Planetary differentiation volatile elements

Ma (Wadhwa et al. 2007 and references therein), which is actually the age of a group of inclusions within chondrites known as calcium-aluminum-rich inclusions (CAIs). The word primitive refers to the fact that the bulk compositions of all chondrites, within a factor of two, are solar in composition for all but the most volatile elements (Weisberg et al. 2006). This fact indicates that chondrites have not been through a planetary melting or differentiation process in their parent body, indicating that they have recorded the materials that were present and the processes that operated within the disk before or during planet formation. 

The siderophile elements (Ir, Au, Re, Ni, etc.) seem to be the most reliable indicators of meteoritic material. These elements concentrate in the metal phase during the planetary melting and are therefore strongly depleted in the surface layers of differentiated planets (e.g. on Earth by a factor of 10-4). However, a number of volatile elements (Ag, Bi, Br, Cd, Ge, Pb, Se, Te, Zn) are so strongly depleted on the lunar surface that some of them can serve as subsidiary indicators of meteoritic matter. 

It is likely that the earliest events in the Earth s mantle were not the product of "normal" mantle convection but rather related to planetary processes such as fractionation within a magma ocean. In this way we can explain the very early differentiation of the Earth (pre-4.5 Ga), proposed on the basis of short-lived Nd-isotopes. Similarly, the extreme volatile element loss from the Earth might be explained in this way. These early processes are thought to have ceased within the first 100 Ma of Earth history (Yokochi Marty, 2005). 

Fig. 14. Uranium is refractory like the rare earths, so that K/U ratios are an analogue for K/La ratios (fig. 11). K and U data are available for a wide variety of solar system material, since both elements are readily determined by gamma-ray spectroscopy. Both are incompatible in igneous processes and so tend to preserve their bulk planetary ratios during differentiation. This diagram illustrates that substantial volatile element depletion was widespread in the inner solar nebula, so that similar variations to K/U in volatile element/rare earth ratios are lo be expected. (From Taylor 1987a.)...

As refractory lithophile elements, the REE play an important role in constraining the overall composition and history of the silicate fraction of planets, which for the terrestrial planets is also termed their primitive mantle (equivalent to the present-day crust plus mantle). Since there is no evidence for significant planetary-scale fractionation of refractory elements during the assembly and differentiation of planetary bodies, it is widely accepted that the primitive mantles of terrestrial planets and moon possess chondritic proportions of the REE. As such, the absolute concentrations of REE (and other refractory elements) in primitive mantles provide an important constraint on the proportions of volatile elements to refractory elements and on the oxidation state (i.e., metal/silicate ratio) of the body. To date, the only major planetary bodies for which REE data are directly available are the Earth, Moon, and Mars, and Taylor and McLennan" recently reviewed these data. 

Some short-lived radionuclides were sufficiently abundant at the start of the solar system to produce variations in the abundance of their daughter isotopes in early-formed objects (Table 10.2). The half-lives of these nuclides are between about 0.1 and 100 Ma (Table 10.2). Hence, the parent isotopes are no longer present today, but they were synthesized in stars shortly before solar system formation and therefore they were present in the early solar nebula. The isotopic record of these nuclides provides information about stellar nucleosynthetic sites active shortly before the birth of the solar system and the time scales over which the early solar system formed and first differentiated. Depending on half-life and chemical affinities of parent and daughter isotopes, extinct radionuclide systems can be used to date processes as diverse as the formation of CAIs and chondrules, volatile element depletion and planetary difierentiation (e.g., core segregation and differentiation of early silicate reservoirs). In particular, they are powerful tools to study the Earth s accretion and core formation [90-92],... 

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