THE BORON SYSTEMATICS OF INTRAPLATE LAVAS: IMPLICATIONS FOR CRUST AND MANTLE EVOLUTION

Abstract

Ocean island basalts (OIBs) possess uniformly low B contents, and lower B/Nb and B/K2O ratios than mid-ocean ridge basalts (MORBs). As with Pb, B enrichments in both MORBs and OIBs are substantially lower than those of arc volcanics or continental rocks. The devolatilization of subducting plates and associated arc magmatism efficiently segregate B into crustal reservoirs and return large volumes of B-depleted material to the deep mantle. Subduction processes (and presumably arc volcanism) have thus played a major role in continental crust formation.While B is depleted in OIBs relative to either MORBs or arc lavas, OIB samples representing EM and HIMU isotopic reservoirs, often ascribed to the effects of ancient subducted materials, cannot be distinguished from other OIBs in terms of B abundances or B ratios. Our results suggest either (1) the differential depletion in B of two distinct mantle reservoirs, one of which now produces MORBs, and the other OIBs or (2) the episodic or continuous mixing of OIB mantle sources with B-depleted subducted materials. The geochemical processes responsible for the isotopic heterogeneity of intraplate lavas may all serve to segregate B from the mantle into crustal rocks and other surface reservoirs.