THERMODYNAMIC ASSESSMENT 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 volcano in the southern Oregon Cascades, north of Mt. McLoughlin, showing bulk phenocryst abundances ranging from 5 to 28 vol%, and a variety of groundmass textures. Compositional analyses of olivine and plagioclase phenocrysts and glomerocrysts allowed for the sequential application of a new D_Ni (olivine-melt) thermometer, a plagioclase-melt hygrometer, and a viscosity model to define olivine-in conditions for a suite of lavas erupted from this edifice. Calculated olivine-in temperatures were compared to results from the anhydrous MELTS model, and the D_Mg model of Beattie (1993). Model results were consistent with experimental data for hydrous arc basalts with respect to temperature (1053–1146 °C), dissolved-H2O contents (0.9–2.4 wt% H2O), and viscosity (1.9–2.2 log10 Pa s), confirming the utility of these models in assessing the thermodynamic properties of mafic, hydrous arc lavas over a broad range in crystallinity, requiring only the completion of bulk geochemical and microprobe analyses. These studies also reinforced the significant and predictable role of water, affecting the compositions of crystals grown during magmatic ascent, and allowed the definition of a reasonable multi-stage eruptive sequence consistent with the degassing of magmas on ascent in the formation of this small-scale basaltic edifice.