PLANETARY MUSINGS

Martian magmas

Agee, CB and DS Draper, Experimental constraints on the origin of Martian meteorites and the composition of the Martian mantle, EPSL, 224, 415-429, (2004) http://dx.doi.org/10.1016/j.epsl.2004.05.022

Interesting experimental study of potential martian magmas with the aim to understand the super-chondritic CaO/Al203 ratios in shergottites. Basic, result is that apparently no one-stage melting process can create both the high CaO and Al203 concentrations as well as the CaO/Al203 ratios in the shergottites. Instead, the authors propose deep melting around 5 GPa (near their experimental conditions) that due to garnet produce appropriate CaO/Al203 ratios, followed by olivine crystallization at lower pressure to up the concentrations. They suggest that either a mantle-plume or magma ocean scenario might account for such a situation. (Note, none of these processes account for the FeO and Mg#'s using the proposed starting materials). They conclude with comments on studies that try to use the moment-of-inertia to constrain the composition of the planet.

Musings:
(1) I suspect that the pervasive upper mantle melting that I and others like Reese, Solomatov, Stevenson, etc have talked about, could be another mechanism for the two-stage differentiation. Normal upwelling (or plumes) generate deep melting, that proceeds to shallower levels where olivine crystallizes.
(2) Agee and Draper are probably right about the moment-of-inertia arguments. We don't know enough about the core at the moment, especially its composition. I would guess that once the high-pressure phase diagram of Fe-FeS is known better we might be able to place a minumum limit on the sulfur content of the core with thermal models, but it may be higher than current estimates because models (including mine) overestimate the melting pt depression of S for lack of data.