Calcium peridotite xenoliths

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 xenoliths 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.

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