| Abstract |
Exceptionally high-grade surface alteration has been observed in hyaloclastite within the caldera of the active Vonarskard central volcano. This surface alteration includes the high-temperature mineral assemblage: actinolite-epidote-wollastonite-quartz (act-ep-wo-qtz), which is assumed to be formed at temperatures above 300?C and pressures above 100 bars. Pits after recently cooled hot springs or mud pits, presumably of early Holocene age, are found within the outcrop, which covers an area of some 2.5 km2. No active geothermal surface manifestations are found within the outcrop itself, while vividly active fumaroles and boiling hot springs field occur in an area of 6 km2 adjacent to the outcrop of the high-grade alteration. The active geothermal field, in the highlands above 950 m a.s.l., is unusual in being characterized by numerous colourful hot springs and permanently running warm streams, instead of the more normal fumarole type of fields, with seasonal variation in hot spring activity and limited runoff in this type of settings. This character of the Vonarskard geothermal field relates to high watertable, which presumably is kept up by heavily altered rocks at shallow depths below the active field, supported by the high-grade alteration outcrop. Seeking explanation for the high-grade surface alteration containing act-ep-wo-qtz on the surface of a volcanically active area, the first one that comes in mind is erosion, which in this case can be ruled out apart from some few metres to tens of metres at the most. The second, and the more likely explanation, is that the high-grade alteration took place on the bottom of a subglacial lake, under at least 1 km thick ice sheet of late Weichselian age. This is the first finding of high grade surface alteration of this type in Iceland. It might be more commonly formed in subglacial settings than hitherto recognized, even globally. Black smokers on the ocean floors are likely candidates for this type of surface alteration. However, our attempt scanning the literature for descriptions of similar curiosities, proved unsuccessful. |