North-Central Section - 54th Annual Meeting - 2020

Paper No. 23-1
Presentation Time: 8:00 AM

APPLICATION OF A NEW DNI (OLIVINE-MELT) GEOTHERMOMETER: A CASE STUDY OF BLUE ROCK SHIELD VOLCANO, JACKSON CO., OREGON


CRABTREE, Stephen, Division of Science and Mathematics, University of Minnesota, Morris, 600 E. 4th St, Morris, MN 56267

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.