The SNC meteorites: basaltic igneous processes on Mars

A group of 31 meteorites (SNC 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 crystallised from basaltic melts. The SNCs can be classified both petrographically and geochemically. We classify the shergottite SNC meteorites on the basis of their 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 oxygen fugacity values (QFM -1.0). Highly Depleted samples - which are mainly olivine-phyric basalts - have low log oxygen fugacity values (QFM -3.5). On the basis of mixing calculations between La/Lu and 87-Sr/86-Sr we favour models linking the correlation between LREE abundances and log oxygen fugacity to mantle heterogeneity rather than contamination by oxidised, LREE-rich crustal fluids. SNC chemistry in general reflects the Fe-rich mantle of Mars (x2 FeO that of the Earth), 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 ponding of large magma bodies) and low viscosities, both of which are consistent with the large scale of many observed martian lava flows

Igneous Differentiation of the Martian Crust

Our understanding about the differentiation of the Mars crust is increasing rapidly as a result of the combination of 129 distinct SNC meteorites, lander and orbiter data. Recent debates have centred on the existence of alkaline versus tholeiitic and silicic magmatism, crystal fractionation versus partial melting controls on melt composition, and the oxidation state of mantle source regions. Recently we have used MSL ChemCam data to show the presence of trachybasalt float rocks, of tholeiitic affinity, in Gale Crater [1]. Other igneous components recorded in Gale sediments suggest the presence of alkaline and silica oversaturated magmatism as well [2,3,4]. When we compare martian datasets, it is apparent that a key primary melt composition in the ancient highlands is basalt with SiO2 45 wt%, Na2O + K2O 3.5 wt%, and high Fe, low Al. Crystal fractionation from this has led to trachybasalt and possibly in extreme cases to rhyolites [4]. The juxtaposition with some likely alkaline source regions is analogous to intraplate magmatism on Earth. Although one martian meteorite (a regolith breccia) shows clasts of alkaline affinity [5,6], the 112 shergottites are silica saturated. We classify them on the basis of their REE abundances, reflecting mantle source compositions rather than crustal contamination [7]. Bulk compositions indicate that their source regions – probably under Tharsis and the Northern Lowlands - were alkali-poor compared to the Ancient Highlands’ basalts. Limited evidence for crystal fractionation has been identified. Here we present the results of new work comparing shergottites, Gale, MER and terrestrial analogue igneous rocks to determine the key controls on martian magmatism. [1] Edwards P.H. et al. (2017) MAPS. DOI: 10.1111/maps.1295. [2] Bedford C. et al. GCA (in rev.). [3] Treiman et al. (2016) doi: 10.1002/2015JE004932. [4] Morris et al. (2016) doi: 10.1073/pnas.1607098113. [5] Santos A. et al. (2015) doi.org/10.1016/j.gca.2015.02.02. [6] MacArthur J. et al. (2017) MetSoc. #6108. [7] Bridges J.C. and Warren P.H. (2006) doi:10.1144/0016-764904-501