Modal abundances minerals

Table 1.3. Major-element composition (%) of the minerals used to calculate the composition of a gabbro with known modal abundances.

Several models for diffusive transport in and among minerals have been discussed in the literature one is the fast grain boundary (FGB) model of Eiler et al. (1992, 1993). The FGB model considers the effects of diffusion between non-adjacent grains and shows that, when mass balance terms are included, closure temperatures become a strong function of both the modal abundances of constituent minerals and the differences in diffusion coefficients among all coexisting minerals. 

Yugami K., Takeda H., Kojima H., and Miyamoto M. (1997) Modal abundances of primitive achondrites and the end-member mineral assemblage of the differentiated trend. Symp. Antarct. Meteorit. 22, 220-222. 

Amphiboles show a wide variety of REE patterns that cannot be uniformly related to modal abundance. Ionov et al. (1997) suggest that where the mineral has equilibrated extensively with silicate melt, LREE-enrichment and convex REE patterns occur. Hence, vein amphiboles from noncratonic xenoliths are enriched in LREE and MREE over HREE and thus have convex REE patterns (e.g.. Figure 28). Disseminated amphiboles have lower MREE/HREE in some instances but those in xenoliths from the W. Eifel region are much more LREE-enriched than amphibole from other locations. Disseminated amphibole can be either LREE enriched or depleted relative to PUM (Figure 28). 

A key feature is that all the nuclides of interest are highly incompatible in common mantle mineral phases (Table 2). Clinopyroxene and garnet (present in normal peridotitic mantle at depths greater than —80 km) are the principal host minerals for uranium and thorium in the solid phase, although even in these phases partition coefficients do not exceed 0.05. An important consideration is the sense of uranium and thorium fractionation imparted by the presence of different minerals. This can be conveniently expressed in terms of Du/ >Th- Minerals with Du/ >Th > 1 retain uranium over thorium and contribute to 23 Th-excesses in a coexisting melt, whereas those with Du/ >Th < 1 help to create 23 3xh-deficits. The effects of different minerals are then weighted by their absolute partition coefficients and modal abundance to control the bulk partition coefficient of the mantle, and thus determine the sense of fractionation of thorium from uranium in a coexisting melt. 

To illustrate the effects of metasomatism on the silicate mineralogy of peridotite xenoliths it is instructive to compare the modal abundances of garnet and clinopyroxene, expected to be the first minerals to be exhausted during partial melting, with indices of melt extraction such as the mg-number of olivine (Fig. 1). The observed abundances of garnet plus clinopyroxene with olivine... 

The acapulcoite-lodranite clan is the prototypical primitive achondrite group. The acapulcoites have modal mineral abundances very much like those of ordinary chondrites, relict chondrules are reported from a few, and their bulk compositions are broadly chondritic (McCoy etal.,l 996 Mittlefehldt et al., 1996, 1998). The lodranites are distinctly nonchondritic. They are generally depleted in... 

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