In order to validate calculated ages of the Martian crust we require precise radiometric dates from igneous rocks where their provenance on the Martian surface is known. Martian meteorites have been dated precisely, but the launch sites are currently unknown. Inferring the formation environment of a correlated suite of Martian meteorites can constrain the nature and complexity of the volcanic system they formed from. The nakhlite meteorites are such a suite of augite-rich rocks that sample the basaltic crust of Mars, and as such can provide unique insights into its volcanic processes. Using electron backscatter diffraction we have determined the shape-preferred and crystallographic-preferred orientation petrofabrics of four nakhlites (Governador Valadares, Lafayette, Miller Range 03346 and Nakhla) in order to understand the conditions under which their parent rocks formed. In all samples, there is a clear link between the shape-preferred orientation (SPO) and crystallographic-preferred orientation (CPO) of augite phenocrysts. This relationship reveals the three-dimensional shape of the augite crystals using CPO as a proxy for SPO, and also enables a quantitative 3dimensional petrofabric analysis. All four nakhlites exhibit a foliation defined by the CPO of the augite axis in a plane, although individual meteorites show subtle textural variations. Nakhla and Governador Valadares display a weak CPO lineation within their axis foliation that is interpreted to have developed in a combined pure shear/simple shear flow regime, indicative of emplacement of their parent rock as a subaerial hyperbolic lava flow. By contrast, the foliation dominated CPO petrofabrics of Lafayette and Miller Range 03346 suggest formation in a pure shear dominated regime with little influence of hyperbolic flow. These CPO petrofabrics are indicative of crystal settling in the stagnant portion of cooling magma bodies, or the flattening area of spreading lava flows. The CPO foliation of Lafayette's is substantially weaker than Miller Range 03346, probably due to its higher phenocryst density causing grain-grain interactions that hindered fabric development. The CPO petrofabrics identified can also be used to determine the approximate plane of the Martian surface and the line of magma flow to within ~20°. Our results suggest that the nakhlite launch crater sampled a complex volcanic edifice that was supplied by at least three distinct magmatic systems limiting the possible locations these rocks could have originated from on Mars.

Nakhlite meteorites are ~1.4 to 1.3 Ga old igneous rocks, aqueously altered on Mars ~630 Ma ago. We test the theory that water-rock interaction was impact driven. Electron backscatter diffraction demonstrates that the meteorites Miller Range 03346 and Lafayette were heterogeneously deformed, leading to localized regions of brecciation, plastic deformation, and mechanical twinning of augite. Numerical modeling shows that the pattern of deformation is consistent with shock-generated compressive and tensile stresses. Mesostasis within shocked areas was aqueously altered to phyllosilicates, carbonates, and oxides, suggesting a genetic link between the two processes. We propose that an impact ~630 Ma ago simultaneously deformed the nakhlite parent rocks and generated liquid water by melting of permafrost. Ensuing water-rock interaction focused on shocked mesostasis with a high density of reactive sites. The nakhlite source location must have two spatially correlated craters, one ~630 Ma old and another, ejecting the meteorites, ~11 Ma ago.

The nakhlite meteorites characteristically contain iddingsite, a hydrous iron–magnesium silicate that formed by aqueous alteration on Mars. Iddingsite is most abundant in Northwest Africa (NWA) 817, and alteration products in this meteorite also have the lowest deuterium/hydrogen ratio of any nakhlite. Taken together, these distinctive properties could be interpreted to show that NWA 817 was altered under different physico‐chemical conditions than the other nakhlites and by liquid water from a separate reservoir. Here this interpretation is tested through a petrographic, mineralogical, chemical, and isotopic study of NWA 817. We find that its iddingsite occurs as olivine‐hosted veins of nanocrystalline smectite and Fe‐oxyhydroxide. Strong similarities in the mineralogy of iddingsite between NWA 817 and other nakhlites suggest that these meteorites were altered under comparable physico‐chemical conditions, with the Fe‐rich composition of NWA 817 olivine grains rendering them especially susceptible to aqueous alteration. Analyses of NWA 817 bulk samples by stepwise pyrolysis confirm that its iddingsite has unusually low deuterium/hydrogen ratios, but owing to terrestrial weathering of this meteorite, the hydrogen isotopic data cannot be used with confidence to infer the origin of Martian aqueous solutions. NWA 817 was most probably altered along with the other nakhlites over a short time period and in a common aqueous system. One interpretation of a correlation between the eruption ages of three of the nakhlites and the chemical composition of their iddingsite is that water originated from close to the surface of Mars and flowed through the nakhlite lava pile under the influence of gravity.

The upcoming Mars Sample Return (MSR) mission aims to deliver small quantities of Martian rocks to the Earth. Investigating these precious samples requires the development and application of techniques that can extract the greatest amount of high quality data from the minimum sample volume, thereby maximising science return from MSR. Atom probe tomography (APT) and transmission electron microscopy (TEM) are two complementary techniques that can obtain nanoscale structural, geochemical and, in the case of atom probe, isotopic information from small sample volumes. Here we describe how both techniques operate, as well as review recent developments in sample preparation protocols. We also outline how APT has been successfully applied to extraterrestrial materials in the recent past. Finally, we describe how we have studied Martian meteorites using TEM and APT in close coordination in order to characterise the products of water/rock interactions in t h e cru st of Ma r s – a k ey sc ie n ce goal of MSR. Our results provide new insights into the Martian hydrosphere and the mechanisms of anhydrous-hydrous mineral replacement. In light of the unique results provided by these tools, APT and TEM should form a crucial part at the culmination of a correlative analytical pipeline for MSR mission materials.

The Martian nakhlite meteorites, which represent multiple events that belong to a single magma source region represent a key opportunity to study the evolution of Martian petrogenesis. Here 16 of the 26 identified nakhlite specimens are studied using coupled electron backscatter diffraction (EBSD) and emplacement end‐member calculations. EBSD was used to determine shape preferred orientation of contained augite (high Ca‐clinopyroxene) phenocrysts by considering their crystallographic preferred orientation (CPO). Parameters derived from EBSD, and energy dispersive X‐ray spectroscopy spectra were used in basic emplacement models to assess their dominant mechanism against three end‐member scenarios: thermal diffusion, crystal settling, and crystal convection. Results from CPO analyses indicate low intensity weak‐moderate CPO. In all samples, a consistent foliation within the <001> axes of augite are observed typically coupled with a weaker lineation CPO in one of the other crystallographic axes. These CPO results agree best with crystal settling being the dominant emplacement mechanism for the nakhlites. Modeled crystal settling results identify two distinguishable groups outside of the model's resolution indicating the presence of secondary emplacement mechanisms. Comparison of the two identified groups against CPO, geochemical, and age parameters indicate random variability between individual meteorites. Therefore, coupled CPO and emplacement modeling results identify an overarching characteristic of a dominant crystal settling emplacement mechanism for the nakhlite source volcano despite exhibiting random variation with each discharge through time.