The oxidation state and mass of the Moon-forming impactor

Jon Wade,Bernard J Wood

doi:10.1016/j.epsl.2016.02.053

Abstract

Abstract Physical simulations of the origin of the Moon have, until recently, centred on impact, about 100 M.yr after the origin of the solar system, of a Mars-like body (10–20% Earth mass) on a near fully-accreted protoEarth. Although this model provides an explanation of the distribution of mass and moment of inertia of the Earth–Moon system it has recently been found that modification of the initial conditions greatly expands the range of permissible impactor masses. Here we take an alternative approach and consider how the oxidation state and mass of the impactor affect the chemical compositions of the product Earth and Moon. We apply the constraints that silicate Moon is richer in FeO than silicate Earth (9–13% as opposed to 8.05%), that their Hf/W ratios are both ∼25 and that they are virtually identical in isotopes of O, Ti, Si, Ni, Cr and W. We then grow protoEarth using a standard accretionary model which yields the correct mantle abundances of Ni, Co, W, Mo, Nb, V and Cr, and add to this body different masses of impactor. The impactor is assumed to be either highly oxidised (∼18% FeO), highly reduced (∼0.3% FeO) or undifferentiated and chondritic. In order to satisfy the isotopic constraints silicate Moon is assumed to be derived principally from silicate protoEarth. We find that an oxidised or chondritic impactor of ∼ 0.15 M E can satisfy the isotopic constraints (most importantly e182W), FeO contents and Nb/Ta of Earth and Moon, but leads to implausibly low Hf/W of ∼ 12 – 16 in silicate Earth and ∼ 4 – 6 in silicate Moon. This is because the Moon requires more impactor mantle, with low Hf/W, than Earth to reach its higher FeO content. In contrast, impact of a similar mass (10–20% M E ) of highly reduced, Mercury-like impactor on an oxidised protoEarth (∼10.7% FeO in mantle) satisfies the isotopic constraints, FeO contents, Nb/Ta and Hf/W of silicate Earth and Moon given a small amount of post-impact re-equilibration of terrestrial mantle with impactor core. The presence of a small S-rich lunar core is consistent with this reduced impactor scenario. We conclude that the geochemical properties of Earth and Moon strongly favour a reduced impactor of 10–20% M E .

Highlights

Models of the conditions of terrestrial accretion and differentiation (e.g. Rubie et al, 2011; Wade and Wood, 2005) rely on the composition of bulk silicate Earth (BSE) (Allègre et al, 1995; McDonough and Sun, 1995), experimental measurements of the partitioning of elements between liquid silicate and segregating liquid metal core and the assumption that refractory elements are in CI chondritic proportions in the bulk Earth
Models differ in detail, there is general agreement that pressures of core segregation became high (30–50 GPa) and some argue for increasing oxidation state during accretion, as expressed by an increasing FeO content of BSE (O’Neill, 1991a, 1991b; Rubie et al, 2011, 2015; Wade and Wood, 2005)
The significant depletions of weakly siderophile V, Cr and Nb in silicate Earth may be due to dissolution of oxygen in the metal at high pressure (Corgne et al, 2009) they are most readily explained by their sequestration in the core during an extended period of accretion under reducing conditions, corresponding to low FeO content of the mantle (Ringwood, 1966; Wood et al, 2008)

Summary

Introduction

Models of the conditions of terrestrial accretion and differentiation (e.g. Rubie et al, 2011; Wade and Wood, 2005) rely on the composition of bulk silicate Earth (BSE) (Allègre et al, 1995; McDonough and Sun, 1995), experimental measurements of the partitioning of elements between liquid silicate and segregating liquid metal core and the assumption that refractory elements are in CI chondritic proportions in the bulk Earth. More recent trace element and isotopic data have shown, that BSE and the Moon are essentially identical in Hf/W (Kleine et al, 2004; König et al, 2011) (Table 1) and in isotopes of O, Cr, Ti, Ni and W (Herwartz et al, 2014; Kleine et al, 2004; Regelous et al, 2008; Trinquier et al, 2008) These latter observations are extremely difficult to reconcile with models of the Moon being dominated by material from the impactor (e.g. Canup and Asphaug, 2001) implying, in agreement with the earlier hypotheses, that Earth and Moon were both made substantially of the same precursor materials. Recent simulations of the Moon-forming impact have expanded the possible range of impactor mass consistent with the physical properties of the Earth–Moon system into two end member scenarios The first of these involves a small impactor (∼2 to 5% of Earth Mass, Cuk and Stewart, 2012), colliding with a rapidly spinning proto-Earth while the second is a collision between two bodies of similar size (Canup, 2012).

Accretion of the protoEarth

Impactor compositions

Chondritic impactors

Implications of presence of a Lunar core

Findings

Conclusions