QUANTIFYING PRE-ERUPTIVE MAGMATIC WATER CONTENT OF EASTERN SNAKE RIVER PLAIN BASALTIC MELTS USING OLIVINE AND PLAGIOCLASE HYGROMETRY WITH IMPLICATIONS FOR MAGMATIC DIFFERENTIATION

Quantifying the pre-eruptive H2O content of magmas aids in assessing volcanic hazards in many geological settings, including distributed volcanic fields like the eastern Snake River Plain (ESRP). Given the inherent challenges with melt inclusion measurements, mineral-melt hygrometry provides an alternative method of quantifying pre-eruptive H2O contents. Here, we applied olivine- and plagioclase-melt hygrometry (standard errors of ~0.5 wt% and ~0.35 wt%, respectively) to ESRP olivine tholeiites at the onset of mineral crystallization (Pu et al., 2021; Waters and Lange, 2015). First, we applied olivine-melt hygrometry to 23 ESRP basalts with 6-12 wt% MgO. High-MgO (>9 wt%) basalts have up to 0.8 wt% H2O. Lower MgO basalts have a linear decrease in H2O content from 2.6 to 0.2 wt% with decreasing MgO. These values represent the H2O contents at the onset of olivine crystallization; olivine is the liquidus mineral in all samples. This trend cannot be explained by fractional crystallization alone, which would increase H2O contents with decreasing MgO. In these samples, WR La/Lu correlates negatively and K2O/P2O5 correlates positively with Mg#, indicating assimilation of gabbroic material. Assimilation of a drier crustal component, such as the gabbroic sill thought to underlie the SRP, could explain the trend of decreasing H2O content and MgO. To further explore ESRP H2O contents, we performed in-situ plagioclase elemental analysis using an electron microprobe and applied plagioclase hygrometry to a subset of 12 samples that span a range of olivine H2O content and MgO. Unlike olivine hygrometry, plagioclase hygrometry calculations require knowledge of the P-T conditions at the time of plagioclase crystallization. Using MELTS, we estimated T assuming P was constant at 1000 bar. The plagioclase hygrometer consistently produces lower H2O contents than the olivine hygrometer for each sample, with a maximum H2O content of 0.9 wt%. Lower plagioclase hygrometry H2O than olivine hygrometry H2O is unexpected. MELTS predicts that plagioclase crystallizes 1-92 °C below olivine in all the samples, no hydrous phase, and that the H2O content is higher upon plagioclase crystallization than olivine crystallization. The inconsistency between olivine and plagioclase hygrometry results requires further investigation.