Spatial Variations in Vent Chemistry at the Lucky Strike Hydrothermal Field, Mid‐Atlantic Ridge (37°N): Updates for Subseafloor Flow Geometry From the Newly Discovered Capelinhos Vent

Abstract

This study aims at characterizing the subseafloor architecture of the Lucky Strike hydrothermal field (LSHF) based on an extensive chemical database of the various vents. Our analysis is motivated by the discovery in 2013 of a new active high‐temperature site, named Capelinhos, approximately 1.5 km east of the LSHF. Capelinhos fluids display particular chemical features with chloride and metal (Fe and Mn) concentrations 2 times lower and 4 times higher, respectively, compared to other vent sites. Trace element partitioning over the entire chlorinity range indicates a single deep fluid source feeding all the venting sites. Applying the Si‐Cl geothermobarometer at Capelinhos, we find phase separation conditions at 435–440 °C and 370–390 bars (2,500–2,800 m below seafloor) consistent with former estimates for the LSHF, while temperatures of fluid‐rock last equilibrium are estimated at ~400 °C for Capelinhos and 350–375 °C for the other sites based on the Fe‐Mn geothermometer. We interpret these discrepancies in thermodynamic conditions beneath the sites in terms of crustal residence time, which are likely related to permeability variations across the hydrothermal upflow zone. We propose that conductive cooling of the upflowing fluids from the phase separation zone to the seafloor, beneath the main field vent sites, lowers the T conditions of last fluid‐rock equilibrium, enabling ~65% of Fe mobilized in the reaction zone to be stored. In comparison, Capelinhos fluids are transported more rapidly from the reaction zone to the seafloor along a high‐angle fracture system. The fluids venting at Capelinhos are more representative of the deeper part of the hydrothermal reaction zone.

Tracking the Lithium and Strontium Isotope Signature of Hydrothermal Plume in the Water Column: A Case Study at the EMSO-Azores Deep-Sea Observatory

“…Indeed, the LSHF hosts about ~30 active hydrothermal vents distributed over its 1 km 2 surface area. At this location, Chavagnac et al (2018a) and Leleu (2017) showed that high-temperature hydrothermal fluids contain DSr concentrations that are either lower or higher than seawater depending on the vent. In particular, the vents located on the western and north-eastern sides of the LSHF contain 94-179 µmol/L of DSr and all the south-eastern vents display 74-81 µmol/L of DSr (light blue boxes in Figure 2A, and Supplementary Table S2; Von Damm et al, 1998;Charlou et al, 2000;Pester et al, 2012;Chavagnac et al, 2018a).…”

“…A better understanding of lithium (Li) and strontium (Sr) oceanic biogeochemical cycles is needed nowadays based on two main issues: 1) their oceanic cycles are still under debate, and 2) they are two economically relevant elements. Li and Sr are widely used to estimate several geochemical processes notably at the water/ rock interface (Huh et al, 1998;Brunskill et al, 2003;Davis et al, 2003) such as continental weathering (Pistiner and Henderson, 2003;Vance et al, 2009;Millot et al, 2010) or hydrothermal alteration of oceanic crust (Barker et al, 2008;Araoka et al, 2016;Chavagnac et al, 2018a). It is generally admitted that dissolved riverine inputs and hydrothermal inputs are the two main sources of Li and Sr to the ocean (Albarède et al, 1981;Palmer and Edmond, 1989;Allègre et al, 2010;Tomascak et al, 2016;von Strandmann et al, 2020).…”

“…Since 2020, Li and Sr have been considered "critical raw materials" by the European Union, i.e., highly important economically but with a high supply risk (European Commission, 2020, Study on the EU's List of Critical Raw Materials-Final Report, 2020). Therefore, several studies investigate the feasibility to extract Li and Sr from seawater (Vikström et al, 2013;Hong et al, 2018;Ryu et al, 2020), and in particular Li from black smokers hydrothermal vents that contain 10-20 times more Li than seawater (European Commission, 2012;Chavagnac et al, 2018a). Yet, the effects of deep-sea mining on the marine environment need to be assessed notably through studies on the hydrothermal role in marine biota and element oceanic biogeochemical cycles.…”

Recovery of hydrothermal vent communities in response to an induced disturbance at the Lucky Strike vent field (Mid-Atlantic Ridge)

“…B. azoricus, B. seepensis, A. lutzi, P. valvatoides and L. atlanticus), which have been previously described as indicator species of cold microhabitats on the Eiffel Tower edifice situated ~ 50 m from Montségur (Sarrazin et al, 2015b). The high similarity between the fauna from the two edifices may be related to their belonging to the same chemistry domain (Chavagnac et al 2018). Among the 43 macrofaunal species identified on Montségur, approximately 74% exhibit a low frequency of abundance (<1%).…”

Fault-controlled deep hydrothermal flow in a back-arc tectonic setting, SE Tyrrhenian Sea

“…Subaerial volcanoes, back-arc systems and mid-ocean ridges have been widely investigated in terms of hydrothermal vents distribution and circulation pattern by bathymetric and backscatter, magnetics, electro-magnetics, gravity and seismic methods 12–15 . High-resolution multibeam bathymetry constrains spatially hydrothermal activity 6,16 , while vent chemistry helps to determine the source of hydrothermal fluids as well as fluid crustal residence time related to rock permeability 17 . However, the lack of direct information, such as drilling, makes numerical modelling an important tool in understanding fluid/rock interactions and hydrothermal circulation within an active tectonic setting 18,19 .…”