Testing of an H-chondrite as a Candidate for the Bulk Martian Mantle Composition

Details Testing of an H-chondrite as a Candidate for the Bulk Martian Mantle Composition Testing of an H-chondrite as a Candidate for the Bulk Martian Mantle Composition

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p11c1239h&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P11C-1239

Other

[5410] Planetary Sciences: Solid Surface Planets / Composition, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [5480] Planetary Sciences: Solid Surface Planets / Volcanism, [6225] Planetary Sciences: Solar System Objects / Mars

Scientific paper

Several models for the bulk composition of the martian mantle have been proposed in the past. Other researchers have used these starting compositions (or similar variations) to model crystallization of the martian mantle from an early, extensive magma ocean. Borg and Draper (2003) used the silicate fraction of the Homestead L5 ordinary chondrite (Mg# of 76.6 and FeO content of ~16 wt%) as their starting composition for the magma ocean, which is similar in composition to the Dreibus and Wänke (1985) model martian mantle composition (Mg# of 75 and FeO content of ~18 wt%). Borg and Draper (2003) determined that garnet must be an early crystallizing phase at the onset of magma ocean crystallization in order to impart a superchondritic CaO/Al2O3 ratio to the residual liquid (aluminum is sequestered into the crystallizing garnet, thus depleting the residual liquid in aluminum and raising the CaO/Al2O3 ratio). Such a process is required to account for the superchondritic CaO/Al2O3 ratios (~1.0 to 1.4 compared to the chondritic value of ~0.80) seen in the shergottite meteorites (martian basaltic meteorites). This ratio should remain relatively unperturbed by subsequent crystallization of olivine ± low-Ca pyroxene. Borg and Draper (2003) also concluded that the L-chondrite starting composition was too FeO-rich to produce the source regions of the shergottite meteorites. They proposed that an H chondrite (Mg# of 80 and FeO content of ~13 wt%) would make a better starting composition for the magma ocean. We conducted high-pressure melting experiments on the Farmville H4 ordinary chondrite from 3 to 17 GPa. We used the compositions of these melts (representing possible compositions of an early martian magma ocean), to constrain the compositions of likely source rocks for the calculated parent liquids of the shergottites. We infer that a magma ocean having a bulk composition like that of Farmville yields source rocks that are a better match to sources of the most primitive martian basalt parent liquids than does a Homestead-composition magma ocean. We therefore conclude that the Dreibus-Wänke model martian mantle (similar in composition to an L chondrite) is too FeO rich to produce the shergottite source rocks. We advocate that an H chondrite is a better candidate for a bulk martian mantle composition.

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