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Pre-Salt Spherulites: Bacterially Induced Initiation of Precipitation

Abstract

Carbonate spherulites form widespread accumulations in the Aptian Pre-Salt deposits in the Campos Basin, offshore Brazil. The calcite spherulites commonly range from 1 to 2 mm in diameter. Petrographic sections through their cores display either a fine-grained nucleus or pore space, and are surrounded by well-developed radiating carbonate crystal cortices. Primary porosity is common between the spherulites whereas some pore spaces are occluded with authigenic precipitates (e.g., dolomite, chert, and megaquartz), other reported sites have a stevensite matric. The spherulites investigated commonly display interpenetration compaction against other spherulites with no inter-spherulitic material. Spherulites from the Pre-Salt are very similar to spherulites from modern hot springs (Yellowstone National Park), cold geyser (Crystal Geyser, Utah), tufa (Searles Lake, California), rock record travertine (Belen, New Mexico), and caliche (West Texas) deposits. Petrographic and SEM analyses of these non-Pre-Salt spherulites show that they are composed of a fine-grained nucleus of carbonate encrusted bacterial bodies and a cortex of radiating crystals of aragonite or calcite. The bacterial colonies in the nucleus induced the precipitation of carbonate, overcoming the inhibition to initiate crystal formation. The radiating crystals comprising aragonitic cortices grew abiotically producing well-formed euhedral crystals with a paucity of included bacterial fossils. Whereas those cortical crystals made-up of calcite commonly contained an abundance of bacterial fossils, indicating that the bacterial colonies contributed to the precipitation of the cortical calcite crystals. The aragonitic crystals probably precipitated rapidly in highly supersaturated conditions (e.g., modern hot springs at Yellowstone) whereas the calcitic cortices precipitated in moderately saturated conditions (e.g., Texas caliche) and thus bacterial growth kept up with cortical growth. The Pre-Salt spherulites most likely also initiated carbonate precipitation around bacterial colonies while the spherulites were afloat in a lacustrine water column. The spherulite-to-spherulite compaction with an absence of inter-spherulite sediment indicates that cortical crystal growth continued while the spherulites were either still afloat or at the sediment-water interface rather than displacively within the sediment and that compaction occurred with burial.