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Title: Volatile compositions of melt inclusions in Coso Range Rhyolite

Abstract

Dissolved water and carbon dioxide concentrations were measured in melt inclusions in quartz phenocrysts from rhyolite domes and lava flows in the Coso volcanic field to determine pre-eruptive volatile content of the magma. Inclusions roughly 50--100 microns in diameter were analyzed for dissolved water and carbon dioxide using Fourier transform infrared spectroscopy (FTIR). The glasses have a total water concentration ranging from four to six weight percent and no detectable carbon dioxide. Vapor bubbles in the inclusions about 10--40 microns in diameter were also analyzed for their volatile composition. The spectra show no evidence for carbon dioxide, although significant water is observed. If the melt inclusions were saturated at the time of entrapment, then crystallization occurred between 1,200 and 2,100 bars pressure or greater than 4 kilometers depth.

Authors:
 [1]
  1. (Caltech, Pasadena, CA (United States))
Publication Date:
OSTI Identifier:
5342970
Alternate Identifier(s):
OSTI ID: 5342970
Report Number(s):
CONF-9305259--
Journal ID: ISSN 0016-7592; CODEN: GAAPBC
Resource Type:
Conference
Resource Relation:
Journal Name: Geological Society of America, Abstracts with Programs; (United States); Journal Volume: 25:5; Conference: 89. annual meeting of the Cordilleran Section and the 46th annual meeting of the Rocky Mountain Section of the Geological Society of America (GSA), Reno, NV (United States), 19-21 May 1993
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; CALIFORNIA; GEOLOGIC FORMATIONS; INCLUSIONS; CHEMICAL COMPOSITION; RHYOLITES; CARBON DIOXIDE; EMISSION SPECTROSCOPY; GEOBAROMETRY; GEOCHEMISTRY; INFRARED SPECTRA; WATER CHEMISTRY; CARBON COMPOUNDS; CARBON OXIDES; CHALCOGENIDES; CHEMISTRY; DEVELOPED COUNTRIES; IGNEOUS ROCKS; NORTH AMERICA; OXIDES; OXYGEN COMPOUNDS; ROCKS; SPECTRA; SPECTROSCOPY; USA; VOLCANIC ROCKS 580000* -- Geosciences

Citation Formats

Blouke, K.J. Volatile compositions of melt inclusions in Coso Range Rhyolite. United States: N. p., 1993. Web.
Blouke, K.J. Volatile compositions of melt inclusions in Coso Range Rhyolite. United States.
Blouke, K.J. Thu . "Volatile compositions of melt inclusions in Coso Range Rhyolite". United States. doi:.
@article{osti_5342970,
title = {Volatile compositions of melt inclusions in Coso Range Rhyolite},
author = {Blouke, K.J.},
abstractNote = {Dissolved water and carbon dioxide concentrations were measured in melt inclusions in quartz phenocrysts from rhyolite domes and lava flows in the Coso volcanic field to determine pre-eruptive volatile content of the magma. Inclusions roughly 50--100 microns in diameter were analyzed for dissolved water and carbon dioxide using Fourier transform infrared spectroscopy (FTIR). The glasses have a total water concentration ranging from four to six weight percent and no detectable carbon dioxide. Vapor bubbles in the inclusions about 10--40 microns in diameter were also analyzed for their volatile composition. The spectra show no evidence for carbon dioxide, although significant water is observed. If the melt inclusions were saturated at the time of entrapment, then crystallization occurred between 1,200 and 2,100 bars pressure or greater than 4 kilometers depth.},
doi = {},
journal = {Geological Society of America, Abstracts with Programs; (United States)},
number = ,
volume = 25:5,
place = {United States},
year = {Thu Apr 01 00:00:00 EST 1993},
month = {Thu Apr 01 00:00:00 EST 1993}
}

