EVIDENCE OF RAPID PHENOCRYST GROWTH AND CONSTRAINTS ON PRE-ERUPTIVE TEMPERATURE AND MELT H2O CONTENT OF BASALTIC FLOW SAMPLES FROM EASTERN SNAKE RIVER PLAIN OLIVINE BASALTS

Olivine is commonly the first mineral to crystallize from a terrestrial basaltic melt and records critical information on the pre-eruptive conditions of basalts. Olivine-melt thermometers can be used to constrain temperature as well as melt H₂O content based on olivine-melt Mg and Ni partitioning, with differing dependence on the presence of H₂O in the melt. These pre-eruptive conditions have broad implications for the interplay between volcanism, extension, and volcanic hazards. Previous studies on the Eastern Snake River Plain (ESRP) basalts have determined the oxygen fugacity (fO₂) to be near QFM, melt H₂O content to be up to 1 wt %, and crystallization temperatures of olivine at 1100-1250 °C, based on olivine-melt Mg thermometry. This study by Idaho State University, in cooperation with the USGS and the Center for Advanced Energy Studies, uses olivine and melt compositions of ESRP basalts to 1) determine if the most Fo-rich olivine is in equilibrium with the melt represented by the whole rock composition; and if so, 2) apply a H₂O independent, olivine-melt Ni thermometer to calculate the temperature at the onset of olivine crystallization. Samples were collected from rock cores from Idaho National Laboratory sites. Various flow facies were sampled to assess possible intraflow variability of olivine-melt crystallization temperatures, olivine populations, and bulk rock compositions.

Petrographic and SEM/BSE observations of olivine phenocrysts display normal zoning and some rapid growth textures. Histograms of Fo# distribution for each sample display normal distributions, with smaller ranges observed at basalt flow margins and wider distributions in flow interiors. The average olivine-melt Fe-Mg exchange coefficient, KD, calculated from the most Mg-rich olivine composition and a melt represented by the whole rock composition, assuming QFM, is 0.30. From these observations we conclude that: (1) the most Mg-rich olivine compositions best represent equilibrium with melt, as represented by bulk rock composition, and (2) all observed olivine phenocrysts grew during rapid ascent of the basaltic magma. The application of the olivine-melt Ni thermometer indicates the onset of olivine crystallization temperatures to be 1116-1197ºC, and melt H₂O contents from 0.46 - 1.65% with an average of 1.13%.