Palagonitization of Volcanic Rocks in Polar Climates: The Case of Deception Island (Antarctica)

Abstract A detailed study of palagonitization in rocks from Deception Island—one of Antarctica's most active volcanoes—has been performed to advance our understanding of this alteration process. A detailed petrographic (optical and SEM), mineralogical (XRD), and mineral and glass spot geochemistry (EDS and EMP) characterization has been conducted on pyroclastic samples. Palagonitization occurred at 80–100°C and involved (a) initial glass to palagonite transformation by congruent glass dissolution and precipitation, followed by (b) palagonite maturation resulting in increasing crystallization into an assemblage of dominant smectite with minor illite, zeolites and Ti‐bearing oxides. During the first stage, an optically amorphous phase is formed with an estimated average density of 1.7–1.8 g/cm 3 and a very early mineralogical control on its composition indicating nucleation at the nm‐scale. Major elements are typically leached except for Ti, which behaves as immobile throughout palagonitization. Palagonite maturation occurs in an open system (variable element depletion and supply) and is controlled by an interplay between crystal nucleation and growth, overall mass balance, and local equilibration between crystals and fluid. Mass balances control palagonite porosity and density. Highly local physicochemical conditions (e.g., fluid chemistry or water‐rock ratio) play a major role in the chemical and mineralogical composition and evolution of palagonite. Variability of these controls at the microscale produces a large variability in palagonite characteristics even at the intraclast scale. Glass composition has not been observed to play a significant role. Textures observed in several samples indicate the contribution of microbial activity to glass alteration.