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

Abstract

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 <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 ~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.

Caleta el Cobre 022 Martian meteorite: Increasing nakhlite diversity

“…Also, the mesostasis shows alteration into iddingsite at the micrometer scale (Fig. 15), which has also been observed in other nakhlites, but rarely studied compared to iddingsite veins in olivine (Treiman 2005;Daly et al 2019). The sulfide and oxide stringers, found inside iddingsite veins cross-cutting olivine, indicate that the fluid responsible for this alteration event on Mars could also have transported reduced sulfur inside the nakhlites, after completion of igneous crystallization of CeC 022.…”

“…Indeed, nakhlites' final cooling would have taken place at the Martian surface over short periods of time from 5 to 184 days (Day et al 2006;Udry and Day 2018), with cooling rates from 1 to 6°C h À1 based on zoning in olivine (Sautter et al 2002) and intercumulus crystals' surface area to volume (Hammer and Rutherford 2005). After emplacement as sill or flow, crystal gravitational settling and compaction, as well as dynamical sorting of a phenocryst-rich magmatic mush (Day et al 2006;Corrigan et al 2015) and flow dynamics (Daly et al 2019) have shaped the final texture of nakhlites, based on petrographic textural evidence.…”

Boom boom pow: Shock-facilitated aqueous alteration and evidence for two shock events in the Martian nakhlite meteorites

“…1 and 2). The deformed regions lie on the plane of the <c> axis girdle distribution reported in the same meteorites by Daly et al ( 4 ), which was interpreted as a foliation derived from a gravity-driven crystal settling in the parent magmatic body.…”

“…The blue polygons indicate the location of areas of altered mesostasis, which also correlate with deformed regions in (B). The red dashed line is the orientation of the foliation described by Daly et al ( 4 ). The white box and labels indicate the sites where high-resolution backscattered electron (BSE) images and energy dispersive x-ray spectroscopy (EDS) images were acquired for Fig.…”

“…They typically comprise elongate, subhedral to euhedral prisms of augite (68 to 81 volume %), phenocrysts of olivine (3 to 17 volume %), and an interstitial fine-grained mesostasis (8 to 22 volume %) ( 1 ). A distinct foliation and lineation petrofabric are defined by the pyroxene phenocrysts ( 2 – 4 ). Crystallization ages of the nakhlites span 93 Ma, from ~1416 to 1322 Ma ago, and indicate that they were emplaced in at least four magmatic events ( 5 ).…”