Glauconitic laminated crusts from hydrothermal alteration of Jurassic pillow-lavas (Betic Cordillera, S Spain): a microbial influence case

Abstract

The green crusts infilling the spaces among pillow-lava bodies from the Median Subbetic (Betic Cordillera, S Spain) are the subject of this textural, mineralogical and geochemical study. The exceptional laminated morphology and mineral composition made it possible to establish the genetic conditions and different phases of the infilling, while also interpreting the possible influence of microbial activity in the origin and growth of the laminated crusts. Two types of crusts were discerned: green laminated crusts and black crusts. The green crusts are mostly composed by glauconite and celadonite, and a minority by smectites, whereas black crust and lens-shaped nappes are saponitic. The record of filaments and coccoid-shaped forms at different scales from the glauconitic crusts indicates the potential implication of chemoorganotrophic microbes in the precipitation of the glauconite and the development of laminated textures under low-temperature hydrothermal conditions. In an early stage, alteration of pillow-lava surfaces with influence of hydrothermal reducing fluids results in black films of saponite and calcite filling the void spaces. During a cooling phase, green laminated crusts composed by glauconite and celadonite grow under oxic conditions due to the circulation and diffusion of oxygenated sea-water along inter-pillow spaces and the chemoorganotrophic microbial activity. A new stage of saponite formation with calcite occurs under higher T and confined-reducing conditions resulting from deposition of marine sediments, and finally calcite and quartz crystallise as the latest product closing the remaining space (or producing geodes) among the pillow-lava bodies.