Trace Elements in Olivine Reveal Lithological and Redox Heterogeneity in the Mantle Source of Carbonated Silicate Melts: Constraints from Cenozoic Melilitites in the West Qinling Orogen, China

Abstract The speciation of carbon in the mantle is predominantly governed by oxygen fugacity (fO2). Under oxidized conditions, carbon is present as carbonate minerals or CO2, which significantly lowers the solidus of mantle lithologies, inducing partial melting and generating carbonatitic to carbonated silicate melts. In contrast, reduced forms such as diamond or graphite remain immobile. Therefore, the formation of carbonated silicate melts and the deep carbon cycling are critically influenced by both the redox state of the mantle and the presence of carbon-rich lithologies. However, systematic studies investigating the lithology and redox state of mantle source remain scarce. Here, we present high-precision major and trace element compositions of olivine phenocrysts from Cenozoic melilitites in the West Qinling orogen, China, to investigate the lithology and redox conditions of the mantle source for carbonated silicate melts. Distinct olivine compositions and redox states are observed among different regions. Olivines from Baiguan and Jiangkou display low Ni, high Ca, elevated Mn/Fe and Mn/Zn ratios, and high fO2 (ΔFMQ +0.55 ± 0.43, 1σ). In contrast, olivines from Baihe exhibit high Ni, low Ca, low Mn/Fe and Mn/Zn ratios, and lower fO2 (ΔFMQ −0.14 ± 0.24, 1σ). Quantitative modeling indicates that the compositional diversity cannot be attributed to fractional crystallization, diffusion, or variations in fO2 during melt evolution. Instead, they point to distinct mantle lithologies: an oxidized, carbonated peridotite source for the Baiguan and Jiangkou melilitites and a reduced pyroxenite source for the Baihe melilitites. Thermobarometric calculations suggest that the West Qinling melilitites formed at pressures of 4.4 ± 0.2 GPa (1σ) and mantle potential temperatures of 1376 ± 27 °C (1σ). When corrected to the source conditions, the fO2 of carbonated peridotite mantle source is ΔFMQ −0.21 ± 0.52 (1σ), corresponding to mantle Fe3+/ΣFe ratios of 7%–10%, significantly higher than that of ambient mantle and lithospheric mantle. These findings suggest that a highly oxidized, carbon-rich mantle source, rather than elevated mantle temperatures, is essential for the generation of carbonated silicate melts. Coupled with stable Mg-Ca-Zn isotope data, we propose that subducted carbonate sediments react with reduced ambient mantle at transition zone depth, either through carbonate melts or in the solid state, producing Fe3+- and carbon-rich peridotites. During subsequent adiabatic ascent, these peridotites undergo redox melting, generating oxidized carbonated silicate melts.