Volatile elements primitive mantle composition

Developing a model for the composition of the Earth and its major reservoirs can be established in a four-step process. The first involves estimating the composition of the silicate Earth (or primitive mantle, which includes the crust plus mantle after core formation). The second step involves defining a volatility curve for the planet, based on the abundances of the moderately volatile and highly volatile lithophile elements in the silicate Earth, assuming that none have been sequestered into the core (i.e., they are truly lithophile). The third step entails calculating a bulk Earth composition using the planetary volatility curve established in step two, chemical data for chondrites, and... 

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. 

The inclusion of the subjects covered in Volume 1 of this Treatise illustrates the recognition that one critical avenue to understanding geo chemistry is to understand the solar environment in which Earth formed. Chapter 2.01 of this volume compares the composition of Earth with that of various primitive meteorite classes and with the spectroscopically determined composition of the Sun. Chemical variability in these meteorites reflects primarily two processes (i) volatility and (u) affinity for metal (the so-called siderophile elements) over silicate (lithophile elements). Perhaps the most surprising outcome of this comparison is that Earth s mantle has a bulk composition that is close to solar, at least for refractory lithophile elements. As detailed in Chapter 2.01, the mantle s most obvious departures from solar composition are its deficiencies in volatile and siderophile elements. The latter is easily understood in that Earth has a large metallic core that extracted the missing siderophile elements from the mantle (Chapter 2.15). 

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