Mantle Plume Volcanism on Presentday Mars Walter S

Mantle Plume Volcanism on Present-day Mars Walter S. Kiefer Lunar and Planetary Institute Talk based in part on Meteoritics and Planetary Science 38: 1815 -1832, Dec. 2003

Young Volcanism on Mars 4 -30 Ma 180 Ma What does the presence of young volcanism imply about thermal structure and the abundance of radioactivity in the martian mantle?

Mars Topography

Models for Young Volcanism on Mars • Mantle plumes – Consistent with point-like, long-lived sources • Mantle heating due to low thermal conductivity of crust – Thermal diffusivity doesn’t vary as much – Olympus Mons is on normal crustal thickness • Thermal effects of very large impact – Known large basins aren’t volcanic

Conceptual Model for Tharsis • Tharsis is a broad plateau containing many volcanic structures. It is NOT a single mantle plume. • The plateau is due to the internally heated component of mantle convection. • Individual large volcanos are fed by mantle plumes. Plumes concentrate in this region because mantle is hotter, allowing boundary layer instabilities to erupt more easily in this region.

Mantle Convection Models • Spherical axisymmetric finite element code. • Partition radioactivity between mantle and crust. • Self-consistent relationship between imposed radioactive heating and mantle temperature, viscosity, and convective vigor. – Explicit, observational tested bound on Ra. • Trace flow streamlines through melting zone.

Parameters Affecting Melt Generation • Plume starting temperature at base of mantle • Partitioning of radioactivity between crust and mantle • Lithosphere thickness above plume • Mantle water content

Mars Dry Mantle Solidus Y 980459 Bertka and Holloway Draper and Agee

Mantle Radioactivity and Magma Production Mantle radioactivity 25 % of the Wänke and Dreibus bulk silicate Mars. Mantle radioactivity 55 % of the Wänke and Dreibus bulk silicate Mars.

Magma Production Rates

Mean Melt Fraction

Mantle Radioactivity Distribution Plum Pudding Model Deep Enriched Layer

Effects of Water Abundance on Melting Temperature Asimow et al. , 2004

Mantle Convection and Isotopic Mixing • The martian mantle has maintained very strong isotopic heterogeneity since 4. 5 Ga – Does this mean that the martian mantle hasn’t been convecting for most of its history? • Reasons the convective mixing efficiency is lower on Mars than on Earth: – Stability of convection cell boundaries over time – Absence of transform faulting and toroidal flow

Ongoing Research • Explore effects of temperature-dependent viscosity – Faster ascent velocity, less cooling while plume rises – Self-consistent model of lid thinning over plume center; testable with new meteorite results on multiple saturation pressures. • Explore effects of moderate amounts of mantle water – Lowers mantle solidus temperature – Lowers viscosity, enhanced convective vigor leads to cooler mantle