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

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 Sr-Nd isotope compositions, suggesting their formation in a mid-ocean ridge setting. The rodingites exhibit variable 826Mg values from -0.61%o to -0.14%o, with an average of -0.37%o & PLUSMN; 0.28%o (n = 19; 2 SD), mostly lower than those of -0.26%o to -0.13%o for the protolith gabbros. Compared to the relatively homogeneous 818O values of 6.1%o-6.7%o for the gabbros, the rodingites show highly variable 818O values from 3.8%o to 9.4%o. 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 degrees C, and has higher 818O values of 8.1%o & PLUSMN; 1.4%o (n = 15) and 826Mg values of -0.32%o & PLUSMN; 0.19%o (n = 12). Group II rodingite consists mainly of hydrogarnet and clinopyroxene, formed at temperatures of 300-400 degrees C, and has lower 818O values of 5.4%o & PLUSMN; 2.5%o (n = 7) and 826Mg values of -0.44%o & PLUSMN; 0.35%o (n = 7). Modeling of the fluid-rock interactions between the protolith gabbro and fluid was undertaken using the Mg-O 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-826Mg fluids produced during serpentinization would result in the Mg-O isotope compositions of the rodingites. Such rodingites with low 826Mg values would generate the geochemical heterogeneity in the mantle wedge overlying the subducting oceanic lithosphere and eventually give rise to Ca-rich, low-826Mg basaltic magmas. The results indicate that low 826Mg values of basaltic rocks cannot be unambiguously ascribed to carbonated mantle sources.