Peridotites highly depleted

Examination of all data for cratonic peridotites (Figure 19) shows similar features to the Lesotho suite (Irvine, 2002). Highly depleted cratonic peridotites generally have low AI2O3 and very low (Pd/Ir) values. Variation in AI2O3 at very low, constant (Pd/Ir) appears characteristic of cratonic peridotites and may be related to the introduction of late-stage diopside by the host kimberlite (Pearson et ai, 2002a Simon et al,... 

A more minor consideration is the role of orthopyroxene. At 1.5 GPa orthopyroxene has Du an order of magnitude less than clinopyroxene, and the small lattice site strongly favors uranium over thorium (Blundy and Wood, 2003a Wood et al., 1999). The Z>u/Dxh for orthopyroxene is consequently high ( 2.5, Table 2) and may influence the bulk partition coefficient despite its low absolute partition coefficients, especially in highly depleted peridotites. 

Figure 17 Comparison of model output for distribution of residue compositions with abyssal peridotites and harzburgites from Oman, (a) Distribution of clinopyroxene compositions in equilibrium with melts at the top of the melting column corresponding to Figure 15. This distribution is sampled uniformly by area (as if we were sampling residues from the top of the column) and is dominated by depleted inter-channel samples, (b) Clinopyroxene compositions in harzburgites from the mantle section of the Oman ophiohte (Kelemen et al., 1995a) superimposed on the range of cpx compositions from abyssal peridotites (Johnson et al., 1990 Johnson and Dick, 1992), showing the predominance of highly depleted samples.

In addition, many peridotites bear the obvious signatures of metasomatism, which re-enriches the rock in incompatible components subsequent to depletion by melt extraction. Where this is obvious (e.g., in reaction zones adjacent to later dikes) it may be avoided easily but often the metasomatism is cryptic, in that it has enriched the peridotite in incompatible trace elements without significantly affecting major-element chemistry (Frey and Green, 1974). Peridotites thus have very variable contents of highly incompatible trace... 

Copper. The abundance of copper in the depleted mantle raises a particular problem. Unlike other moderately compatible elements, there is a difference in the copper abundances of massive peridotites compared to many, but not all, of the xenolith suites from alkali basalts. The copper versus MgO correlations in massive peridotites consistently extrapolate to values of 30 ppm at 36% MgO, whereas those for the xenoliths usually extrapolate to <20 ppm, albeit with much scatter. A value of 30 ppm is a relatively high value when chondrite normalized ((Cu/Mg)N = 0.11), and would imply Cu/Ni and Cu/Co ratios greater than chondritic, difficult to explain, if true. However, the copper abundances in massive peridotites are correlated with sulfur, and may have been affected by the sulfur mobility postulated by Lorand (1991). Copper in xenoliths is not correlated with sulfur, and its abundance in the xenoliths and also inferred from correlations in basalts and komatiites points to a substantially lower abundance of 20 ppm (O Neill, 1991). We have adopted this latter value. 

Figure 17 Summary fields of chondrite-normalized REE patterns for whole-rock peridotites and cUnopyroxenes for peridotite xenoliths. Noncratonic whole-rock peridotites are either LREE-depleted (type lA least common) or LREE-enriched (type IB most common). Data sources from Stosch and Seek (1980), Stosch and Lugmair (1986), Menzies et al (1985). Clinopyroxenes from these rocks also show LREE enrichment or depletion. Cratonic peridotite whole rocks are ubiquitously LREE-enriched. Low-T (granular) suite show greater LREE/HREE compared to high-T (sheared) suite and this is reflected in the more LREE-enriched clinopyroxene compositions in the low-T suite. Data sources from Shimizu (1975), Nixon et al. (1981), and Irvine (2002). Low-T whole-rock suite includes 19 samples...

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