Boron Isotopic Composition of the Troodos Ophiolite

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Hydrothermal reactions at mid-ocean ridges have a profound influence on the chemical and isotopic composition of seawater and the oceanic crust, involving chemical fluxes of many elements exceeding those from global river discharge. Despite its importance, fundamental aspects of these hydrothermal processes such as the extent of reaction at various temperatures and fluid flow paths are not well constrained. In addition, subduction of the altered crust results in the reinjection of elements derived from the hydrosphere back into the mantle. The bulk chemical transfer rates and the chemical signature of these recycled materials is poorly constrained. Recently it has been demonstrated that there are large boron isotopic fractionations associated with the alteration of the oceanic crust (Spivack and Edmond, 1987). This exchange exerts a primary control on the boron isotopic composition of seawater (~40 per mil enriched in the 11B/10B ratio relative to continental and oceanic igneous rocks|) and imparts a boron isotopic signature to the crust that should be useful as a tracer of subducted volatiles, and diagnostic in evaluating the nature of hydrothermal processes including fluid reaction histories, temperatures of reaction and flow paths. We propose to conduct a boron isotope study of the Troodos ophiolite that is aimed at two goals: 1) establishing a first order boron isotope profile for the altered oceanic crust and 2) developing the background data required for the future exploitation of boron isotopes in the study of hydrothermal systems. To these ends we intend to determine (B) and 11B in whole rocks and vein separates that are mineralogically and chemically well characterized. Alteration temperatures and fluid evolution wii be constrained by oxygen and strontium isotope data.