Oxygen isotopic evidence for accretion of Earth's water before a high-energy Moon-forming giant impact

The Earth-Moon system likely formed as a result of a collision between two large planetary objects. Debate about their relative masses, the impact energy involved, and the extent of isotopic homogenization continues. We present the results of a high-precision oxygen isotope study of an extensive suite of lunar and terrestrial samples. We demonstrate that lunar rocks and terrestrial basalts show a 3 to 4 ppm (parts per million), statistically resolvable, difference in D17O. Taking aubrite meteorites as a candidate impactor material, we show that the giant impact scenario involved nearly complete mixing between the target and impactor. Alternatively, the degree of similarity between the D17O values of the impactor and the proto-Earth must have been significantly closer than that between Earth and aubrites. If the Earth-Moon system evolved from an initially highly vaporized and isotopically homogenized state, as indicated by recent dynamical models, then the terrestrial basalt-lunar oxygen isotope difference detected by our study may be a reflection of post-giant impact additions to Earth. On the basis of this assumption, our data indicate that post-giant impact additions to Earth could have contributed between 5 and 30% of Earth's water, depending on global water estimates. Consequently, our data indicate that the bulk of Earth's water was accreted before the giant impact and not later, as often proposed

An oxygen isotope study of Wark–Lovering rims on type A CAIs in primitive carbonaceous chondrites

Calcium–aluminium-rich Inclusions(CAIs) and the thin Wark–Lovering (WL) rims of minerals surrounding them offer a record of the nature of changing conditions during the earliest stages of Solar System formation. Considerable heterogeneity in the gas composition in the immediate vicinity of the proto-Sun had previously been inferred from oxygen isotopic variations in the WL rim of a CAI from Allende (Simon et al., 2011). However, high precision and high spatial resolution oxygen isotope measurements presented in this study show that WL rim and pristine core minerals of individual CAIs from meteorites that had experienced only low degrees of alteration or low grade metamorphism (one from Léoville (reduced CV3), two in QUE 99177 (CR3.0) and two in ALHA 77307 (CO3.0)) are uniformly 16O-rich. This indicates that the previously observed variations are the result of secondary processes, most likely on the asteroid parent body, and that there were no temporal or spatial variations in oxygen isotopic composition during CAI and WL rim formation. Such homogeneity across three groups of carbonaceous chondrites lends further support for a common origin for the CAIs in all chondrites. 16O-poor oxygen reservoirs such as those associated with chondrule formation, were probably generated by UV photo-dissociation involving self-shielding mechanisms and must have occurred elsewhere in outer regions of the solar accretion disk

One of the earliest refractory inclusions and its implications for solar system history

A ∼175 µm refractory inclusion, A-COR-01 from one of the least altered carbonaceous chondrites, ALHA 77307 (CO3.0), has been found to bear unique characteristics that indicate that it is one of the first solids to have formed at the very birth of the solar system while isotopic reservoirs were still evolving rapidly. Its core is composed mainly of hibonite and corundum, the two phases predicted to condense first from a gas of solar composition, and like many common types of Calcium-, Aluminium-rich Inclusions (CAIs) is surrounded by a rim of diopside. Core minerals in A-COR-01 are very 16O-rich (Δ17OCore = -32.5 ± 3.3 (2SD) ‰) while those in the rim display an O isotopic composition (Δ17ORim = -24.8 ± 0.5 (2SD) ‰) indistinguishable from that found in the vast majority of the least altered CAIs. These observations indicate that this CAI formed in a very 16O-rich reservoir and either recorded the subsequent evolution of this reservoir or the transit to another reservoir. The origin of A-COR-01in a primitive reservoir is consistent with the very low content of excess of radiogenic 26Mg in its core minerals corresponding to the inferred initial 26Al/27Al ratio ((26Al/27Al)0 = (1.67 ± 0.31) × 10-7), supporting a very early formation before injection and/or homogenisation of 26Al in the protoplanetary disk. Possible reservoir evolution and short-lived radionuclide (SLRs) injection scenarios are discussed and it is suggested that the observed isotope composition resulted from mixing of a previously un-observed early reservoir with the rest of the disk

The origin of water in the primitive Moon as revealed by the lunar highlands samples

