The Iceland Deep Drilling Project (IDDP) is a long-term study of high-temperature hydrothermal systems on the Reykjanes Peninsula, where the Mid-Atlantic Ridge emerges on to the SW tip of Iceland. The IDDP is a collaborative effort, by a consortium of Icelandic power companies and the Icelandic government, to investigate if utilizing supercritical geothermal fluids would improve the economics of power production from geothermal fields. Over the next decade this will involve drilling a series of wells >4 km deep, to reach temperatures ~450°C. The deepest of these wells so far was completed at 3.1 km in February 2005. The rocks penetrated consist of Holocene basaltic lavas, subglacial hyaloclastites, marine sediments, submarine pillow basalts, and diabase dikes. In 2006, the IDDP will rotary drill and spot core this, or another candidate well, to 4.0 km, and in 2007, the IDDP will deepen the borehole from 4.0 km to 5.0 km, using continuous wireline coring. Such deep, hot wells present both technical challenges and opportunities for important scientific studies. For example, preliminary analyses of rock samples and fluids from the existing geothermal wells indicate that the shallow geothermal system is complex, as indicated by paragenetic relations and strong compositional zoning in calc-silicate minerals, such as epidote. Calculation of local equilibria between calc-silicates and calcite suggests that the CO₂ content of the geothermal fluids increased during the evolution of this geothermal system. Zoned hydrothermal amphiboles at 3.1 km depth include tschermakitic hornblende (~13 wt. % Al₂O₃), suggesting temperatures in the upper 300°C range. Similarly, analyses of hydrogen isotopic ratios of epidotes and amphiboles currently underway indicate that meteoric water has mixed with seawater during the evolution of the Reykjanes geothermal system. The Reykjanes Peninsula is a superb location for scientific investigations of the deeper levels of a high enthalpy geothermal resource. Coring below 4.0 km is designed to penetrate into supercritical fluids which couple black smoker hydrothermal systems with their magmatic heat sources. Supercritical fluids have greatly enhanced rates of mass transfer and chemical reaction. Such environments have never before been available for comprehensive direct study and sampling. These investigations will be a very important contribution to global science and have clear connections to the studies of ridge-hotspot interactions by the Integrated Ocean Drilling Program. The broader implications of the IDDP are twofold; scientifically it will permit a quantum leap in our understanding of active hydrothermal processes that are important on a global scale, and secondly, if the industrial aims are successful, the resulting technology could have a major impact on improving the economics of high-temperature geothermal resources worldwide. The IDDP has welcomed participation by an international group of scientists that will investigate and test models of the coupling of hydrothermal and magmatic processes. The status of the project is reported at http://www.iddp.is.

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