Mantle Metasomatism in Mars: Evidence from Bulk Chemical Compositions of Martian

Bulk compositions of martian meteorite basalts suggest that they formed from a highly depleted mantle that was variably metasomatised and enriched in incompatible elements. These results are consistent with radio-isotope results Bulk chemical compositions of basaltic rocks retain clues and tracers to their origins and histories. Interpretations of bulk compositions are not so straight-forward as once envisioned [1], because real-world magmatic processes can be far from theoretical simple models like one-stage partial melting or closed-system fractional crystallization [e.g., 2]. Yet, bulk chemistry can shed a broad (if dim) light on Martian basalt petrogenesis that complements the sharply focussed illumination of radio-isotope systematics. History: Starting ~20 years ago, bulk compositions of martian basalts have been used to constrain the composition of the martian mantle and its basalt petrogensis [3,4]. Treiman [5] delineated two groups of incompatible elements; abundances of elements in each group are strongly correlated, but abundances between groups are not correlated (Table, Fig. 1). Group 1 elements are highly incompatible in silicate mineral/melt fractionation; and Group 2 was inferred to be moderately incompatible. Several peculiarities (notably P, S, and Se being in Group 2) were inexplicable. Table 1. Incompatible Element Groups [5]. Group 1 Group 2 La, Ce, Nd, Th, U, Ta, Nb, W, K, Rb, Cs, Ba Ti, Sm, Eu, HREE, P, Al, Ga, Na, S, Se, Cu, Sc, Hf