The Northwest Africa (NWA) 5790 meteorite: A mesostasis‐rich nakhlite with little or no Martian aqueous alteration

Tim Tomkinson,Darren F Mark,Katherine J Dobson,Martin R Lee,Ian A Franchi

doi:10.1111/maps.12424

Abstract

AbstractNorthwest Africa (NWA) 5790 is the most recently discovered member of the nakhlite group. Its mineralogy differs from the other nakhlites with a high abundance mesostasis (38.1 ± 3.6 vol%) and scarcity of olivine (4.0 ± 2.2 vol%). Furthermore, zoning of augite phenocrysts, and other petrographic and chemical characteristics suggest that NWA 5790 samples the chilled margin of its parent lava flow/sill. NWA 5790 contains calcite and rare clay minerals that are evidence for its exposure to liquid water. The calcite forms a cement to coatings of dust on the outer surface of the find and extends into the interior of the meteorite within veins. The presence of microbial remains within the coating confirms that the dust and its carbonate cement are terrestrial in origin, consistent with the carbon and oxygen isotope composition of the calcite. The clay minerals are finely crystalline and comprise ~0.003 vol% of the meteorite. δD values of the clay minerals range from −212 ± 109‰ to −96 ± 132‰, and cannot be used to distinguish between a terrestrial or Martian origin. As petrographic results are also not definitive, we conclude that secondary minerals produced by Martian groundwaters are at best very rare within NWA 5790. The meteorite has therefore sampled a region of the lava flow/sill with little or no exposure to the aqueous solutions that altered other nakhlites. This isolation could relate to the scarcity of olivine in NWA 5790 because dissolution of olivine in other nakhlites by Martian groundwaters enhanced their porosity and permeability, and provided solutes for secondary minerals.

Highlights

The meteorites Allan Hills (ALH) 84001, Northwest Africa (NWA) 7034, and the SNC clan are samples of the crust of Mars, and together provide unique insights into the planet’s evolution (e.g., Treiman et al 2000; Treiman 2005; McSween et al 2009; Basu Sarbadhikari et al 2011; Agee et al 2013; Humayun et al 2013; Cartwright et al 2014)
Previous work on NWA 5790 and our own observations show that it is dominated by hundreds of micrometer to a few millimeter size grains of augite, olivine, and titanomagnetite, between which is the mesostasis (Figs. 2a and 2b)
The polished block was point counted three times, and the values for modal mineralogy obtained are close to the mean of the X-ray computed tomography (XCT) slices (Table 1)

Summary

Introduction

The meteorites Allan Hills (ALH) 84001, NWA 7034 (paired with NWA 7533), and the SNC clan (shergotittes, nakhlites, and chassignites) are samples of the crust of Mars, and together provide unique insights into the planet’s evolution (e.g., Treiman et al 2000; Treiman 2005; McSween et al 2009; Basu Sarbadhikari et al 2011; Agee et al 2013; Humayun et al 2013; Cartwright et al 2014). NWA 5790 was recovered from the Sahara desert in 2009, and shortly thereafter recognized as the eighth nakhlite (Jambon et al 2010; Weisberg et al 2010); it is possibly paired.

Methods

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Discussion

Conclusion