Temperature-dependent distribution of Cr between olivine and pyroxenes in lherzolite xenoliths

Abstract

The temperature effect on the exchange reaction Cr2O3(ol)=Cr2O3(px) was studied for coexisting olivine and both clino and ortho pyroxenes. The distribution of Cr between olivine and clinopyroxene in 31 coarse garnet lherzolites and 10 porphyroclastic garnet lherzolites from kinberlites, and in 17 coarse spinel lherzolites from basalts, obeys a van't Hoff relation (c.f. Stosch 1981) with the Wells two-pyroxene temperature: T(Kelvin)=8,787 (In DCr+ 2.87) where DCr(opx/ol)=wt.% Cr(clinopyroxene)/Cr(olivine). An analogous exchange for olivine and orthopyroxene with 0.7–1.6 wt.% Al2O3 in 41 garnet lherzolites from kimberlites shows considerable scatter about the following relation: T(Kelvin)=5,540/(ln Dcr+1.86) where Dcr(opx/ol)= wt.% Cr(orthopyroxene)/Cr(olivine). Spinel lherzolites and a garnet lherzolite from the Malaita alnoite do not obey the second relation. For orthopyroxene with 2.5–5.1 wt.% Al2O3, Dcr(opx/ol) is 1.7 to 3 times higher, and for 0.1 wt.% Al2O3 is 2 times lower than for the garnet lherzolites. Experimental calibration is needed, especially to check the possible effect of Al on Dcr(opx/ol).