Magnesium–oxygen isotope constraints on the origin of rodingites in oceanic lithosphere

Abstract

Rodingite is produced by Ca-rich fluid metasomatism of mafic igneous rocks in oceanic lithosphere. Despite its importance for the composition of subduction zone magmas, its formation mechanism still remains obscure. To address this issue, we investigated the major-trace element and Mg–O–Sr–Nd isotope compositions of rodingites in serpentinites from the Xigaze ophiolite, southern Tibet. Coexisting gabbros and rodingites show consistently depleted SrNd isotope compositions, suggesting their formation in a mid-ocean ridge setting. The rodingites exhibit variable δ26Mg values from −0.61‰ to −0.14‰, with an average of −0.37‰ ± 0.28‰ (n = 19; 2 SD), mostly lower than those of −0.26‰ to −0.13‰ for the protolith gabbros. Compared to the relatively homogeneous δ18O values of 6.1‰–6.7‰ for the gabbros, the rodingites show highly variable δ18O values from 3.8‰ to 9.4‰. The rodingites can be subdivided into two groups based on mineral assemblages and formation conditions. Group I rodingite is prehnite-dominated and hydrogarnet-free, formed at temperatures of 200–300 °C, and has higher δ18O values of 8.1‰ ± 1.4‰ (n = 15) and δ26Mg values of −0.32‰ ± 0.19‰ (n = 12). Group II rodingite consists mainly of hydrogarnet and clinopyroxene, formed at temperatures of 300–400 °C, and has lower δ18O values of 5.4‰ ± 2.5‰ (n = 7) and δ26Mg values of −0.44‰ ± 0.35‰ (n = 7). Modeling of the fluid–rock interactions between the protolith gabbro and fluid was undertaken using the MgO isotope compositions of the serpentinizing fluid inferred from the associated serpentinites. Based on the modeling results, we propose that the metasomatism of gabbros by Ca-rich, low-δ26Mg fluids produced during serpentinization would result in the MgO isotope compositions of the rodingites. Such rodingites with low δ26Mg values would generate the geochemical heterogeneity in the mantle wedge overlying the subducting oceanic lithosphere and eventually give rise to Ca-rich, low-δ26Mg basaltic magmas. The results indicate that low δ26Mg values of basaltic rocks cannot be unambiguously ascribed to carbonated mantle sources.