OXYGEN ISOTOPE COMPOSITIONS OF OLIVINE FROM QUATERNARY MAFIC VENTS OF THE GOLDEN TROUT VOLCANIC FIELD, KERN PLATEAU, CA

The Golden Trout Volcanic Field (GTVF) is a Quaternary volcanic field located at an average elevation of 2,600 m on the Kern Plateau in the southern Sierra Nevada. Approximately 0.04 km³ of olivine-phyric basalt, trachy-basalt, basaltic trachy-andesite, and basaltic andesite magma has erupted from four vents in the GTVF between ~743 and ~55 ka, which corresponds to an average eruption rate of ~0.06 km³/Myr. The number of vents, eruption volume, and eruption rate in the GTVF is orders of magnitude less than contemporaneous and neighboring alkalic and sub-alkalic basaltic volcanic fields like Big Pine and Coso, which exist ~60 km north and ~30 km east, respectively. One possible explanation for the pronounced differences in the evolution of these mafic volcanic fields relates to crustal thickness. Whereas geophysical surveys suggest a Moho depth of ~35 km beneath the GTVF, the Moho beneath the Big Pine and Coso volcanic fields is thought to be only ~25 km deep. Perhaps differences in eruption volume and eruption rate between GTVF and neighboring volcanic fields like Coso and Big Pine occur because rising mafic magma spends more time traveling through thicker crust and ends up stalling out due to density and/or thermal limitations. To test this hypothesis, we measured via laser fluorination δ¹⁸O values of olivine phenocrysts from GTVF and compared them to existing δ¹⁸O measurements from Big Pine and Coso. Initial results yield higher than mantle δ¹⁸O values of 5.88 - 6.52 ‰ from Fo_74-88 olivine sampled from Groundhog Cone, the youngest vent in the GTVF. These values are 1 ‰ higher than any Sierran peridotites but agree with high δ¹⁸O values obtained previously from older GTVF vents, suggesting a high δ¹⁸O mantle source and steady input from assimilated crust over time. Interestingly, GTVF δ¹⁸O values are only <1 ‰ higher than those from Big Pine, suggesting a similar mantle source and assimilated input of Sierra Nevada crust despite differences in crustal thickness. Our findings are inconsistent with the idea that differences in eruption volume and eruption rate between GTVF and neighboring volcanic fields are the result of differences in crustal thickness.