The isotopic composition of seawater boron (11B/10B) is highly enriched (~40 per mil) relative to fresh mantle derived rocks. Initial work indicates that this enrichment is primarily the result of large isotopic fractionations associated with the alteration of the oceanic crust. In addition, the boron isotopic composition of the altered crust is dominated by seawater boron that appears to be distinct from the mantle and may be composed of compositionally distinct domains. As a result, it appears that boron isotopes have a great potential for elucidating a number of geochemical and geophysical problems associated with the alteration of the oceanic crust. These include the role of subducted volatiles in island-arc and ocean island volcanism, the evolution of hydrothermal fluids and secular changes in seawater/crust exchange. This study of seawater/oceanic crust boron isotope exchange has the following specific goals: 1) to determine the distribution of boron and boron isotopes within the altered crust, 2) to quantify the net flux and isotopic composition of boron extracted from seawater into the oceanic crust and 3) to develop the essential background data that are required for the future exploitation of boron isotopes as a diagnostic tool in the study of hydrothermal systems.
