Abstract
Evidence from iron meteorites indicates that a large number of differentiated planetesimals formed early in Solar System history. These bodies should have had well-developed olivine-rich mantles and consequentially such materials ought to be abundant both as asteroids and meteorites, which they are not. To investigate this “Great Dunite Shortage” we have undertaken a geochemical and oxygen isotope study of main-group pallasites and dunitic rocks from mesosiderites.Oxygen isotope analysis of 24 main-group pallasites (103 replicates) yielded a mean Δ17O value of −0.187±0.016‰ (2σ), which is fully resolved from the HED Δ17O value of −0.246±0.014 (2σ) obtained in our earlier study and demonstrates that both groups represent distinct populations and were derived from separate parent bodies. Our results show no evidence for Δ17O bimodality within the main-group pallasites, as suggested by a number of previous studies.Olivine-rich materials from the Vaca Muerta, Mount Padbury and Lamont mesosiderites, and from two related dunites (NWA 2968 and NWA 3329), have Δ17O values within error of the mesosiderite average. This indicates that these olivine-rich materials are co-genetic with other mesosiderite clasts and are not fragments from an isotopically distinct pallasite-like impactor. Despite its extreme lithologic diversity the mesosiderite parent body was essentially homogeneous with respect to Δ17O, a feature best explained by an early phase of large-scale melting (magma ocean), followed by prolonged igneous differentiation.Based on the results of magma ocean modeling studies, we infer that Mg-rich olivines in mesosiderites formed as cumulates in high-level chambers and do not represent samples of the underlying mantle. By analogy, recently documented Mg-rich olivines in howardites may have a similar origin.Although the Dawn mission did not detect mesosiderite-like material on Vesta, evidence linking the mesosiderites and HEDs includes: (i) their nearly identical oxygen isotope compositions; (ii) the presence in both of coarse-grained Mg-rich olivines; (iii) both have synchronous Lu-Hf and Mn-Cr ages; (iv) there are compositional similarities between the metal in both; and (v) mesosiderite-like material has been identified in a howardite breccia. The source of the mesosiderites remains an outstanding question in meteorite science.The underrepresentation of olivine-rich materials amongst both asteroids and meteorites results from a range of factors. However, evidence from pallasites and mesosiderites indicates that the most important reason for this olivine shortage lies in the early, catastrophic destruction of planetesimals in the terrestrial planet-forming region and the subsequent preferential loss of their olivine-rich mantles.
Highlights
A well-known and yet still poorly understood problem in both meteorite science and remote sensing studies of asteroids, is the apparent under representation of olivine-rich differentiated materials (Chapman, 1986; Bell et al, 1989; Burbine et al, 1996; Mittlefehldt et al, 1998; Scott et al, 2010)
Evidence from iron meteorites indicates that a large number of such fully differentiated bodies formed very early in Solar System history, possibly as little as 0.1 Myr after CAIs (Markowski et al, 2006; Kruijer et al, 2012, 2014)
We have investigated the petrography, geochemistry and oxygen isotope composition of a coarse-grained, olivine-rich clast from the Vaca Muerta mesosiderite, a dunite clast from the Mount Padbury mesosiderite, previously described by McCall (1966), and two dunite clasts from the Lamont mesosiderite described by Boesenberg et al (1997)
Summary
A well-known and yet still poorly understood problem in both meteorite science and remote sensing studies of asteroids, is the apparent under representation of olivine-rich differentiated materials (Chapman, 1986; Bell et al, 1989; Burbine et al, 1996; Mittlefehldt et al, 1998; Scott et al, 2010). Estimates of the number of distinct differentiated asteroids from which we have core samples vary between 26 and 60 (Scott, 1972; Burbine et al, 1996; Mittlefehldt et al, 1998; Haack and McCoy, 2005; Chabot and Haack, 2006; Wasson, 2013a; Benedix et al, 2014). Olivine-rich asteroids that are thought to be from differentiated bodies (A-types) are one of the rarest types in the main belt (Burbine et al, 1996; DeMeo et al, 2009, 2014; Burbine, 2014)
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