Calcium mantle composition

Zhu and MacDougall (1998) showed that calcium isotope composition of the shells was both species and temperamre dependent and that river water is depleted by 2%o relative to seawater. The temperature dependency of calcium isotope fractionation enables this tracer to be a potential paleotem-peramre proxy (Nagler et al., 2000). The seawater Cal" Ca secular variations are indicated by the results of De La Rocha and De Paolo (2000) for the last 160 Myr, and by the data of Zhu (1999) for the entire 3.4 Gyr of earth history. The latter indicate that, in analogy to strontium isotopes (Veizer and Compston, 1976), the Archean samples have C Ca ratios similar to the earth mantle, with the crustal-like values first appearing at the Archean/Proterozoic transition. However, in contrast to the strontium isotope trend, the calcium isotope ratios appear to dip towards mantle values also at —1.6 Gyr ago. 

Murakami et al. (2002) studied a natural peridotite composition (with 7.5-13.5 wt.% H2O) at 25.5 GPa and 1,600-1,650 °C. They measured water contents in their run products by SIMS. They found magnesium-rich perovskite and ferropericlase to have —2,000 ppm H2O and calcium-rich perovskite to have —4,000 ppm H2O. A lower mantle consisting of 79 wt.% Mg-perovskite, 16 wt.% ferropericlase, and 5 wt.% Ca-perovskite could contain 2,100 ppm H2O, which when integrated over the mass of the lower mantle yields a reservoir —5 times greater than the oceans. They compare this to the transition zone, which can store nearly six oceans worth of water, despite its smaller volume, because of the greater solubility of water in wadsleyite and ringwoodite (—3.3 X 10" ppm and... 

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