Magma differentiation fractionates Mo isotope ratios: Evidence from the Kos Plateau Tuff (Aegean Arc)

We investigated high temperature Mo isotope fractionation in a hydrous supra-subduction volcano-plutonic system (Kos, Aegean Arc, Greece) in order to address the debate on the delta Mo-98/95 variability of the continental crust. In this igneous system, where differentiation is interpreted to be dominated by fractional crystallization, bulk rock data from olivine basalt to dacite show delta Mo-98/95 ratios increasing from + 0.3 to + 0.6% along with Mo concentrations increasing from 0.8 to 4.1 mu g g(-1). Data for hornblende and biotite mineral separates reveal the extraction of light Mo into crystallizing silicates, with minimum partition coefficients between hornblende-silicate melt and biotite-silicate melt of 0.6 and 0.4 delta Mo-98/95, respectively. Our data document significant Mo isotope fractionation at magmatic temperatures, hence, the igneous contribution to continental runoff is variable, besides probable source-related variability. Based on these results and published data an average continental delta Mo-98/95 of + 0.3 to + 0.4% can be derived. This signature corresponds more closely to the average of published data of dissolved Mo loads of large rivers than previous estimates and is consistent with an upper limit of delta Mo-98/95 = 0.4% of the Earth's upper crust as derived from the analysis of molybdenites. (C) 2013 Elsevier B.V. All rights reserved.