Environmental and Compositional Controls on the Texture and Composition of Palagonitized Hyaloclastites

Palagonitization is a widespread geochemical process in which sideromelane is converted to hydrous alteration products (palagonite and a variety of authigenic minerals) in subaqueous environments that produce volcanic glass. Recognizing that an 'aging' process may influence the textural and compositional properties of palagonite over time, we have been investigating the direct effects of specific environmental conditions (e.g. glass and fluid composition, fluid/rock ratio) on palagonite formation. For this study, hyaloclastite samples inferred to have palagonitized in relatively low fluid/rock ("closed") environments (submarine volcano flanks, submarine volcaniclastic basins) as well as from relatively high fluid/rock ("open") environments (tuff cones, subaqueous and subglacial eruption sites) were subjected to petrographic, electron microprobe, and LA-ICP-MS analysis. Palagonite rind textures are broadly consistent at each sampled site, but there is a wide range of textural variation between the sites. Rind thickness in all our samples varies from 0.05 to 1.0 mm, and the thicker rinds are generally from localities where it is believed that the aging of palagonite glass has occurred over a relatively long time. Authigenic zeolites are much more common in samples palagonitized in lower fluid/rock environments. Inferred water content of palagonite in our samples varies from 17 to 37 weight percent and appears to be highest from tuff cones, intermediate from subaqueous eruption sites, and lowest from submarine volcaniclastic basins. In submarine volcaniclastic sandstones sampled offshore of Kilauea and Mauna Loa volcanoes, palagonite rind compositions from adjacent grains of alkalic and tholeiitic sideromelane are distinguishable and reflect the original glass composition. Palagonite REE patterns parallel those of adjacent sideromelane, but the REE concentrations are higher in the palagonite and increase systematically towards the outermost palagonitized rim. Gradients in the Sr-isotopic composition of sideromelane palagonitized in submarine environments follow a mixing relationship between seawater and sideromelane initial values, and imply that palagonitization is a process that proceeds inwards with time. These observations suggest that the initial glass and fluid composition, as well as the fluid/rock ratio of the palagonitization environment, influence the textural and compositional properties of palagonite, and that palagonites formed in relatively low vs. high fluid/rock environments have distinguishable characteristics.