Institut für Planetologie, Wilhelm-Klemm-Str. 10, D-48149 Münster
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The elevated abundances of highly siderophile elements (HSE) in the Earth's mantle are thought to reflect the late accretion of primitive material (the “late veneer”) to the mantle after core formation was complete. However, it is currently not known whether the late-accreted material is genetically linked to the main building material of the Earth, in which case it may consist of the leftover planetesimals remaining after the main accretion phase of terrestrial planets, or if, alternatively, the late veneer derives from an exotic source of planetesimals (perhaps even comets) unrelated to the building blocks of Earth. Distinguishing between these two contrasting possibilities is important for understanding the origin and composition of the late veneer, its relation to the formation of the Earth, and, more generally, the dynamics of the late stages of terrestrial planet formation.
This issue can be addressed using nucleosynthetic isotope anomalies, which arise through the heterogeneous distribution of one or several presolar components in the solar protoplanetary disk, and constitute a distinctive tracer of potential genetic relationships among planetary bodies.
The proposed study will focus on using nucleosynthetic Ru and Mo isotope anomalies in chondrites and lunar samples (1) to evaluate whether the late veneer and the main building blocks of the Earth share the same genetic heritage, (2) to establish a link (if any) between known groups of meteorites and late-accreted materials, and (3) to identify any changes in the origin and composition of these materials over time.
Some of the same samples will be analysed in subprojects A1, A2, and B1 for their age and chemical composition. The combined data, therefore, will constrain the chemical inventory and origin of the late-accreted material, and any temporal change thereof, with unprecedented detail.
Elevated abundances of the highly siderophile elements (HSE) in the Earth’s mantle are thought to reflect the late accretion of primitive, broadly chondritic material (the “late veneer”) after the putative giant Moon-forming impact and the end of core formation on Earth. However, exactly when late accretion occurred and how the late-accreted material was mixed into the Earth’s mantle is not well known. For instance, HSE analyses of lunar rocks suggest that the relative mass fraction of late-accreted material is an order of magnitude lower for the Moon than for the Earth.
This disparity may mean that the Moon received a disproportionally lower share of late-accreted material, perhaps because late accretion was stochastic and consisted of only a few large impactors. However, this disparity may also mean that the major period of late accretion preceded the giant Moon-forming impact, such that only a small amount of late-accreted material was added after the giant impact. Therefore, this means that the giant impact did not remove all these previously accumulated HSEs from the Earth’s mantle.
Another related question is whether the giant impact erased from the Earth’s mantle previously generated chemical and isotopic heterogeneities. The small 182W/184W enrichments in some Archaean samples suggest that this may not have been the case; instead, they suggest that early-generated chemical and isotopic heterogeneities were preserved during the giant impact. However, these 182W enrichments may alternatively result from a heterogeneous distribution of the late veneer in Earth’s mantle, in which case these signatures could all be post–giant impact features.
To unravel these complexities, we propose to perform high-precision W isotope measurements on terrestrial (mainly Archaean) and lunar samples. By precisely determining the lunar 182W/184W, we will be able to better constrain the time of late accretion relative to the formation of the Moon. In addition, measuring the 182W on terrestrial samples will shed new light on how the late veneer was distributed within the Earth, and it will also help to distinguish between exogenous and indigenous origins of 182W variations in terrestrial samples. In order to correctly interpret the W isotope data for the terrestrial samples, they will be supplemented with Nd and Os isotope data as well as HSE abundance data. These combined data will make it possible to unravel the different processes involved in generating the 182W heterogeneities in the early Earth’s mantle.