The recent discoveries of hydrogen (H) bearing species on the lunar surface and in samples derived from the lunar interior have necessitated a paradigm shift in our understanding of the water inventory of the Moon, which was previously considered to be a ‘bone-dry’ planetary body. Most sample-based studies have focused on assessing the water contents of the younger mare basalts and pyroclastic glasses, which are partial-melting products of the lunar mantle. In contrast, little attention has been paid to the inventory and source(s) of water in the lunar highlands rocks which are some of the oldest and most pristine materials available for laboratory investigations, and that have the potential to reveal the original history of water in the Earth–Moon system. Here, we report in-situ measurements of hydroxyl (OH) content and H isotopic composition of the mineral apatite from four lunar highlands samples (two norites, a troctolite, and a granite clast) collected during the Apollo missions. Apart from troctolite in which the measured OH contents in apatite are close to our analytical detection limit and its H isotopic composition appears to be severely compromised by secondary processes, we have measured up to ~2200 ppm OH in the granite clast with a weighted average δD of ~-105±130‰, and up to ~3400 ppm OH in the two norites (77215 and 78235) with weighted average δD values of -281±49‰ and -27±98‰, respectively. The apatites in the granite clast and the norites are characterised by higher OH contents than have been reported so far for highlands samples, and have H isotopic compositions similar to those of terrestrial materials and some carbonaceous chondrites, providing one of the strongest pieces of evidence yet for a common origin for water in the Earth–Moon system. In addition, the presence of water, of terrestrial affinity, in some samples of the earliest-formed lunar crust suggests that either primordial terrestrial water survived the aftermath of the putative impact-origin of the Moon or water was added to the Earth–Moon system by a common source immediately after the accretion of the Moon

Insight into the silicate and organic reservoirs of the comet forming region

Cometary interplanetary dust particles (IDPs), collected in Earth’s stratosphere, currently represent the best way to sample outer Solar System primordial dust. The fine-grained (sub-μm) minerals of IDPs show some strong similarities to the textures expected for primary condensates from the solar nebula. In this study we have analysed a set of IDPs for combined bulk carbon, nitrogen and hydrogen isotopes, and high precision oxygen isotopes by NanoSIMS 50L. This study concentrates on combining isotopic analyses, including high precision oxygen, on the silicate and organic components within the same sample on fine-grained primordial materials. Oxygen isotope analyses reveal that some IDPs are more 16O-rich than any bulk meteorite compositions, extending to O isotope compositions in between chondritic- and solar-like values (δ17O = -20‰, δ18O = -20‰). The 16O-rich IDPs display more primitive organic signatures than the chondritic-like 16O-poor IDPs but, rather interestingly, they also have lower presolar grain abundances. The 16O-poor signatures probably indicate an abundant component of chondritic-like material, processed in the inner protoplanetary disk. In order to explain the association of relatively processed material (silicates and organics) with high presolar grain abundances we propose a model whereby the initial dust component is 16O-rich (solar-like) and originally present homogenously throughout the protoplanetary disk. Outward radial transport, potentially associated with aerodynamic sorting, led to an influx of processed silicates from the inner Solar System with chondritic-like isotopic signatures. Comet accretion that occurred late, or at smaller heliocentric distance, included higher abundances of this chondritic-like component and hence had more 16O-poor O isotope compositions. Presolar grains, whose extreme isotopic signatures were destroyed during early homogenisation of solar nebula dust, continue to accrete onto the protoplanetary disk with time such that the later-formed comets, those with a higher proportion of processed silicates within their source, also had time to accumulate a higher abundance of presolar grains. A similar pattern is observed in the organics, with decreasing δD and C/H, from increased exchange with nebula gas, in IDPs that contain more 16O-poor chondritic-like material

Investigating Raman variation across large cluster interplanetary dust particles

Interplanetary dust particles (IDPs) collected in the stratosphere are dust from comets and asteroids that have arrived in the Earth’s stratosphere via Poynting-Robertson effect after being ejected from their parent body. Chondritic porous IDPs display a range of features indicative of a very primitive nature and are generally believed to originate from comets. While our knowledge is somewhat limited about the internal and near-surface processes affecting non-ice materials within a cometary body, those processes that have occurred will have been different, and probably less pervasive compared to those occurring on the asteroidal meteorite parent bodies. The abundant organic material present in primitive IDPs may be the result of formation processes that occurred across a large volume of the protoplanetary disk. However, the study of organic material in particles only a few to a few tens of microns across is challenging. Laser Raman microscopy offers a rapid and potentially non-destructive approach for determining some important general characteristics of the organic matter in IDPs. While a number of studies have been conducted to date, the number of IDPs analysed remains relatively small (a few dozen) – particularly when the number of potential parent bodies is considered or the huge volume of the protoplanetary disk that may have contributed to the formation of comets. Nine IDPs were selected at the Cosmic Dust Laboratory at Johnson Space Center from five large cluster particles on collectors L2005 and L2006. Cluster particles represent large IDPs that broke into smaller pieces on impact with the collector. These particles can be >100 μm across but the individual particles within them are typically 5-15 μm in size. This offers the opportunity to investigate the variability of the Raman signature of the organic matter on a number of scales. This study is the first part of a larger, on-going project integrating the Raman, mineralogy (ASEM) and isotopic signatures (NanoSIMS) of the particles

Helium isotopes in early Iceland plume picrites: constraints on the composition of high ³He/⁴He mantle

