| Idaho’s eastern Snake River Plain (ESRP) is an active volcanic province that hosts Idaho
National Laboratory and the underlying Snake River Plain Aquifer. Therefore, this study
addresses the ongoing need to advance our understanding of petrogenesis and architecture of
subsurface basaltic lava flows in two parts. I first assessed whole-rock geochemical variability in
a single large (45 km2) flow group named Sixmile Butte across 8 different coreholes, 2 surface
exposures, and 32 samples. This work demonstrated that the Sixmile Butte flow group has
significant spatial and stratigraphic geochemical variability, which I attributed to systematic
differences in olivine and plagioclase fractional crystallization and assimilation of a gabbroic
mid-crustal sill previously inferred to be present in the middle crust. Despite this variability, it
was still possible to identify geochemical outliers and revise the Sixmile Butte flow group
correlation in corehole USGS139. I also used olivine and plagioclase mineral-melt hygrometry to
quantify minimum magmatic H2O content of 13 flow groups that span a representative
compositional range of ESRP basalts (6.3-13.4 wt.% MgO). Olivine hygrometry H2O contents
are 0.1-2.6 (± 0.5) wt. %. In some flow groups, H2O contents decrease systematically with lower
whole-rock MgO wt. %, which I attributed to assimilation of the dry mid-crustal gabbroic sill.
Plagioclase hygrometry reproduces these H2O contents within error. However, because olivine is
the liquidus mineral in these basalts, the challenges of assessing temperature, pressure, and
equilibrium conditions during plagioclase crystallization makes the application of plagioclase
hygrometry in ESRP basalts less straightforward than the olivine hygrometer.
Key words: Sixmile Butte lava flow, whole-rock geochemistry, flow group correlation, olivine-
melt thermometry, olivine-melt hygrometry, plagioclase-melt hygrometry |