Surtsey: Basalt alteration by seawater in a low-temperature geothermal system

• Main Author: Prause, Simon
• Other Authors: Tobias Björn Weisenberger, Jarðvísindadeild (HÍ), Faculty of Earth Sciences (UI), Verkfræði- og náttúruvísindasvið (HÍ), School of Engineering and Natural Sciences (UI), Háskóli Íslands, University of Iceland
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Iceland, School of Engineering and Natural Sciences, Faculty of Earth Sciences 2023
• Subjects: Jarðefnafræði; Surtseyjargosið; Surtsey; Basalt; Doktorsritgerðir; Ocean Island
• Online Access: https://hdl.handle.net/20.500.11815/4174

The chemical interaction between seawater and basalt at low temperatures (≤150°C) ranks among the more significant factors controlling the composition of seawater and the oceanic crust. However, the time-integrated geochemical and petrographic outcomes of this process remain understudied. This work investigates the effects of time, temperature, crystallinity, and water-rock ratio on seawater-basalt interaction through a combination of petrographic and geochemical analyses of drill-cores obtained in 1979 and 2017 from the volcanic ocean island of Surtsey as well as through geochemical modeling. The rates of basaltic glass palagonitization and formation of secondary minerals are positively correlated with temperature. Palagonitized glass undergoes maturation from mostly amorphous to increasingly crystalline clay mineral-rich textures. Direction and magnitude of the element flux associated with the different stages of glass alteration are distinct for most elements. Rare earth elements, Hf, Ta, Zr, Nb and Y remain immobile, whereas FeO and TiO2 are slightly mobile during palagonite maturation. The entire process of basaltic tuff alteration at Surtsey indicates a net source of dissolved Ca, Al and SiO2 and a net sink of dissolved Mg and Na for seawater. Geochemical modeling of the alteration process suggests that crystalline and glassy basalts progress through a similar series of reactions, characterized by initial dissolution of glasses and primary minerals, followed by formation of phyllosilicates, oxides, hydroxides, carbonates, sulfates, sulfides and eventually zeolites. Water-rock ratio and pH are identified as the main factors determining the dominant mineral assemblage, whereas temperature and rock crystallinity are less important for the outcome of alteration. Einn af ríkjandi þáttum sem stýra samsetningu sjós og úthafsskorpu er efnafræðilegt samspil sjós og basalts við lágan hita (≤150°C). Jarðefnafræði og þau efnahvörf sem hafa áhrif á þetta ferli hins vegar ekki vel skilgreind. Í þessu verki voru áhrif ...