnetite or in the calculated temperatures. To produce a smooth ilmenite + titanite + melt + fluid is required by the temperature plot as a function of pressure, the following Al-in-hornblende barometer. Two Fe Ti oxide minerals procedure was used. Temperatures were calculated for can take the place of one oxide and titanite, and the Ti pressures between 50 and 1000 MPa (0 5 and 10 kbar) at contents of the melt and hornblende will still be buffered a compositional uncertainty of (molar endmember ( Johnson & Rutherford, 1989). composition). For this range of pressures, temperature typically varied by 100 C. If temperature did not vary smoothly, successively smaller uncertainties were spe- Pre-eruptive H 2 O contents and the presence of a vapor phase cified, leading to smaller adjustments in composition and A free vapor phase is one of the requirements of the Al- less variability in temperature. The result is a generally in-hornblende barometer. Evidence from melt inclusions smooth curve with temperature rising with pressure at suggests that magmas of at least three dome groups were about 7 11 C per 100 MPa (per 1 kbar). saturated with an H 2 O-rich fluid. Four pristine, quartz- Two-feldspar temperatures [referenced to 200 MPa hosted melt inclusions from the Coso rhyolite samples (2 kbar)] range from 680 to 775 C, save for the oldest have been analyzed for pre-eruptive H 2 O and CO 2 dome at 630 C. Two-feldspar temperatures show no contents by Fourier transform infrared spectroscopy correlation with dome age or group; this further suggests (FTIR) (Blouke, 1993; Newman et al., 1993). CO 2 was that some or much of the feldspar may not be strictly below the 100 ppm detection limit for all the inclusions phenocrystic. At present, we consider the two-oxide and bubbles (S. Newman, personal communication, thermometer in QUILF more reliable for these rocks 1998). One inclusion from dome 10 (group 3) contained than any of the two-feldspar thermometers. 6 2 wt % total H 2 O, which would be saturated at lithostatic pressures less than >270 MPa [2 7 kbar; program of Holloway & Blank (1994), which utilizes the Two-pyroxene temperatures Burnham model for H 2 O and the Stolper model for CO 2 solubilities]. Two inclusions from the relatively Temperatures were calculated for several domes using the porphyritic (2% crystals) dome 5 (group 4) contained 4 5 two-pyroxene thermometer in the QUILF 4 1 software of and 5 2 wt % total H 2 O [saturated at pressures less than Andersen et al. (1993). Uncertainties are ±30 C (Frost > MPa ( kbar)], and both also contained & Lindsley, 1992). These temperatures are nearly always large bubbles equal to >7 10% of the inclusion volume. higher than the other temperature determinations, often An inclusion from dome 4 (group 7) contained 6 4wt% unrealistically so (up to 900 C) with respect to the mintotal H 2 O [saturated at pressures <290 MPa (2 9 kbar)] eralogy. In domes with multiple populations of pyroxenes, and a large bubble. The bubbles showed significant water temperature determinations using pyroxene pairs with but no detectable CO 2 (Blouke, 1993). Melt inclusions high mg-numbers are C higher than those using from dome 28 (group 2) were also examined; many are the low mg-number pyroxene pairs; the high mg-number crystalline, but of those still glassy, some contain bubbles pairs apparently grew in a more mafic, higher-temthat make up as much as 10% of the inclusion s volume. perature magma. The large bubble volumes observed in the samples from group 2, 4, and 7 domes, compared with shrinkage Al-in-hornblende pressures bubbles of only a few volume percent (Roedder, 1979), imply that a free vapor phase was probably present We used the Anderson & Smith (1995) T-dependent during trapping of the melt inclusions. formulation of the Al-in-hornblende barometer. The Circumstantial evidence that a vapor phase is now original Al-in-hornblende barometer (Hammarstrom & present in the Coso magma reservoir was provided by 164

17 MANLEY AND BACON COSO RHYOLITE THERMOBAROMETRY the 1992 Landers 7 3 M w earthquake in the Mojave below FMQ, they were low in temperature and in Desert of California. This earthquake triggered longlived, total Fe, unlike the anorogenic granites. The hornblende small-magnitude local seismicity at a number of phenocrysts were apparently fully buffered (see below) in active volcanic areas in the western USA, including Coso, a system with elevated H 2 O contents at water-saturated Long Valley caldera, The Geysers, and Yellowstone conditions. A low oxygen fugacity by itself does not caldera (Hill et al., 1993; Roquemore & Simila, 1994). appear to preclude the conditions necessary for Al-in- Linde et al. (1994) suggested that the details of the long- hornblende barometry to be successful in these rocks. lived seismicity and inflation at Long Valley are best explained by the shaking loose of existing vapor bubbles. These bubbles then rose (advected) through the magma Hornblende populations reservoir and expanded because of the decreasing conblende, one relatively rich in Fe (in dome groups 2, 3, The Coso rhyolites contain two populations of horn- fining (lithostatic) pressure; this would have increased the pressure on the walls of the magma reservoir and/or and 7), the other relatively rich in Mg (all dome groups increased the volume of the