Magnesium silicate chimneys at the Strytan hydrothermal field, Iceland as analogues for prebiotic chemistry at alkaline submarine hydrothermal vents on the early Earth

The Strytan Hydrothermal Field (SHF) in Iceland, is of great interest as a member of the class of alkaline submarine hydrothermal vent systems that are leading contenders for the places where life on Earth may have begun. In this work we analyse the composition, structure, and mineralogy of samples from hydrothermal chimneys at the SHF. We find that the chimney mineral precipitates are composed of Mg-silicates including clays of the saponite-stevensite group (high Mg and Si, low Fe and Al); Ca-carbonates and Ca-sulfates. The chimney samples were porous / permeable structures with porosity evident at the macro and micro scale; pores were visible down to ~1 µm in size and possibly smaller. The chimney interiors as observed with SEM (Scanning Electron Microscopy) and X-ray CT (computed tomography) scanning exhibited complex structures with high internal surface area. EDX (energy-dispersive X-ray spectroscopy) analysis showed an increase in the Mg/Si ratio with oxygen content in the external area of the chimney. Chemical garden analogue experiments in the laboratory produced similar Mg-silicate chimneys with porous internal structures, indicating that injection-precipitation experiments can be high-fidelity analogues for natural hydrothermal chimneys at the SHF. The SHF chimneys may be able to host similar prebiotic reactions as proposed for other alkaline vents (e.g. the Lost City Hydrothermal Field); the additional presence of silicates and clays in the SHF may also facilitate prebiotic chemistry favoured on clays and silica gels. Analysis of the fluid dynamics shows that these chimneys are intermediate in growth rate compared to faster black smokers and the slower Lost City. The SHF is therefore a prebiotic vent analogue for worlds such as early Earth and Mars that may have had basalt-hosted alkaline hydrothermal systems.