The Critical Role of Fluid-Mediated Diffusion in Anomalous Fe-Mg-O Isotope Fractionations in Ultramafic Rocks of Ophiolites


Su B., Chen C., Xiao Y., Robinson P. T. , Liu X., Wang J., ...More

JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, vol.126, no.4, 2021 (Journal Indexed in SCI) identifier identifier

  • Publication Type: Article / Article
  • Volume: 126 Issue: 4
  • Publication Date: 2021
  • Doi Number: 10.1029/2020jb020632
  • Title of Journal : JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH
  • Keywords: Chromite, Fe-Mg-O isotopes, fluid immiscibility, olivine, ophiolite, POZANTI-KARSANTI OPHIOLITE, GEOLOGICAL REFERENCE MATERIALS, PODIFORM CHROMITE DEPOSITS, SOUTHERN TURKEY, SUBDUCTION INITIATION, PERIDOTITE XENOLITHS, OCEANIC LITHOSPHERE, KIZILDAG OPHIOLITE, LUOBUSA OPHIOLITE, PARENTAL MAGMAS

Abstract

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 Pozanti-Karsanti 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 delta Fe-56 ranging from -0.125 parts per thousand to 0.101 parts per thousand and -0.261 parts per thousand to 0.191 parts per thousand, respectively, but with heavier olivine than coexisting chromite in most samples. In contrast, olivine records homogeneous Mg isotopic compositions with delta Mg-26 values of -0.36 parts per thousand to -0.26 parts per thousand, whereas chromite grains have variable but relatively high delta Mg-26 values ranging from -0.02 parts per thousand to 0.45 parts per thousand. The delta O-18 values of olivine range from 4.65 parts per thousand to 5.86 parts per thousand and co-vary with delta Li-7 (-5.43 parts per thousand to 16.65 parts per thousand), delta Fe-56, and delta Mg-26, 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 delta Fe-56, 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 delta Li-7 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.