Potassium isotopic evidence for a high-energy giant impact origin of the Moon

Abstract

The Earth-Moon system has unique chemical and isotopic signatures compared with other planetary bodies; any successful model for the origin of this system therefore has to satisfy these chemical and isotopic constraints. The Moon is substantially depleted in volatile elements such as potassium compared with the Earth and the bulk solar composition, and it has long been thought to be the result of a catastrophic Moon-forming giant impact event. Volatile-element-depleted bodies such as the Moon were expected to be enriched in heavy potassium isotopes during the loss of volatiles; however such enrichment was never found. Here we report new high-precision potassium isotope data for the Earth, the Moon and chondritic meteorites. We found that the lunar rocks are significantly (>2σ) enriched in the heavy isotopes of potassium compared to the Earth and chondrites (by around 0.4 parts per thousand). The enrichment of the heavy isotope of potassium in lunar rocks compared with those of the Earth and chondrites can be best explained as the result of the incomplete condensation of a bulk silicate Earth vapour at an ambient pressure that is higher than 10 bar. We used these coupled constraints of the chemical loss and isotopic fractionation of K to compare two recent dynamic models that were used to explain the identical non-mass-dependent isotope composition of the Earth and the Moon. Our K isotope result is inconsistent with the low-energy disk equilibration model, but supports the high-energy, high-angular-momentum giant impact model for the origin of the Moon. High-precision potassium isotope data can also be used as a 'palaeo-barometer' to reveal the physical conditions during the Moon-forming event.

The Origin of the Moon Within a Terrestrial Synestia

“…Equilibration between moonlets and the vapor of the synestia would produce an isotopically heavy Moon compared to Earth, but since the equilibration occurs at relatively high pressures and temperatures, the degree of fractionation is expected to be small. Recently, a potassium isotope fractionation has been measured between lunar and terrestrial samples that is consistent with condensation at high pressures and temperatures (Wang & Jacobsen, ). Further development of our model could make predictions as to the degree of fractionation of other elements.…”

“…They also suggested that the lunar volatile element depletion could be explained if the material that formed the observable Moon was a partial condensate of disk material. Wang and Jacobsen () recently reported that the potassium isotopes of the Moon are heavier than BSE, which supports the idea of partial condensation. However, the model presented by Canup et al () does not quantitatively explain the magnitude, nor pattern, of moderately volatile element depletion observed for the Moon.…”

Lunar Cumulate Mantle Overturn: A Model Constrained by Ilmenite Rheology

“…The bulk U and Th abundances of the present day are taken as 25.7 and 102.8 ppb (Th/U = 4), respectively (e.g., Taylor, 1982). The Moon is highly depleted of the volatile element K (Albarede et al, 2014;Jones & Palme, 2000;Wang & Jacobsen, 2016). We neglect the K contribution to the thermal budget in most of our calculations.…”

The effect of ilmenite viscosity on the dynamics and evolution of an overturned lunar cumulate mantle

“…The proportion of KREEP-bearing cumulates, flowing with the IBC, is also determined by the overturn process. The Moon is highly depleted in the volatile element K [Wang and Jacobsen, 2016;Albarede et al, 2014;Jones and Palme, 2000]. The bulk U and Th abundances are used to estimate the heat generation rate in KREEP.…”

“…The bulk U and Th contents at the present-day are taken to 25.7 ppb and 102.8 ppb (Th/U = 4) [e.g., Taylor, 1982]. The Moon is highly depleted in the volatile element K [Wang and Jacobsen, 2016;Albarede et al, 2014;Jones and Palme, 2000]. Our test model shows that K content (based on K/U ratio of 2500 [Taylor, 1982]) does not affect the instability of IBC-rich layer ( Table S1 in the supporting information).…”