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The Critical Role of Fluid‐Mediated Diffusion in Anomalous Fe‐Mg‐O Isotope Fractionations in Ultramafic Rocks of Ophiolites
Author(s) -
Su BenXun,
Chen Chen,
Xiao Yan,
Robinson Paul T.,
Liu Xia,
Wang Jing,
Uysal Ibrahim,
Bai Yang,
Sun Yang
Publication year - 2021
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2020jb020632
Subject(s) - chromite , olivine , geology , ophiolite , chromitite , ultramafic rock , geochemistry , fractional crystallization (geology) , mineralogy , mantle (geology) , paleontology , tectonics
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 δ 56 Fe 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 δ 26 Mg values of −0.36‰ to −0.26‰, whereas chromite grains have variable but relatively high δ 26 Mg values ranging from −0.02‰ to 0.45‰. The δ 18 O values of olivine range from 4.65‰ to 5.86‰ and co‐vary with δ 7 Li (−5.43‰ to 16.65‰), δ 56 Fe, and δ 26 Mg, 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 δ 56 Fe, 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 δ 7 Li 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.