magma reservoir (Sahagian except 2). The two populations are seen most easily if & Proussevitch, 1992). At Long Valley, the number of plotted against Fe/(Fe + Mg), where a value of 0 6 earthquakes triggered by the Landers event, and the separates them ( Fig. 5). Several domes contain horn- amplitude of the deformation measured at stations inside blende grains from both populations. and outside the caldera (Linde et al., 1994), imply that a Other major element systematics are also different significant volume of existing bubbles was involved, which for the two populations. Covariation plots of all major indicates that the entire system contains a free vapor elements against Fe/(Fe + Mg) show that most Fe-rich phase. On the basis of similarities in triggered seismicity hornblendes have restricted compositional ranges ( Fig. at Coso and Long Valley, Roquemore & Simila (1994) 5a d), suggestive of compositional buffering by other concluded that the triggered seismicity beneath the Coso phases, which is required for barometry. This is not seen volcanic field also was due to advective overpressure, in the Mg-rich hornblendes, where analyses of a number suggesting that the Coso magmatic system is at present of hornblende grains from a single dome, and sometimes vapor saturated. multiple analyses of a single grain, define linear trends showing significant variation in one or more elements Oxygen fugacity (Fig. 5a). These grains were not buffered: they grew in a liquid the bulk composition of which was strongly The plutons used by Hammarstrom & Zen (1986) for controlled by the growth of other phases, such as feldspars. their empirical barometer are known to have crystallized Many tie-lines are subparallel, reflecting control by the at fairly high f O 2, close to that of the nickel nickel oxide same phase or assemblage. This information also shows (NNO) buffer. The experiments performed by Johnson that the Mg-rich hornblendes did not grow in the Coso & Rutherford (1989) and Schmidt (1992), on which the rhyolite magma, which varies little in major element present version of the barometer (Anderson & Smith, composition across the dome groups (Duffield et al., 1980). 1995) is based, were also buffered at NNO. Anderson & Back-scattered electron (BSE) imaging ( Figs 6 and 7) Smith (1995) discussed the effect of lower f O 2 levels using reveals other differences between the populations. Mgrich data from a suite of plutons that crystallized at f O 2 hornblendes can be zoned or unzoned, euhedral, conditions below NNO and closer to FMQ (fayalite embayed, or glomerocrystically intergrown with other magnetite quartz). These are the Proterozoic anorogenic phases; most are also moderately to intensely fractured. granites that occur in a broad swath running from the Fe-rich hornblendes are unzoned and euhedral; only a southwestern to the northeastern USA. These granites few are fractured; some have partially skeletal exterior have the required mineral assemblage (although dissolved morphologies ( Fig. 7d) and/or irregular glass inclusions water contents are unknown) for applying the barometer, (Figs 6f, 7b and d) that imply, respectively, rapid growth but their calculated Al-in-hornblende equilibrium pres- (Roedder, 1979; Swanson & Fenn, 1986) and no extended sures are both unreasonably large and much greater than residence time before eruption (Manley, 1996). pressures indicated from barometry on country rock in On the basis of the compositions, zoning features, and contact with the plutons. On that basis, Anderson & morphologies, the Mg-rich hornblendes are interpreted Smith (1995) suggested that silicic magmas with oxygen as xenocrystic to the rhyolite magma, probably syneruptively fugacity much below NNO are not amenable to Al-inhornblende stirred-in from an andesitic or dacitic magma barometry. The bulk compositions of ano- or from a relatively early crystallized mush (Nakada et rogenic granites (especially high total Fe) may also play al., 1994), presumably beneath the rhyolite magma in an important role, as might the probable low H 2 O the reservoir. The Fe-rich hornblendes appear to be contents of their magmas. Although the Coso magmas phenocrysts that grew in the rhyolite magma now rep- apparently equilibrated at f O 2 conditions at and just resented by the Coso domes. That the compositionally 165

18 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 1 JANUARY 2000 Hornblende plagioclase temperatures Temperatures were also calculated with the pressuredependent hornblende plagioclase thermometer of Blundy & Holland (1990) and Holland & Blundy (1994). In three cases, hornblende-plagioclase P T trends lie at higher pressures and temperatures than indicated by the two-oxide and Al-in-hornblende methods, but in two other cases the results from all three methods converge within the uncertainties. Fig. 5. Selected elements vs Fe/(Fe + Mg) showing compositional variation in Coso hornblendes (see Table 3). Plotted points are individual microprobe point analyses. (a) Total Al in formula vs Fe/(Fe + Mg), showing division of hornblendes into xenocrysts (Mg-rich), phenocrysts, and buffered phenocrysts [shaded and shown in more detail in (d)]. Continuous tie-lines connect analyses on a single grain; dotted tie-lines connect analyses from different minerals in the same sample (one line is curved