Linking the Chassigny meteorite and the Martian surface rock Backstay: Insights into igneous crustal differentiation processes on Mars

Abstract

Abstract-In order to use igneous surface lithologies to constrain Martian mantle characteristics, secondary processes that lead to compositional modification of primary mantle melts must be considered. Crystal fractionation of a mantle-derived magma at the base of the crust followed by separation and ascent of residual liquids to the surface is common in continental hotspot regions on Earth. The possibility that this process also takes place on Mars was investigated by experimentally determining whether a surface rock, specifically the hawaiite Backstay analyzed by the MER Spirit could produce a known cumulate lithology with a deep origin (namely the assemblages of the Chassigny meteorite) if trapped at the base of the Martian crust. Both the major cumulus and melt inclusion mineral assemblages of the Chassigny meteorite were produced experimentally by a liquid of Backstay composition within the pressure range 9.3 to 6.8 kbar with bulk water contents between 1.5 and 2.6 wt%. Experiments at 4.3 and 2.8 kbar did not produce the requisite assemblages. This agreement suggests that just as on Earth, Martian mantle-derived melts may rise to the surface or remain trapped at the base of the crust, fractionate, and lose their residual liquids. Efficient removal of these residual liquids at depth would yield a deep low-silica cumulate layer for higher magmatic water content; at lower magmatic water content this cumulate layer would be basaltic with shergottitic affinity.

Experimental and crystal chemical study of the basalt–eclogite transition in Mars and implications for martian magmatism

“…On Mars, the plagioclase-out transition occurs at approximately 75 km (>1.0 GPa) and the basalt-eclogite transition occurs at approximately 200 km ($2.6 GPa) (Borg and Draper, 2003;Elkins-Tanton et al, 2003, 2005Sleep, 1994 and references therein). These mineralogic transitions in the martian interior correspond well with estimated thicknesses for the martian crust and lithosphere (McCubbin et al, 2008;Nekvasil et al, 2007Nekvasil et al, , 2009. Relative thickness estimates for the primary and secondary crusts are under-constrained, however, upper and lower limits have been established.…”

Water and the composition of Martian magmas

“…3.5 Ga) basalts in Gusev crater. Although the Gusev rocks are alkali basalts while the shergottites are tholeiitic, their textural and chemical properties suggest that hydrous magmatism (Nekvasil et al, 2009;McSween et al, 2006) occurred early in martian history. Tuff et al (2013) proposed that the Gusev basalts derived from oxidized mantle, …”

Melting of the primitive martian mantle at 0.5–2.2 GPa and the origin of basalts and alkaline rocks on Mars

“…Irvine and Backstay are saturated with olivine on the liquidus (e.g. Nekvasil et al, 2009). Therefore, the crystallization of a large amount of plagioclase should be accompanied by significant olivine fractionation, which is inconstant with constant and elevated MgO contents.…”

“…Alternatively, we suggest that low-degree melts, produced from a mantle not anomalously enriched, can fractionate and eventually crystallize to form rocks similar to Jake_M. Experiments performed from the composition of Backstay illustrate how residual liquids rich in Al 2 O 3 and poor in MgO can be derived from martian alkali basalts (Nekvasil et al, 2009). …”