Alteration progress within the Surtsey hydrothermal system, SW Iceland – A time-lapse petrographic study of cores drilled in 1979 and 2017

The evolution of hydrothermal alteration in glassy and variably palagonitized tuff, erupted as tephra in 1963–1964 on Surtsey, an island built in the offshore extension of Iceland's southeast rift zone, is documented through a comparative petrographic study of samples from drill cores recovered in 1979 and 2017. Time-lapse alteration within the low-temperature meteoric to seawater dominated hydrothermal system of the volcano is characterized in terms of secondary mineral contents, alteration rates and alteration style with depth. Between 1979 and 2017 palagonitization and cementation by secondary minerals has progressed into previously poorly altered parts of the system, leading to increased consolidation of the basaltic tephra. Alteration rates range between 1.05 and 42.5 μm·yr−1 for palagonitization of glass and 0.4–8.33 μm·yr−1 for pseudomorphic olivine replacement by clay minerals over a temperature interval of 47–140 °C. Five distinct zones of alteration style, distinguished through alteration mineralogy, development of authigenic phases over time, as well as degree of alteration are described. Alteration of basaltic tephra at Surtsey volcano is defined by an early stage of phillipsite and clay mineral formation, followed by a later stage of analcime and tobermorite formation as well as replacement of phillipsite below the water table in zone 2 between 65.4 and 138.4 m. Only minor advancement of alteration is detected in zone 3 between 138.4 and 150.3 m depth, where the primary tephra remains largely unpalagonitized and unconsolidated. In contrast, from 1979 to 2017 alteration has increased between 150.3 and 177.8 m depth, in zone 4, leading to rapid and extensive glass and olivine alteration. The quantification of these time-lapse hydrothermal alteration processes at Surtsey provides an important reference for studies of the evolution of young oceanic islands hosting hydrothermal systems