Calcium peridotites

Calcium- rich (anorthite) Ultramatic (kormatlite/ peridotite)... 

O Hara, M.J., 1963. Distribution of iron between coexisting olivines and calcium-poor pyroxenes in peridotites, gabbros, and other magnesian environments. Am. J. Sci., 261 32-46. 

Hart and Zindler (1986) also based their estimate on chondritic ratios of RLE. They plotted Mg/Al versus Nd/Ca for peridotites and chondritic meteorites. The two refractory elements, neodymium and calcium, approach chondritic ratios with increasing degree of fertility. From the intersection of the chondritic Nd/Ca ratio with observed peridotite ratios. Hart and Zindler (1986) obtained an Mg/Al ratio of 10.6 (Table 2). 

Cadmium. Cadmium appears to be compatible or very mildly incompatible, similar to zinc. Almost nothing is known about which minerals it prefers. From a crystal-chemical view, cadmium has similar ionic radius and charge to calcium, but a tendency to prefer lower coordination due to its more covalent bonding with oxygen (similar to zinc and indium). Cadmium in spinel Uierzolites varies from 30 ppb to 60 ppb (BVSP) and varies in basalts from about 90 ppb to 150 ppb (Hertogen et al., 1980 Yi et al., 2000). Cd/Zn is —10 in peridotites (BVSP) and the continental cmst (Gao etal., 1998), and —1.5 X 10 in basalts (Yi etai, 2000). We adopt the mean of these ratios (1.2 X 10" ). 

Secondary alteration processes are also responsible for element deviations, particularly the strong depletion of calcium and sodium relative to aluminum in some abyssal peridotites, as well as part of the scattered variations of silicon, magnesium, calcium, and sodium. Serpentinized, orogenic and ophiolitic peridotites may also be strongly depleted in calcium and sodium. However, these samples are rare in the selected database for peridotite massifs, which includes only moderately serpentinized samples. 

Figures 3(d)-(f) compares modes observed in four well-characterized on-craton xenolith suites (n = 189) with degree of depletion. When compared to the off-craton samples, trends are far more scattered for olivine and orthopyroxene, and a significant population of samples are orthopyr-oxene-rich, as originally remarked by Boyd (1989). The trend for garnet is remarkably regular and uniform, whereas many samples contain far more clinopyroxene than expected for their level of depletion. This excess clinopyroxene may be of exsolution origin, or introduced to the rock after its original formation as a residue (Canil, 1992 Shimizu, 1999 Simon et al, 2003). Comparison with trends expected from peridotite melting models is complicated by the fact that orthopyroxene is replaced at the solidus by a low calcium clinopyroxene, and is a product of the melting reaction at P > 3 GPa (Walter, 1998). The mean and median modes of off-craton and on-craton xenoliths from Figure 3 are summarized in Table 2.

Figure 5 Covariation of CaO with Ct203 in garnets from a large database (n = 900) from a wide variety of xenoUths in kimberlites and other alkaline rocks. Note the positive correlation of calcium and chromium in peridotite garnets. Garnets below the line are harzbur-gitic, whereas those above it are Iherzolitic or wehrlitic.

Kohler T. and Brey G. P. (1990) Calcium exchange between olivine and clinopyroxene calibrated as a geothermobarometer for natural peridotites from 2-60 kb with applications. Geochim. Cosmochim. Acta 54, 2375-2388. 

The commonly used model for calcium-carbo-natite genesis involves partial melting of a carbonate-bearing peridotite, forming a carbonate-rich silicate melt that upon ascent produces immiscible calcium-rich carbonate and magnesium-rich silicate melts (e.g., Lee and Wyllie,... 

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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