Komatiites constrain molybdenum isotope composition of the Earth's mantle

In order to estimate the Mo isotope composition and Mo abundance in the Bulk Silicate Earth (BSE), a total of thirty komatiite samples from five localities on three continents were analyzed using an isotope dilution double spike technique. Calculated Mo concentrations of the emplaced komatiite lavas range from 25 +/- 3 to 66 +/- 22 ng/g, and the inferred Mo concentrations in the deep mantle sources of the komatiites range between 17 +/- 4 and 30 +/- 12 ng/g, with an average value of 23 +/- 7 ng/g (2SE). This average value represents our best estimate for the Mo concentration in the BSE; it is identical, within the uncertainty, to published previous estimates of 39 +/- 16 ng/g, but is at least a factor of 2 more precise. The Mo isotope compositions of the komatiite mantle sources overlap within uncertainty and range from delta Mo-98 = 0.04 +/- 0.28 to 0.11 +/- 0.10 parts per thousand, with an average of 0.04 +/- 0.06 parts per thousand (2SE). This value is analytically indistinguishable from published Mo isotope compositions of ordinary and enstatite chondrites and represents the best estimate for the Mo isotope composition of the BSE. The inferred delta Mo-98 for the BSE is therefore lighter than the suggested average of the upper continental crust (0.3 to 0.4 parts per thousand). Thus, from the mass balance standpoint, a reservoir with lighter Mo isotope composition should exist in the Earth's mantle; this reservoir can potentially be found in subducted oceanic crust. The similarity of delta Mo-98 between chondritic meteorites and estimates for the BSE from this study indicates that during the last major equilibration between Earth's core and mantle, i.e., the one that occurred during the giant impact that produced the Moon, chemical and isotopic equilibrium of Mo between Fe metal of the core and the silicate mantle was largely achieved. (C) 2015 Elsevier B.V. All rights reserved.