Azores Hot Vents – The quest for deep sea sediment-hosted hydrothermal vents on the Azores Plateau

The Azores are a magmatically active region in the central North Atlantic. Despite ongoing magmatic activity on and offshore the archipelago, only a few shallow hydrothermal vents are known in the region. During the “Azores Tephra” cruise M 141 on the German research vessel Meteor in September 2017 we carried out pore fluid analyses of 28 gravity cores in the deep sea of the Azores Plateau. The pore fluids were analyzed for total alkalinity, ammonium, methane and other major elements such as Mg, SO4, Ca, Cl, B and Sr and allowed, for the first time a thorough analysis of sub-seafloor geochemical processes in this area of intense tectonic and volcanic activity.

The present status of geochemical analyses suggests that there are two different types of hydrothermally altered seepage systems: (1) In most places acidic hydrothermal fluids seem to percolate through the sediments. The majority of the systems shows a typical low Mg/SO4 high Ca signature of the fluids. Helium data suggest that venting above sill intrusions are diffusive. (2) Pore fluids of a sediment core taken close to a volcanic cone, show that the fluid composition is significantly different from typical Mid-Oceanic hydrothermal systems. Porewater Mg, SO4, and total alkalinity (TA) concentrations are significantly higher compared to seawater and a nearby reference core. The most straightforward way of interpreting these excursions is the re-dissolution of the metastable mineral Caminite (MgSO4 0.4Mg(OH)2 0.2H2O). Caminite is known from mineral assemblages with e.g. anhydrite and forms only under specific conditions such as high fluid temperatures and in altered oceanic crust with only few fresh basaltic glass present. Isotope measurements of δ34S, δ26Mg, 87Sr/86Sr, δ44∕42Ca and δ30Si provide additional evidence for Caminite as a source for Mg, SO4 and TA.

The re-dissolution of the Caminite is interpreted as a sign of cooling temperatures, which may indicate a waning state of the hydrothermal system. To date, no signs of extensive Caminite formation and/or dissolution have been reported. The data however imply that element recycling through Caminite might play a presently unrecognized role in element budgets of hydrothermal systems.