PLAINS VOLCANISM IN THE EASTERN SNAKE RIVER PLAIN: QUANTITATIVE MEASUREMENTS OF PETROLOGIC CONTRIBUTIONS TO TOPOGRAPHY WITH COMPARISONS TO MARS

Topographic profiles of Quaternary eastern Snake River Plain (ESRP) monogenetic shield volcanoes are used to infer compositional changes in magma supply. Many low-volume, 5-25 km diameter, olivine tholeiite shields have unique steep-sided vent regions comprised of shelly pahoehoe, spatter, and breached spatter ramparts, which are manifested as elevated “caps” atop the normal shield profiles projected from the surrounding low-angle flanks. The caps represent a dramatic change in eruptive style, most likely related to viscosity changes as the local magma supply degassed. Other shields have much less dramatic or even insignificant increase in slope near the vents. Shields with elevated caps typically have very coarsely diktytaxitic textures with large plagioclase grains, whereas medium-to-fine sub-ophitic textures prevail in the shields without caps. Topographic GPS and DEM profiles show a link between steep-sided vent caps and coarsely diktytaxitic textures. One possible explanation is that magma feeding these shields is chemically more evolved than the magma supply to shields with low-profile vents, leading to larger variation in magmatic properties. Chemical analyses from Circular Butte, a representative ~1Ma coarsely diktytaxitic shield on the ESRP, and related subsurface samples from drill core display significant geochemical variation with stratigraphy. Chemical variability from a base level of ~130 m below the distal shield flanks is exemplified by increases in average P₂O₅, TiO₂, FeO, and La. Concomitant decreases in Cr, MG#, and other compatible elements occur in the same sequence, reflecting extensive fractionation of magma during the eruptive cycle. Analyses of flow group sequences sampled from Lava Ridge, a low-profile eruptive system with finer textured basalt and no significant summit slope increase, indicate lower P₂O₅, TiO₂, FeO, La, and higher MgO, MG#, and Cr, indicating much less chemical evolution and more primitive magma compositions. These ESRP data suggest that topographic profiling, along with remotely sensed chemical data, can be used to determine the state of chemical evolution and possible incompatible element enrichment near vent areas on lava plains regions of Mars.