The Geochemistry of Hydrothermal Vent Waters from Milos Island, Hellenic Volcanic Arc

Abstract

The Hellenic Volcanic Arc is a tectonically and volcanically active part of the earth's crust, situated above the subduction zone of the African plate under the Aegean. The arc extends from the Methana peninsula (NE Peloponese) through the greek islands Milos, Santorini, Kos, Nisiros and Yali to the Bodrum peninsula, Turkey (e.g. Fytikas et at., 1986). Hydrothermal venting has been reported from Nisiros, Yali (Varnavas and Cronan, 1991), Santorini (Varnavas and Cronan, 1988) and Milos (Fytikas, 1989). The island of Milos, at the central west part of the volcanic arc, is dominated by volcanic and voIcaniclastic rocks, with the exception of minor occurrences of metamorphic basement rocks and Neogene sediments outcropping at the south part of the island, underneath the volcanics (Fytikas et al., 1986). Submarine and, later, sub-aerial extrusive sequences formed during a prolonged period of volcanic activity (3.5 to 0.1 Ma, Fytikas, 1989). Geochemically, the Milos volcanics show a calcalkaline character and comprise andesites, dacites and ryolites (Fytikas et al., 1986). Hot water springs and fumaroles occur at several localities around the island. In submarine settings, hot fluids exit through the sandy seafloor, often in the form of a mixed liquid/ gas phase; as a result, venting can be located due to the visually impressive presence of streams of gas bubbles. In the most hydrothermally active south-east part of the island (Palaeohori bay), extensive areas of bacteria mats are associated with submarine hydrothermal brine seeps (Fitzsimons et al., 1997). Field observations and sampling