The SNC meteorites: basaltic igneous processes on Mars

A group of 32 meteorites, the SNC (Shergotty, Nakhla, Chassigny) group, was derived from Mars as a product of 4-7 ejection events, probably from Tharsis and Elysium-Amazonis. The SNCs either have basaltic mineralogy or some are ultramafic cumulates crystallized from basaltic melts. The SNCs can be classified both petrographically and geochemically. We classify the shergottite SNC meteorites on the basis of their light rare earth element (LREE) depletion into highly depleted, moderately depleted and slightly depleted. The slightly depleted samples (which are mainly but not exclusively aphyric basalts) show high log(10)fO(2) values (QFM -1.0, where QFM is quartz-fayalite-magnetite). Highly depleted samples, which are mainly olivine-phyric basalts, have low log(10)fO(2) values (QFM -3.5). On the basis of mixing calculations between La/Lu and Sr-87/Sr-86 we favour models linking the correlation between LREE abundances and log(10)fO(2) to mantle heterogeneity rather than contamination by oxidized, LREE-rich crustal fluids. SNC chemistry in general reflects the Fe-rich mantle of Mars (which contains twice as much FeO as the Earth's mantle), the late accretion of chondritic material into the mantle, and possibly the presence of a plagioclase-rich magma ocean, which acted to variably deplete the mantle in Al. The high FeO contents of the SNC melts are associated with high melt densities (allowing the pending of large magma bodies) and low viscosities, both of which are consistent with the large scale of many observed martian lava flows.