Causes for recent fluid venting at the Cretaceous Henry Seamount, Canary Archipelago
Mineralogy, Petrology and Geochemistry
Final Report Abstract
The circulation of heated seawater through oceanic crust and seamounts (submarine volcanoes) is globally important for cooling of the lithosphere, chemical exchange between crust and ocean, and marine ecosystems. Henry Seamount near El Hierro (Canary Islands) is an example for an ancient (126 Myrs old) but recently hydrothermally active volcano; it was investigated in detail during R/V METEOR cruise M146 in 2018 and a follow-up cruise with the Spanish R/V Ángeles Alvariño one year later. Using data from TV-sled surveys and water sampling we have mapped the distribution of the following indicators for past and present fluid discharge at the seamount: 1) massive occurrences of shells from vesicomyid clams that live exclusively at fluid venting sites; 2) occurrence of stained sediment, microbial mats, barite precipitates, and cemented sediment; and 3) chemical anomalies in the seawater (decreased oxygen reduction potential, increased methane contents) that indicate the presence of reduced species. Three regions of past to present hydrothermally activity could be identified in the central area of the seamount, and one at its southeastern flank. Radiocarbon ages of shells fall into two groups of 0.7-4.1 kyrs and 15.4-22.4 kyrs before present. This suggests two distinct phases of fluid flow activity, each giving rise to deep-sea chemosynthetic communities. One surprising finding of M146 was the recovery of basaltic sand (ash) and dispersed basaltic rock fragments <4 cm in size from the summit area of Henry Seamount. Glass from the dominant ash type has high sulfur contents, indicating that this ash must have been erupted at depth. Our data imply rejuvenated volcanic activity in the summit area of the seamount, suggesting that magmatic heat released by volcanic pulses was the driver for at least two phases of ephemeral hydrothermal activity. Magmatic activity long after formation of Henry Seamount is also indicated by acoustic blanking zones in the seismic data obtained during M146, which are likely related to magmatic intrusions and fluids released from them. The chemical and radiogenic isotope compositions of the ash samples suggests a direct relationship of the rejuvenated volcanic activity to the Canary hotspot, which feeds current volcanism at El Hierro and La Palma. In contrast to the dominant ash type, other basaltic samples from the seamount have low sulfur contens and are unlikely to have been erupted from Henry Seamount. Their overall similarity to volcanic rocks from El Hierro suggests an origin from this island. We infer that the basaltic fragments were transported from El Hierro to Henry Seamount by a giant flank collapse toward the southeast, which occurred at ca. 145-175 ka. This is a surprising result and indicates a wider dispersion of El Hierro's flank collapse deposits than hitherto believed. If the development of hydrothermal circulation and chemosynthetic communities at Henry Seamount was indeed a consequence of single magmatic pulses, then similar scenarios might be envisaged for many other volcanic seamounts in the deep ocean basins. As exploration of the deep sea is still very limited, Henry Seamount may be a rare example of a common feature rather than a special case.
Publications
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Hydrothermal Activity at a Cretaceous Seamount, Canary Archipelago, Caused by Rejuvenated Volcanism. Frontiers in Marine Science, 7.
Klügel, Andreas; Villinger, Heinrich; Römer, Miriam; Kaul, Norbert; Krastel, Sebastian; Lenz, Kai-Frederik & Wintersteller, Paul
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Enhanced weathering in the seabed: Rapid olivine dissolution and iron sulfide formation in submarine volcanic ash. American Mineralogist, 107(9), 1668-1680.
Kahl, Wolf-Achim; Klügel, Andreas; Bach, Wolfgang & Murshed, Mangir
