Iron isotope variations in spinel peridotite xenoliths from North China Craton: implications for mantle metasomatism

Byline: Xinmiao Zhao (1), Hongfu Zhang (1), Xiangkun Zhu (2), Suohan Tang (2), Yanjie Tang (1) Keywords: Iron isotopes; Mantle xenoliths; Lithospheric mantle; Mantle metasomatism; North China Craton Abstract: Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in [delt[a].sup.57]Fe are in a range of -0.25 to 0.14[per thousand] for olivine, -0.17 to 0.17[per thousand] for orthopyroxene, -0.21 to 0.27[per thousand] for clinopyroxene, and -0.16 to 0.26[per thousand] for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on '[delta]-[delta]' plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr , (La/Yb).sub.N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites. Author Affiliation: (1) State Key Laboratory of Lithopheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, 100029, Beijing, China (2) Laboratory of Isotopic Geology, MLR, Institute of Geology, Chinese Academy of Geological Sciences, 100037, Beijing, China Article History: Registration Date: 26/10/2009 Received Date: 12/12/2008 Accepted Date: 26/10/2009 Online Date: 16/11/2009 Article note: Communicated by F. Poitrasson. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0461-y) contains supplementary material, which is available to authorized users. An erratum to this article can be found at http://dx.doi.org/10.1007/s00410-010-0488-0