Origin of rodingites in the Xigaze ophiolite, SW Tibet: Constraints from Ca and Sr isotopes

Rodingites, extremely calcium-rich but sodium- and silicon-poor rocks forming through fluid-rock reactions, are crucial for understanding the oceanic cycling of materials. However, the source of calcium in rodingites remains debated. Here, we investigate whole rock Ca and Sr isotopic compositions of samples collected across a continuous section of a rodingite dyke from the Xigaze ophiolite, SW Tibet. The rodingite dyke exhibits progressive metasomatism from core to margin, defining central, transitional, and marginal zones. The dyke displays a wide range of δ^44/40Ca values from 0.28 ‰ to 1.63 ‰ but limited variation in Sr isotope (⁸⁷Sr/⁸⁶Sr_(i) = 0.70308–0.70438). The slightly metasomatized central rodingites exhibit lower δ^44/40Ca values (0.53 ‰ to 0.68 ‰) than MORB, whereas the moderately metasomatized transitional rodingites display significantly lighter Ca isotope compositions (δ^44/40Ca = 0.28 ‰ to 0.61 ‰). The intensively metasomatized marginal rodingites show the highest δ^44/40Ca values (0.64 ‰ to 1.63 ‰). The transverse profile of δ^44/40Ca values across the rodingite dyke reveals clear oscillatory zoning, suggesting multiple fluid sources for Ca during rodingitization. In the early-stage, the mafic dyke was metasomatized by a carbonate-bearing fluid characterized by low δ^44/40Ca values, which originated from the mixing of deserpentinization-related fluids with marine carbonates during subduction. Subsequently, the dyke experienced more intense metasomatism by a high-δ^44/40Ca serpentinizing fluid, leading to additional Ca input and an elevation of δ^44/40Ca values. The gradual increase in ⁸⁷Sr/⁸⁶Sr_(i) ratios from the central zone to the marginal zone indicates that the primary fluids were derived from seawater-triggered serpentinization. This study proposes that during the initial stages of subduction fluids released by serpentinite dehydration dissolved sedimentary carbonates on the subducted oceanic slab. As subduction progressed, extensive dehydration of the surrounding serpentinites produced high-δ^44/40Ca serpentinizing fluids, which further elevated both δ^44/40Ca and ⁸⁷Sr/⁸⁶Sr_(i) values of the dyke. The highly variable δ^44/40Ca values of rodingites from diverse tectonic settings support the model involving multiple fluid contributions with distinct Ca isotopic compositions during rodingitization.