APPLICATION OF OLIVINE HYGROMETRY TO EASTERN SNAKE RIVER PLAIN BASALTS: IMPLICATIONS FOR THE VOLATILE BUDGET DURING MAGMATIC DIFFERENTIATION

Quantifying magmatic volatile contents and identifying the processes responsible for transporting volatiles from source to surface provides critical insights about volcanic systems. Most of our existing knowledge on pre-eruptive volatile contents comes from direct measurements of glassy melt inclusions, which relies on the availability and fidelity of melt inclusions in erupted lavas. However, these are scarce in basaltic lava flows, which makes it challenging to obtain information on pre-eruptive volatile content of basalts from Yellowstone and the Snake River Plain (SRP). A new olivine hygrometer, which harnesses the different olivine-melt partitioning behaviors of Mg and Ni, provides a new opportunity to estimate the H₂O content of the melt at the onset of olivine crystallization in SRP and YS basalts. Here, we present olivine-melt hygrometry results from 23 eastern SRP olivine tholeiites with whole-rock (WR) MgO compositions from 6 to 12 wt.%. The melt H₂O content of the high-MgO samples (12-8.5 wt.% MgO) increases with decreasing WR MgO content; this could be explained by fractional crystallization. The highest calculated H₂O content was 2.6 wt.%, observed at 9.2 wt.% WR MgO. The low-MgO samples (8.5-6 wt.% MgO) have decreasing melt H₂O content with decreasing MgO content, indicating a potentially drier component was incorporated during magma differentiation and evolution towards lower MgO compositions. These results are consistent with previous results from melt inclusion measurements from three SRP basalts. Other recent applications of olivine hygrometry and melt inclusion measurements on Yellowstone basalts also appear to have a similar trend. This new information about the evolution of SRP magmatic volatile contents advances our understanding of the magmatic differentiation and transportation processes in the most primitive parts of the Yellowstone-SRP volcanic system.