The large ion lithophile element (LILE) contents of island-arc tholeiites are dominated by the contribution from the subducted slab, whereas those of continental margin rocks are dominated by that from the mantle wedge. It is therefore argued that the LILE abundances in the destructive-margin rocks that are most depleted in high field strength elements offer the best indication of the LILE flux from the subducted slab, and that the slab contributes ∼25% of the Sr in average new crust. The remainder is derived from the mantle wedge, which is the principal site of crust generation and therefore must be replenished about every 75 m.y. Sr isotope arguments suggest that <5% of the Sr in new crust is from recycled continental crust with average crustal 87Sr/86Sr ratios. In the model outlined here, ≤5% of the Rb, Ba, and K in new crust is from the subducted slab, which is in contrast to previous suggestions that as much as 90% of the K in new crust might be from recycled continental material. As little as 3% of the Sr in subducted crust is required for the slab-derived flux, which further implies that most of the LILE in oceanic crust are recycled deep into the upper mantle.

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