Melting in the martian mantle: Shergottite formation and implications for present‐day mantle convection on Mars

Abstract

Abstract-Radiometric age dating of the shergottite meteorites and cratering studies of lava flows in Tharsis and Elysium both demonstrate that volcanic activity has occurred on Mars in the geologically recent past. This implies that adiabatic decompression melting and upwelling convective flow in the mantle remains important on Mars at present. I present a series of numerical simulations of mantle convection and magma generation on Mars. These models test the effects of the total radioactive heating budget and of the partitioning of radioactivity between crust and mantle on the production of magma. In these models, melting is restricted to the heads of hot mantle plumes that rise from the core-mantle boundary, consistent with the spatially localized distribution of recent volcanism on Mars. For magma production to occur on present-day Mars, the minimum average radioactive heating rate in the martian mantle is 1.6 × 10 −12 W/kg, which corresponds to 39% of the Wänke and Dreibus (1994) radioactivity abundance. If the mantle heating rate is lower than this, the mean mantle temperature is low, and the mantle plumes experience large amounts of cooling as they rise from the base of the mantle to the surface and are, thus, unable to melt. Models with mantle radioactive heating rates of 1.8 to 2.1 × 10 −12 W/kg can satisfy both the present-day volcanic resurfacing rate on Mars and the typical melt fraction observed in the shergottites. This corresponds to 43-50% of the Wänke and Dreibus radioactivity remaining in the mantle, which is geochemically reasonable for a 50 km thick crust formed by about 10% partial melting. Plausible changes to either the assumed solidus temperature or to the assumed core-mantle boundary temperature would require a larger amount of mantle radioactivity to permit present-day magmatism. These heating rates are slightly higher than inferred for the nakhlite source region and significantly higher than inferred from depleted shergottites such as QUE 94201. The geophysical estimate of mantle radioactivity inferred here is a global average value, while values inferred from the martian meteorites are for particular points in the martian mantle. Evidently, the martian mantle has several isotopically distinct compositions, possibly including a radioactively enriched source that has not yet been sampled by the martian meteorites. The minimum mantle heating rate corresponds to a minimum thermal Rayleigh number of 2 × 10 6 , implying that mantle convection remains moderately vigorous on present-day Mars. The basic convective pattern on Mars appears to have been stable for most of martian history, which has prevented the mantle flow from destroying the isotopic heterogeneity.

The evolution of a heterogeneous Martian mantle: Clues from K, P, Ti, Cr, and Ni variations in Gusev basalts and shergottite meteorites

“…Crater densities suggest volcanic activity has occurred as recently as ∼ 10 Ma (Lucchitta, 1987;Berman and Hartmann, 2002;Hartmann, 2005). In addition, significant uplift of the Tharsis volcanic region is likely buoyed by both magmatic crustal thickening and hot mantle upwelling (Harder and Christensen, 1996;Kiefer, 2003). Together, these observations suggest that processing of the Martian mantle by convection and melt extraction is ongoing to the present day, although it is not nearly as vigorous as inside the Earth.…”

“…Thermal models by Kiefer (2003) suggest that in order for mantle convection to support the Tharsis bulge uplift today, ∼ 50% of radioactive elements (including K) presently reside in the mantle. If all heat-producing elements are in PM domains, then PM makes up at least 10 to 15% of present total mantle volume.…”

“…In order to sustain convection and volcanism in the geologically recent past (e.g., Harder and Christensen, 1996;Lucchitta, 1987), roughly half of all the heat-producing elements (K, U and Th) must remain in the mantle (Kiefer, 2003). High pressure phase equilibria experiments at upper mantle to core-mantle boundary conditions (2 to 23.5 GPa) on WD composition mantle by Bertka and Fei (1997) indicate that the Martian upper mantle contains olivine, orthopyroxene, clinopyroxene, and garnet, which transitions to majoritic garnet and γ-spinel in the lower mantle.…”

Chemical compositions of martian basalts (shergottites): Some inferences on b; formation, mantle metasomatism, and differentiation in Mars

“…are critically important for understanding Mars' thermal structure and the formation of martian basalts (Kiefer 2003). A few differences exist between the G1 assignments here and in Treiman et al (1986).…”

Mantle convection controls the observed lateral variations in lithospheric thickness on present‐day Mars

“…where T is the dimensional temperature, T 0 is the nondimensional temperature, Z is the depth, DT is the superadiabatic temperature difference (1600 K), and T ad is the adiabatic gradient in the mantle (0.18 K/km) [Kiefer, 2003]. T S is the average surface temperature, 220 K.…”