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Major element and oxygen isotope geochemistry of vapour-phase garnet from the Topopah Spring Tuff at Yucca Mountain, Nevada, USA

R. J. Moscati and C. A. Johnson
Major element and oxygen isotope geochemistry of vapour-phase garnet from the Topopah Spring Tuff at Yucca Mountain, Nevada, USA
Mineralogical Magazine (August 2014) 78 (4): 1029-1041

Abstract

Twenty vapour-phase garnets were studied in two samples of the Topopah Spring Tuff of the Paintbrush Group from Yucca Mountain, in southern Nevada. The Miocene-age Topopah Spring Tuff is a 350 m thick, devitrified, moderately to densely welded ash-flow tuff that is zoned compositionally from high-silica rhyolite to latite. During cooling of the tuff, escaping vapour produced lithophysae (former gas cavities) lined with an assemblage of tridymite (commonly inverted to cristobalite or quartz), sanidine and locally, hematite and/or garnet. Vapour-phase topaz and economic deposits associated commonly with topaz-bearing rhyolites (characteristically enriched in F) were not found in the Topopah Spring Tuff at Yucca Mountain. Based on their occurrence only in lithophysae, the garnets are not primary igneous phenocrysts, but rather crystals that grew from a F-poor magma-derived vapour trapped during and after emplacement of the tuff. The garnets are euhedral, vitreous, reddish brown, trapezohedral, as large as 2 mm in diameter and fractured. The garnets also contain inclusions of tridymite. Electron microprobe analyses of the garnets reveal that they are almandine-spessartine (48.0 and 47.9 mol.%, respectively), have an average composition of (Fe (sub 1.46) Mn (sub 1.45) Mg (sub 0.03) Ca (sub 0.10) )(Al (sub 1.93) Ti (sub 0.02) )Si (sub 3.01) O (sub 12) and are comparatively homogeneous in Fe and Mn concentrations from core to rim. Composited garnets from each sample site have delta (super 18) O values of 7.2 and 7.4 per mil. The associated quartz (after tridymite) has delta (super 18) O values of 17.4 and 17.6 per mil, values indicative of reaction with later, low-temperature water. Unaltered tridymite from higher in the stratigraphic section has a delta (super 18) O of 11.1 per mil which, when coupled with the garnet delta (super 18) O values in a quartz-garnet fractionation equation, indicates isotopic equilibration (vapour-phase crystallization) at temperatures of approximately 600 degrees C. This high-temperature mineralization, formed during cooling of the tuffs, is distinct from the later and commonly recognized low-temperature stage (generally 50-70 degrees C) of calcite, quartz and opal secondary mineralization, formed from downward-percolating meteoric water, that locally coats fracture footwalls and lithophysal floors.


ISSN: 0026-461X
EISSN: 1471-8022
Coden: MNLMBB
Serial Title: Mineralogical Magazine
Serial Volume: 78
Serial Issue: 4
Title: Major element and oxygen isotope geochemistry of vapour-phase garnet from the Topopah Spring Tuff at Yucca Mountain, Nevada, USA
Affiliation: U. S. Geological Survey, Denver, CO, United States
Pages: 1029-1041
Published: 201408
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 53
Accession Number: 2015-002473
Categories: Isotope geochemistryMineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
N36°00'00" - N39°07'60", W118°15'00" - W115°00'00"
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201502
Program Name: USGSOPNon-USGS publications with USGS authors
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