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

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 D_Ni(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.