Pyroxenes mantle

The outer 1.5 mm is a mantle of polycrystalline melilite whose Mg content increases radially inwards, as does the abundance of included spinel. The interior of the inclusion is predominantly coarsely crystalline Ti-Al-pyroxene, melilite, and anorthite, all containing euhedral crystals of spinel. The entire inclusion is bounded by a fine-grained rim of complex mineralogy. The inclusion is 1.5 cm in diameter. 

Figure 5. A plot of A Mg vs. 5 Mg for terrestrial Mg materials. Within best estimates of uncertainties (cross) all of the data lie in the region bounded by equilibrium and kinetic mass fractionation laws. Waters, carbonates, and organic Mg (chlorophyll) have higher A Mg values than mantle and crustal Mg reservoirs represented by mantle pyroxene, loess, and continental basalts. The difference in A Mg values is attributable to episodes of kinetic mass fractionation.

Pyroxenes (as feldspars) occupy a position that is chemically central in the realm of rock compositions (Robinson, 1982). They are therefore found ubiquitously and virtually in any kind of paragenesis developing in the P-T-Xconditions typical of earth s mantle and crust. Table 5.31 hsts the main occurrences. 

Samples 6 and 7 in table 5.32 are from the Zabargad peridotite (Red Sea) and are representative of the chemistry of upper mantle pyroxenes (Bonatti et al., 1986). The absence of Fe203 in these samples is due to the fact that microprobe analyses do not discriminate the oxidation state of iron, which is thus always expressed as FeO. It must be noted here that the observed stoichiometry (based on four oxygen ions) is quite consistent with the theoretical formula and that no Fe is required to balance the negative charges of oxygen. 

Table 5.32 Compositions (in weight %) of natural pyroxenes (samples 1-5 from Deer et al., 1983 samples 6 and 7 from Bonatti et al., 1986) (1) enstatite from a pyroxenite (2) ferrosilite from a thermometamorphic iron band (3) hedembergite (4) chromian augite from a gabbroic rock of the Bushveld complex (5) aegirine from a riebeckite-albite granitoid (6) diopside from a mantle peridotite (7) enstatite from a mantle peridotite. ...

Because Li isotopes may be used as a tracer to identify the existence of recycled material in the mantle, systematic studies of arc lavas have been undertaken (Morignti and Nakamura 1998 Tomascak et al. 2000 Leeman et al. 2004 and others). However, most arc lavas have 5 Li values that are indistinguishable from those of MORB. Thus Li seems to be decoupled from other fluid mobile elements, becanse Li can partition into Mg-silicates (pyroxene, olivine) in the mantle (Tomascak et al. 2002). 

Water in the mantle is fonnd in different states as a fluid especially near sub-duction zones, as a hydrous phase and as a hydroxyl point defect in nominally anhydrous minerals. 8D-values between -90 and -110%c have been obtained by Bell and Ihinger (2000) analyzing nominally anhydrous mantle minerals (garnet, pyroxene) containing trace quantities of OH. Nominally anhydrous minerals from mantle xenoliths are the most D-depleted of all mantle materials with 5D-values 50%c lower than MORE (O Leary et al. 2005). This difference may either imply that these minerals represent an isotopically distinct mantle reservoir or that the samples analyzed have exchanged hydrogen dnring or after their ascent from the mantle (meteoric/water interaction ). 

Backscattered electron image of a Type CAI in the Allende CV3 chondrite. This object crystallized from a melt and has an outer mantle of melilite (Mel) surrounding an inner zone of pyroxene (Px) and anorthite (An). Tiny grains of spinel (Sp) are abundant in the inner portion of the inclusion. Image courtesy of A. Krot. 

The formation of the Moon s crust, composed primarily of feldspar (the rock is called anorthosite) illustrates how physical fractionation can occur during differentiation. Early in its history, a significant portion of the Moon was melted to form a magma ocean. The first minerals to crystallize, olivine and pyroxene, sank because of their high densities and formed an ultramafic mantle. Once feldspar began to crystallize, it floated and accumulated near the surface to produce the crust. 

Although Cr2+ ions are rare and unstable in terrestrial minerals, their presence is suspected in olivines and pyroxenes from the Earth s Mantle and the Moon (Bums, 1975a Smith, 1971). Crystal field spectra exist for these silicates, as well as other synthetic Cr2+-bearing phases, and parameters are summarized in table 5.12. Just one spin-allowed transition, corresponding to 5Eg - 5T2g, might... 

The olivine spinel phase transition Experimental phase equilibrium studies have confirmed deductions from seismic velocity data that below 400 km, olivine and pyroxene, the major constituents of Upper Mantle rocks, are transformed to denser polymorphs with the garnet, y-phase (spinel) and P-phase (wadsleyite) structures (fig. 9.2). In transformations involving olivine to the P- or y-phases, transition pressures... 

Radiative heat transport through olivine has been discussed extensively (e.g., Fukao et al., 1968 Shankland, 1970 Schatz and Simmons, 1972 Scharmeli, 1979 Shankland et al., 1979). The radiative thermal conductivity, Kt of forsteritic olivine increases with rising temperature and would contribute to heat flow in the Upper Mantle (Shankland et al., 1979). However, values of Kt for olivine are considered to be rather low to satisfactorily explain the dissipation of the Earth s internal heat by radiation and lattice conduction alone. Note, however, that Fe2 CF transitions in almandine, pyroxenes (M2 site) and, perhaps, silicate perovskites absorb strongly in the wavelength range 1,250 to... 

The pyroxenes are the most abundant minerals, after olivine, in perido-tites, which are the dominant constituents of the upper mantle. It is not surprising, therefore, that there has been considerable interest in the mechanical properties of the pyroxenes, and a review has recently been given by Doukhan et al. (1986). The orthorhombic pyroxenes deform by slip and by a shear transformation that produces monoclinic lamellae (one or a few unit cells thick) parallel to (100). Coe and Kirby (1975) and McLaren and Etheridge (1976) have shown that the shear transformation is achieved by the glide of partial dislocations of b = 0.83[001] in (100), which leave partial dislocations of b = 0.17[001] terminating the shear lamellae. The dominant slip system is (100) [001]. Recent TEM observations by van Duysen, Doukhan, and Doukhan (1985) suggest that the dislocations associated with this slip system may be dissociated into four partials and that the slip system (100) [010] may also be activated. These observations are discussed in Section 9.9.1. 

Silicates and oxides in pallasites Olivine (F080-90). chromite, low-Ca pyroxene (En83 9i) Coarse-grained, rounded to angular Cumulates from core-mantle boundary... 

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