Qi-Shuai Huang

Iron and magnesium isotopic constraints on the origin of chemical heterogeneity in podiform chromitite from the Luobusa ophiolite, Tibet

We present high-precision measurements of iron (Fe) and magnesium (Mg) isotopic compositions of olivine, orthopyroxene, and chromite separates from harzburgites, dunites, and chromitites in the mantle section of the Luobusa ophiolite, southern Tibet, to investigate the origins of podiform chromitite. Two harzburgites in the Zedong ophiolite, southern Tibet, are also reported for comparison. The olivine and orthopyroxene in the Luobusa and Zedong harzburgites have similar Fe and Mg isotopic compositions, with Fe-56 values ranging from 0 to +0.083 in olivine, from -0.034 to +0.081 in orthopyroxene and Mg-26 values ranging from -0.25 parts per thousand to -0.20 parts per thousand in olivine, from -0.29 parts per thousand to -0.26 parts per thousand in orthopyroxene, respectively. The olivines of two dunites from the Luobusa display small Fe and Mg isotopic variations, with Fe-56 values of +0.014 parts per thousand and +0.116 parts per thousand and Mg-26 values of -0.21 parts per thousand and -0.29 parts per thousand. All chromites in the Luobusa chromitites have lighter Fe isotopic compositions than the coexisting olivines, with Fe-56 values ranging from -0.247 parts per thousand to +0.043 parts per thousand in chromite and from -0.146 parts per thousand to +0.215 parts per thousand in olivine (Fe-56(Chr-Ol)=-0.294 to -0.101 parts per thousand). The chromite Mg-26 values span a significant range from -0.41 parts per thousand to +0.14 parts per thousand. Large disequilibrium Fe and Mg isotope fractionation between chromite and olivine, as well as positive correlation of chromite Fe-56 values with their MgO contents, could be attributed to Fe-Mg exchange between chromite and olivine. In the disseminated chromitites, the higher modal abundances of olivine than chromite would result in a more extensive Fe-Mg exchange, whereas chromite in the massive chromitite where olivine is rare could not be affected by this process.