The influence of crustal recycling on the molybdenum isotope composition of the Earth's mantle

Several studies have suggested that the Earth's upper mantle is slightly enriched in light molybdenum isotopes relative to bulk Earth, defined by chondrites, but there is no consensus on the presence of this subtle but potentially notable signature. To establish better whether or not the Mo/Mo of Earth's upper mantle is indeed sub-chondritic, we have analysed hand-picked glasses of depleted (i.e. chondrite normalised La/Sm<1) mid-ocean ridge basalts (MORB) from the Pacific, Atlantic and Indian ocean basins. The mean Mo isotope composition of our depleted MORB relative to reference NIST SRM 3134 (?Mo) is -0.22±0.03? (95% confidence interval, c.i.) compared to a value of -0.15±0.01? (95% c.i.) for bulk Earth. Our high precision analyses of the U/U activity ratios of these samples are within uncertainty of unity, which rules out the effect of possible secondary, sea-floor processes as the dominant cause of their low ?Mo. We further report experimental data showing that sulphide liquid has ?Mo 0.25±0.01? lower than basaltic silicate liquid at 1400 °C. This fractionation is too small to significantly alter the Mo isotope composition of basalts relative to their sources during melting or differentiation. Our MORB data show that resolvably sub-chondritic Mo isotope compositions are common in the upper mantle. Moreover, an appropriately weighted average ?Mo of depleted and enriched MORB, taken from this study and the literature, yields an estimated mantle value of -0.20±0.01?, indicating that the upper mantle as a whole is sub-chondritic. Since prior work demonstrates that core formation will not create a residual silicate reservoir with a sub-chondritic ?Mo, we propose that this feature is a result of recycling oceanic crust with low ?Mo because of Mo isotope fractionation during subduction dehydration. Such an origin is in keeping with the sub-chondritic Th/U and low Ce/Pb of the depleted mantle, features which cannot be explained by simple melt extraction. We present mass balance models of the plate tectonic cycle that quantitatively illustrate that the ?Mo of the Earth's mantle can be suitably lowered by such oceanic crustal recycling. Our Mo isotope study adds to the notion that the depleted mantle has been substantially modified by geodynamic cycling of subduction-processed oceanic crust.