Amphibole as a witness of chromitite formation and fluid metasomatism in ophiolites

Abstract

This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

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

“…The ∆ 26 Mg Chr‐Ol value was thus enlarged in this process (Figure 7b). Orthopyroxenes in ophiolitic harzburgites may have undergone modifications similar to those of olivine (Chen et al., 2015; Pan et al., 2021; Xiao et al., 2016), and the harzburgites in the vicinity of the podiform chromitites would likely contain newly crystallized chromite from the fluids, in addition to those in the melt residue. Thus, immiscible fluids play a critical role in ion‐exchange between olivine and chromite.…”

“…In many ophiolitic harzburgites, olivine Fo and chromite Cr# do not strictly follow the olivine‐spinel mantle array defined by Arai (1994), and are incompatible with estimates from trace elemental and isotopic compositions of bulk rocks and clinopyroxenes. Thus, these discrepancies have been ascribed to various metasomatic processes (e.g., Dick & Bullen, 1984; Kelemen et al., 1992; Pan et al., 2021). Essentially, mantle‐derived melts should theoretically be more enriched in Fe and heavy Fe isotopes than the residue because during partial melting Fe is less refractory than Mg, and compared to Fe 2+ , Fe 3+ is more incompatible and prefers accommodation of heavy isotopes (Dauphas, Craddock, et al., 2009; Weyer & Ionov, 2007).…”

“…Mantle peridotites in ophiolites represent oceanic lithospheric mantle, and normally experience melt extraction and melt/fluid modification (e.g., Kelemen et al, 1992;Pan et al, 2021;Su et al, 2018Su et al, , 2019Su et al, , 2021Xiong et al, 2015). The degrees of melt extraction are mainly evaluated by using compositions of refractory olivine and chromite.…”

“…In this process, olivine grains crystallizing from the melts are preferentially attached to the fluid-melt interface (Matveev & Ballhaus, 2002;McDonald, 1965), and they formed dunite envelopes around podiform chromitite (Figure 10c). The fluids cannot be structurally accommodated in chromite and when released from the pools they penetrate and modify the surrounding dunite and harzburgite (Pan et al, 2021;Su et al, 2020Su et al, , 2021 (Figures 10c and 10d).…”

“…Because of high water contents, the fluids may hydrate or alter the olivine crystals (Pan et al, 2021;Su et al, 2019Su et al, , 2020Su et al, , 2021. The extent of fluid-olivine reaction depends on fluid volumes that are related to chromite concentrations; more chromite grains in the pool cause more fluids to be collected and subsequently released.…”