Spinel melting reactions

As another example, at 10 kbar, melting of a spinel peridotite has the following melting reaction in mass units (Kinzler and Grove, 1992)... 

The trend for orthopyroxene with depletion is more constant, whereas clinopyroxene and spinel show good positive correlations (Figures 3(a)-(c)). In melting reactions on the solidus, clinopyroxene contributes the most to the melt phase (Figure 2) and thus is consumed most rapidly with depletion until it is exhausted after —20% melting. 

The formation of replacive pyroxenites can be explained by melt-consuming reactions at pressure and temperature conditions close to the peridotite solidus. Ronda represents a situation where the reaction was associated with relaxation of thinned and thermally eroded subcontinental lithosphere (Garrido and Bodinier, 1999). Upon cooling of the melting domain developed during the thermal erosion (Lenoir et al., 2001), the interstitial melt became saturated in pyroxene ( aluminous phases) and reacted with olivine to produce secondary cpx, opx, and spinel... 

The modal abundances of ohvine, orthopyroxene, clinopyroxene and spinel observed for spinel peridotites from six well-characterized olf-craton xenolith suites are plotted against a depletion index in Figures 3(a)-(c). The amount of ohvine correlates negatively with degree of depletion, as expected because olivine is a product of the reaction that produces melt at the solidus (Figure 2) (see Chapter 2.08). The number of samples compiled (n = 143) may not be completely representative, but there is nonetheless a suspicious population gap at 2 wt.% AI2O3. 

In the oceanic setting, spinel Iherzolite xenoliths from Pali (Hawaii) have olivine 5 0 values of 5.09-5.12 per mil, typical of olivines from other oceanic and continental mantle rocks (Ducea et al., 2002). In contrast, olivines from plagioclase peridotites are enriched by 0.5 per mil. This is interpreted to be due to the formation of plagioclase by reaction with or crystallization from melts intruding the Pacific lithospheric mantle. 

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