Evolution of palagonite: Crystallization, chemical changes, and element budget

The structural and chemical evolution of palagonite was studied as a functi on of glass composition, alteration environment, and time by applying a ran ge of analytical methods (electron microprobe, infrared photometry, atomic force microscopy, X-ray fluorescence, and X-ray diffraction). Palagonitizat ion of volcanic glass is a continuous process of glass dissolution palagoni te formation, and palagonite evolution, which can be subdivided into two di fferent reaction stages with changing element mobilities. The first stage i s characterized by congruent dissolution of glass and contemporaneous preci pitation of "fresh," gel-like, amorphous, optically isotropic, mainly yello wish palagonite. This stage is accompanied by loss of Si, Al, Mg, Ca, Na, a nd K, active enrichment of H2O and the passive enrichment of Ti and Fe. The second stage is an aging process during which the thermodynamically unstab le palagonite reacts with the surrounding fluid and crystallizes to smectit e. This stage is accompanied by uptake of Si, Al, Mg, and K from solution a nd the loss of Ti and H2O. Ca and Na are still showing losses, whereas Fe r eacts less consistently, remaining either unchanged or showing losses. The degree and direction of element mobility during palagonitization was found to vary mainly with palagonite aging, as soon as the first precipitation of palagonite occurs. This is indicated by the contrasting major element sign atures of palagonites of different aging steps, by the changes in the direc tion of element mobility with palagonite aging, and by the general decrease of element loss with increasing formation of crystalline substances in the palagonite. Considering the overall element budget of a water-rock system, the conversion of glass to palagonite is accompanied by much larger elemen t losses than the overall alteration process, which includes the formation of secondary phases and palagonite aging. The least evolved palagonitized m afic glass studied has undergone as much as 65 wt% loss of elements during palagonite formation compared to similar to 28 wt% element loss during bulk alteration. About 33 wt% element loss was calculated for one of the more e volved, in terms of the aging degree, rocks studied, compared to almost no loss for bulk alteration.