Sulfur peridotite xenoliths

Interesting differences in sulfur isotope compositions are observed when comparing high-S peridotitic tectonites with low-S peridotite xenoliths (Fig. 3.7). Tec-tonites from the Pyrenees predominantly have negative 5 " S-values of around —5%c, whereas low-S xenoliths from Mongoha have largely positive 8 S-values of up to - -l%o. Ionov et al. (1992) determined sulfur contents and isotopic compositions in... 

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. 

In contrast, peridotites metasomatized by small melt fractions show enrichment in platinum and palladium and elevated (Pd/Ir) . Bulk mineral separate PGE-Re analyses of two fertile xenoliths from southeastern Australia indicate less than 6% of the whole-rock PGE budget resides in either silicate or oxide phases and further implicates sulfides and alloys as the main controls of PGE-Re abundance. Comparison of sulfide versus whole-rock budgets by Lorand and Alard (2001) demonstrates the dominance of sulfide as the main PGE host in relatively fertile peridotites. This confirms the results of earlier studies of xenolith PGE mass balance (Hart and Ravizza, 1996 Mitchell and Keays, 1981) plus xenolith-derived and diamond inclusion sulfide studies (Jagoutz et al, 1979 Pearson et al, 1998b). As with cratonic xenoliths, sulfur-PGE and major-element-PGE correlations in more depleted noncratonic peridotites indicate that I-PGEs are probably not hosted entirely by sulfide (Lee, 2002). 

The systematics of sulfur isotopes in mantle xenoliths have been reviewed by Kyser (1990). Most sulfide data have so far been obtained by in situ analyses using SIMS or laser probe and this is less prone to alteration effects than whole-rock analyses. Chaussidon et al. (1989) found that considerable sulfur isotope variation exits in mantle minerals (S S —5 to 8 per mil), which they attributed to fractionation between residual sulfide and the melt during melting. However, it is megacrysts that show most variation in their data set, possibly due to magmatic processes while sulfide from the garnet peridotites has a much more restricted range, of between —1 per mil and - -4 per mil, typical of mantle values. Wilson et al. (1996) found elevated in peridotites from Dish Hill, which they proposed was due to metasomatic introduction of subducted cmstal sulfur. 

Sulfur is almost always present in mantle-derived magmas and mantle samples as sulfide, which has been documented from mantle xenolith suites, abyssal peridotites, peridotite massifs, and diamonds (Meyer and Brookins, 1971 Desborough and Czamanske, 1973 Frick, 1973 Vakhrushev and Sobolev, 1973 Bishop et al., 1975 De Waal and Calk, 1975 Meyer and... 

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