Could sedimentary carbonates be recycled into the lower mantle? Constraints from Mg isotopic composition of Emeishan basalts

Abstract

Whether or not sedimentary carbonates can be recycled into the lower mantle through subduction remains unclear. To further elucidate this issue, we investigate the Mg isotopic composition of Permian basalts from the Emeishan large igneous province (ELIP). Emeishan basalts can be divided into two major groups: the low-Ti and high-Ti basalts, which exhibit distinct major, trace element, and Sr–Nd–Pb isotopic compositions. However, they both possess mantle-like Mg isotopic compositions with δ26Mg values of −0.33 to −0.19‰ and −0.35 to −0.19‰, respectively. Both the low-Ti and high-Ti basalts have experienced compositional evolution, e.g., fractional crystallization or crustal contamination, because their Mg# (39.8–61.1) values are significantly lower than that of primary melt (~72.0). However, their δ26Mg values do not vary with Mg#, indicating that the Mg isotopic compositions of the basalts were inherited from their mantle source. The low-Ti basalts are enriched in light rare earth elements with low εNd(t) values (mostly ranging from −10.5 to −8.8) and low Ce/Pb, high Ba/Th ratios, suggesting that they were derived from the lithospheric mantle that was metasomatized by slab-derived fluids. Their mantle-like Mg isotopic composition suggests that the metasomatism of slab-derived fluids did not affect their Mg isotopic composition. This is consistent with most of the reported Arc basalts with mantle-like Mg isotopic compositions. On the other hand, the high-Ti basalts display an OIB-like trace element pattern and positive εNd(t) values (+0.2~+2.1), which were possibly derived from a mantle plume that originated from the lower mantle. Their mantle-like Mg isotopic composition indicates that sedimentary carbonates recycled into the lower mantle beneath the ELIP were limited (<2%).