Molybdenum isotope evidence for recycled oceanic crust in the mantle sources of continental intraplate mafic lavas, Emeishan Large Igneous Province

The chemical and lithological heterogeneities of Earth's mantle in oceanic settings have long been attributed to the presence of recycled materials such as subducted crust, oceanic or subcontinental lithosphere mantle and sediment, as evidenced by Mid-Ocean Ridge Basalts (MORB) and Ocean Island Basalts (OIB). However, the extent to which recycled materials contribute to mantle heterogeneity in continental settings remains uncertain. Here we present systematic Sr-Nd-Pb-Mo isotope data for the Pingchuan picritic porphyries and associated mafic rocks (basalts and gabbros) in the central Emeishan Large Igneous Province, southwestern China, to resolve this issue. In contrast to the gabbro samples that show continental crust contamination in variations of delta Mo-98/95 (relative to NIST SRM 3134, -0.35 +/- 0.03 to 0.03 +/- 0.02 parts per thousand), the Pingchuan picritic porphyries and basalts show a large range of delta Mo-98/95 (-0.32 +/- 0.01 to -0.15 +/- 0.06 parts per thousand) and broadly negative trend between delta Mo-98/95 and Ce/Mo ratios, which cannot be explained by post-magmatic alteration, crustal contamination, fractional crystallization, olivine accumulation, metasomatism or fluid-activity in the mantle source. They were most likely derived from incongruent melting of a heterogeneous mantle source, i.e., a deep mantle with chondritic-like delta Mo-98/95 (-0.15 +/- 0.01 parts per thousand) containing recycled oceanic crust (+ sediment) with light delta Mo-98/95 (< -0.32 parts per thousand). The basalts display low delta Mo-98/95 (-0.32 +/- 0.01 to -0.24 +/- 0.04 parts per thousand), high Sr-87/Sr-86(i) (0.7046 to 0.7059), TiO2, Sm/Yb, Dy/Yb and Ce/Mo, pointing to characteristics of melts produced by MORB-type eclogite (+ sediment) at relatively low degrees of partial melting. With increasing degrees of partial melting, melts produced by peridotite (Sr-87/Sr-86(i) = 0.7035 to 0.7043, chondritic-like delta Mo-98/95 = -0.18 +/- 0.03 to -0.15 +/- 0.06 parts per thousand) come to dominate, resulting in low TiO2, Sm/Yb, Dy/Yb and Ce/Mo of the Pingchuan picritic porphyries. From a global perspective, continental basalts also plot on the negative trend defined by the OIB datasets (delta Mo-98/95 vs. Ce/Mo). As such, Mo isotope may provide a robust tracer of recycled oceanic crust in shaping mantle heterogeneities that occur in both oceanic and continental settings.