The primary economic objective of the Iceland Deep Drilling Project (IDDP) is to find 450-600°C supercritical geothermal fluids at drillable depths. The Reykjanes geothermal system is a seawater recharged hydrothermal system, although fluid composition has evolved through time as a result of changing proportions of meteoric water influx as well as differing pressure and temperature conditions imposed by glaciation (Arnorsson, 1995; Frišleifsson et al., 2003). We have measured the Sr compositions in epidote and anhydrite as well as S and O isotopic composition of anhydrite in the Reykjanes geothermal system in order to better constrain the geometry and processes by which seawater contributes to the generation of hydrothermal fluid. Anhydrite is widespread in shallow portions of the Reykjanes system to about 1500 m, and after a depositional hiatus appears again around 2650 m depth. This may suggest either a transient incursion of cold seawater into the system at depth, or possibly a more permanent cooling at depth in the system. A zone of pyroxene hornfels facies contact metamorphism has also been observed at this depth (Marks et al., 2008), and the reappearance of anhydrite could possibly indicate heating and oxidation of previously precipitated sulfides. Analysis of the spot core successfully recovered by IDDP in November 2008 will provide an excellent opportunity to further investigate the fluid-rock interaction at depth within this well.

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