BORON CONTENT AND ISOTOPIC COMPOSITION OF OCEANIC BASALTS - GEOCHEMICAL AND COSMOCHEMICAL IMPLICATIONS

Ion microprobe determination of boron content and deltaB-11 values has been performed for a set of 40 oceanic basalt glasses (N-MORB, E-MORB, BABB and OIB) whose chemical characteristics (major and trace elements and isotopic ratios) are well documented. Boron contents, determined at +/-10% relative, range from 0.34 to 0.74 ppm in N-MORB, whereas E-MORB, BABB and OIB extend to higher concentrations (0.5-2.4 ppm). After correction for crystal fractionation, this range is reduced to 0.5-1.3 ppm. N-MORB and E-MORB also exhibit different B/K ratios, 1.0 +/- 0.3 x 10(-3) and 0.2 to 1.4 x 10(-3) respectively. This can be interpreted as resulting from the incorporation into the upper mantle of a K-rich and B-poor component (e.g., subducted oceanic crust having lost most of its initial boron). DeltaB-11 values range between -7.40 +/- 2 and +0.6 +/- 2 parts per thousand, with no significant difference between N-MORB, E-MORB, OIB or BABB. The Hawaiian samples define a strong linear correlation between boron contents, deltaB-11 values, MgO and water contents and deltaD values. This is interpreted as resulting from assimilation-fractionation processes which occurred within a water-rich oceanic crust, and which produced high deltaB-11 values associated with high deltaD values. The low level of B-11 enrichment in the upper mantle constrains the amount of boron reinjected by subduction to a maximum of about 2% of the boron present in the subducted slab. This in turn corresponds to a maximum net Boron transfer of about 3 x 10(10) g/a towards the surface reservoirs. Finally, a boron content of 0.25 +/- 0.1 ppm is estimated for the bulk silicate Earth (i.e., primitive mantle), corresponding to a depletion factor relative to Cl chondrites of about 0.15 and suggesting that B was moderately volatile upon terrestrial accretion.