X-ray and electron microanalyses of a martian breccia and comet 81p/wild 2 samples

X-ray spectroscopy and electron microscopy have been used to study a martian breccia meteorite and grains from Comet 81P/Wild 2 retrieved by NASA’s Stardust mission. Martian meteorite Northwest Africa (NWA) 8114 (paired with NWA 7034) allows examination of the thermal history of a regolith near an impact crater on Mars. Transmission electron microscopy (TEM) reveals that some pyroxene clasts are porous and have partially re-crystallised, forming magnetite and a K-bearing feldspathic glassy material with relict pyroxene. Fe-K X-ray absorption spectroscopy (XAS) shows the pyroxene to be oxidised, with up to 25% Fe3+/ΣFe. A partially re-crystallised augite clast gave a 40Ar-39Ar maximum age of 1.13 Ga to 1.25 Ga, inferred to correspond to the impact heating event causing pyroxene breakdown, being held above 700 °C for at least seven days in an oxidising regolith environment. A Fourier cooling model indicates that a regolith of at least five metres depth would provide these temperatures. Low temperature hydrous alteration formed goethite, identified with X-ray diffraction (XRD), XAS and Fourier transform infrared spectroscopy (FTIR). Fe-K XAS and XRD identified magnetite and olivine in terminal grains 1 and 2 respectively of Stardust track #187 and provisionally iron sulphide, magnetite and pyroxene in tracks #188, #189 and #190. Magnetite provides evidence for aqueous alteration on a cometary parent body. The same preparation techniques with carbonaceous chondrites yielded the identification of two magnetite and three olivine particles, suggesting dense magnetite is preferentially preserved during capture. TEM analysis of Stardust terminal grains identifies chondrule-like fragments with pyroxenes, olivines and feldspars, and iron sulphides that were originally pyrrhotite, but have lost volatile sulphur due to heating during the capture process. The combination of XRD, FTIR and Fe-K XAS are a powerful non-destructive method for mineralogical identification on a micron scale for small planetary samples.