Tracing crustal recycling mechanisms using Mo isotopes in high-Mg andesites

Abstract High-Mg andesites (HMAs) are key magmatic markers of crust-mantle interaction, yet the mechanisms driving this process remain elusive. This study presents Mo isotope data for Eocene (post-collision) HMAs from central Tibet, showing high δ98Mo values comparable to arc basalts, arc crustal xenoliths, and melts from undetached Tibetan crust, but distinct from deeply subducted oceanic or continental crust. Coupled with their adakitic signatures and seismic imaging, these HMAs are interpreted as products of interactions between lower crustal drip melts and peridotite. We propose that negligible Mo isotope fractionation occurs during the delamination and subsequent melting of lower continental crust, unlike the substantial fractionation observed in subducting slabs, where heavy Mo is lost during dehydration along distinct pressure-temperature paths. Thus, Mo isotopes in HMAs offer a promising avenue for resolving the longstanding conundrum on whether crustal materials are recycled back into the mantle through subduction or delamination.