TEXTURAL STUDY OF TABLE LEGS BUTTE AND QUAKING ASPEN BUTTE, EASTERN SNAKE RIVER PLAIN, IDAHO: AN ANALOG TO SMALL SHIELD VOLCANOES ON MARS

Mars Orbiter Laser Altimeter (MOLA) data reveal numerous small, low-profile shields with prominent summit caps in the Tempe Terra region of Mars. Terrestrial examples of this morphology exist on the eastern Snake River Plain (ESRP), Idaho, host to many late Pliocene to Holocene tholeiitic basalt shields built up over short monogenetic eruptive periods. These shields generally have low distal slopes (1-2°), but some shields, such as Table Legs Butte and Quaking Aspen Butte, have gentle (4-6°) medial slopes that sharply transition to steep proximal slopes (20°-30°), forming a summit cap. We evaluate the effects of lithology and geochemistry on volcanological controls, such as change in eruptive style (i.e. from fissure to single vent) or eruptive rate that are responsible for this shield morphology. Table Legs Butte lava flows exhibit well-developed coarse diktytaxitic texture and Quaking Aspen Butte flows exhibit a less-developed coarse diktytaxitic texture; shields without summit caps are generally non-diktytaxitic. GIS/GPS topographic profiling indicates a positive correlation between diktytaxitic coarsening and summit steepness. The diktytaxitic texture implies high volatile content, possibly related to fractionated pockets of source magma enriched in H₂O and/or CO₂. Basalts from Table Legs Butte contain over 25% skeletal plagioclase laths (up to 2cm); only 5% skeletal plagioclase laths (<1cm) are present in Quaking Aspen Butte lava flows. Both shields are low (<8%) in olivine (<1.5cm), which form glomerocrysts and cumulophyric textures with plagioclase. Zoned plagioclase (<2%) at both localities show resorbed cores, and the groundmasses contain a dendritic network of Fe-Ti oxides (primarily skeletal ilmenite) and augite. Significant differences between the summit and flank textures on either shield are not observed. Trace element analyses further show that flank and summit regions cannot be geochemically separated. These results suggest that shield morphology is controlled by properties intrinsic to magma composition. Continued analysis of distal flows, and possible analysis of melt inclusions for volatiles, will lead to a better understanding of magma genesis and volcanic processes of small shields on the ESRP and Mars.