Hydrothermal activity in Tertiary Icelandic crust: Implication for cooling processes along slow-spreading mid-ocean ridges

Abstract:

Known hydrothermal activity along the Mid–Atlantic Ridge is mostly high–temperature venting, controlled by volcano–tectonic processes confined to ridge axes and neotectonic zones ~15km wide on each side of the axis (e.g. TAG or Snake Pit). However, extensive exploration and discoveries of new hydrothermal fields in off–axis regions (e.g. Lost City, MAR) show that hydrothermalism may, in some areas, be dominated by off–axis venting. Little is known about nature of such systems, including whether low–temperature “diffuse” venting dominates rather than high–temperature black-smokers. This is particularly interesting since such systems may transport up to 90% of the hydrothermal heat to the oceans.

In this study we use Icelandic hot springs as onshore analogues for off–shore hydrothermal activity along the MAR to better understand volcano-tectonic controls on their occurrence, along with processes supporting fluid circulation. Iceland is a unique laboratory to study how new oceanic crust cools and suggests that old crust may not be as inactive as previously thought. Our results show that Tertiary (>3.3 Myr) crust of Iceland (Westfjords) has widespread low–temperature hydrothermal activity. Lack of tectonism (indicated by lack of seismicity), along with field research suggest that faults in Westfjords are no longer active and that once sealed, can no longer support hydrothermal circulation, i.e. none of the hot springs in the area occur along faults. Instead, dyke margins provide open and permeable fluid migration pathways. Furthermore, we suggest that the Reykjanes Ridge (south of Iceland) may be similar to Westfjords with hydrothermalism dominated by off–axis venting. Using bathymetric data we infer dyke positions and suggest potential sites for future exploration located away from neotectonic zone. We also emphasise the importance of biological observations in seeking for low-temperature hydrothermal activity, since chemical or optical methods are not sufficient.