Geochemical transect through a travertine mount: A detailed record of CO 2 -enriched fluid leakage from Late Pleistocene to present-day – Little Grand Wash fault (Utah, USA)

Abstract : Active and fossil endogenic travertine mounts scattered along the Little Grand Wash fault are studied as records of Quaternary CO2-enriched fluid leakage. This study focusses on a particular area where a fossil mount formed in a near-surface setting by successive circulation/sealing episodes from Late Pleistocene to Mid-Holocene and where a modern surface travertine is still being formed by a CO2-enriched fluid source. The fossil mount is composed of horizontal and vertical veins whereby the vertical veins recorded numerous cycles of circulation/sealing/dissolution events and were used as conduits for the CO2- enriched fluid circulation from the depth to the surface or along sub-horizontal fractures where successive precipitation events are recorded. The modern travertine is being built at the surface by successive eruption of Crystal Geyser, an anthropic geyser active since the 1930's. d13C and d18O signatures and U/Th datings, ranging from 11.5 ky till present-day allows calibrating in detail the CO2 enriched fluid leakage along a single fault segment and in a post glacial context, as last glaciations in the study area took place 15 ky ago. The dataset shows a high decrease of the oxygen stable isotope values till about 6 ky, then the variations reflect a constant range until present-day. This tends to restrain the period of local increase of the meteoric water input in the aquifer that is sourcing the CO2- enriched water. The fossil travertine represents a 7 ky-long record of CO2 leakage above a natural reservoir, from Late Pleistocene to Mid-Holocene. The flux of CO2 leakage through time and the total escaping volume have been computed and appears to be low in comparison with an anthropogenic leak provoked, for instance, by a non-sealed well.