Elemental changes and alteration recorded by basaltic drill core samples recovered from in situ temperatures up to 345°C in the active, seawater‐recharged Reykjanes geothermal system, Iceland

Abstract

Hydrothermal activity results in element exchanges between seawater and oceanic crust that contribute to many aspects of ocean chemistry; therefore, improving knowledge of the associated chemical processes is of global significance. Hydrothermally altered basaltic drill core samples from the seawater‐recharged Reykjanes geothermal system in Iceland record elemental gains and losses similar to those observed in samples of hydrothermally altered oceanic crust. At Reykjanes, rocks originally emplaced on the seafloor were buried by continued volcanism and subsided to the current depths (>2250 m below surface). These rocks integrate temperature‐dependent elemental gains and losses from multiple stages of hydrothermal alteration that correspond to chemical exchanges observed in rocks from different crustal levels of submarine hydrothermal systems. Specifically, these lithologies have gained U, Mg, Zn, and Pb and have lost K, Rb, Ba, Cu, and light rare earth elements (La through Eu). Alteration and elemental gains and losses in lithologies emplaced on the seafloor can only be explained by a complex multistage hydrothermal alteration history. Reykjanes dolerite intrusions record alteration similar to that reported for the sheeted dike section of several examples of oceanic crust. Specifically, Reykjanes dolerites have lost K, Rb, Ba, and Pb, and gained Cu. The Reykjanes drill core samples provide a unique analog for seawater‐oceanic crust reactions actively occurring at high temperatures (275–345°C) beneath a seafloor hydrothermal system.

The Iceland Deep Drilling Project at Reykjanes: Drilling into the root zone of a black smoker analog

“…The concentration of relatively immobile trace elements indicates that these rocks are related to the trace element enriched category of Gee et al (1998), similar to the tholeiitic lava flows that cover most of the Reykjanes peninsula. The pattern of alteration-induced gains and losses of elements recorded in the RN-17B core, relative to an unaltered precursor, suggests emplacement on the seafloor and passive alteration by seawater, followed by subsidence, burial, and alteration at progressively higher temperature conditions (Fowler and Zierenberg, 2016b).…”

“…The cores are composed of a series of fineto coarse-grained dolerite intrusions of basaltic composition (example in Fig. 13F), interpreted to come from a sheeted dike complex (Fowler and Zierenberg, 2016b). The location of RN-30 rocks immediately below the Skálafell shield volcano suggests at least some of the intrusions may be feeder dikes for that edifice, but confirmation studies are needed.…”

“…13D and E), that were originally vesicular glassy basalt, hyaloclastite, and volcanic siltstone, but these rocks are extremely altered with extensive leaching and silicification. The rocks are cut by quartz, epidote, and anhydrite veins (Fowler and Zierenberg, 2016b) and may be an example of rechargerelated alteration in a major geothermal aquifer. While the chemistry of the RN-30 cores is compatible with a single-stage of alteration at the in-situ conditions, the slough blocks have undergone a complex prograde alteration history, like that in RN-17B rocks (Fowler and Zierenberg, 2016b).…”

The Iceland Deep Drilling Project 4.5 km deep well, IDDP-2, in the seawater-recharged Reykjanes geothermal field in SW Iceland has successfully reached its supercritical target

“…We were using an IDDPdesigned 8 1 2 coring tool, with 9.7 m long core barrel (Skinner et al, 2010), which had yielded good core recoveries in RN-17B, an inclined 8 1 2 hole from 2800 m depth, and also from three successive core runs below a 9 RN-30 at Reykjanes from similar depth to RN-17B (Fowler et al, 2015;Fowler and Zierenberg, 2016). Table 2 gives an overview of the core recovery in 10 core runs with the IDDP 8 1 2 coring tool and 3 successive core runs with the 6 Baker Hughes tool at the bottom of IDDP-2, beneath the 7 liner.…”

“…Data for DSDP/ODP/IODP holes from Alt et al (2010); Anderson et al (1982);and Marks et al (2010). Data for shallower portions of Reykjanes from Marks et al (2010);Fowler et al (2015);and Fowler and Zierenberg (2016). chilled margins recovered between 4634.20 and 4656.00.…”