Gabbroic Shergottite Northwest Africa 6963: An intrusive sample of Mars

Abstract

Meteorite Northwest Africa (NWA) 6963 was classified as a basaltic shergottite based on mineralogy, but here we show that it is a gabbroic rock with a quartz-alkali feldspar intergrowth that represents a late-stage granitic melt. NWA 6963 contains clinopyroxene and maskelynite grains up to 5 mm in length, with minor ferroan olivine, spinel, ilmenite, merrillite, apatite, Fe-sulfides, and high-Si glass. NWA 6963 also contains areas of quartz and alkali-feldspar intergrowths up to ~1 mm in size. Based on mineral abundances and textural analysis, we suggest that NWA 6963 is an intrusive rock similar to a terrestrial gabbro. Infiltration of the martian crust by young gabbroic bodies would suggest that estimates of crustal composition, density, and thickness based on the surface chemistry alone would be problematic and the martian crust may be even more heterogenous than is seen from orbit alone. Investigations of crater walls, where intrusive crustal rocks would be exposed, are needed to discover the launch sites of the shergottites and the full heterogeneity of the martian crust.

Formation of Evolved Rocks at Gale Crater by Crystal Fractionation and Implications for Mars Crustal Composition

“…Our results show that, from the different starting melt compositions, fractionation of orthopyroxene, plagioclase, and diopside as well as some minor olivine can lead to the felsic compositions (Mg# = 19–38) found in Gale crater during the first 800 sols (Table ). The modeling presented in this study also agrees with studies on the NWA 6963 meteorite, the only shergottite yielding granitic compositions consisting of quartz, plagioclase, and K‐feldspar (Filiberto et al, ; Gross & Filiberto, ). These authors suggested that these highly felsic compositions likely formed in pockets through fractional crystallization of a basaltic magma with NWA 6963 composition.…”

Shergottite Northwest Africa 6963: A Pyroxene‐Cumulate Martian Gabbro

“…More specifically, NWA 6963 was originally classified as a basaltic shergottite based on similarities of the mineralogy, chemistry, and textures with Shergotty—the type specimen of basaltic shergottites. Additional preliminary investigations into the textures of the rock led to resubclassification of the rock as a gabbro because of similarity of the textures and crystal sizes to those of terrestrial and lunar gabbros (Filiberto et al, ).…”

“…For this study we used the same 1.201 g part slice of the meteorite from Martin Altmann and Stefan Ralew of Chladni's Heirs and cut and polished a section of it into a thick section (Filiberto et al, ). The same thick section from our previous study was used for the mineral chemical analyses, shape‐preferred orientation (SPO), crystal size distribution (CSD), and spatial distribution patterns (SDP).…”

“…However, original investigations of basaltic shergottites suggested that their textures were consistent with terrestrial diabases (e.g., Brearley, ; Smith & Hervig, ). Further, a recent preliminary investigation into the textures of NWA 6963 (originally classified as a basaltic shergottite) has suggested that basaltic shergottites can be gabbroic in nature and represent intrusive magmas (Filiberto et al, ). Another shergottite, NWA 7320, with a gabbroic texture including accumulated plagioclase has recently been discovered as well (Udry et al, ).…”

“…The original investigation of NWA 6963 was largely a qualitative comparison of grain sizes and shapes between NWA 6963 and terrestrial and lunar gabbros (Filiberto et al, ) and did not provide mineral or bulk chemistry, quantitative textural analyses, or spectroscopic measurements. Therefore, we combine mineralogy, petrology, quantitative textural analyses pyroxenes, and spectral properties of gabbroic shergottite NWA 6963 in the current study to constrain whether it formed in a near‐surface flow or crystallized at greater depth as a gabbroic or hypabyssal intrusion.…”