Isotope Planetology

The quest for in-depth knowledge of the formation and early evolution of the Earth-Moon system is a cornerstone of the planetary sciences. Virtually all chemical studies that address these key questions rely on the availability of extremely ancient rock samples (>4 billion years ago). On Earth's surface, active plate tectonics, weathering, and volcanism have destroyed nearly all ancient samples. Samples from the Moon are sufficiently old but strongly limited in quantity and cover only a small portion of the lunar surface. The Moon is thought to have formed from the residue of an impact (or impacts) into the proto-Earth. There are two schools of thought as to when this occurred, one advocating an "old Moon" forming prior to 100 million years after solar system formation (SSF) and one supporting a "young Moon" forming later than 100 million years after SSF. This debate and the clear definition of the processes involved have continued unabated for the 50 years since lunar samples were first returned by the Apollo missions. A key to deepening our knowledge of these issues resides in understanding the extinct 182Hf-182W decay system in lunar and terrestrial rocks. To address this question, we analyzed a suite of 29 lunar samples from the Apollo missions to better understand the elemental Hf and W budgets of the moon. We used new high-precision, high field strength element (HFSE) analyses, combined with isotopic and experimental partitioning data in line with the lunar magma ocean (LMO) model. Through these methods it is possible to observe lunar mantle-wide heterogeneities in ratios of highly incompatible elements such as U/W, which are traditionally assumed to be invariant. This observation, in conjunction with 182W isotope data for lunar rocks, supports the hypothesis of a Moon covered by a magma ocean after its formation. Crystallization and mixing of this LMO produced different hybrid cumulate sources; thus forming the sources of the distinct rock types found in the lunar sample suite. Under the low oxygen fugacity conditions during lunar mantle partial melting, the low-Ti mare basalt source preferentially retains tungsten (W) over hafnium (Hf). The measured Hf/W values of low-Ti mare basalts thus provide a minimum for the Hf/W of the low-Ti source and by extension of the silicate Moon. We find that the Hf/W of the silicate Moon should lie between 30 to 50, significantly higher than the silicate Earth's modeled Hf/W of 25.8. Combined with a recently reported “global, uniform” 182W excess in lunar samples, we find that in-situ decay of 182Hf, in the time range between 40 to 60 million years after SSF is a superior explanation of the lunar 182W excess instead of a previously suggested disproportionate “late accretion” of extraterrestrial material to the Moon and the Earth. Our finding lends clear support for an "old Moon." We expanded our work on lunar samples to include the KREEP-rich gabbroic meteorite Northwest Africa (NWA) 6950. This meteorite yields new insight into the history of the KREEP reservoir which formed as the final residual melt of the LMO. A previous study had dated the meteorite to 3100 million years ago through Pb-Pb dating of baddeleyite grains. This marks the NWA 6950 meteorite to be the youngest KREEP-like sample available and thus decisive for constraining lunar evolution. We obtained Lu-Hf, Sm-Nd, and Rb-Sr mineral isochrons for this meteorite. Through Lu-Hf dating we found an age of 3103 ± 39 million years ago, perfectly overlapping the Pb-Pb age and underpinning the significance of this meteorite’s isotope systematics to anchor the evolution of KREEP. A Sm-Nd isochron of clean, hand-picked minerals yielded a compatible age of 3052 ± 57 million years ago. Inclusion of all mineral fractions that might have suffered later disturbance yields a young Sm-Nd isochron age of 2900 ± 200 million years ago that is closely akin to previous ages found via Ar-Ar (2800), Rb-Sr (2900), and Sm-Nd (2900) which dates younger resetting. In addition, the Rb-Sr isochron provides an even younger age of ca. 1450 million years ago, although this may bear no geological relevance. The significance of finding these young ages becomes clear considering that several Sm-Nd and Rb-Sr studies aimed to date related meteorites whose history might thus have been characterized incorrectly. The initial εHf of NWA 6950 is the youngest anchor of the KREEP evolution line, from which we determined a time of KREEP formation at 4514 million years ago, or ca. 55 million years after SSF. We therefore found, through an entirely different line of research, independent support for an "old Moon" formation. To calibrate this methodology, we investigated multiple peridotites from the West Eifel volcanic field of Germany that exhibit similarly low abundances of Lu, Hf, Sm, Nd, Rb, and Sr. For this project, three different ion exchange separation techniques were investigated as part of the calibration. Mineral isochrons of Lu-Hf, Sm-Nd, and Rb-Sr all provided a functionally modern age, indicative of a resetting event during the Quaternary. We also found that whole rock, host rock, and mineral compositions argue against equilibration of the host magma and the peridotite xenoliths. The observation that whole rock samples plot off the horizontal isochrons, in contrast, is explained by melt infiltration and grain boundary entrainment which likely postdated the resetting of the isochrons. One peridotite examined in a companion study supervised by myself (M.M. Thiemens) yielded four distinct ages. The Lu-Hf system was reset by a Quaternary age event, while the Hf isotope signature was highly radiogenic, indicative of differentiation from a modern mantle source between 1.22 and 1.76 Ga. Rb-Sr isochron data yielded an age of ca. 635 Ma, and a Sm-Nd age of 235 Ma corresponds with regional uplift. Our findings reveal that fine scaled isotope investigations are potent tools to unravel evolutionary complexities. The wealth of fine scaled information gained from the Eifel peridotite xenoliths once again underlines the stark contrast between the extremely dynamic evolution of the Earth’s lithosphere and mantle when compared to the largely static lunar evolution following LMO crystallization

