BORON ISOTOPE GEOCHEMISTRY OF THE OCEANIC-CRUST FROM DSDP/ODP HOLE-504B

Boron contents and boron isotopic compositions were determined for the uppermost 1.3 km section of typical 6.2 Ma oceanic crust from DSDP/ODP Hole 504B, Costa Rica Rift, Galapagos Spreading Center. Both the boron content and the delta-B-11 value in the oceanic crust are controlled by two types of alteration: (1) low-temperature alteration (0 to 60-degrees-C; Zones I and II) and (2) high-temperature hydrothermal alteration (200 to 400-degrees-C; Zones III and IV). Basalts subjected to the low-temperature alteration are characterized by their relatively high boron contents (0.69 to 19.3 ppm) and high delta-B-11 values (+2.2 to + 10.6 parts per thousand), indicating uptake of boron into secondary phases in equilibrium with seawater or evolved seawater. Hydrothermally altered basalts contain less abundant boron (0.17 to 0.52 ppm) and relatively constant delta-B-11 values (-0.1 to 1.0 parts per thousand). Although basalts from the upper part of these hydrothermal zones (< 1300 mbsf) show equilibrated boron content and delta-B-11 value with aqueous fluid, effective leaching of boron from basalt is predominant in the lower part (< 1300 mbsf). Original boron content and delta-B-11 value of the Hole 504B MORB were 0.35 ppm and + parts per thousand, respectively. The present data provide fundamental information in understanding of the distribution of boron and boron isotopes in the oceanic crust.