Understanding of origin of the Main Group Pallasites from large area mineral investigations of the Sericho Meteorite

The class of meteorites known as Pallasites continue to present a riddle in terms of their origin due to their mineralogy composed of olivine grains suspended in a Fe-Ni metal matrix. It has previously been thought that this unique texture could only arise from the core mantle boundary of a planetesimal. However, recent nanoscale magnetic measurements have demonstrated that this origin is physically impossible. We are currently undertaking a systematic large area survey of multiple pallasites to understand the origin of these enigmatic meteorites. In this report we present our initial mineralogical and textural results for the Sericho meteorite, a main group pallasite. Using automated stitching we collected energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) maps across the entire polished meteorite section. Analysis of our large area surveys shows that the matrix was composed of 60% troilite and 30% kamacite. The kamacite matrix contains 100 500 m sized schreibersite crystals and 1 2 mm taenite cloudy zones. Between the kamacite and troilite regions of matrix, there are iron oxides rims that also contain few 1 3 mm schreibersite grains. A 5mm whitlockite is located between olivine crystals and in contact with kamacite and troilite. The olivine crystals embedded within this matrix are euhedral and 1 5 mm in size. The olivine crystals also contain a network of fractures that are infilled with iron oxide. The olivine crystals also exhibit Fe and Mn enriched rims where in contact with the matrix. Observations of the grain edges and distribution of olivine crystals reveals that in some cases, the shapes of some olivine crystals are complementary like a jigsaw. This suggest that the olivine grains were initially situated close together, and the kamacite was later injected along grain boundaries pushing olivine crystals apart. During injection of the matrix, the temperature was likely below the melting point of olivine as the olivine preserves a euhedral shape, the temperature likely cooled relatively quick as the olivine is out of equilibrium with the matrix evidenced by the Fe-rich rims of the olivine. After the Sericho fell to Earth, terrestrial water got into the olivine fractures rusting the metal to form iron oxide.