Garnets mineral residues

Figure 4.11 Monte-Carlo simulation (100 trials) of error propagation for La/Yb fractionation in residual melts by clinopyroxene-garnet removal from a basaltic parent magma (see text for parameter description and distributions used). Top mineral-liquid partition coefficients for La and Yb. Bottom variations of the La/Yb ratio as a function of the fraction F of residual melt.

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. 

Recasting the major-element compositions of experimental residues and mantle samples into a common set of mineral components effectively eliminates diff erences in temperatures and pressures of final equilibration, as the normalization essentially re-equilibrates all compositions to a common temperature and pressure. Here, we use the high P-T spinel and garnet Iherzolite... 

Figure 5 Normative garnet Iherzolite mineral abundances (wt.%) in batch partial melt extraction residues (0% to >50%) from fertile peridotite as a function of Mg from 3 GPa to 7 GPa (based on the experimental data of Walter (1998) as parametrized by Walter (1999b) for melting of fertile peridotite KR4003 normative mineral compositions are calculated using the garnet Iherzolite normative algorithm of Kelemen (1992)).

Normal mantle melting, such as that which leads to the formation of oceanic crust and oceanic islands, produces magma of basaltic to picritic composition and leaves a residue consisting of olivine, orthopyroxene, chnopyroxene and an aluminous phase. The compositions of minerals in this residue, and their relative proportions, are unlike those in old subcontinental hthosphere Mg-Fe ratios of the ferromagnesian minerals are too low, and the amount of chnopyroxene and spinel or garnet is too high. 

The lanthanide abundance pattern for Shergotty (table 9, fig. 7) is unique among meteorites and indicates a complex prehistory in the Martian mantle from which they appear to have been derived, as basalts, by partial melting. The enriched pattern of the heavy lanthanides (Gd-Lu) resembles that of pyroxenes (the parent rocks appear to have been pyroxene cumulates). It provides no evidence that garnet was a residual phase in the source from which these basalts were derived, for, if so, the reciprocal pattern would be displayed. Leaching experiments show that most of the lanthanides are contained in accessory phases (whitlockite and apatite) rather than in the major mineral phases. 

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