Chondrite mixing model

The origin of the components that were accreted to make up the planets is the subject of intense discussion. Chondrite-mixing models attempt to build the planets using known chondritic materials. These models are constrained by the mean densities, moments of inertia, and, to the extent that they are known, the bulk chemical and isotopic compositions of the planets. Mars and 4 Vesta can be modeled reasonably well by known types of chondritic material (Righter et al., 2006). However, the Earth seems to have formed, at least in part, from materials that are not represented in our collections of chondritic meteorites (see below). 

Heterogeneous accretion models for the formation of the Earth advocate the initial accretion of refractory, less-oxidized components that make up the bulk of the planet (some 50-80%), followed by the accretion of a lower-temperature, more oxidized component (e.g., perhaps comparable to carbonaceous chondrites). The overall nature of the initially refractory material is not well characterized, but it could have affinities to ordinary or enstatite chondrites. These two-component mixing models seek to reconcile the observational constraints from chemical and isotopic studies of the silicate Earth. As of early 2000s, we do not have sufficient data to identify in detail the nature of these two components of accretion if they existed. 

Workers often speak of chondritic compositional models for the terrestrial planets, but some use the phrase to denote C chondrites and some take it to mean a mix of chondrite classes or material mineralogically similar to chondrites, but not exactly the same as any single class . This confusion is a perennial source of confusion. Caveat lector. 

Ringwood (1979) first proposed these models but the concept was more fully developed by Wanke (1981). In Wanke s model, the Earth accretes by heterogeneous accretion with a mixing ratio A B — 85 15. Most of component B would be added after the Earth had reached about two thirds of its present mass. The oxidized volatile-rich component would be equivalent to Cl carbonaceous chondrites. However, the reduced refractory rich component is hypothetical and never has been identified in terms of meteorite components. 

Hudson GB, Kennedy BM, Podosek FA, Hohenberg CM (1989). The early solar system abundance of " " Pu as inferred from the St. Severin chondrite. Proc 19th Lunar Planet Sci Conf, p 547-557 Hunt DL, Kellogg LH (2001) Quantifying mixing and age variations of heterogeneities in models of mantle convection role of depth-dependent viscosity. J Geophys Res 106 6747-6759 Hunten DM, Pepin RO, Walker JCG (1987) Mass fractionation in hydrodynamic escape. Icaras 69 532-549 Hurley PM, Rand JR (1969) Pre-drift continental nuclei. Science 164 1229-1242... 

Isotopic studies involving MC-ICP-MS revealed evidence for a heterogeneous distribution of Zr, Ti, Mo, and Ni isotopes in the solar system [49, 53-55, 59, 61]. Variations in relative isotopic abundances of Zr and Ti between carbonaceous chondrites and terrestrial samples provide evidence for a heterogeneous distribution of CAIs (Table 10.1) as carriers of these nucleosynthetic anomalies. This conclusion has implications for mixing processes in the solar nebula and accretion models of planets. 

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