Evolution of fluid–rock interaction in the Reykjanes geothermal system, Iceland: Evidence from Iceland Deep Drilling Project core RN-17B

Abstract

We describe the lithology and present spatially resolved geochemical analyses of samples from the hydrothermally altered Iceland Deep Drilling Project (IDDP) drill core RN-17B. The 9.3 m long RN-17B core was collected from the seawater-dominated Reykjanes geothermal system, located on the Reykjanes Peninsula, Iceland. The nature of fluids and the location of the Reykjanes geothermal system make it a useful analog for seafloor hydrothermal processes, although there are important differences. The recovery of drill core from the Reykjanes geothermal system, as opposed to drill cuttings, has provided the opportunity to investigate evolving geothermal conditions by utilizing in-situ geochemical techniques in the context of observed paragenetic and spatial relationships of alteration minerals. The RN-17B core was returned from a vertical depth of~2560 m and an in-situ temperature of~345°C. The primary lithologies are basaltic in composition and include hyaloclastite breccia, fine-grained volcanic sandstone, lithic breccia, and crystalline basalt. Primary igneous phases have been entirely pseudomorphed by calcic plagioclase + magnesium hornblende + chlorite + titanite + albitized plagioclase + vein epidote and sulfides. Despite the extensive hydrothermal metasomatism, original textures including hyaloclastite glass shards, lithic clasts, chilled margins, and shell-fragment molds are superbly preserved. Multi-collector LA-ICP-MS strontium isotope ratio ( 87 Sr/ 86 Sr) measurements of vein epidote from the core are consistent with seawater as the dominant recharge fluid. Epidote-hosted fluid inclusion homogenization temperature and freezing point depression measurements suggest that the RN-17B core records cooling through the two-phase boundary for seawater over time to current in-situ measured temperatures. Electron microprobe analyses of hydrothermal hornblende and hydrothermal plagioclase confirm that while alteration is of amphibolite-grade, it is in disequilibrium and the extent of alteration is dependent upon protolith type and water/rock ratio. Alteration in the RN-17B core bares many similarities to that of Type II basalts observed in Mid-Atlantic Ridge samples.

Geochemical bias in drill cutting samples versus drill core samples returned from the Reykjanes Geothermal System, Iceland

“…3D). In contrast to the RN-17B cuttings, the RN-17B core is more pervasively altered with albitized igneous plagioclase, actinolite after igneous clinopyroxene, and near complete replacement of the pillow interior to chlorite + hornblende (Fowler et al, 2015). The drill cuttings immediately above the core do include a few fragments of epidote, actinolite, pyrite and titanite veins similar to those common throughout the RN-17B core (Fowler et al, 2015;Fig.…”

“…Geothermics (2016), http://dx.doi.org/10.1016/j.geothermics.2016.02.007 Sigurgeirsson et al (2011). c Fowler et al (2015).…”

“…The spot cores were recovered from true vertical depths ranging between ∼2245 m and ∼2570 m and in situ temperatures ranging from ∼250 to 345 • C (Friðleifsson et al, 2005;Friðleifsson and Richter, 2010;Friðleifsson et al, 2014;Fowler et al, 2015). The purpose of this study is to compare wholerock geochemical results from the Reykjanes drill core samples to vertically adjacent drill cutting samples to understand the limitations of using drill cutting samples to elucidate subsurface lithology and alteration processes during geothermal energy exploration, and for addressing questions about seawater-crust elemental exchanges that are of interest to the seafloor hydrothermal research community.…”

“…The 9.3 m RN-17B core was recovered at an in situ temperature of 345 • C, a TVD of ∼2560 m (2798.6-2807.9 m down hole depth), and is composed of basalt pillows, hyaloclastite and lithic breccia, and volcanic sandstone (Friðleifsson et al, 2005;Friðleifsson and Richter, 2010;Fowler et al, 2015). RN-30 consists of three sequential cores totaling 22.5 m. The RN-30 cores were recovered at an in situ temperature of 345 • C, a TVD of ∼2240 m (2510.3-2532.8 m down hole depth), and are composed of a series of fine and coarse crystalline basalt intrusions with an ophitic texture.…”

“…Elevated CaO is also potentially hosted in epidote, which is common in cuttings from the intervals studied (Helgadóttir et al, 2009;Sigurgeirsson et al, 2011). In the case of RN-17B, elevated SiO 2 likely reflects the high proportion of quartz logged in the cutting samples (Helgadóttir et al, 2009), in contrast to the RN-17B core samples where quartz is entirely absent (Fowler et al, 2015).…”