Early Moon formation inferred from hafnium–tungsten systematics

0DecretOANoAutActifinfo:eu-repo/semantics/inPres

Zinc, carbon, and oxygen isotopic variations associated with the Marinoan deglaciation

International audienceThe "Snowball Earths" were cataclysmic events during the late Neoproterozoic's Cryogenian period (720-635 Ma) in which most, if not all, of Earth's surface was covered in ice. Paleoenvironmental reconstructions of these events utilize isotopic systems, such as Δ17O and barium isotopes of barites. Other isotopic systems, such as zinc (Zn), can reflect seawater composition or environmental conditions (e.g., temperature changes) and biological productivity. We report here a multi-isotopic C, O, and Zn data set for carbonates deposited immediately after the Marinoan glaciation (635 Ma) from the Otavi Group in northern Namibia. In this study, we chemically separated calcite and non-calcitic carbonate phases, finding isotopically distinct carbon and oxygen isotopes. These could reflect changes in the source seawater composition and conditions during carbonate formation. Our key finding is largescale Zn isotopic variations over the oldest parts of the distal foreslope cap carbonate sections. The magnitude of variation is larger than any found throughout post-snowball cap carbonates to date, and in a far shorter sequence. This shows a heretofore undiscovered difficulty for Zn isotopic interpretations. The primary Zn sources are likely to be aeolian or alluvial, associated with the massive deglaciation related run-off from the thawing continent and a greater exposed surface for atmospheric aerosol entrainment. The samples with the lightest Zn isotopic compositions (δ66Zn < 0.3 ‰) potentially reflect hydrothermally sourced Zn dominating the carbonates' Zn budget. This finding is likely unique to the oldest carbonates, when the meltwater lid was thinnest and surface waters most prone to upwelling of hydrothermally dominated Snowball Earth brine. On the other hand, local variations could be related to bioproductivity affecting the Zn isotopic composition of the seawater. Similarly, fluctuations in sea-level could bring the depositional site below and above a redoxcline, causing isotopic variations. These variations in Zn isotope ratios preclude the estimation of a global Zn isotopic signature, potentially indicating localized resumption of export production

Potassium isotope systematics of the LL4 chondrite Hamlet: Implications for chondrule formation and alteration

