Molybdenum isotopes record dehydrated slab components input to arc magmatism in subduction zones

Abstract

The traditional model that attributes the genesis of basaltic arc magma to the partial melting of metasomatized mantle wedges is increasingly being challenged by evidence highlighting the critical role of dehydrated oceanic crust. Molybdenum (Mo) isotopes (expressed as δ98/95Mo, relative to the NIST SRM 3134 standard) from subarc mantle-derived mafic cumulate rocks offer a novel perspective on this issue. This study reports δ98/95Mo values with whole-rock Sr-Nd isotopes for the Late Cretaceous Milin juvenile lower crustal mafic–ultramafic cumulates in the Gangdese belt, Tibet. The Milin samples exhibit low initial (87Sr/86Sr)i ratios ranging from 0.70392 to 0.70454 and depleted εNd(t) values between 3.26 and 4.45. These samples display a significant variation in δ98/95Mo values (–0.64 to –0.05 ‰), with a lower mean value of –0.38 ‰ compared to depleted mantle values (–0.21 ± 0.02 ‰). The light Mo isotopes show no correlations with MgO content, Sr-Nd isotopes, or whole-rock hornblende content, suggesting that observed light Mo isotopes are associated with dehydrated oceanic crust rather than crustal processes (crustal contamination or fractional crystallization). The positive correlation between Ba/Th and Ba/La ratios and δ98/95Mo values indicates the overprinting of subduction fluids. According to the Mo-Sr-Nd isotopic mixing model, the Milin mantle source incorporated minor subduction fluids (∼1 %), reduced sediment melts (∼1 %) and less than 30 % dehydrated oceanic crust melts, which leads to its heterogeneity and significant variation in the light Mo isotopes within the Milin lower crustal mafic–ultramafic cumulates. We suggest that such lower crust with oceanic crust melts in their source can serve as an important light Mo isotopic reservoir. Integrating the Mo isotopic features in global subduction zones, we propose that the thermal structure of the subduction zones controls the input of dehydrated oceanic crust melts into basaltic arc magmas, resulting in the predominant participation of oceanic crust in the basaltic arc magma genesis within the hot arc and back-arc regions.