Mantle xenoliths in Northeast China record deep carbon cycling triggered by subduction of the Pacific Plate

Earth science studies have focused on the deep mantle carbon cycle and its geodynamic effects. However, the way in which carbonated silicate melts modify the mantle remains poorly constrained. In this study, we report petrographic, mineral major and trace element, and Sr isotopic data for a suite of peridotite and pyroxenite xenoliths from the Shulan and Yitong areas of Northeast China. These data are used to investigate how carbonated silicate melts modify the mantle. The olivine, orthopyroxene, and clinopyroxene in the Shulan harzburgite and lherzolite xenoliths have relatively high Mg^# values (90−93), and the clinopyroxenes have low (La/Yb)_N ratios (0.49−0.61), implying that they may be post-partial melting residues of mantle previously modified by silica-rich melts, which is consistent with the light rare earth element (LREE)-depleted REE patterns of the clinopyroxenes. The olivine, orthopyroxene, and clinopyroxene in the Shulan (olivine) websterite xenoliths also have high Mg^# values (90−94) that are indicative of a mantle origin. However, the clinopyroxenes have high (La/Yb)_N (1.43−65.8), Ti/Eu (3504−5255), and Ca/Al (5.13−9.59) ratios, a positive correlation between Sm/Hf and Zr/Hf ratios, and high LREE contents, which suggest carbonated silicate metasomatism. This inference is also consistent with the replacement of olivine and orthopyroxene by clinopyroxene. In contrast, the Mg^# values (78−86) of the olivines in the Yitong orthopyroxenite, wehrlite, websterite, and clinopyroxenite xenoliths are lower than those of the corresponding minerals from peridotites. This result, combined with the variably LREE-enriched and high field strength element (HFSE)−depleted patterns (e.g., Nb, Ta, Zr, and Hf) of the clinopyroxenes, suggest that these pyroxenites could have crystallized from mantle-derived melts. Compared with the Shulan peridotites, the Yitong xenoliths record higher temperatures (1028−1310 °C) and higher (La/Yb)_N (1.05−5.30) and Ti/Eu (2624−6567) ratios, and a positive correlation between La and Sr, which reflect the occurrence of carbonated silicate melts in the lithospheric mantle. This is also supported by the estimated equilibrium melt compositions obtained from clinopyroxene. Thus, we propose that the different types of Yitong xenoliths are the crystallized products of silica-rich to carbonated silicate melts. The low ⁸⁷Sr/⁸⁶Sr ratios (0.70345−0.70488) of the clinopyroxenes in the wehrlites and pyroxenites from Shulan and Yitong, along with the timing of formation of the Northeast Asian big mantle wedge (ca. 20 Ma) and geochemical characteristics of the Cenozoic basalts and high-Mg andesites in Northeast China, indicate that the carbonated silicate melts were derived from the partial melting of recycled carbonatized oceanic crust and ancient recycled crustal material in the mantle transition zone. This implies that the lithospheric mantle along the Yilan−Yitong faults experienced not only crystallization and the accumulation of silica-rich and carbonated silicate melts that formed the Yitong wehrlite and pyroxenite xenoliths, but also subsequent modification by carbonated silicate melts that formed the Shulan (olivine) websterite xenoliths. These mantle xenoliths record deep carbon cycling triggered by the subduction of the Pacific Plate.