Role of hidden deeply subducted slabs in mantle depletion

The distribution of Nb and Ta among the continental crust, the depleted mantle and subducted slabs and mass balance of bulk-silicate-earth require that a significant mass of deeply subducted oceanic slabs is hidden in the lower mantle. The characteristic Nb/Ta ratios of continental crust (ca. 12) and eclogitic oceanic crustal slabs (ca. 33) likely result from element fractionation during slab dehydration. This is because the most efficient mechanism of fractionating the chemical twins Nb and Ta is by chemical complexing in hydrous solutions. The near-chondritic present-day Nb/Ta ratio of the depleted mantle (ca. 17) suggests that similar proportions of Nb and Ta have been recycled from both the continents and oceanic slabs over geological time. Furthermore, the Nb and Ta contents of a worldwide suite of MORB samples are very well-correlated (r = 0.996), which shows that the depleted mantle is very well-mixed for high-field-strength elements. Long-term existence of chemically distinct oceanic crust or sediment reservoirs in the depleted mantle is therefore unlikely. This strongly suggests that high Nb/Ta slabs, which are required to balance the low Nb/Ta continental lithosphere, are hidden in the lower mantle. There is no evidence that these deeply subducted slabs are a significant source component of ocean-island basalts. Rather, the chondritic Nb/Ta ratio of the majority of ocean-island basalts suggests that these rocks are derived either from an undepleted portion of the lower mantle, and/or from the depleted mantle. (C) 2000 Elsevier Science B.V. All rights reserved