Deformation indicator in the upper mantle based on quantitative analysis of kinetic demixing in chromian spinel

Chromian spinel is one of the few minerals in which kinetic demixing can be induced by diffusion creep; thus it is an useful indicator of deformation conditions in the upper mantle. Cr-Al demixing textures in chromian spinel were reported for naturally deformed peridotites by Ozawa (1989) and produced experimentally in samples deformed in the diffusion creep regime (Suzuki et al., 2009). In this study, we focus on quantitative analysis of deformation conditions of chromian spinels and its host rocks using peridotites from the Oman ophiolite. We analyzed 7 samples -- three from dunites, three from harzburgites, and one from chromitite. Spinel grains in dunites and harzburgites are discrete, surrounded by olivine and pyroxene, while those in chromitite contact each other. Based on elemental distribution maps, an increase of Cr and a decrease of Al occur at phase and/or grain boundaries normal to the shortening direction, while an increase of Al and a decrease of Cr exist normal to the elongation direction. The stress distribution indicated by the Cr-Al demixing textures is consistent with that determined from the surrounding minerals. We obtained Cr# (=Cr/(Cr+Al)) gradients in grains from line profiles of Cr and Al measured with an electron microprobe. Demixing extended from phase and/or grain boundaries into grain interiors. The differential stresses during diffusion creep of chromian spinels were estimated using the composition gradient of demixing as proposed by Dimos et al. (1988) with the assumption of steady state creep. Strain rates were estimated using the flow law obtained by Suzuki & Kohlstedt (2010), and time scales of deformation were calculated from the strain rate and strain with the assumption of equiaxed initial grains. The estimated stresses in three types of rocks were clearly distinguishable, with stresses in dunites being the smallest and stresses in chromitite the largest. The stresses recorded in chromian spinels (70-100 MPa in harzburgites) are much greater than those estimated from the recrystallized grain size of the surrounding olivine. This observation suggests that spinel grains recorded deformation conditions at an earlier stage than that recorded in olivine.