Plate-tectonic mechanisms drive the recycling of oceanic crust into Earth’s mantle, generating chemical and lithological heterogeneities that are believed to significantly influence plume-related magmatism on a global scale. Here, we present new constraints on the interaction between mantle plumes and recycled oceanic crust, based on integrated Os-Hf-O-Mg isotopes and whole-rock geochemical data (major and trace elements and platinum group elements PGEs). These analyses were performed on late Paleozoic picrites with mid-ocean-ridge basalt (MORB) affinity and picrites/basalts with oceanic-island basalt (OIB) affinity from the Changning-Menglian belt in southwestern China. Both MORB- and OIB-type picrites have Zn/Fe ratios 12 and 187Os/188Os ratios (0.113−0.131) similar to typical mantle values, suggesting a predominantly peridotite mantle source. In contrast, over half of the OIB-type basalts show elevated Zn/Fe ratios (12) and 187Os/188Os ratios (up to 0.513) that deviate from typical mantle values, indicating a contribution from pyroxenite-derived melts. Zircons from these OIB-type basalts exhibit mantle-like εHf(t) values (+1.4 to +7.9) but crust-like δ18O values (+3.2‰ to +5.2‰), supporting the involvement of high-temperature altered oceanic crust in their source. These basalts also exhibit atypical PGE signatures marked by strong depletions in Pd and Ru relative to Ir, consistent with the incorporation of residual oceanic crust that previously experienced release of oxidizing fluids during plate subduction. Notably, the OIB-type basalts record systematically lower δ26Mg values (−0.57‰ to −0.31‰) compared to mantle peridotite (−0.25‰ ± 0.04‰). This light Mg isotope signature, coupled with elevated Hf/Sm ratios, indicates metasomatism of the mantle source by dolomitic melts derived from recycled carbonate-bearing oceanic crust. Thermal modeling yields anomalously high and overlapping mantle melting temperatures for MORB-type (1433−1542 °C) and OIB-type (1434−1678 °C) samples, supporting a cogenetic, plume-driven melting regime. Our results demonstrate that recycled oceanic crust was entrained by an ascending plume beneath the Paleo-Tethys Ocean, revealing a slab-plume interaction mechanism that contributes to the chemical and isotopic diversity observed in mantle sources. These findings enhance current understanding of deep Earth material cycling and the role of Paleo-Tethys evolution in supercontinent cycles.
Zhang et al. (Wed,) studied this question.