Fig. 2.
The fraction of Cr remaining in the bulk silicate Moon relative to the BSE, fCr, as a function of the δ53/52Cr isotopic difference between the Moon and BSE, Δ53CrMoon-Earth. The curves correspond to values of fCr and Δ53CrMoon-Earth calculated by liquid–vapor equilibrium between CrO2(g) and CrO(l) at various temperatures (1,500 K, blue; 1,600 K, green; 1,700 K, yellow; and 1,800 K, red) and oxygen fugacities (dashed lines, ΔFMQ+1, ΔFMQ, and ΔFMQ-1). Values of fCr are calculated from Eq. 2 and depend on temperature, pressure, and fO2, where the total pressure is that in the gas above a BSE composition (13). Values of Δ53CrMoon-Earth are given by Eq. 4. The dark gray field demarcates the permissible range of fCr and Δ53CrMoon-Earth defined by sample data; curves passing through this field satisfy both constraints.