C3 - Preservation of 182W heterogeneities in the Earth's mantle – implications for the timing and nature of late accretion

Research areas: Isotope geochemistry, cosmochemistry

Summary

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.