LOW SHIELDS ON EARTH AND MARS: A COMPARATIVE MODEL OF PETROGENESIS AND VOLCANIC EVOLUTION

A petrogenetic model of magma genesis and low shield evolution is developed by comparing volcanoes at or near Tempe Terra, Pavonis Mons, Syria Planum, and the type area for plains-style volcanism, the eastern Snake River Plain (ESRP) of Idaho. Constraints are derived from surface geomorphology, dimensional topography, geologic settings, age relations, and geochemical and isotopic systematics of ESRP basalts. Petrologic studies of basaltic shergottites provide constraints for magma genesis on Mars. Surface roughness and topography of low shields and cones on Mars, determined from MOC, THEMIS, and MOLA data, compare well to shield geomorphology on Earth determined by field mapping and DEM analysis. Volcanism dominated by low-viscosity, non-explosive fissure eruptions produces coalescent shields with overall slopes less than 1.5°. Monogenetic shield volumes of ~5 – 50 km³ reflect small magma batches generated within extensional near-plume or post-plume geologic terranes. Geochemical signatures, as well as oxygen fugacity and isotopic constraints of both ESRP tholeiitic basalts and Mars basaltic shergottites indicate similar processes in magma genesis. Trace element variations cannot be produced from a common parental magma by crustal assimilation and fractional crystallization. Trace element trends reflect variable mixing between enriched and depleted (or primitive) components in source regions, and each shield is derived from a separate magma batch. A well-constrained model for ESRP basalts is thus believed to be applicable to the petrogenesis of low shields on Mars. Low-volume magma batches, generated in a thermally-weakened lithospheric source (near- or post-plume), intrude into the crust and evolve to produce cumulate piles with late-stage trapped liquids. Successive primary magmas intrude and mix with the layered precursors, becoming variably enriched in trace elements and volatiles. Monogenetic eruptions are short, lasting months to years, yielding multiple lava flow lobes and geochemical variability derived at the source. Unless fresh primary magma is supplied, the eruption rate rapidly subsides with pressure loss as liquid is removed from the central mixing zone. Tests for geochemical enrichment and variability in low shields on Mars could be used to confirm or deny this hypothesis.