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DOI	10.1029/2020JB020632
	The Critical Role of Fluid-Mediated Diffusion in Anomalous Fe-Mg-O Isotope Fractionations in Ultramafic Rocks of Ophiolites
	Su B.-X.; Chen C.; Xiao Y.; Robinson P.T.; Liu X.; Wang J.; Uysal I.; Bai Y.; Sun Y.
发表日期	2021
ISSN	21699313
卷号	126 期号:4
英文摘要	A number of bulk-rock and mineral Fe and Mg isotope data have been increasingly reported from ophiolitic rocks, but their anomalous fractionations are still not well addressed. This study presents new Fe, Mg, and O isotopes and published Li isotope data of olivine and/or chromite in chromitite, dunite, and harzburgite from the Pozantı-Karsantı ophiolite (southern Turkey) to reveal the mechanisms of inter- and intra-mineral isotope fractionation during chromitite formation. In the studied samples, both olivine and chromite have highly variable δ56Fe ranging from −0.125‰ to 0.101‰ and −0.261‰ to 0.191‰, respectively, but with heavier olivine than coexisting chromite in most samples. In contrast, olivine records homogeneous Mg isotopic compositions with δ26Mg values of −0.36‰ to −0.26‰, whereas chromite grains have variable but relatively high δ26Mg values ranging from −0.02‰ to 0.45‰. The δ18O values of olivine range from 4.65‰ to 5.86‰ and co-vary with δ7Li (−5.43‰ to 16.65‰), δ56Fe, and δ26Mg, particularly at lithological scale. These isotopic features cannot be explained solely by subsolidus diffusion between olivine and chromite, or by partial melting and fractional crystallization. Instead, they require fluids as a diffusive medium for high efficient element exchange between chromite and olivine. The fluids were likely the hydrous fluids that were collected on chromite grain surfaces during crystallization and released later from chromite piles during precipitation. The fluid-olivine reaction preferentially transferred light Fe from olivine grains resulting in elevated δ56Fe, but caused small O and Mg isotopic variations in the olivine. The Li diffusion depends on Li concentration gradient between olivine and fluid and olivine/fluid ratio, which can account for large δ7Li variations in olivine between different rock types. As these reactions proceeded, they induced elemental and isotopic variations in the fluids. These fluids could continuously penetrate the harzburgites through the dunite envelopes around the chromitites, producing some of the observed anomalous geochemical features of the harzburgites. Thus, we conclude that the hydrous fluids acted as critical media for ion exchange between chromite and olivine and for significant modification of the host harzburgites. © 2021. American Geophysical Union. All Rights Reserved.
英文关键词	Chromite; Fe-Mg-O isotopes; fluid immiscibility; olivine; ophiolite
语种	英语
来源期刊	Journal of Geophysical Research: Solid Earth
文献类型	期刊论文
条目标识符	http://gcip.llas.ac.cn/handle/2XKMVOVA/187215
作者单位	Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China; Innovation Academy for Earth Science, Chinese Academy of Sciences, Beijing, China; College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, China; State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China; Department of Geological Engineering, Karadeniz Technical University, Trabzon, Turkey
推荐引用方式 GB/T 7714	Su B.-X.,Chen C.,Xiao Y.,et al. The Critical Role of Fluid-Mediated Diffusion in Anomalous Fe-Mg-O Isotope Fractionations in Ultramafic Rocks of Ophiolites[J],2021,126(4).
APA	Su B.-X..,Chen C..,Xiao Y..,Robinson P.T..,Liu X..,...&Sun Y..(2021).The Critical Role of Fluid-Mediated Diffusion in Anomalous Fe-Mg-O Isotope Fractionations in Ultramafic Rocks of Ophiolites.Journal of Geophysical Research: Solid Earth,126(4).
MLA	Su B.-X.,et al."The Critical Role of Fluid-Mediated Diffusion in Anomalous Fe-Mg-O Isotope Fractionations in Ultramafic Rocks of Ophiolites".Journal of Geophysical Research: Solid Earth 126.4(2021).