PRIMITIVE BORON ISOTOPE COMPOSITION OF THE MANTLE

Boron isotope ratios are homogeneous in volcanic glasses of oceanic is land basalts [-9.9 +/- 1.3 per mil, relative to standard NBS 951 (defi ned by the National Bureau of Standards)], whereas mid-oceanic ridge b asalts (MORBs) and back-are basin basalts (BABBs) show generally highe r and more variable ratios. Melts that have assimilated even small amo unts of altered basaltic crust show significant variations in the boro n isotope ratios. Assimilation may thus account for the higher boron r atios of MORBs and BABBs. A budget of boron between mantle and crust i mplies that the primitive mantle had a boron isotope ratio of -10 +/- 2 per mil and that this ratio was not fractionated significantly durin g the differentiation of the mantle.