Modification of mantle rocks by plastic flow below spreading centers: Fe isotopic and fabric evidence from the Luobusa ophiolite, Tibet

Abstract

Abstract Combined Fe isotopic and olivine fabric analyses have been conducted on the nodular chromitites from the Luobusa ophiolite, Tibet, to study the physico-chemical effects of plastic flow in modifying chromitites and their associated mantle peridotites below spreading centers. The Luobusa nodular chromitites are generally composed of fresh dunitic matrices and well-aligned chromite nodules. Olivine grains in both the dunitic matrices and chromite nodules underwent pressure solution, and those in the dunitic matrices have prevalent Fe-rich stripes parallel to their kink bands and sometimes show lattice dislocation-induced preferred orientations of (1 0 0). Chromite grains in the chromite nodules have Cr# ∼ 75–80 and δ56Fe values from −0.122 to 0.067. The Fo and δ56Fe values of olivine in the dunitic matrices vary from 96 to 99 and 0.165 to 0.294, respectively, distinguishably higher than those of olivine in ordinary ophiolitic dunites (mostly 90–95 and 0.050–0.150). The calculated Mg-Fe exchange temperatures between olivine in the dunitic matrices and chromite in the chromite nodules range from 700 to 900 °C, overall higher than those for ordinary dunites and chromitites (∼600–800 °C). Combined with numerical modeling of diffusion of Fe in olivine, our study reveals that the abnormally high Fo and δ56Fe values of olivine in the dunitic matrices of nodular chromitites were not controlled by their parental magmas but caused by significant loss of Fe from olivine under subsolidus conditions. Intensive plastic deformation in nodular chromitites largely promoted the migration of Fe from olivine and facilitated the Mg-Fe exchange between the chromite nodules and dunitic matrices, maximizing the Fo and δ56Fe values of olivine. However, such an effect is expected to be negligible in chromite-barren dunites and harzburgites because such rocks do not have enough chromite to generate effective chemical-potential gradients for Fe migration. The high Fo values of olivine in nodular and massive chromitites require fast cooling rates (∼0.05–0.1 °C/yr) in the surrounding mantle, consistent with settings below intermediate-fast spreading ridges. Boudinization of small chromitite bodies in less competent dunites under 900–1200 °C is a potential mechanism for generating the nodular texture, also accounting for the en-echelon or bead-like occurrences of chromitite bodies in ophiolites. Effective mantle flow below spreading centers plays an important role in shaping the textures and compositions of mantle rocks, explaining the absence of deformation-related nodular chromitites in layered mafic-ultramafic intrusions.