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Peridotites mineral residues

These studies highlight the importance of finding other ways to more systematically evaluate whole-rock peridotite Re-Os isotope model ages. Ideally, Re-Os dating studies on peridotites should be carried out on primary sulphides included in minerals such as olivine. However, as already indicated, most cratonic peridotites are residues of large degrees of melting. The removal... [Pg.70]

Assuming that half a percent liquid is trapped with the residue, equating the concentration in the rock to that in the solid residue will result in a severe bias for incompatible elements with Df<0.01 and in pure nonsense for elements with Dt<0.001. A typical example is represented by rare-earth elements in peridotites. Even separated clinopyroxenes can be suspected to have incorporated most of the REE from whichever trace amounts of liquid happened to be trapped in the cooling rock. If the rest of the minerals do not take any REE, it is left to the reader as an exercise to show that the concentration in clinopyroxene after uptake of incompatible elements is related to that in the clinopyroxene from a liquid-free residual peridotite through... [Pg.520]

Osmium isotopes currently provide the strongest case for mineral-to-mineral disequilibrium, and for mineral-melt disequilibrium available from observations on natural rocks. Thus, both osmium alloys and sulfides from ophiolites and mantle xenoliths have yielded strongly heterogeneous osmium isotope ratios (Alard et al., 2002 Meibom et al., 2002). The most remarkable aspect of these results is that these ophiolites were emplaced in Phanerozoic times, yet they contain osmiumbearing phases that have retained model ages in excess of 2 Ga in some cases. The melts that were extracted from these ophiolitic peridotites contained almost certainly much more radiogenic osmium and could, in any case, not have been in osmium-isotopic equilibrium with all of these isotopically diverse residual phases. [Pg.768]

Boyd (1989) compiled modes of major minerals in peridotite xenoliths from on and off-craton localities and compared the results with mantle residues from oceanic mantle represented by... [Pg.886]

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. [Pg.938]

Figure 4 Normative spinel Iherzolite mineral abundances (wt.%) in batch partial melt extraction residues (0-25%) from fertile peridotite (composition 8, Table 1) as a function of Mg (molar Mg/(Fe + Mg)) at 0.5 GPa, 1 GPa, and 2 GPa, based on the melting model of Kinzler and Grove (1992a, 1993). Normative mineral compositions are calculated using the spinel Iherzolite normative algorithm of Kelemen et ai, (1992). Figure 4 Normative spinel Iherzolite mineral abundances (wt.%) in batch partial melt extraction residues (0-25%) from fertile peridotite (composition 8, Table 1) as a function of Mg (molar Mg/(Fe + Mg)) at 0.5 GPa, 1 GPa, and 2 GPa, based on the melting model of Kinzler and Grove (1992a, 1993). Normative mineral compositions are calculated using the spinel Iherzolite normative algorithm of Kelemen et ai, (1992).
Major-element compositions of most lithospheric peridotites reflect an origin as melt-residues. However, as with parent-daughter isotope ratios, compilation of their strontium and neodymium mineral isotopic compositions reveals that very few samples show the characteristics of ancient melt residues (Figure 37). Osmium isotopes are the exception and dominantly reflect ancient extraction of high Re/Os melts, leaving rhenium-depleted residues to develop time inte-... [Pg.225]


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Peridotites

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