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  • Thirty-eight separate domes and flows of phenocryst-poor, high-silica rhyolite of similar major element chemical composition were erupted over the past 1 m.y. from vents arranged in a crudely S-shaped array atop a granitic horst in the Coso Range, California. Most of the extrusions are probably less than about 0.3 m.y. old. The area is one of Quaternary basaltic volcanism and crustal extension. The central part of the rhyolite field is characterized by high heat flow, low apparent resistivity, and substantial fumarolic activity indicative of an active geothermal system. The immediate source of heat for the surficial geothermal phenomena is probablymore » a silicic magma reservoir that may still contain molten or partially molten material at a depth of at least 8 km beneath the central part of the rhyolite field. Outlying rhyolite extrusions probably reflect the presence of feeder dikes emanating from the reservoir beneath the central region. Azimuths of dikes appear to be parallel to the regional tectonic axis of maximum horizontal compression, analogous to some dike-fed flank eruptions on basaltic shields and andesitic strato-volcanoes. The areal extent of a magma reservoir and the present total heat content of the silicic magma system at Coso may be less than was previously estimated. However, the area is still considered to be one of significant geothermal potential.« less
  • Gas concentrations and ratios in 110 analyses of geothermal fluids from 47 wells in the Coso geothermal system illustrate the complexity of this two-phase reservoir in its natural state. Two geographically distinct regions of single-phase (liquid) reservoir are present and possess distinctive gas and liquid compositions. Relationships in soluble and insoluble gases preclude derivation of these waters from a common parent by boiling or condensation alone. These two regions may represent two limbs of fluid migration away from an area of two-phase upwelling. During migration, the upwelling fluids mix with chemically evolved waters of moderately dissimilar composition. CO{sub 2} richmore » fluids found in the limb in the southeastern portion of the Coso field are chemically distinct from liquids in the northern limb of the field. Steam-rich portions of the reservoir also indicate distinctive gas compositions. Steam sampled from wells in the central and southwestern Coso reservoir is unusually enriched in both H{sub 2}S and H{sub 2}. Such a large enrichment in both a soluble and insoluble gas cannot be produced by boiling of any liquid yet observed in single-phase portions of the field. In accord with an upflow-lateral mixing model for the Coso field, at least three end-member thermal fluids having distinct gas and liquid compositions appear to have interacted (through mixing, boiling and steam migration) to produce the observed natural state of the reservoir.« less
  • Six melt inclusions and their sanidine host crystals from the Bishop Tuff were analyzed by ion microprobe for the concentrations of Ba and Sr. Host crystals were analyzed 30 [mu]m away from the melt inclusions. The analyses yield an average sanidine/melt-inclusion partition coefficient for Ba of 28.4 [+-] 4.1 and for Sr of 11.35 [+-] 0.74. These values are significantly larger than those produced by analyses of mineral separates and matrix glasses, which largely fall between 4 and 7 for Ba and 1 and 4 for Sr. The new results are further confirmed by the concentration ratios of these twomore » elements in the rims of the late Bishop sanidine phenocrysts and late Bishop matrix glasses. The new findings are considered closer to the real phenocryst/metal partition coefficients than those produced by bulk analyses of crystal separates and matrix glass, because ion microprobe analysis eliminates or at least greatly reduces the error introduced by the existence of chemical zonations and impurities. Melt inclusions are better representatives of the melt in equilibrium with the phenocrysts than the matrix glass, because matrix glass is subject to various chemical changes after formation of the phenocrysts. Modifications of chemical concentrations in melt inclusions by boundary layer buildup during melt inclusion formation has been shown to be negligible for most elements. These new and considerably larger sanidine/melt partition coefficients for Ba and Sr shed new light on the debate over origin of high-silica rhyolites with extremely low concentrations of Ba and Sr. Using these new partition coefficients, only about 40% fractional crystallization is required to produce a rhyolitic magma with 2 ppm Ba from a magma that originally had 2,000 ppm Ba.« less
  • The preeruptive volatile gradient that was present in the magma which produced the Lower Bandelier Tuff (LBT) is determined on the basis of an analysis of the H{sub 2}O, Cl, and F contents of melt inclusions (MIs) in LBT phenocrysts. The trace element contents of bulk pumice and MIs are measured in order to facilitate interpretation of the pristine nature of the MIs. The data show that there was a large gradient in the H2O content (hence density) of the magma between an H2O-saturated cap and the body of the chamber. The trace element analysis of the MIs and bulkmore » rocks show that after the water gradient evolved, the chamber underwent about 40 percent eutectic fractional crystallization and was then intruded by a second rhyolitic magma at some time prior to eruption. 75 refs.« less