Ureilites are meteorites that represent mantle restites of a planetesimal likely disrupted before the magma ocean stage and then reaccreted. Historically, it was speculated that evaporation shifts the Zn isotope ratios in ureilites toward heavier compositions. The fact that the ureilite parent body (UPB) is depleted in some moderately volatile elements (MVEs) makes ureilites an appealing target to study isotopic fractionation by evaporation in the early Solar System. Here, we show that Fe and Zn isotope ratios of bulk ureilites and their metal and silicate components rather record metal melting and extraction of Fe-FeS melts in the UPB, which also resulted in isotopic disequilibrium between the silicate and metal parts. This finding underlines that the isotopic evolution of MVEs in the early Solar System is not only affected by evaporation, but also by planetary differentiation processes due to the chalcophile and/or siderophile behaviour of many MVEs. It shows that to avoid interpretational bias due to undersampling of planetesimal reservoirs in meteorite collections, and to distinguish planetary differentiation from evaporation, isotopic compositions of MVEs should be combined with common geochemical proxies.
Abstract The processes of planetary accretion and differentiation, whereby an unsorted mass of primitive solar system material evolves into a body composed of a silicate mantle and metallic core, remain poorly understood. Mass‐dependent variations of the isotope ratios of non‐traditional stable isotope systems in meteorites are known to record events in the nebula and planetary evolution processes. Partial melting and melt separation, evaporation and condensation, diffusion, and thermal equilibration between minerals at the parent body (PB) scale can be recorded in the isotopic signatures of meteorites. In this context, the acapulcoite–lodranite meteorite clan (ALC), which represents the products of thermal metamorphism and low‐degree partial melting of a primitive asteroid, is an attractive target to study the processes of early planetary differentiation. Here, we present a comprehensive data set of mass‐dependent Fe, Zn, and Mg isotope ratio variations in bulk ALC species, their separated silicate and metal phases, and in handpicked mineral fractions. These non‐traditional stable isotope ratios are governed by mass‐dependent isotope fractionation and provide a state‐of‐the‐art perspective on the evolution of the ALC PB, which is complementary to interpretations based on the petrology, trace element composition, and isotope geochemistry of the ALC. None of the isotopic signatures of ALC species show convincing co‐variation with the oxygen isotope ratios, which are considered to record nebular processes occurring prior to the PB formation. Iron isotopic compositions of ALC metal and silicate phases broadly fall on the isotherms within the temperature ranges predicted by pyroxene thermometry. The isotope ratios of Mg in ALC meteorites and their silicate minerals are within the range of chondritic meteorites, with only accessory spinel group minerals having significantly different compositions. Overall, the Mg and Fe isotopic signatures of the ALC species analyzed are in line with their formation as products of high‐degree thermal metamorphism and low‐degree partial melting of primitive precursors. The δ 66/64 Zn values of the ALC meteorites demonstrate a range of ~3.5‰ and the Zn is overall isotopically heavier than in chondrites. The superchondritic Zn isotopic signatures have possibly resulted from evaporative Zn losses, as observed for other meteorite parent bodies. This is unlikely to be the result of PB differentiation processes, as the Zn isotope ratio data show no covariation with the proxies of partial melting, such as the mass fractions of the platinum group and rare earth elements.
Our study provides the most comprehensive dataset for high-precision radiogenic isotopes of lead (Pb) in blood for the western European population. It investigates their potential for elucidating the contribution of soil Pb to blood Pb using a human biomonitoring survey involving 81 adults and 4 children living in the urban area of Liège (Belgium). Soils in the area show moderate (median of 360 mg/kg) to high (95th percentile of 1000 mg/kg) Pb concentrations, due to former metal processing activities. Blood lead levels (BLL) measured in the study population are, on average, quantitatively consistent with a ∼ 20 % increase due to the exposure to Pb from soils, as estimated by a single-compartment biokinetic model. Consistently, its isotopic composition does not represent an endmember that fully accounts for the variability of Blood lead isotope (BLI) compositions measured in the study population. While some individuals show more thorogenic BLI ratios (relatively more enriched in 208Pb), which could be consistent with a greater exposure to local soils and/or by their country of birth, the BLI data mostly follow a trend roughly parallel to the European Standard Lead Pollution (ESLP) line, within the European leaded gasoline field, even two decades after the withdrawal of this source. Differences in BLI are probably associated with factors related to the presence of Pb in dwellings (pipes, paint) and drinking water distribution system, suggesting that the anthropogenic Pb in use, relevant to human exposure, may contain ore components of different origins, including the Australian Pb ore signature.
The 26 Al- 26 Mg short-lived chronometer has been widely used for dating ancient objects in studying the early Solar System. Here, we use this chronometer to investigate and refine the geological history of the asteroid 4-Vesta. Ten meteorites widely believed to come from Vesta (4 basaltic eucrites, 3 cumulate eucrites and 3 diogenites) and the unique achondrite Asuka 881394 were selected for this study. All samples were analyzed for their δ 26 Mg∗ and 27 Al/ 24 Mg ratios, in order to construct both whole rock and model whole rock isochrons. Mineral separation was performed on 8 of the HED's in order to obtain internal isochrons. While whole rock Al-Mg analyses of HED's plot on a regression that could be interpreted as a vestan planetary isochron, internal mineral isochrons indicate a more complex history. Crystallization ages obtained from internal 26 Al- 26 Mg systematic in basaltic eucrites show that Vesta's upper crust was formed during a short period of magmatic activity at 2.66 - 0.58 + 1.39 million years (Ma) after Calcium-Aluminum inclusions (after CAI). We also suggest that impact metamorphism and subsequent age resetting could have taken place at the surface of Vesta while 26 Al was still extant. Cumulate eucrites crystallized progressively from 5.48 - 0.60 + 1.56 to >7.25 Ma after CAI. Model ages obtained for both basaltic and cumulate eucrites are similar and suggest that the timing of differentiation of a common eucrite source from a chondritic body can be modeled at 2.88 - 0.12 + 0.14 Ma after CAI, i.e. contemporaneously from the onset of the basaltic eucritic crust. Based on their cumulate texture, we suggest cumulate eucrites were likely formed deeper in the crust of Vesta. Diogenites have a more complicated history and their 26 Al- 26 Mg systematics show that they likely formed after the complete decay of 26 Al and thus are younger than eucrites. This refined chronology for eucrites and diogenites is consistent with a short magma ocean stage on 4-Vesta from which the basaltic eucrites rapidly crystallized. In order to explain the younger age and the complex history of diogenites, we postulate that a second episode of magmatism was possibly triggered by a mantle overturn. We bring a refined chronology of the geological history of Vesta that shows that the asteroid has known a more-complex differentiation than previously thought.
