Chemical Budget of Late Accretion

In project area B we will test the hypothesis that a few large planetesimals accreted soon after the formation of the Moon as the Earth’s late veneer....

The late veneer comprises the same type of material as some of the late volatile-rich building materials of the terrestrial planets.

To test this hypothesis we will (1) assess the genetic heritage of the late-accreted material in comparison to meteorites and inferred building materials of the Earth, (2) determine the chemical composition of the late veneer, and (3) investigate the extent to which the late veneer delivered volatiles to the Earth and Moon. The five projects will investigate these issues from various angles, using different but complementary approaches. Specifically, the projects will address the following issues:

  1. Does the late-accreted material derive from the same or a distinct population of bodies as meteorites? B1, B3, B5
  2. Is the late veneer genetically linked to the main building materials of the Earth and other terrestrial planets? B3, B5 (A3)
  3. What was the chemical composition of the Earth’s late veneer and was it volatile-rich? B1, B2, B4 (A3)
  4. What is the volatile element budget of the Earth and Moon and how does it compare to that of the late veneer? B4, B2, B1, B5
  5. How were volatile and siderophile elements redistributed during the last giant impact, late accretion and differentiation? B1, B2, B4 (C1, C2, C4, C5)

 

Projects B1 and B3 will investigate the provenance of late accreted material through chemical and isotopic analyses. In B1 the chemical composition of lunar impact rocks will be used to assess whether the late-accreted material can be linked to any group of chondrites and/or differentiated meteorites. B3 will address the same issue, but using nucleosynthetic isotope anomalies in highly siderophile elements to establish a link (if any) between the late-accreted material and known groups of meteorites and the main building materials of the Earth. Combined, the proposed studies will determine the type (B1) and origin (B3) of the late-accreted material and as such will provide new and firm constraints on whether the late veneer is derived from the same or a distinct population of bodies as meteorites (see also A3). B5 will address this issue from a different angle, by investigating volatile-rich rock fragments in meteorite breccias. These fragments are different from known meteorites. They may represent volatile-rich material, perhaps from the outer solar system, which likely was scattered into the inner solar system at various times during terrestrial planet formation. Taken together, the investigation of meteorite breccias (B5), bulk meteorites (B3) and lunar samples (B1, B3) will provide novel insights and a comprehensive assessment of the type and provenance of the material added to the Earth and Moon during late accretion.


Projects B1, B2 and B4 will also assess the fraction of volatile elements delivered to the Earth and Moon during late accretion. The investigation of specific siderophile volatile elements in lunar impact rocks will provide some of the most direct constraints on the volatile content of the late-accreted material (B1), while B2 will disentangle contributions of core formation versus late accretion for the budget of highly siderophile and siderophile volatile elements in the Earth's mantle. Comparing terrestrial and lunar volatile budgets to the estimated late veneer composition from B1 and B2 will make it possible to quantify the fraction of these highly volatile elements added during late accretion and during the main stage of Earth’s accretion. Comparing the volatile element systematics of the Earth and Moon to those of volatile-rich rock fragments in meteorite breccias (B5) will help to assess if these fragments represent viable sources of volatiles in the terrestrial planets.