Two styles of melt-peridotite interactions in the upper mantle revealed by mantle xenoliths from northeastern China

To better constrain the mechanisms of interactions between different types of melts and mantle peridotites in the upper mantle of the big mantle wedge, we studied petrology, mineral chemistry, and temperatures recorded by major element and rare earth element (REE) for mantle xenoliths from the Wangqing area in the eastern part of northeastern China, and compared these data with those for xenoliths from the adjacent Jiaohe and Shuangliao areas. The Wangqing xenoliths, including olivine clinopyroxenite, wehrlite, orthopyroxenite, and websterite, generally contain clinopyroxene that is surrounding or replacing olivine. Mineral compositions of the xenoliths from northeastern China vary along melt-peridotite reaction trends, where the Wangqing xenoliths have lower mineral Mg#'s (77-85 in olivine and 81-87 in clinopyroxene) than the Jiaohe and Shuangliao peridotite and pyroxenite xenoliths. In addition, clinopyroxenes in the Wangqing samples have moderately elevated rare earth element contents and (La/Yb)N and Sr-87/Sr-86 ratios (0.7035-0.7046), within the range of those for clinopyroxenes in the Jiaohe and Shuangliao samples. The REE temperatures of the Wangqing samples (1024-1253 C-degrees) are higher than those of the Jiaohe and Shuangliao samples (844-1120 C-degrees). Based on the differences in mineral chemistry and temperature, we conclude that the Jiaohe and Shuangliao peridotites and pyroxenites were mainly derived from lithospheric mantle accreted from carbonated regions of the asthenosphere, whereas the Wangqing wehrlites and pyroxenites were formed by reaction of lithospheric mantle peridotites with hot carbonated silicate melts. These results provide evidence for two different types interactions between of carbonate -related melts and mantle peridotite, which are important for understanding the chemical evolution of the upper mantle in the big mantle wedge of northeastern Asia.