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

Ocean island basalts (OIBs) possess uniformly low B contents, and lowe r 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 are volcanics or continental rocks. The devolatilization of s ubducting plates and associated are magmatism efficiently segregate B into crustal reservoirs and return large volumes of B-depleted materia l to the deep mantle. Subduction processes (and presumably are volcani sm) have thus played a major role in continental crust formation. Whil e B is depleted in OIBs relative to either MORBs or are lavas, OIB sam ples representing EM and HIMU isotopic reservoirs, often ascribed to t he effects of ancient subducted materials, cannot be distinguished fro m other OIBs in terms of B abundances or B ratios. Our results suggest either (1) the differential depletion in B of two distinct mantle res ervoirs, one of which now produces MORBs, and the other OIBs or (2) th e episodic or continuous mixing of OIB mantle sources with B-depleted subducted materials. The geochemical processes responsible for the iso topic heterogeneity of intraplate lavas may all serve to segregate B f rom the mantle into crustal rocks and other surface reservoirs.