Chromium mantle rocks

Liang Y. and Elthon D. (1990) Evidence from chromium abundances in mantle rocks for extraction of picrite and komatiite melts. Nature 343, 551-553. 

Chromium is a compatible element in the Earth s mantle, and tends be present in much greater concentrations in mafic igneous rocks than in felsic ones (Faure 1991). Ultramafic rocks often contain over 1,000 ppm Cr and can generate environmental problems when they weather (Robertson 1975 Robles and Armienta 2000). Granites may contain less than 20 ppm Cr, whereas shales contain roughly 90 ppm. 

Schoenberg et al. (2008) presented the first set of Cr isotope data for rocks and Cr(ll) rich ores. Mantle derived rocks and chromite ores from layered intrusions have a uniform Cr/ Cr isotope ratio very close to the certified Cr standard NIST SRM 979. The Cr isotope composition of hydrothermal lead chromates is substantially heavier (S Cr from 0.6 to 1.0%o) than the rocks from which the chromium was leached. 

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... 

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