to avoid analyses from other samples). (b) Ti in formula vs Fe/(Fe + Mg). (c) CaO vs Fe/(Fe + Mg). (d) Total Al in formula vs Fe/(Fe + Mg) of buffered phenocrysts, showing grouping of analyses by dome or group. Error bars in (d) show analytical uncertainties. Intensive parameters by dome group Individual two-oxide temperature determinations using the QUILF software range from 705 to 790 C, and oxygen fugacity values [referenced to 200 MPa (2 kbar) pressure] range from 14 7to 16 9 log units on and just below the FMQ buffer at these temperatures (Fig. 4). Bacon & Metz (1984) reported temperatures between 775 and 825 C, also at and just below FMQ, for a few oxide pairs from the Coso rhyolites, using the graphical method of Buddington & Lindsley (1964). Pressures were calculated with the Al-in-hornblende barometer at temperatures calculated by two-oxide thermometry. These pressure estimates range from 140 to 270 MPa ( kbar; km depth) and decrease from the older to younger domes. Depths were calculated from the pressure values assuming a basement (granitic and metamorphic rock) density of 2700 kg/m 3 (Plouff & Isherwood, 1980), which translates to 100 MPa (1 kbar) per 3 78 km depth. Combining Al-in-hornblende pressures with calculated two-oxide temperatures yields estimated variations in pressure and temperature of the uppermost portion of the Coso rhyolite magma reservoir over the last 0 6 My. The magma tapped by any given eruption would have resided at or very near the reservoir roof, because the dome eruptions were low-energy, small-volume events, and draw-down of the magma in the reservoir would have been minimal. Thermobarometric determinations have the combined uncertainties of microprobe analyses and those inherent in the thermodynamic solution models restricted subset of Fe-rich hornblendes ( Fig. 5d) are buffered phenocrysts is corroborated by the Al content (examples in Fig. 8). Uncertainties (accuracies) for inof coexisting biotite ( Table 5), which is about twice that dividual pressure determinations are ±50 MPa in the hornblende. This relationship was also seen in (±0 5 kbar); uncertainties for temperature deexperimental runs ( Johnson & Rutherford, 1989) used terminations from the two-oxide thermometer are to confirm and extend the Al-in-hornblende barometer ±25 C, and from both the two-feldspar and two-pyr- ( M. J. Rutherford, personal communication, 1998). In oxene thermometers are ±30 C. Relative temperature addition, the apparent late growth of the hornblende differences between individual domes are significantly phenocrysts implies that the other observed phases were less than these values, particularly for the Fe Ti oxide already present to buffer the hornblende during growth. thermometer, as the precision of the analyses can be as The bimodality in Fe/(Fe + Mg) of the buffered Fe- much as an order of magnitude better than the reported rich hornblende phenocrysts, where a value of 0 83 uncertainties in some well-constrained cases. separates the two groups ( Fig. 5d), must reflect slight differences in the environment of crystal growth Dome Group 1; 1 04 ± 0 02 Ma perhaps near the reservoir wall vs floating free in the magma. Dome 38 is the only group 1 dome; it is cut by faults, highly eroded, and displays no non-hydrated glass. Two- 166

19 MANLEY AND BACON COSO RHYOLITE THERMOBAROMETRY Fig. 6. Back-scattered electron images showing zoning features of selected hornblende grains in epoxy mounts. On the left are xenocrysts (Mgrich) and on the right are phenocrysts (Fe-rich), most with attached or included glass (gl). (a) (d) Regular and contrast-enhanced images of hornblende grains from dome 13 (group 3): (a) and (c) xenocryst displaying complex zoning and a bright (Fe-rich?) rim; inclusions are oxide minerals; (b) and (d) phenocryst without zoning. (e) and (f ) Contrast-enhanced images of (e) an unzoned xenocryst and (f ) an unzoned phenocryst, both from dome 26 (group 7). 167

20 JOURNAL OF PETROLOGY VOLUME 41 NUMBER 1 JANUARY 2000 Fig. 7. Back-scattered electron images showing morphologies of selected hornblende grains in epoxy mounts. On the left are xenocrysts (Mg-rich) and on the right are phenocrysts (Fe-rich). Contrast has been moderately enhanced in all images; none of the grains appear zoned. (a) Xenocryst fragment showing fracturing and possible alteration along cleavages; dome 20 (group 5). (b) Phenocryst with a euhedral termination and highly irregular glass inclusions; dome 28 (group 2). (c) Fractured xenocrystic glomerocryst of hornblende and other phases; dome 34 (group 5). (d) Phenocryst showing growth faces, skeletal morphology, and irregular glass inclusions and external projections; dome 21 (group 7). feldspar thermometry yields 628 C [referenced to indicates a magma temperature of >740 C; at this 200 MPa (2 kbar)], which seems unreasonably low. temperature, Al-in-hornblende barometry indicates a pressure of 270 MPa (2 7 kbar), which corresponds to Dome Group 2; 0 59 ± 0 02 Ma 10 2 km depth (Fig. 8a). Two-feldspar and two-pyroxene The sole group 2 dome is the Devils Kitchen dome temperatures agree well with these P T estimates; horn- (dome 28). It has the greatest crystal content of all the blende plagioclase temperatures are consistent within Coso domes (15%). QUILF two-oxide thermometry reasonable limits of uncertainties. 168