Fluid inclusion analysis of twinned selenite gypsum beds from the Miocene of the Madrid basin (Spain). Implication on dolomite bioformation

This research work is centred on continental lacustrine gypsum deposits of Miocene age cropping out in the easternmost part of the Madrid Basin. These gypsum deposits, accumulated in a continental saline lake, are characterized by a spectacular, distinctive Christmas-tree morphology and a peculiar dolomite replacement. A combination of microscopic (petrography and scanning electron microscopy) and analytical techniques (fluid inclusion microthermometry, X-ray energy dispersive spectroscopy and X-ray diffractometry) was used in order to study the crystallographic distribution and the composition of the fluid inclusions within the gypsum. The objectives were to characterize the continental brine from which the mineral precipitated, and to detect mineral and element traces that could indicate early diagenetic processes altering the gypsum deposits. Data from primary fluid inclusions indicated that gypsum precipitated from an aqueous fluid (lake water) of low to moderate total salinity (between 20 and 90 g/L NaCl). Secondary fluid inclusions represent interstitial lake brine in contact with gypsum, slightly enriched in total salt content as crystal formation proceeded. Textural, ultrastructural and microanalytical analysis indicate that the presence of dolomite precipitates inside the gypsum layers is related to the microbial colonization of the gypsum deposits and the biomineralization of the cell walls and extracellular polymeric substances around the cells. Our investigation emphasizes necessity of a multidisciplinary approach to assess geobiological processes.