Reaction relationships of chrome-spinels in igneous rocks ? further evidence from the layered intrusions of Rhum and Mull, Inner Hebrides, Scotland

Abstract

Compositions of chrome-spinels in peridotites from the layered igneous rocks of Rhum and of the Ben Buie intrusion, Mull, show evidence of post-depositional reaction relationships with the cumulus olivine and/or the intercumulus liquid. Some seam-forming spinels in the Ben Buie intrusion are highly aluminous, more so than those in the Rhum intrusion. Occasional zoned spinels, in both intrusions, show enrichment in Al towards their rims. The evidence suggests that the spinel reaction trend involving Cr-Al exchange proceeds in the direction of Al-enrichment. Equilibrium between chrome-spinel and cumulus olivine appears to have been attained rarely, thus limiting the application of the olivine-spinel geothermometer.