Spinel 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 14. Inter-mineral Fe isotope fractionations among olivine and clinopyroxene from spinel peridotite mantle xenoliths. Data are from Zhu et al. (2002) ( ) and Beard and Johnson (2004) ( ). In the study by Beard and Johnson (2004), the difference in the Fe isotope composition between clinopyroxene and olivine is larger as a function of their 5 Fe values, suggesting disequilibrium fractionation.

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 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... 

Partitioning of iron in post-spinel phases in the Lower Mantle... 

The transformation of olivine to spinel which delineates the onset of the Transition Zone in the Mantle is isochemical. However, in the Lower Mantle the (Mg,Fe)2Si04 stoichiometry appears to be unstable with respect to denser oxide structure-types. One post-spinel transformation that has been the subject of numerous investigations is the spinel to periclase plus perovskite transition... 

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