Primitive olivine‐phyric shergottite NWA 5789: Petrography, mineral chemistry, and cooling history imply a magma similar to Yamato‐980459

Abstract

Abstract-Knowledge of Martian igneous basaltic compositions is crucial for constraining mantle evolution, including early differentiation and mantle convection. Primitive magmas provide direct information about their mantle source regions, but most Martian meteorites either contain cumulate olivine or crystallized from fractionated melts. The recently discovered Martian meteorite Northwest Africa (NWA) 5789 is an olivine-phyric shergottite. NWA 5789 has special significance among the Martian meteorites because it appears to represent one of the most magnesian Martian magmas known, other than Yamato (Y) 980459. Its most magnesian olivine cores (Fo 85 ) are in Mg-Fe equilibrium with a magma of the bulk rock composition, suggesting that the bulk represents a magma composition. Based on the Al ⁄ Ti ratio of its pyroxenes, we infer that the rock began to crystallize at a high pressure consistent with conditions in Mars' lower crust ⁄ upper mantle. It continued and completed its crystallization closer to the surface, where cooling was rapid and produced a mesostasis of radiating sprays of plagioclase and pyroxene. The mineralogy, petrology, mineral chemistry, and bulk rock composition of NWA 5789 are very similar to those of Y-980459. The similarities between the two meteorites suggest that NWA 5789 (like Y-980459) represents a primitive, mantle-derived magma composition. They also suggest the possibility that NWA 5789 and Y-980459 formed in the same lava flow. However, based on the mineralogy and texture of its mesostasis, NWA 5789 must have cooled more slowly than Y-980459. NWA 5789 will help elucidate the igneous geology and geochemistry of Mars.

Crystallization history of enriched shergottites from Fe and Mg isotope fractionation in olivine megacrysts

“…While some shergottites have been recognized as possible parental melts (e.g. Musselwhite et al, 2006;Gross et al, 2011), NWA 1068 has been suspected to contain up to 20% of accumulated olivine (Filiberto et al, 2010), which represents the total fraction of olivine in NWA 1068 (Barrat et al, 2002). However, this test is only valid if we assume perfect fractional crystallization with no solid-liquid partial equilibration by diffusion during crystal growth.…”

“…Numerous authors used the term xenocryst to refer to the megacrysts (McSween and Jarosewich, 1983;Wadhwa et al, 2001;Barrat et al, 2002;Goodrich, 2003;Koizumi et al, 2004;Herd, 2006;Shearer et al, 2006;Filiberto et al, 2010) while others preferred the term phenocryst (Mikouchi et al, 2001;Taylor et al, 2002;Koizumi et al, 2004;Musselwhite et al, 2006;Shearer et al, 2008Shearer et al, , 2013Usui et al, 2008;Basu Sarbadhikari et al, 2009Peslier et al, 2010;Filiberto and Dasgupta, 2011;Gross et al, 2011Gross et al, , 2013Balta et al, 2013Balta et al, , 2015Liu et al, 2016). Xenocrysts are generally defined as disequilibrium crystals with no genetic relationship to the magma.…”

“…Shergottites, which are basalts or gabbros, are the most common type of martian meteorites. Among shergottites, the olivine-phyric group (Goodrich, 2002) is the most primitive, and some samples are arguably representative of liquids produced by partial melting of the mantle and not significantly affected by fractional crystallization or assimilation (Musselwhite et al, 2006;Basu Sarbadhikari et al, 2009;Peslier et al, 2010;Gross et al, 2011). These samples record magmatic processes that occurred over a large range of pressure and provide constraints on the depth and composition of their mantle sources and the shallower conditions of crystallization.…”

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

“…Some shergottites are believed to represent primitive melts (Gross et al, 2013(Gross et al, , 2011Musselwhite et al, 2006;Peslier et al, 2010). However, they exhibit a significant depletion in aluminum when compared to virtually all other analyses of martian rocks (McSween et al, 2009).…”

“…To produce this level of heterogeneity from the PMM, parts of the mantle must have been affected by more than 50 wt.% melting during one or several events. However, such refractory reservoirs are expected to produce melts with lower concentration of incompatible elements relative to the primitive depleted shergottites (Gross et al, 2011;Usui et al, 2008). The crystallization of a magma ocean might more easily produce mantle sources with elevated Mg# (Elkins-Tanton et al, 2003).…”