Mineralogy and geochemistry of clay samples from active hydrothermal vents off the north coast of Iceland

Active hydrothermal vent sites were sampled during 1997 in a series of submersible dives at the active Grimsey (GHF) and Kolbeinsey (KHF) hydrothermal fields off the north coast of Iceland. This study focuses on secondary clay minerals which were formed in two different settings. The GHF is characterized by the presence of clay minerals precipitated within active chimneys. By contrast, the KHF is characterized by the presence of secondary clay minerals, which are the products of hydrothermal alteration of lava fragments. Based on XRD, electron microprobe and ICP-MS analyses, the dominant clay mineral in both hydrothermal fields is saponite (Mg-rich smectite). Chlorite and chlorite–smectite mixed-layer minerals also occur at the KHF. The Mg-rich nature of saponite from the GHF chimneys suggests intense Mg metasomatism in the mixing zone where hydrothermal fluids interact with seawater at temperatures of 250 °C. Saponite formation resulted in the additional uptake of Cu, Zn, and Pb. Enrichment in Ba is evident in the almost pure saponite from the KHF. Based on oxygen isotope data, the saponite formation at the KHF occurred at 148 °C, which is close to the maximum measured fluid temperature of 131 °C in this field.