A detailed study of the geochemistry of a new suite of early Iceland plume picrites shows that extremely high 3He/4He ratios (up to 50 Ra) are found in picrites from Baffin Island and West Greenland. High 3He/4He picrites display a wide range in 87Sr/86Sr (0.70288–0.70403), 143Nd/144Nd (0.51288–0.51308) and incompatible trace element ratios (e.g. La/Smn = 0.5–1.6). These overlap the complete range of compositions of mid-ocean ridge basalts and most northern hemisphere ocean island basalts, including Iceland. Crustal contamination modelling in which high-grade Proterozoic crustal basement rocks for the region are mixed with a depleted parent cannot account for the trend displayed by the Baffin Island and West Greenland picrites. This rules out the possibility that the incompatible trace element, Sr and Nd isotope range of the high 3He/4He picrites is due to crustal contamination. The compositional range at high 3He/4He is also inconsistent with derivation from a primordial-He-rich reservoir that is a residue of ancient mantle depletion. This implies that the composition of the high 3He/4He mantle cannot be determined simply by extrapolating ocean island basalt He–Sr–Nd–Pb–Os isotope data. The apparent decoupling of He from trace element and lithophile radiogenic isotope tracers is difficult to attain by simple mixing of a high-[He], high 3He/4He reservoir with various depleted and enriched He-poor mantle reservoirs. The possibility that primordial He has diffused into a reservoir with a composition typical of convecting upper mantle cannot be ruled out. If so, the process must have occurred after the development of existing mantle heterogeneity, and requires the existence of a deep, primordial He-rich reservoir

Using apatite to unravel the origin of water in ancient Moon rocks

There have been a limited number of studies investigating the hydrogen isotopic composition of water in samples representing the lunar highlands. This is surprising considering highlands lithologies comprise a large proportion of the returned Apollo samples and are some of the oldest and most pristine samples of the Moon. As such they potentially hold the geological record of the earliest water in the Moon and may ultimately help us decipher the origin of lunar water. We have investigated the δD-H2O systematics of apatite in an Apollo 14 and four Apollo 17 rocks using a Cameca NanoSIMS 50L ion probe. The data were corrected for the contribution of background H2O, its associated D/H ratio, and for spallation effects. Our results indicate that apatites in Apollo 17 troctolite (76535) and granulite (79215) do not preserve magmatic δD-H2O characteristics. Instead, they seem to have recorded the volatile compositions of various metasomatic alteration agents. In the case of the troctolite, metasomatism has likely altered apatite to merrillite, whereas, in the case of the granulite, merrillite has been altered to produce secondary apatite. Consequently, apatite in these samples is not a useful tracer of the original source of lunar interior water. Granite 14303 and two norites (77215 and 78235) collectively display a range in apatite H2O content from 700 to ~ 2000 ppm, and a weighted average δD of -160 ± 74 ‰. After careful consideration of the potential secondary processes that may have altered the indigenous δD signature of these apatites, we conclude that these apatites do indeed preserve their magmatic H-isotopic compositions. By extension, they also record the δD signature of their Mg-rich source regions in the lunar interior. This δD signature is in good agreement with a recent estimate for the δD of the source region of the Ti-rich pyroclastic glasses (Füri et al., 2014. Icarus 229), and is comparable to estimates of the H-isotopic composition of the Earth’s mantle (Lécuyer et al., 1998. Chem. Geol. 145) and the δD of bulk CI-chondrites (Alexander et al., 2012. Science 337). This dataset supports the hypothesis for a common-origin for water in the Earth-Moon system (Füri et al., 2014. Icarus 229; Saal et al., 2013. Science 340)

Igneous and shock processes affecting chassignite amphibole evaluated using chlorine/water partitioning and hydrogen isotopes

Amphibole in chassignite melt inclusions provides valuable information about the volatile content of the original interstitial magma, but also shock and postshock processes. We have analyzed amphibole and other phases from NWA 2737 melt inclusions, and we evaluate these data along with published values to constrain the crystallization Cl and H2O content of phases in chassignite melt inclusions and the effects of shock on these amphibole grains. Using a model for the Cl/OH exchange between amphibole and melt, we estimate primary crystallization OH contents of chassignite amphiboles. SIMS analysis shows that amphibole from NWA 2737 currently has 0.15 wt% H2O. It has lost ~0.6 wt% H2O from an initial 0.7–0.8 wt% H2O due to intense shock. Chassigny amphibole had on average 0.3–0.4 wt% H2O and suffered little net loss of H2O due to shock. NWA 2737 amphibole has δD ≈ +3700‰; it absorbed Martian atmosphere-derived heavy H in the aftermath of shock. Chassigny amphibole, with δD ≤ +1900‰, incorporated less heavy H. Low H2O/Cl ratios are inferred for the primitive chassignite magma, which had significant effects on melting and crystallization. Volatiles released by the degassing of Martian magma were more Cl-rich than on Earth, resulting in the high Cl content of Martian surface material