Fluid-fluxed melting of juvenile lower crust traced by molybdenum isotopes

Fluid-fluxed melting is considered to be an important process in reworking continental crust. However, the process is challenging to identify because of complex magma differentiation processes. This study uses novel molybdenum (Mo) isotopes to address this issue because they are sensitive to fluid activity. Early Paleozoic adakitic rocks from the Dabie orogen, China, were analyzed for whole-rock Mo isotopes as well as elements and Sr−Nd isotopes to identify the magma sources and the influence of fluids during melting. The rocks are characterized by high and highly varied δ⁹⁸Mo values (−0.13‰ to 0.74‰, median = 0.22‰), which cannot be explained by fractional crystallization (which can only cause a maximum of 0.06‰ fractionation). Sr−Nd isotopes indicate that these rocks probably originated from melting of juvenile basements, but the higher δ⁹⁸Mo values relative to those of adjacent coeval mafic rocks and their positive correlation with Na₂O/K₂O ratios indicate the influx of external fluids with heavy Mo isotope compositions. Meanwhile, the correlation between δ⁹⁸Mo values and Sr/Y ratios and fluid-immobile elements suggests that the Dabie adakitic rocks are more likely to have originated from fluid-fluxed melting induced by fluid addition. This conclusion is supported by Ti-in-zircon thermometry and phase equilibria modeling. Our work presents compelling evidence for fluid-fluxed melting in generating adakitic rocks and highlights that Mo isotopes can act as an effective tracer for fluid activity during crustal reworking.