Martian Meteorites and Spacecraft Data: Synergy or Mismatch?

Abstract:

Martian basaltic meteorites sample more sites than rovers have but, with only a few exceptions, they consist young (Amazonian) igneous rocks. Mars rovers mostly have sampled old (Late Hesperian) sedimentary rocks, although Gusev rocks analyzed by Spirit constitute the best-characterized igneous province on Mars. Because we have only one sedimentary meteorite – NWA 7034, a regolith breccia composed of igneous clasts – we will focus on comparing igneous rocks.

Basaltic rocks analyzed so far on the planet’s surface are more alkaline than the meteorites. Limited trace element ratios from rover APXS also indicate differences in composition from meteorites. Orbital geochemical and mineralogical data are generally consistent with a basaltic surface, showing localized exposures of sedimentary rocks. Compositional differences may arise from melting and fractionation at different depths, under different redox states, or with different water contents.

The various data sets do show some linkages, however. Major mineral compositions are similar (and different from Earth), and rocks tend to be oxidized. Although no hydrous igneous minerals occur (except apatite), many parent magmas contained water that was mostly lost on ascent. It is surprising, then, that calc-alkaline rocks have not been found.

Highly fractionated rocks are uncommon, but tephrites and mugearites occur in Gusev, Gale, and NWA 7034. Feldspathic rocks discovered by remote sensing and encountered at Gale are not yet understood.

The geochemically distinct magmas represented by different classes of martian meteorites and surface rocks demonstrate that the mantle sources for magmas are heterogeneous. These differences are primary, likely inherited from a magma ocean, since there is no crustal recycling. With only one source of data (either meteoritic or spacecraft), we would have a greatly simplified and incorrect view of martian petrogenesis.