Refractory mantle proportions

The two elements calcium and aluminum are RLEs. The assumption is usually made that aU RLEs are present in the primitive mantle of the Earth in chondritic proportions. Chondritic (undifferentiated) meteorites show significant variations in the absolute abundances of refractory elements but have, with few exceptions discussed below, the same relative abundances of lithophile and siderophile refractory elements. By analogy, the Earth s mantle abundances of refractory lithophile elements are assumed to occur in chondritic relative proportions in the primitive mantle, which is thus characterized by a single RLE/Mg ratio. This ratio is often normalized to the Cl-chondrite ratio and the resulting ratio, written as (RLE/Mg)N, is a measure of the concentration level of the refractory component in the Earth. A single factor of (RLE/Mg) valid for all RLEs is a basic assumption in this procedure and will be calculated from mass balance considerations. 

Fig. 2. Calculated proportions of refractory residue formed during mantle melting, (a) Radial profiles for cylindrical symmetry and the radial distribution of residue, of source and of geometric volume (b) the McKenzie Bickle (1988) relationship between temperature and local degree of melting (c) and (d) the distribution as function of temperature and degree of melting. It should be noted that because the two horizontal scales are not linearly related, the distribution maximum around 35-40% melting corresponds to the fiattest part of the McKenzie Bickle curve.

A possibly troubling feature of our models is that we implicitly predict a secular trend in the isotopic composition of carbonates. This would occur because the carbonate subducted would presumably have the isotopic composition of sea water, whereas CO2 currently outgassed at midocean ridges is isotopically lighter. One way to avoid this problem is to presume that carbonates and reduced carbon are subducted in the same proportions as they are produced. Because the mantle emits some reduced gases and subducts oxidized ferric iron (Lecuyer Ricard 1999), some reductant must be subducted lest the continents grow secularly more reduced. The natural candidate, perhaps the only viable candidate, is reduced carbon, which is notably refractory and already known to be subducted. This is a reasonable expectation, and it is consistent with what is known of arc volcanoes today (see above), but it is unmotivated by our model. 

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. 

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