Missing molybdenum and the composition of the continental crust inferred from molybdenum isotopes

Abstract Accurately constraining the molybdenum (Mo) isotope composition (δ 98/95 Mo) of Earth’s major reservoirs is essential for understanding its evolution. However, δ 98/95 Mo of the continental crust (CC), particularly the middle and lower crust, remains poorly constrained. Here we show the Mo isotope data for the Gangdese arc section in combination with published data from ultramafic-mafic, intermediate and felsic intrusions, representing the lower, middle and upper CC, respectively, constrain variability within the CC. Mass balance calculations using several crustal depth models generate an average δ 98/95 Mo of the bulk CC of −0.116 ± 0.011‰ (2 s.d.), resolvably heavier than the bulk silicate Earth. Global scale mass balance modeling demonstrates that the Mo isotope compositions of the CC and the depleted mantle are presently near balanced. Lower crustal delamination is an additional mechanism capable of contributing to the subchondritic Mo isotope composition of the depleted mantle. Over the course of Earth’s history, new crustal growth and destruction have reached dynamic equilibrium.