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Abstract
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.
Highlights
Seafloor hydrothermal fields often contain polymetallic massive sulfides, chimneys, mounds, and their host rocks
Most of the δ56Fe values (− 1.96 to +0.11‰) of the sulfide minerals are within the range of the vent fluids, but they are significantly lower than those of the mid-ocean ridge basalts (MORBs) and back-arc basin basalts (BABBs), suggesting that the Fe in the sulfides was mainly derived from the fluids
This is consistent with the varying fluid temperatures, which ranged from high to low during the pyrite formation (Fig. 7a) (Abraitis et al, 2004; Keith et al, 2016). These values are slightly lower than the δ56Fe values of seawater (− 0.88 to +0.10‰; Rouxel and Auro, 2010) and are similar to the δ56Fe values of hydrothermal fluids (− 1.85 to − 0.14‰; Sharma et al, 2001; Severmann et al, 2004; Rouxel et al, 2008; Moeller et al, 2014) (Fig. 3). All these findings indicate that the fluids may be a source of the light Fe isotopic compositions of the sulfides, and that 54Fe is more likely to be preferentially incorporated into pyrite facies during mixing between seawater and hydrothermal fluids
Summary
Seafloor hydrothermal fields often contain polymetallic massive sulfides, chimneys, mounds, and their host rocks (e.g., basalts). Both high (>300 ◦C) and low temperature (
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