| Abstract |
The Reykjanes geothermal system is located on the western tip of the Reykjanes Peninsula, approximately 50 km southwest of Reykjavík, Iceland. The Reykjanes geothermal fluid is seawater, chemically modified through boiling and interaction with basaltic host rock, and bares many similarities to mid ocean ridge (MOR) hydrothermal fluids. The Reykjanes geothermal system is the proposed location of the next Iceland Deep Drilling Project (IDDP) well, IDDP-2. The IDDP-2 effort proposes to return fluid and rock samples from the super-critical reaction zone of a sea-floor style hydrothermal system for the first time. Previous studies of seafloor hydrothermal systems provide an analog for conditions that might be encountered during drilling of IDDP-2, however, to use MOR hydrothermal systems as an analog for Reykjanes (and vice versa) we need to understand the key differences. We have developed protocols to sample and measure rare earth element (REE) concentrations in high salinity geothermal fluids from Reykjanes. We have also measured REE in-situ in epidote precipitated from geothermal fluids in Reykjanes drill cutting and drill core samples using Laser Ablation ICP-MS (LA-ICP-MS). Additionally we present results for REE analysis of wholerock samples from the Reykjanes RN-17B IDDP drillcore. Results indicate that 1) REE in Reykjanes fluid samples are enriched in light REE and have large positive Eu anomalies relative to the basaltic host rock, 2) REE in Reykjanes fluids are approximately two orders of magnitude less enriched in REE than has been observed in a range of seafloor hydrothermal vent fluids, and show relative light REE enrichment 3) The REE content of epidote ranges from the maximum observed for whole rock samples to below the method detection limit, and 4) Chondrite normalized REE concentrations in epidote fall on a spectrum between wholerock REE and seafloor hydrothermal fluid REE, and are dissimilar to Reykjanes fluids sampled at the well head. Our data suggests that Reykjanes geothermal fluids sampled at the wellhead have undergone significant REE loss, and the chondrite normalized REE pattern of Reykjanes geothermal fluids at depth is better reflected by hydrothermal epidote. We suggest the REE content of Reykjanes fluids at depth are more similar to MOR fluids, and that the REE content of Reykjanes fluids is derived from localized fluid-rock interaction, and is predominantly controlled by seawater/rock ratio and temperature. LA-ICP-MS analysis of epidote precipitated in veins sequentially over time may provide a means to identify favorable high water/rock ratio and high temperature zones at depth in the Reykjanes geothermal system. Because of the minimal sample preparation and analytical procedures offered by LA-ICP-MS, we will be able to analyze samples in near real time and inform IDDP of high temperature reaction zones that may be encountered while drilling into the supercritical zone of a hydrothermal system prior to attempting to produce hydrothermal fluids from the well. |