Wulong: A New Main Group Pallasite

Abstract

As the fragments of differentiated asteroids in the early solar system, pallasites record deep-mantle and core processes. As such, studying the origin of pallasites could help us understand the formation and differentiation processes governing the evolution of planets. Wulong pallasite was found in Inner Mongolia, China around 1992, but didn't get much attention until 2020. In this study, we reported the detailed petrography and mineralogy, oxygen isotopic composition, and metal phase chemistry of Wulong pallasite. Wulong pallasite is mainly composed of olivine ($\sim$ 72.7 vol.%), Fe-Ni metal ($\sim$ 26.2 vol.%), troilite ($\sim$ 0.6 vol.%), schreibersite ($\sim$ 0.4 vol.%), and chromite ($\sim$ 0.1 vol.%) etc. Olivine (Fa${}_{12.5\pm 0.2}$) and chromite (Al$\#$$\sim$15.2 $\pm$ 0.9, Fe$\#$$\sim$ 67.9 $\pm$ 1.5) compositions are homogeneous, and no chemical zoning is observed. Siderophile element contents of a metal phase are Ni, 7.71 wt.%; Ga, 17.8 $\mu$g/g; and Ge, 26.2 $\mu$g/g. The mean $\delta$${}^{18}$O value is 3.04 $\pm$ 0.01‰, $\Delta$${}^{17}$O value is $-$0.18 $\pm$ 0.01‰. In terms of the Fa value of olivine, metal phase chemistry, and oxygen isotope ratios, Wulong could be a new member of main group pallasite.