Crustal xenoliths in the 6220 BP Sوfell tuff-cone, south Iceland: Evidence for a deep, diatreme-forming, Surtseyan eruption

Three types of xenoliths were collected from a 80 m high vertical section of the Sæfell tuff-cone on Heimaey, Iceland. These types are: alkali basaltic (Type I), sedimentary (Type II) and cpx-bearing basalts (Type III). They are distinct in hand specimen, thin section and geochemistry. Near the base of the eruptive succession only juvenile material and Type I xenoliths are present. The middle section has Type II xenoliths in addition to Type I. Xenoliths of Type III first appear in the uppermost xenolith-rich horizons, where they are found together with all other types. Juvenile Sæfell clasts are present in various amounts throughout the investigated section. A downward migration of explosion foci during the eruption is established based on the distribution of the different types of xenoliths in the tuff-cone (i.e. the appearance of Type I–Type II–Type III xenoliths with increasing stratigraphic height) in combination with the observed relations in basement stratigraphy beneath Heimaey. In the beginning of the eruption a combination of high eruption rate and abundant seawater resulted in shallow-seated explosions depositing only juvenile material and Type I xenoliths. The downward migration of explosion foci is suggested to be the result of lowered influx of seawater (due to the formation of a cone that reached 50–75 m a.s.l.) in combination with lowered eruption rate. As the basement stratigraphy beneath Heimaey is well documented by a 1565 m deep drill-hole, the depth of the explosions can be constrained to vary between less than 170 m in the beginning of the eruption and exceeding 820 m in the final phases. This is much deeper than previously reported for any Surtseyan tuff-cone forming eruption and consistent with the downward penetration of a diatreme.