TOPOGRAPHY, GEOCHEMISTRY AND VOLCANOLOGY OF ESRP BASALTIC SHIELDS STUDIED AS ANALOGS TO MARS PLAINS-STYLE VOLCANOES

Morphologic analysis of small basaltic plains-style volcanoes on the eastern Snake River Plain (ESRP) reveal similarities in dimensions and vent spacing to volcanic fields on Mars located at Tempe Terra, Syria Planum, and near Pavonis Mons. Small volcanoes in these regions are shield-like in topography with low-angle slopes, but they have convex upward flank profiles and prominent elevated summit regions. Quaternary ESRP basalts, erupted long after the passage of the Yellowstone hotspot, suggest minimal or marginal involvement of an active mantle plume. Similarly, martian plains-style volcanic regions are located adjacent to, but not within, larger plume-related magmatic provinces, suggesting near-plume or post-plume associations. Geochemistry and physical volcanology of ESRP basalts, studied as planetary analogs to Mars, imply a system with variably evolved magma batches and possibly multiple magmatic sources. Chemical variability is extensive (MgO = ~11.2 – 4.6 wt.%; La = ~8 – 60 ppm) in over 500 analyses of samples representing ~40 individual eruptions. Magma evolution models suggest that sequences of sub-volcanic mafic intrusions fractionate to evolved compositions, which can be assimilated by subsequent, less-evolved melts. Olivine equilibrium temperatures in high-MgO representatives are tightly constrained to temperatures above 1200 °C, and support the origin of magma from upper mantle or lower crust. Low-MgO samples represent lower temperature magmas that produced lava flows with high incompatible element abundances and coarse diktytaxitic textures composed of interlocking plagioclase crystals and large pore spaces. These results support an extensive and diverse post-hotspot system of small mafic intrusions beneath the ESRP.