Localized shock melting in lherzolitic shergottite Northwest Africa 1950: Comparison with Allan Hills 77005

Abstract

The lherzolitic Martian meteorite Northwest Africa (NWA) 1950 consists of two distinct zones: 1) low-Ca pyroxene poikilically enclosing cumulate olivine (Fo70-75) and chromite, and 2) areas interstitial to the oikocrysts comprised of maskelynite, low- and high-Ca pyroxene, cumulate olivine (Fo68-71) and chromite. Shock metamorphic effects, most likely associated with ejection from the Martian subsurface by large-scale impact, include mechanical deformation of host rock olivine and pyroxene, transformation of plagioclase to maskelynite, and localized melting (pockets and veins). These shock effects indicate that NWA 1950 experienced an equilibration shock pressure of 35-45 GPa. Large (millimeter-size) melt pockets have crystallized magnesian olivine (Fo78-87) and chromite, embedded in an Fe-rich, Al-poor basaltic to picro-basaltic glass. Within the melt pockets strong thermal gradients (minimum 1 C/μm) existed at the onset of crystallization, giving rise to a heterogeneous distribution of nucleation sites, resulting in gradational textures of olivine and chromite. Dendritic and skeletal olivine, crystallized in the melt pocket center, has a nucleation density (1.0 x 10^3 crystals/mm^2) that is two orders of magnitude lower than olivine euhedra near the melt margin (1.6 x  10^5 crystals/mm^2). Based on petrography and minor element abundances, melt pocket formation occurred by in situ melting of host rock constituents by shock, as opposed to melt injected into the lherzolitic target. Despite a common origin, NWA 1950 is shocked to a lesser extent compared to Allan Hills (ALH) 77005 (45-55 GPa). Assuming ejection in a single shock event by spallation, this places NWA 1950 near to ALH 77005, but at a shallower depth within the Martian subsurface. Extensive shock melt networks, the interconnectivity between melt pockets, and the ubiquitous presence of highly vesiculated plagioclase glass in ALH 77005 suggests that this meteorite may be transitional between discreet shock melting and bulk rock melting.