Understanding the emplacement of Martian volcanic rocks using petrofabrics of the nakhlite meteorites

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 and quantitatively, 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 3D SPO, and also enables a quantitative 3-dimensional petrofabric analysis. All four nakhlites exhibit a foliation defined by the CPO of the augite < c > axis in a plane, although individual meteorites show subtle textural variations. Nakhla and Governador Valadares display a weak CPO lineation within their < c > 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 similar to 20 degrees. 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. (C) 2019 Elsevier B.V. All rights reserved.