Trace element volatility and the conditions of liquid-vapor separation in the proto-lunar disk

Abstract

The Moon is thought to have formed from material ejected by a giant impact that took place at the end of Earth's accretion. The material ejected to space generated a large hot structure where material beyond the Roche limit accreted to form the Moon. It has long been known that the Moon is characterized by abundances in moderately volatile elements (MVE) lower than that of the Earth, while more recent studies have established that the concentrations in refractory elements are similar to the bulk Silicate Earth. The thermodynamic conditions that prevailed after this impact are poorly known and understanding the origin of the Moon-Earth differences in MVE requires a knowledge of the volatility of elements under these conditions. In this study, we reexamine the volatility of a large set of geochemically relevant elements and attempt to determine the P-T conditions under which volatiles were putatively separated from the liquid material. Our model predicts very different condensation temperatures due to higher pressures, compared with the conditions of the Solar Nebula and we extend the values of these temperatures to a wide number of trace elements (Se, Ag, Pt, Mo, W, Zn, Sn, Sb, Rb, Cs, U, Th, Cr, Ni, Co, Ga, Ge, Cu, and P). Our modeling shows that the observed lunar compositions cannot be explained by a single set of P and T conditions. Rather, it is best explained by a mixture between high-temperature condensates (~4000 K) and low temperature condensates (2000-2500 K). An important constraint is that for the low temperature condensates, liquid metal must have been stable and this is crucial for matching the abundance of volatile siderophile elements in the bulk Moon.