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.

We report a detailed method analyzing K isotopes in high-precision using Neptune Plus MC-ICP-MS in cold plasma and conduct an inter-laboratory comparison.

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Available online xxxx Editor: B. Marty Keywords: iron isotopes achondrite ureilite coreUreilite meteorites are achondrites that are debris of the mantle of a now disrupted differentiated asteroid rich in carbon. They provide a unique opportunity to study the differentiation processes of such a body. We analyzed the iron isotopic compositions of 30 samples from the Ureilite Parent Body (UPB) including 29 unbrecciated ureilites and one ureilitic trachyandesite (ALM-A) which is at present the sole large crustal sample of the UPB. The δ 56 Fe of the whole rocks fall within a restricted range, from 0.01 to 0.11h, with an average of +0.056 ± 0.008h, which is significantly higher than that of chondrites.We show that this difference can be ascribed to the segregation of S-rich metallic melts at low degrees of melting at a temperature close to the Fe-FeS eutectic, and certainly before the onset of the melting of the silicates (<1100 • C), in agreement with the marked S depletions, and the siderophile element abundances of the ureilites. These results point to an efficient segregation of S-rich metallic melts during the differentiation of small terrestrial bodies.

We present new high precision iron isotope data (d 56 Fe vs. IRMM-014 in per mil) for four groups of achondrites: one lunar meteorite, 11 martian meteorites, 32 howardite-eucrite-diogenite meteorites (HEDs), and eight angrites. Angrite meteorites are the only planetary materials, other than Earth/Moon system, significantly enriched in the heavy isotopes of Fe compared to chondrites (by an average of +0.12& in d 56 Fe). While the reason for such fractionation is not completely understood, it might be related to isotopic fractionation by volatilization during accretion or more likely magmatic differentiation in the angrite parent-body. We also report precise data on martian and HED meteorites, yielding an average d 56 Fe of 0.00 ± 0.01&. Stannern-trend eucrites are isotopically heavier by +0.05& in d 56 Fe than other eucrites. We show that this difference can be ascribed to the enrichment of heavy iron isotopes in ilmenite during igneous differentiation. Preferential dissolution of isotopically heavy ilmenite during remelting of eucritic crust could have generated the heavy iron isotope composition of Stannern-trend eucrites. This supports the view that Stannern-trend eucrites are derived from main-group eucrite source magma by assimilation of previously formed asteroidal crust.These new results show that iron isotopes are not only fractionated in terrestrial and lunar basalts, but also in two other differentiated planetary crusts. We suggest that igneous processes might be responsible for the iron isotope variations documented in planetary crusts.

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At ocean spreading ridges, circulation of seawater through rock at elevated temperatures alters the chemical and isotopic composition of oceanic crust. Samples obtained from drilling into ocean floor and from ophiolites have demonstrated that certain isotope systems, such as18O/16O and87Sr/86Sr, are systematically modified in hydrothermally altered oceanic crust. Although K is known to be mobile during hydrothermal alteration, there have not yet been any K-isotope analyses of altered oceanic crustal materials. Moreover, the41K/39K of seawater was recently found to be significantly higher than that of igneous rocks, so the addition of seawater K to oceanic crust would be expected to generate41K/39K variations in affected rocks. Here, we report high-precision41K/39K measurements for samples from the Bay of Islands ophiolite, and we document large variations in41K/39K, covarying with previous determinations of87Sr/86Sr. Our data indicate that analytically resolvable41K/39K effects arise in oceanic crust as a result of hydrothermal alteration. This finding raises the possibility that41K/39K can be used as an effective tracer of oceanic crust recycled into the mantle, as a diagnostic criterion by which to identify ancient fragments of oceanic crust, and as a constraint on the flux of K between oceanic crust and seawater.