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

Resumo

The green crusts infilling the spaces among pillow-lavas from the Lower Jurassic Median Subbetic (Betic Cordillera, S Spain) are the subject of this textural, mineralogical and geochemical study. Their 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 crust 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 crusts and lens-shaped nap-pes 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. During an early stage, alteration of pillow-lava margins caused by hydrothermal reducing fluids re-sulted in black films of saponite and calcite filling the void spaces. During a cooling phase, green laminated crusts composed by glauconite and celadonite grew under oxic conditions due to both circulation and diffusion of oxygenated sea-water along inter-pillow spaces, and the chemoorganotrophic microbial activity. A new stage of saponite formation with calcite occurred under higher T and confined/reducing conditions resulting from deposition of marine sediments; finally, calcite and quartz crystallised as the latest product closing the remaining space (or producing geodes) among the pillow-lava bodies.