Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p33b1451r&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P33B-1451
Physics
3611 Thermodynamics (0766, 1011, 8411), 5430 Interiors (8147), 5480 Volcanism (6063, 8148, 8450), 8147 Planetary Interiors (5430, 5724, 6024), 8150 Plate Boundary: General (3040)
Scientific paper
We present models of convection in the Martian mantle in which particular attention is paid to water and its effects on partial melting, melt extraction, and viscosity over timescales of several billion years. We have combined the mantle convection program STAG3D (e.g. Tackley 1998) with a parameterized thermodynamic model of martian mantle mineralogy and carried out calculations of convection in a two-dimensional compressible model of the planet's mantle. As the depth to the base of the mantle is known only to within a few hundred kilometers, models with core-mantle boundary depths of ~1700 and ~2000 km, which essentially cover the range of possible values, are considered. The models are heated from below by a cooling core and from within by the radioactive decay of 40K, 232Th, 235U, and 238U. Near-fractional hydrous melting is included in a parameterized form by modifying the dry solidus of martian peridotite by means of a simple method to include the solidus-lowering effect of water in low concentrations; the exhaustion of phases is also taken into account in a simple form. Melting reduces the concentration and changes the distribution of both the water and the heat-producing radionuclides in the mantle. Different initial water and radionuclide contents and distributions are assumed and allowed to evolve with time as a consequence of melting, dehydration, and the secular expansion of the ringwoodite stability field. The viscosity of the mantle is dependent on temperature, pressure, water content, and the amount of retained melt and therefore also undergoes a secular change as the mantle cools and becomes depleted. Preliminary models indicate the existence of a two-layer convection regime at least in the early stage of planetary evolution where the upper layer undergoes dehydration and depletion of radionuclides much sooner than the lower. Depending on the assumed water content, the dehydration of the zone of melt generation can lead to the formation of a high-viscosity layer in the upper part of the mantle whose thickness reaches several hundreds of kilometers. This high-viscosity layer is sandwiched between a crust/lithosphere and -- at least initially -- a lower mantle layer, both of which have higher radionuclide and volatile contents. The existence of a high-viscosity layer in the upper part of the mantle may be an alternative, non-thermal explanation for the high effective elastic lithosphere thickness beneath the northern polar deposits which was recently deduced from SHARAD observations.
Ruedas Thomas
Solomon Stanley C.
Tackley Paul J.
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