Garnet peridotite

Beattie P (1993b) Uranium-thorium disequihbria and partitioning on melting of garnet peridotite. Nature 363 63-65... 

Table 2. Experimentally determined mineral/melt partition coefficients (x 10 ) for U, Th, and Ba in clinopyroxene and garnet and calculated bulk partition coefficients (xlO ) for a garnet peridotite source.

Figure 15. 6 Fe of garnet versus that of coexisting clinopyroxene for eclogites, garnet peridotites, and garnet pyroxenite. Six of the seven samples scatter about a A FCq,j.g, of +0.3%o over a wide range in 6 Fe values measured in the minerals. Error bars are 1-SD of the mean as determined by two or more replicate analyses of the mineral. Modified from Beard and Johnson (2004).

McGregor I. D. (1974). The system Mg0-Al203-Si02 Solubility of AI2O3 in enstatite for spinel and garnet peridotite composition. Amer. Mineral, 59 110-119. 

Mysen B. O. (1977a). Partitioning of nickel between liquid, pargasite and garnet peridotite minerals and concentration limit of behavior according to Henry s law at high pressure and temperature. Amer. Jour. ScL, 278 217-243. 

Mysen B. O. (1977b). Experimental determination of cesium, samarium and thulium partitioning between hydrous liquid, garnet peridotite minerals and pargasite. Carnegie Inst. Wash. Yb., 76 588-594. 

The distribution coefficients of the parent and daughter nuclides are important in U-series disequilibria. If the parent has a higher distribution coefficient than the daughter, its extraction from the matrix is retarded and the daughter/parent activity ratio in the melt can be greater than 1. Experiments have shown that Dy > Dj for garnet peridotites but Du spinel peridotites at low pressures. Due to experimental difficulties, Dro and Dpa are not directly known but are inferred to be very small. Therefore, it is reasonable to assume Dj, >Di and Du > Dp for both garnet peridotites and spinel peridotites. 

Fig. 5.8. (A) Variation of ( Th/ U) in the extracted melt as a function of M (kg/mVyear) for a fixed (0.3%) and X (2.4%) produced by melting of a garnet peridotite source. The source-normalized ThAJ represents the Th/U ratio in the extracted melt normalized to its source Th/U ratio and is calculated using Eq. (5.50) for stable...

While all spinel-lherzolite facies suites show remarkably similar compositional trends as a function of depletion, some garnet peridotite xenoliths in kimberlites and lamproites from ancient cratonic lithospheric keels show signih-cantly different trends (e.g., see Boyd, 1989 Chapters 2.05 and 2.08). Most of these xenoliths are extremely depleted extrapolation of the trends back to the PM MgO of 36.7% gives similar concentrations of Si02, EeO AI2O3, and CaO to the spinel Iherzolites (O Neill and Palme, 1998) the difference in their chemistry is due to a different style of melt extraction, and not a difference in original mantle composition. 

Brueckner and Medaris (2000) reviewed the main occurrences of orogenic garnet peridotites and pointed to their diversity, even within individual HP/UHP terranes. They attributed these differences to different initial environments and different evolutions during the tectonic cycles that ultimately exposed them at the surface, and proposed a classification based on these criteria. The garnet peridotites are first... 

Mantle garnet peridotites represent mantle rocks exhumed to the surface along active continental margins or continent-continent sutures. These peridotites may be classified in three main groups (modified after Bmeckner and Medaris, 2000) ... 

When Beni Bousera and Ronda are excluded, the tme orogenic garnet peridotites have not been much studied from the geochemical point of view. As they witness deep mantle processes that are hardly accessible to the observation otherwise, these rocks represent an exciting field of investigation for the future. 

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