Open AccessIron and magnesium isotopic constraints on the origin of chemical heterogeneity in podiform chromitite from the Luobusa ophiolite, Tibet
Author(s) -
Xiao Yan,
Teng FangZhen,
Su BenXun,
Hu Yan,
Zhou MeiFu,
Zhu Bin,
Shi RenDeng,
Huang QiShuai,
Gong XiaoHan,
He YongSheng
Publication year - 2016
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1002/2015gc006223
Subject(s) - chromite , olivine , ophiolite , chromitite , geology , geochemistry , mantle (geology) , peridotite , mineralogy , paleontology , tectonics
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 δ 56 Fe values ranging from 0‰ to +0.083‰ in olivine, from −0.034‰ to +0.081‰ in orthopyroxene and δ 26 Mg values ranging from −0.25‰ to −0.20‰ in olivine, from −0.29‰ to −0.26‰ in orthopyroxene, respectively. The olivines of two dunites from the Luobusa display small Fe and Mg isotopic variations, with δ 56 Fe values of +0.014‰ and +0.116‰ and δ 26 Mg values of −0.21‰ and −0.29‰. All chromites in the Luobusa chromitites have lighter Fe isotopic compositions than the coexisting olivines, with δ 56 Fe values ranging from −0.247‰ to +0.043‰ in chromite and from −0.146‰ to +0.215‰ in olivine (Δ 56 Fe Chr‐Ol = −0.294 to −0.101‰). The chromite δ 26 Mg values span a significant range from −0.41‰ to +0.14‰. Large disequilibrium Fe and Mg isotope fractionation between chromite and olivine, as well as positive correlation of chromite δ 56 Fe 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.