Suporn Boonsue

Shock-Induced Phase Transformations in Melt Pockets within Martian Meteorite NWA 4468

ABSTRACT Northwest Africa (NWA) 4468 is a 675 g Martian meteorite recovered from the Western Sahara in 2006. It is an olivine basaltic shergottite, comprising clinopyroxene, olivine, maskelynite, chromite, ilmenite, Ca-phosphate polymorphs, troilite, and pyrrhotite. The sample shows both bulk (maskelynite formation) and localized shock effects (pressure-temperature excursions in veins and melt pockets). Here we report the presence of high-pressure polymorphs in melt pockets. We have discovered three phosphate polymorphs in the melt pockets (chlorapatite, merrillite, and tuite). Their crystal structure has been confirmed by Raman analysis. The mineral assemblages of the melt pockets are indicative of solid-state transformations, nucleation, and recrystallization at high temperatures and pressures. The stability fields of these minerals are used to infer the pressure and temperature conditions reached locally in the melt pockets as 23 GPa and 2000°C.

Shock-induced kelyphite formation in the core of a complex impact crater

We present a compositional and textural analysis of shock-induced microtextures in garnet porphyroblasts in migmatitic garnet–cordierite–biotite paragneisses from the centre of the Vredefort impact structure, South Africa. Detailed imaging and major element analysis of deformation features in, and adjacent to, the garnet porphyroblasts record a complex, heterogeneous distribution of shock effects at the microscale. As the most competent silicate mineral in the assemblage, with the highest Hugoniot Elastic Limit and a wide pressure–temperature stability field, the porphyroblastic garnet preserves a more diverse shock deformation response compared to minerals such as quartz and feldspar, which underwent more comprehensive shock metamorphism and subsequent annealing. The garnet porphyroblasts display pre-impact fractures that are overprinted by later intra-granular Hertzian and distinctive planar fractures associated with the impact event. Shock-induced strain localization occurred along internal slip planes and defects, including pre-existing fractures and inclusion boundaries in the garnet. Symplectitic (kelyphitic) coronas commonly enclose the garnet porphyroblasts, and inhabit intra-granular fractures. The kelyphite assemblage in fractures with open communication beyond garnet grain boundaries is characterized by orthopyroxene—cordierite—sapphirine. Conversely, the kelyphite assemblage in closed-off intra-granular fractures is highly variable, comprising spatially restricted combinations of a secondary garnet phase with a majoritic component, Al-rich orthopyroxene, sapphirine and cordierite. The impedance contrast between garnet porphyroblasts and their inclusions further facilitated the formation of shock-induced features (Al-rich orthopyroxene coronas). Together, the textural and mineralogical data suggest that these features provide a record of oscillatory shock perturbations initiated under confining pressure beneath the transient crater floor. This occurred as the shocked rock volume underwent post-shock expansion, forming the core of the central uplift, and was followed by variable textural re-equilibration. This study thus provides a microtextural and mineralogical perspective of the shock regime within confined crust immediately prior to and during central uplift formation.

A Small Surface-Based Infrared Spectrometer for the Exploration of the Moon and Other Planetary Bodies

A concept for a small broadband Fourier transform spectrometer mounted on a surface platform such as rover for a lunar exploration mission is presented. The characteristics and performance of the instrument are given.