Here, we apply recently developed high-precision K isotope analyses to individual components of the LL4 chondrite Hamlet in order to investigate key processes which occurred during chondrite formation. The K isotopic compositions of all Hamlet chondrules range from -1.36 parts per thousand to -0.24 parts per thousand delta K-41 while the matrix and bulk samples show ranges of -0.89 parts per thousand to -0.80 parts per thousand and -0.86 parts per thousand to -1.08 parts per thousand delta K-41, respectively. This range of delta K-41 values is significantly less than what was seen by in situ K isotopic analysis of Semarkona and Bishunpur chondrules, a likely effect of the different chondrite petrologic types, analytical artifacts in the SIMS analyses, and chondrule rim effects. Strong evidence for secondary parent-body alteration effects within Hamlet suggests its K fractionation and distribution are dominantly controlled by these processes. Interestingly, the strong correlation between delta K-41 and chondrule mass suggests that chondrule size played a significant role in the K isotopic distribution within Hamlet. This trend is likely a result of either inherited initial differences in the chondrule K isotopic ratios which were not completely overprinted or mechanisms involved in the metamorphism processes creating variations. This K isotope correlation with chondrule mass could also be suggestive of chondrule-forming nebular processes; nevertheless, it is currently unable to definitively favor any specific model. The K isotopic similarities between Hamlet and bulk ordinary chondrites suggest that all LL chondrites, if not all ordinary chondrites, may have formed via the same processes. Nevertheless, analysis of more pristine chondrules from chondrites of lower metamorphic grade is required to further assess any nebular processes of chondrule formation

Early Moon formation inferred from hafnium-tungsten systematics

The date of the Moon-forming impact places an important constraint on Earth's origin. Lunar age estimates range from about 30 Myr to 200 Myr after Solar System formation. Central to this age debate is the greater abundance of W-182 inferred for the silicate Moon than for the bulk silicate Earth. This compositional difference has been explained as a vestige of less late accretion to the Moon than to the Earth after core formation. Here we present high-precision trace element composition data from inductively coupled plasma mass spectrometry for a wide range of lunar samples. Our measurements show that the Hf/W ratio of the silicate Moon is higher than that of the bulk silicate Earth. By combining these data with experimentally derived partition coefficients, we found that the W-182 excess in lunar samples can be explained by the decay of the now extinct Hf-182 to W-182. Hf-182 was only extant for the first 60 Myr after the Solar System formation. We conclude that the Moon formed early, approximately 50 Myr after the Solar System, and that the excess W-182 of the silicate Moon is unrelated to late accretion

Unravelling lunar mantle source processes via the Ti isotope composition of lunar basalts

Formation and crystallisation of the Lunar Magma Ocean (LMO) was one of the most incisive events during the early evolution of the Moon. Lunar Magma Ocean solidification concluded with the coeval formation of K-, REE- and P-rich components (KREEP) and an ilmenite-bearing cumulate (IBC) layer. Gravitational overturn of the lunar mantle generated eruptions of basaltic rocks with variable Ti contents, of which their δ49Ti variations may now reflect variable mixtures of ambient lunar mantle and the IBC. To better understand the processes generating the spectrum of lunar low-Ti and high-Ti basalts and the role of Ti-rich phases such as ilmenite, we determined the mass dependent Ti isotope composition of four KREEP-rich samples, 12 low-Ti, and eight high-Ti mare basalts by using a 47Ti-49Ti double spike. Our data reveal significant variations in δ49Ti for KREEP-rich samples (+0.117 to +0.296 %) and intra-group variations in the mare basalts (-0.030 to +0.055 % for low-Ti and +0.009 to +0.115 % for high-Ti basalts). We modelled the δ49Ti of KREEP using previously published HFSE data as well as the δ49Ti evolution during fractional crystallisation of the LMO. Both approaches yield δ49TiKREEP similar to measured values and are in excellent agreement with previous studies. The involvement of ilmenite in the petrogenesis of the lunar mare basalts is further evaluated by combining our results with element ratios of HFSE, U and Th, revealing that partial melting in an overturned lunar mantle and fractional crystallisation of ilmenite must be the main processes accounting for mass dependent Ti isotope variations in lunar basalts. Based on our results we can also exclude formation of high-Ti basalts by simple assimilation of ilmenite by ascending melts from the depleted lunar mantle. Rather, our data are in accord with melting of these basalts from a hybrid mantle source formed in the aftermath of gravitational lunar mantle overturn, which is in good agreement with previous Fe isotope data.SCOPUS: ar.jinfo:eu-repo/semantics/publishe