X-Ray Diffraction mineral analysis of continental modern stromatolites from the Socompa Lake, Argentina Puna

POIRÉ, DANIEL G.; GENAZZINI, CECILIA I.; LÓPEZ AGOSTI, V; ARROUY, M.J; GARCÍA, PABLO J.; FARÍAS, M.E

San Pablo

Congreso; 6th Latin American Congress of Sedimentology; 2013

University of S?ao Paulo, Brazil

Stromatolites forming at an altitude of 3570 m at the shore of a volcanic lake Socompa, Argentinean Andes, have been reported in the last three years. The water at the site of stromatolites formation is alkaline, hypersaline, rich in inorganic nutrients, very rich in arsenic, and warm (20-24 ◦C) due to a hydrothermal input. The stromatolites do not lithify, but form broad, rounded and low-domed bioherms (up to 24x80 cm in size) dominated by diatom frustules and aragonite micro-crystals agglutinated by extracellular substances. Neighbouring domes tend to coalesce into bigger domed biostromes. Vertical sections display visually clear stratification. The surface continuously exposed to air is covered by a white-pinkish crust, whereas the surface intermittently exposed to water due to waves is green-yellow. Below this cover is a 0.5-1.5 mm thick dark-green layer, which gradually fades away and disappears at depths of 3-5 mm. Deeper parts are characterized by alternating light-brown (5-20 mm thick) and dark-brown (0.5-1.5 mm thick) layers. A 91 centimetres core taken from the top-surface of these stromatolites shows a very complete growing record of the stromatolites layers along the core just interrupted between 9 cm and 26 cm by ash fall deposition. XRD mineral analyses have allowed recognise aragonite, calcite, quartz, feldspars and amorphous silica. From the top to the bottom a very clear trend of the transformation from aragonite to calcite has been detected. Isotopic analyses in order to date this transformation and get the stromatolite growing rate are being performed.