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 δ¹¹B value in the oceanic crust are controlled by two types of alteration: 1. (1) low-temperature alteration (0 to 60°C; Zones I and II) and 2. (2) high-temperature hydrothermal alteration (200 to 400°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 δ¹¹B values (+2.2 to +10.6%.), 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 δ¹¹B values (-0.1 to +1.0%.). Although basalts from the upper part of these hydrothermal zones (< 1300 mbsf) show equilibrated boron content and δ¹¹B value with aqueous fluid, effective leaching of boron from basalt is predominant in the lower part (>1300 mbsf). Original boron content and δ¹¹B value of the Hole 504B MORB were 0.35 ppm and +0.2%., respectively. The present data provide fundamental information in understanding of the distribution of boron and boron isotopes in the oceanic crust.