Thermochemical Controls on the X, 410, and 660 Discontinuities in the Central Mediterranean

Abstract The mantle beneath the Central Mediterranean is thermally and compositionally heterogeneous, as indicated by fragmented and locally stagnant slabs in tomographic images and HIMU‐like anorogenic magmatism. Mantle discontinuities are sensitive to both temperature and composition, and thus provide a way to quantify these heterogeneities. We investigate mantle discontinuities using 1798 high‐quality teleseismic P receiver functions. P‐to‐S converted phases are identified at individual stations through back‐azimuth stacking across multiple frequency bands and converted from time to depth using locally constrained velocity models. Discontinuity depth, sharpness, and impedance contrasts are used to infer mantle temperature and composition. Robust converted phases are detected at three major discontinuities: the X, 410, and 660. The X discontinuity is at an average depth of 272 8 km. The 410 averages 399 4 km, making it shallower than the global reference depth, while the 660 is deeper at 669 4 km. All three discontinuities are relatively sharp (thickness 15 5 km). The X reflects silica‐rich phase transitions linked to HIMU‐like anorogenic volcanism. Synthetic tests show that comparable PXs and P410s amplitudes imply high basalt fractions (∼70%–80%), well above pyrolitic values (∼18%). Variations in 410 and 660 depths produce significant mantle transition zone thickening. We infer thermal anomalies from the 410 topography. Yet, temperature alone cannot explain large 660 depressions of ∼30 10 km. The 660 depth, sharpness, and P410s/P660s amplitude ratios indicate a strong compositional control, with post‐garnet transition in basalt‐enriched material likely contributing to the observed deepening and aiding slab stagnation.