Transformation of silica-rich to carbonated silicate melts in a big mantle wedge revealed by geochemistry of mantle xenoliths

Carbonated silicate melts transport oxidized carbon from the deep to shallow mantle, and thus play an important role in the deep carbon cycle. However, it is unclear how carbonated silicate melts are formed. Here, we report whole-rock major and trace element, and Mg-Zn-Fe isotope, and in situ olivine and clinopyroxene Mg isotope data for peridotites and pyroxenites in Cenozoic alkaline basalts from Shulan and Yitong, Northeast China, to reveal the formation mechanisms of carbonated silicate melts. As residues of the primitive mantle, most of the Shulan and Yitong (spinel) harzburgite and lherzolite have relatively lower δ26Mg and higher δ66Zn than the mantle values, and show LREE-enriched patterns. These features indicate that these peridotites have been modified by carbonated silicate melt, transforming them into carbonated peridotites. The Shulan websterites exhibit metasomatic textures, and most of their whole-rock and in situ δ26Mg values are lower than the mantle values, which also supports the presence of a carbonated lithospheric mantle. The Yitong cumulate orthopyroxenites, representing silica-rich melts, have lower δ26Mg and higher δ66Zn values than mantle peridotites, implying that recycled carbonate has been involved in these silica-rich melts. The Yitong cumulate wehrlites and (olivine) websterites display major element compositions similar to those of carbonated peridotite melts, coupled with low whole-rock and clinopyroxene δ26Mg values, high δ66Zn and δ57Fe values, indicating the occurrence of carbonated silicate melts. The transition from orthopyroxenites to (olivine) websterites is marked by the decreasing of their whole-rock δ26Mg, δ57Fe values and Zn/Fe ratios and increasing of their δ66Zn values, along with decreasing of their crystallization temperatures, suggesting that silica-rich melts were gradually transformed into carbonated silicate melts via continuous interaction with carbonated peridotite. Our findings demonstrate that silica-rich melts from the stagnant slab can extract carbon from pre-existing carbonated mantle within the big mantle wedge.