50 Petrology, Mineralology and Economic Geology

Tuesday, 21 March 2017
8:00 AM - 12:00 PM
Lawrence Welk Room, Mezzanine Level

THERMODYNAMIC ASSESSMENT OF THE MAGMATIC HISTORY OF BLUE ROCK SHIELD VOLCANO, JACKSON CO., OREGON:  APPLICATION OF A NEW DNI (OLIVINE-MELT) GEOTHERMOMETER AND OTHER MODELS

Paper number: 50-2
Program: Technical Sessions
Time: 8:20 AM – 8:40 AM

Authors

Abstract

Blue Rock is a basaltic shield in the southern Oregon Cascades, north of Mt. McLoughlin, showing bulk phenocryst lodes (>500 μm) ranging from 5-28 vol%, and a variety of groundmass textures. Compositional analyses of olivine and plagioclase phenocrysts and glomerocrysts have allowed for the sequential application of a new thermometer, a plagioclase hygrometer, and a viscosity model to a suite of lavas erupted from this edifice. Where first-grown phenocryst compositions were confidently defined, results were consistent with experimental data for hydrous arc basalts with respect to temperature (1030-1172°C), dissolved-H2O contents (0.8-4.1 wt% H2O), and viscosity (1.6-2.1 log10 Pa·s). This confirmed the utility and precision of these models in assessing the thermodynamic properties of mafic, hydrous arc lavas over a broad range in crystallinity, while requiring only the completion of bulk geochemical and microprobe analyses to define original compositional parameters. Where first-grown plagioclase was not well-defined, otherwise-anomalous results from the DNi(olivine-melt) thermometer were explicable through analyses of sample petrography and field-based relationships. Altogether, these analyses allowed the definition of a multi-stage eruptive sequence consistent with the degassing of magma on ascent in the formation of this small-scale basaltic edifice.

Geological Society of America Abstracts with Programs. Vol. 49, No. 2

doi: 10.1130/abs/2017NE-291578

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