Orthopyroxene excess

Fig. 92. Equilibrium of olivine with pyroxene in rocks with excess silica in the absence of fluid. Figures on curves indicate maximum iron content of orthopyroxene, in mol.%.

Fig. 93. Equilibrium of magnesian-iron minerals in rocks with excess silica. A. In silicate iron-formations (aqueous fluid. Cum + Px -t- OH- Q association). B. In carbonate iron-rich rocks (carbonic acid fluid. Car -(- Px -I- 01 -t- Q association). Figures indicate maximum iron content of orthopyroxene in association with quartz and olivine.

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.

The effects of post melt-depletion interaction with fluid or melt components in the lithospheric mantle has been extensively documented (e.g. Menzies Hawkesworth 1987, and references therein Harte et al. 1993 Pearson 1999 ) and it is widely accepted that these phenomena dominate the minor element geochemistry of cratonic peridotites. Extensive studies of the effect of metasomatism on the major element chemistry of lithospheric peridotites have also been made (Boyd Mertzman 1987 Keleman et al. 1992, 1998 Walter 1999). To date, most of the discussion has centred around the apparent excess of orthopyroxene, especially in Kaapvaal peridotites. However, major and trace element studies show that it is likely that the abundances of garnet and clinopyroxene are also grossly affected (Burgess Harte 1999 Shimizu 1999). 

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