Volatile loss following cooling and accretion of the Moon revealed by chromium isotopes

Abstract

SignificanceWith the exception of volatile elements, which are strongly depleted and isotopically fractionated, the Moon has chemical and isotopic signatures that are indistinguishable from Earth’s mantle. Reconciliation of these properties with Moon formation in a high-energy giant impact invokes evaporative loss of volatile elements, but at conditions that are poorly known. Chromium isotopic fractionation is sensitive to temperature variations and liquid–gas equilibration during evaporation. We measure an isotopic difference between Earth’s mantle and the Moon, consistent with the loss of a Cr-bearing, oxidized vapor phase in equilibrium with the proto-Moon. Temperatures of vapor loss required are much lower than predicted by recent models, implying that volatile elements were removed from the Moon following cooling rather than during a giant impact.

High‐Precision Measurement of Stable Cr Isotopes in Geological Reference Materials by a Double‐Spike TIMS Method

“…Recently, studies have found that partial melting of the mantle may cause a small but detectable (~ 0.4‰ of δ 53/52 Cr NIST SRM 979 ) stable Cr isotopic fractionation (Schoenberg et al , Xia et al ). The oxygen fugacity variation may dominate the mass‐dependent chromium stable isotope fractionation in high‐temperature processes (Bonnand and Halliday , Shen et al , Sossi et al ). Thus, stable Cr isotopic fractionation may be observed between mantle and basalts, given the different Cr(II)/Cr(III) ratios of mantle melts and basaltic melts (Schoenberg et al ).…”

“…Recently, Xia et al () suggested that the δ 53/52 Cr NIST SRM 979 values of fresh fertile peridotites are −0.14 ± 0.12‰ on average and the variation of δ 53/52 Cr NIST SRM 979 values of mantle peridotites is caused by partial melting. The δ 53/52 Cr NIST SRM 979 value of BSE was given as −0.11 ± 0.11‰ by analysing komatiites (Sossi et al ). For the planetary processes, Moynier et al () suggested that there could be Cr isotopic fractionation during core segregation, as the δ 53/52 Cr NIST SRM 979 values of meteorites (− 0.2‰ to ∼ − 0.4‰) are lighter than that of the BSE.…”

“…Chromium is typically present in nature in three valence states -Cr(II), Cr(III) and Cr(VI). Hexavalent chromium Cr(VI), as either a chromate (alkaline pH) (CrO 4 2-) or a hydrogen chromate (acidic pH) ion (HCrO 4 analysing komatiites (Sossi et al 2018). For the planetary processes, Moynier et al (2011) suggested that there could be Cr isotopic fractionation during core segregation, as the d 53/ 52 Cr NIST SRM 979 values of meteorites (-0.2‰ to -0.4‰) are lighter than that of the BSE.…”

Factors affecting chromium isotope measurements using the double‐spike method

“…Recent studies did not find any Cr isotope difference between chondrites and BSE. 20,21 The very recent study of Sossi et al 22 In contrast, the double-spike technique has several advantages over the SSB method 17,[27][28][29] : (1) it can monitor and correct for the short-term drift in mass fractionation in the mass spectrometer; (2) it can minimize the effects of the difference in matrix composition between the sample and the standard; and (3) by adding the double spike before the sample preparation, mass fractionation caused by the purifying procedure can also be corrected even when the recovery of Cr is not 100%. Because mass-dependent isotopic fractionation occurs during mass spectrometry measurements, the "true" isotopic composition of the double spike cannot be measured directly.…”

“…Recent studies did not find any Cr isotope difference between chondrites and BSE . The very recent study of Sossi et al confirms Cr isotope fractionation between the Earth and the Moon, and attributes it to the Cr partitioning into an oxygen‐rich vapor phase in equilibrium with the proto‐moon.…”