Mathematics – Logic
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p31c0150l&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P31C-0150
Mathematics
Logic
5418 Heat Flow, 5430 Interiors (8147), 6063 Volcanism (5480, 8450), 6225 Mars, 8121 Dynamics: Convection Currents, And Mantle Plumes
Scientific paper
The low crater densities on some volcanic units on Mars and the young radiometric ages of the igneous shergottite meteorites both provide evidence for geologically recent volcanism on Mars. In turn, this helps constrain the thermal structure of the martian mantle and the nature of present-day mantle convection. Kiefer (Meteoritics Planet. Sci. 38, 1815-1832, 2003) explored the implications of these constraints using spherical axisymmetric finite element simulations with depth-dependent viscosity and radioactive heating that was partitioned between mantle and crust. Here, we extend these models to the more realistic case of a temperature-dependent Arrhenius viscosity law. We test our models against the geologically inferred recent volcanism rate and the geochemically inferred range of melt fractions in the shergottites. We also use bounds on the surface heat flux inferred from gravity modeling and on the heat flux out of the core inferred from the absence of a magnetic dynamo. We have systematically explored the parameter space defined by thermal Rayleigh number (106- 108, defined at the bottom temperature), activation energy (E=100-300 kJ/mole), and fraction of total radioactive heating that is retained in the mantle (30-60%). As expected, increasing E increases both the thickness of the upper thermal boundary layer and the mean internal temperature. Although the hotter interior temperature would favor higher magma production, the thicker boundary layer limits the extent of pressure- release melting. This second effect dominates, and thus increasing E tends to decrease both the magma production and the convective heat fluxes. For values of E inferred from lab studies of olivine, a thermal Ra exceeding 2x106 best fits the observational constraints. These results support the previous conclusion that present-day mantle convection on Mars remains relatively vigorous. An important limitation of the earlier study was that the inferred heat flux out of the core was relatively large and suggested the likelihood of a present-day geodynamo. The non-uniformly thicker lithosphere in the new models results in higher mantle temperatures. This significantly reduces the heat flux from the core into the mantle, in agreement with the absence of a present-day magnetic dynamo.
Kiefer Walter Scott
Li Qian
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