Chromium peridotites

In detail, however, the picture is not so simple. All mantle peridotites (whether massive peridotites or xenoliths) are metamorphic rocks that have had a complex subsolidus history after melt extraction ceased. As well as subsolidus recrystallization, peridotites have undergone enormous amounts of strain during their emplacement in the lithosphere. Massive peridotites show modal heterogeneity on the scale of centimeters to meters, caused by segregation of the chromium-diopside suite of dikes, which are then folded back into the peridotite as deformation continues. The net result is more or less diffuse layers or bands in the peridotite, which may be either enriched or depleted in the material of the chromium-diopside suite, i.e., in climopyroxene and orthopyroxene in various proportions, minor spinel, and sulfide. This process should cause approximately linear correlations of elements versus MgO, broadly similar to, but not identical with, those caused by melt extraction. Indeed, there is... 

Figure 7 Plots of Ti02 (anhydrous wt.%), nickel, and chromium (anhydrous ppm) versus AI2O3 (anhydrous wt.%) in whole-rock orogenic, ophiolitic, and abyssal peridotites. The symbols and the data sources are the same as in...

In a given facies, the sodium and titanium content of clinopyroxene shows a negative correlation with depletion (or chromium content) and can increase substantially in metasomatized xenoliths. The substimtion of sodium, chromium, and aluminum has a complex relationship with T, P and degree of depletion but in general, clinopyroxene from higher P samples contains more sodium. Representative analyses for different peridotite facies are shown in Table 4. 

Figure 5 Covariation of CaO with Ct203 in garnets from a large database (n = 900) from a wide variety of xenoUths in kimberlites and other alkaline rocks. Note the positive correlation of calcium and chromium in peridotite garnets. Garnets below the line are harzbur-gitic, whereas those above it are Iherzolitic or wehrlitic.

The Cr-diopside series is the most abundant type of xenolith found in alkali basalts. Amphibole is uncommon in samples of this series, but rare examples have been found from locations across the world (see review by Kempton, 1987). The amphibole is typically a chromium-rich pargasite and has been observed to constitute up to 6% of the mode. Commonly, these amphiboles have partially broken down, a process interpreted to be a response to the incorporation of the xenolith into the ascending host magma. Phlogopite seems to be less commonly observed in spinel peridotites, but is present along with amphibole in some suites (Kempton, 1987 and references therein). In other suites, phlogopite is the only hydrous phase present (Francis, 1987 Canil and Scarfe, 1989). 

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