Physics – Geophysics
Scientific paper
Jan 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002usnc.conf...23h&link_type=abstract
Unmixing the SNCs: Chemical, Isotopic, and Petrologic Components of the Martian Meteorites, p. 23-24
Physics
Geophysics
Mars Surface, Metamorphism (Geology), Planetary Crusts, Geophysics, Planetary Geology, Planetary Mantles, Neodymium Isotopes, Tungsten Isotopes, Southern Hemisphere, Precambrian Period, Shergottites, Radioactive Isotopes, Peridotite, Anomalies
Scientific paper
The existence of a planet-wide early magma ocean on Mars is supported by a growing base of petrochemical and geophysical observations 1) The parent liquids to the SNC meteorites are significantly depleted in Al2O3 and CaO relative to terrestrial basalts. Only terrestrial komatiites, the products of more than 30% melting of the Archean mantle and boninites, wet melts of the mantle wedge in island arc regions, have similar low Al2O3 and CaO contents. Mare basalts and picrite glasses on the Moon have similar geochemical depletions, and the major element compositions of very low Ti mare basalts bear a striking resemblance to the Shergotty parent magmas. What these terrestrial and lunar magmas have in common is that the parent magmas last equilibrated with a mantle severely depleted in magmaphile elements. The boninites and mare basalts, in particular, last coexisted with a mantle residue of olivine and orthopyroxene. In the lunar case the mantle was a product of crystallization from a magma ocean whereas the harzburgite parent mantle for boninites was a residuum to previous melting events that eliminated diopside from the mantle. 2) W-182 and Nd-142 anomalies date the fractionation of the core and mantle, respectively, within about 50- 100 million years of the origin of the solar system. The large heavily crated Martian crust and the absence of large scale recycling suggests strongly that the crust was also a product of this ancient global differentiation and has experienced only modest volcanic activity, particularly in the southern hemisphere, in subsequent epochs. Whole rock Rb-Sr systematics appear to record this planet wide differentiation at about 4.5 Ga 3) The Nd-143 composition of the Martian mantle is significantly more depleted than the terrestrial mantle and even the cumulate source regions of mare basalts on the Moon. Only Archean lithosphere on earth has the extreme positive and negative epsilon values so characteristic of the Martian mantle. Continental lithosphere, by definition, is stable and has withstood the homogenizing effects of mantle convection. The extreme epsilon values reflect ancient depletion events and subsequent metasomatic perturbations. The data is consistent with the early differentiation of a Martian magma ocean producing a buoyant crust, dense core and a complementary stratified cumulate mantle. The stratified cumulate is likely to be gravitationally unstable, at least, in the shallowmost stratigraphic levels where more iron-rich cumulates overlie dense magnesian cumulates. Under these unstable conditions, solid state differentiation would have carried dense, iron-rich and relatively cool cumulates into the Martian interior ultimately resulting in a lower mantle that is denser and compositional more evolved than the upper mantle. This lower mantle would also contain varying amounts of heat producing radioactive elements.
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