Re-assessment of the effect of fractional crystallization on Mo isotopes: Constraints from I-type granitoids and their enclosed mafic magmatic enclaves

In recent years, considerable progress has been made on the study of Mo isotopes in high-temperature geological processes. However, it is still controversial whether Mo isotope fractionation occurs during magmatic differentiation. Here we reassess the effect of magma differentiation on Mo isotope fractionation using published data in conjunction with our new analysis of Mo isotopes in well-characterized I-type granitoids and their enclosed mafic magmatic enclaves (MMEs) from two plutons in the North Qilian Orogen. The MMEs in each pluton, representing earlier cumulates with greater modal proportions of amphibole and biotite, show similar Mo elemental and isotopic compositions to their host granitoids. The absence of covariation of Mo isotopes with Dy/Dy*, Fe2O3 and K/Rb in both plutons further indicates that fractional crystallization of amphibole, biotite and Fe3+-rich minerals does not fractionate Mo isotopes in these granitoids. On the basis of re-assessment of published and our newly obtained Mo isotope data, we find that there is no clear evidence for Mo isotope fractionation during fractional crystallization in igneous rock. Instead, Mo isotopes of our samples correlate positively with Sr isotopes and Rb/La ratios, and negatively with Nd isotopes and Ce/Pb ratios, indicating clearly that magma source compositional variation controls the variation of Mo isotope compositions of these granitoids.