Iron isotope evidence for multistage melt-peridotite interactions in the lithospheric mantle of eastern China

Iron isotopic data of the Cenozoic alkaline basalts and peridotite xenoliths entrained in the Cenozoic and Mesozoic basalts, eastern China, are reported to further constrain the Fe isotopic composition of the mantle and investigate the behavior of Fe isotopes during mantle processes. Nine Cenozoic alkaline basalts show limited variation in delta Fe-57 that are within the ranges of oceanic basalts, which indicate homogeneous Fe isotope composition of the basalts. The Iherzolites from the South China Block also display a limited Fe isotopic variation delta Fe-57 of 0.041 to 0.077 parts per thousand and an average of 0.060 parts per thousand +/- 0.029 parts per thousand. (2 SD). This Fe isotopic composition is similar to that of the bulk silicate Earth, indicating that these samples represent mantle residues that underwent just limited degrees of partial melting. In contrast, both the mantle peridotites and their mineral separates from the North China Craton exhibit an extremely large Fe isotopic variation with delta Fe-57 ranging from -1.0029 parts per thousand. to 0.232 parts per thousand. This may reflect heterogeneous Fe isotopic compositions of the lithospheric mantle beneath the North China Craton. An average delta Fe-57 (-0.066 parts per thousand) calculated from all the mantle xenoliths in this study is obviously lower than the average value (0.198 parts per thousand) of delta Fe-57 for the Cenozoic alkaline basalts, consistent with the previous observation that basalts generally have heavier Fe isotopes than mantle peridotites. In individual minerals, delta Fe-57 of the olivines (-0.997 to 0.154 parts per thousand) are systematically lighter than those of the coexisting pyroxenes (-0.718 to 0.169 parts per thousand for orthopyroxene and -0.642 to 0.304 parts per thousand for clinopyroxene, respectively). Phlogopite of apparently metasomatic origin has the heaviest iron isotopic composition amongst the mineral phases with delta Fe-57 of 0.302 to 0.376 parts per thousand. Extreme Fe isotope variation in these peridotites, in particular for the wehrlites, was probably produced by multistage melt-peridotite interactions, consistent with the petrological observation that some of the xenoliths analyzed such as wehrlites were the products of extensive mantle metasomatism. Positive correlations between delta Fe-57 and other major and trace element indicators of metasomatism such as CaO and Rb further support the above suggestion. These characteristics suggest that mantle metasomatism or melt-rock interaction can significantly modify Fe isotopes and play an important role in producing Fe isotopic heterogeneity of the lithospheric mantle. Thus, Fe isotopes can provide important information concerning melt-rock interaction and evolution of the lithospheric mantle. (C) 2011 Elsevier B.V. All rights reserved.