Basaltic glass alteration in the Surtsey hydrothermal system

Figure 1. Alteration of volcanic glass. A) Back scattered electron image shows an altered glass rim in a basaltic lapillus (SE-01 core, 22.0 m). B) Profile shows enrichment and depletion of major element oxides. MgO concentrations in the altered (palagonitized) glass are comparable to those in the volcanic glass (sideromelane). However, MgO shows progressive enrichment towards the surface of the lapillus while SiO2, Al2O3 and CaO show moderate depletion and Na2O and K2O show greater depletion. Sdml: sideromelane. PG: palagonitized glass. Php: phillipsite zeolite.

At a larger scale, the early release of SiO2, Al, Mg and Ca resulted in depletion of these elements within the basalt and increased concentrations of these elements in fluids within the pores of the rock. At more advanced stages of alteration, however, a new uptake of SiO2, Al, Mg, Ca and Na occurred as mineralization processes produced clay minerals and various mineral cements in the basaltic rock. 

Overall, the study finds that submarine alteration of basaltic tuff at Surtsey volcano results in a long-term source of dissolved Ca, Al and SiO2 to seawater and a long-term sink, or loss, of Mg and Na from the seawater and back into the altered basalt. Figure 2 shows annual molar flux estimates for major elements per kg bulk rock, based on bulk rock mass balance and the age of the hydrothermal system at the time of drilling of each core, in 1979 and 2017. The flux of elements from the basaltic rock to the hydrothermal fluid (negative values) and from the hydrothermal fluid back into the altered basalt (positive values) vary over time and the local degree of alteration in the hydrothermal system.

Figure 2. Annual molar flux estimates for major elements per kg bulk rock, based on bulk rock mass balance and the age of the hydrothermal system at the time of drilling of the cores, SE-01 in 1979 and SE-02b in 2017. Positive values indicate flux from the hydrothermal fluid into the rock and negative values denote fluxes from the rock to the hydrothermal fluid. Green circles: SE-01, Red squares: SE-02b.

Figure 3. Fluid temperature and alteration zones with time, depth and occurrence of altered basaltic glass (palagonite), described with three different types in the subaerial and submarine deposits. Mineral abbreviations: after Warr (2021): Php: phillipsite. Anl: analcime. Tbm: tobermorite. Anh: anhydrite. Gp: gypsum. Cal: calcite. PG: palagonitized glass. Olx: olivine alteration. White circles in the alteration log indicate samples taken for both major and trace element analysis. Modified from Prause et al. (2020).

The study also affirms that alteration of basaltic glass at Surtsey volcano is a multi-stage process, which proceeds at different rates in the different sectors of  the  hydrothermal system. Overall, the low-temperature hydrothermal alteration of basaltic tuff at Surtsey acts as a source for seawater Ca, aqueous silica and aluminum and represents an important sink for dissolved sodium and magnesium. The findings may have implications for seawater crust chemical exchange in seamounts.

References

Warr, L.N., 2021,  IMA–CNMNC approved mineral symbols: Mineralogical Magazine, 85, 291–320,  doi.org/10.1180/mgm.2021.43 

Prause, S., T. B. Weisenberger, P. Cappelletti, C. Grimaldi, C. Rispoli, K. Jónasson, M. D. Jackson, M. T. Gudmundsson, 2020, Alteration progress within the Surtsey hydrothermal system, SW Iceland – A time-lapse petrographic study of cores drilled in 1979 and 2017: Journal of Volcanology and Geothermal Research, 106754, doi.org/10.1016/j.jvolgeores.2019.106754.

Citation

Prause, S., T. B. Weisenberger, B. I. Kleine, P. Monien, C. Rispoli, A. Stefánsson, 2022, Alteration of basaltic glass within the Surtsey hydrothermal system, Iceland – Implication to oceanic crust seawater interaction: Journal of Volcanology and Geothermal Research, 429, 107581, doi:10.1016/j.jvolgeores.2022.107581.

Figure 1. Alteration of volcanic glass. A) Back scattered electron image shows an altered glass rim in a basaltic lapillus (SE-01 core, 22.0 m). B) Profile shows enrichment and depletion of major element oxides. MgO concentrations in the altered (palagonitized) glass are comparable to those in the volcanic glass (sideromelane). However, MgO shows progressive enrichment towards the surface of the lapillus while SiO2, Al2O3 and CaO show moderate depletion and Na2O and K2O show greater depletion. Sdml: sideromelane. PG: palagonitized glass. Php: phillipsite zeolite.

Figure 2. Annual molar flux estimates for major elements per kg bulk rock, based on bulk rock mass balance and the age of the hydrothermal system at the time of drilling of the cores, SE-01 in 1979 and SE-02b in 2017. Positive values indicate flux from the hydrothermal fluid into the rock and negative values denote fluxes from the rock to the hydrothermal fluid. Green circles: SE-01, Red squares: SE-02b.

Figure 3. Fluid temperature and alteration zones with time, depth and occurrence of altered basaltic glass (palagonite), described with three different types in the subaerial and submarine deposits. Mineral abbreviations: after Warr (2021): Php: phillipsite. Anl: analcime. Tbm: tobermorite. Anh: anhydrite. Gp: gypsum. Cal: calcite. PG: palagonitized glass. Olx: olivine alteration. White circles in the alteration log indicate samples taken for both major and trace element analysis. Modified from Prause et al. (2020).