Extremely heavy Ca isotope compositions of rodingites in the oceanic lithosphere formed by fluid-rock interaction: Implications for the discrimination of light Mg isotope components

Abstract

Rodingite is a crucial component of the altered oceanic lithosphere and is an extremely Ca-rich rock (CaO content reaching ca. 30 wt%). However, its Ca isotope composition remains unclear. Furthermore, rodingite and carbonate represent two common Ca-rich reservoirs in the oceanic lithosphere characterized by light Mg isotope compositions. Currently, it remains unclear how these two rock reservoirs contribute to the light Mg isotope composition of some igneous rocks. In this study, we present the Ca isotope compositions of rodingites and their protolith gabbros from the Xigaze ophiolites in southern Tibet. The gabbros exhibit relatively homogeneous δ^44/40Ca values ranging from 0.70 to 0.87 ‰, which are comparable to the MORB composition, while rodingites display significantly higher δ^44/40Ca values ranging from 0.92 to 1.89 ‰. Based on formation temperature conditions and mineral assemblages, rodingites can be categorized into two groups that exhibit contrasting correlations with respect to the ⁸⁷Sr/⁸⁶Sr isotope ratios, bulk Sr contents and Sr/Ca ratios. Group I δ^44/40Ca values show a positive correlation with (⁸⁷Sr/⁸⁶Sr)_i but negative correlations with Sr/Ca ratios and Sr contents. Group II δ^44/40Ca values exhibit a negative correlation with (⁸⁷Sr/⁸⁶Sr)_i but positive correlations with Sr/Ca ratios and Sr contents. The opposing relationships between δ^44/40Ca values and geochemical compositions for Group I and II rodingites suggest Ca additions from different fluid sources and fluid compositions during rodingitization. Initially, seawater predominates the metasomatic fluids during the early stages of rodingitization, however, serpentinizing fluid that dissolves clinopyroxene and orthopyroxene becomes dominating as the fluid-rock reaction progresses. The results indicate that fluid-rock reactions occurring during rodingitization control the Ca isotope composition of the resulting rodingites. Rodingite represents a previously unrecognized reservoir of high Ca contents and δ^44/40Ca values in the altered oceanic lithosphere, which could significantly influence mantle Ca isotope compositions through oceanic slab subduction. In addition, due to the distinct δ^44/40Ca values between rodingite and carbonate-related components, Ca isotopes may serve as an effective tool for distinguishing these light Mg isotope components in the oceanic lithosphere, which is critical for utilizing CaMg isotopes as powerful tracers in crust-mantle interaction and carbonate recycling processes.