Mg–K isotopic compositions deciphering mantle heterogeneity beneath southeast Tibet

Abstract

<p>The Tibet peaked due to the incorporation of slab materials and intensive mantle-crust interaction. It remains uncertain regarding the differentiated mantle enrichment and elemental cycling process in a post-collisional setting. Here, we use stable Mg-K isotopic data from young (<5 Ma) arc-type and OIB-type basalts—spatially and genetically linked to slab subduction and stagnation—to quantify the origin, proportion, and transport mechanism of materials contributing to mantle heterogeneity. Both basaltic types from southeast Tibet possess homogeneous and low δ²⁶Mg values (−0.39 ± 0.03 ‰ to −0.36 ± 0.04 ‰) compared to the normal mantle range (δ²⁶Mg = −0.25 ± 0.04 ‰); Mg–Sr–Nd–Pb isotopic mixing models suggest that isotopically light marine carbonates were incorporated into the mantle wedge and mantle transition zone (MTZ). In terms of the isotopic dataset of post-collisional magmatic rocks, pervasive carbonatitic metasomatism served as the deep carbon source that enhanced mantle heterogeneity. Compared to the mantle-like δ⁴¹K (−0.49 ± 0.04 ‰ to −0.35 ± 0.02 ‰) of arc-type samples that re-homogenized in the wedge, the OIB-type lavas have variable δ⁴¹K values (−0.73 ± 0.03 ‰ to −0.54 ± 0.02 ‰) lower than those of primitive mantle (δ⁴¹K = −0.42 ± 0.08 ‰), suggesting an increased release of isotopically light K from dehydrated Neo-Tethyan slab residues that stagnated in the MTZ. Geophysical evidence supports that these deep-seated metasomatic agents triggered wet upwelling flows atop the MTZ and transported surficial materials back via volcanism. Critically, this study constrains the origin of mantle heterogeneity and material circulation triggered by various contributions of the subducted plate.<br></p>