Geochemical characterization of deep-sea sediments on the Azores Plateau – From diagenesis to hydrothermal activity

Highlights • Overview on geochemical composition of pore water and solid phase of sediments on the Azores Plateau. • Evidence for deep marine hydrothermal activity on the Azores Plateau. • Pore water data suggest ongoing anaerobic oxidation of methane and carbonate recrystallization. The Azores Plateau is an active magmatic region in the Central North Atlantic Ocean. In this study, we present a comprehensive data set of major element compositions and 87Sr/86Sr ratios of pore waters from surface sediments (0–9 mbsf) of the Azores Plateau. Based on distinct geochemical signatures we can separate normal marine from hydrothermally affected sediments. Normal marine sediments can further be differentiated by their ash content. Pore waters of ash rich gravity cores (GCs) do not show any deviations from seawater values except of a minor increase in Sr. In contrast, ash poor GCs generally show a trend for decreasing Ca with increasing depth, accompanied by a minor SO4 decrease and a more pronounced Sr increase. We suggest that these deviations are caused by processes such as anaerobic oxidation of methane and carbonate recrystallization. At four additional sample locations we observed a decrease in Mg and SO4 accompanied by a Ca increase in the pore waters, a pattern typical for hydrothermal fluids. The existence of hydrothermal systems in this region are corroborated by multi-channel seismic data, suggesting that sill or dyke intrusions are present in the subsurface close to the core locations. Overall, our observations offer preliminary indications for the existence of submarine hydrothermal systems on the Azores Plateau away from the Mid- Atlantic Ridge

Insights into pore fluid geochemistry of the Azores Plateau: From early diagenesis to hydrothermal vents

During the 2017 research cruise M 141 “Azores Tephra” of the German R/V Meteor to the Azores Plateau and Terceira Rift, pore water geochemistry of sediments from 27 gravity cores were analyzed for major elements and isotopic composition of 87Sr/86Sr, �34S, �26Mg, �18O, �D, �30Si, and �11B. We use this data set to characterize fluid sources of active hydrothermal vents, as well as the fluid evolution during early diagenesis. In general, pore fluids show a down-core decrease in Ca, while Na and K show no significant changes. A main component of the sediments are ash layers from volcanoes in the Azores archipelago. Early alteration of these ashes may result in a Ca-rich zeolite formation, and thus represent a potential sink of Ca. In contrast, Sr concentrations typically increase significantly, accompanied by rather constant 87Sr/86Sr ratios. Such a pattern has frequently been observed in similar pelagic sediments and is referred to calcium carbonate recrystallization within carbonate oozes. Until now, only one submarine hydrothermal vent is known in the Terceira Rift at the D. Joao de Castro seamount. Our findings indicate that at least one further vent is active in the rift. Fluids here are significantly different from typical Mid-Oceanic hydrothermal systems. Pore fluids are marked by elevated total alkalinity, Mg, and SO4 concentrations while Ca stays at background values. The most straightforward way to interpret this finding in combination with the isotopic results is a re-dissolution of a hydrothermally formed magnesium-sulfatehydroxide- hydrate, a meta-stable mineral previously reported once from the East Pacific Rise. In addition to that, we identified 5 further locations on the Azores Plateau with indications for hydrothermal activity. Here, down-core Mg and SO4 decrease is accompanied with an increase in Ca concentration, which follows the trend of typical Mid-Oceanic hydrothermal systems. Seismic data suggest that close to the core locations magmatic intrusions are present in the subsurface. The geochemical findings together with the evidence of a heat source in the subsurface lead to the assumption that hydrothermal seepage is active on the entire Azores Plateau

Genesis of high Mg and SO4 hydrothermal fluids in the Terceira Rift

Typical Mid-Oceanic ridge hydrothermal systems are in general significantly enriched in Ca due to leaching from the magmatic basement (albitization of anorthite), while Mg and SO4 are quantitatively removed because of Mgrich smectite and anhydrite formation and further processes. In the Terceira Rift (TR), a hyper-slow spreading center in the Central North Atlantic Ocean, hydrothermal fluid venting is known to occur only at shallow intertidal water depths around the volcanogenic Azores Archipelago. Here, we show for the first time that hydrothermal fluid venting is active in the eastern TR at water depths of 2800 m. Pore fluids of a sediment core taken close to a volcanic cone, however, show that the fluid composition is significantly different from typical Mid-Oceanic hydrothermal systems. Pore water 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, which are generally met at the TR. Isotope measurements of �34S, �26Mg, 87Sr/86Sr, �88=86Sr, �44=42Ca and �30Si provide additional evidence for caminite as a source for Mg, SO4 and TA. The redissolution 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. Our study implies that element recycling through caminite might play a presently unrecognized role in element budgets of hydrothermal systems

Caminite: An underestimated metastable Mg reservoir in hydrothermal recharge zones?

Mid-Oceanic ridge hydrothermal systems are in general significantly enriched in Ca due to leaching from the oceanic basement (albitization of anorthite), while Mg and SO4 are quantitatively removed by Mg-rich smectite and anhydrite formation respectively. During Meteor expedition 141 in September 2017 we sampled, close to a volcanic outcrop in the Terceira Rift (Azores, Central North Atlantic Ocean), pore fluids with a significantly different composition compared to typical Mid-Oceanic hydrothermal systems. Pore water Mg, SO4, and total alkalinity (TA) concentrations are significantly higher compared to seawater and a nearby reference core, while Ca concentrations stay at low values. The most straightforward way of interpreting the observed deviations is the dissolution of the prior hydrothermally formed high temperature (> 240°C) mineral caminite (MgSO4·0.25Mg(OH)2·0.2H2O). This interpretation is corroborated by a thorough investigation of fluid isotope systems (δ26Mg, δ30Si, δ34S, δ44/42Ca, and 87Sr/86Sr). 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 little fresh basaltic glass present, which are generally met in the Terceira Rift. To date, no signs of extensive caminite formation and/or dissolution have been reported, caminite has only been once described before in the nature by Haymon and Kastner (1986). Our study is the first indication of an abundant occurrence of caminite. The results imply that element recycling through caminite might play a presently unrecognized role in element budgets of hydrothermal systems. Haymon, R. M., and Kastner, M., 1986, Caminite; a new magnesium-hydroxide-sulfate-hydrate mineral found in a submarine hydrothermal deposit, East Pacific Rise, 21 degrees N: American Mineralogist, v. 71, no. 5-6, p. 819-825