HR: 1340h
AN: H13F-1646    [Abstracts]
TI: Learning about crustal CO2 migration and leakage using natural analogues
AU: * battani, A
EM: anne.battani@ifp.fr
AF: IFP, 1 & 4 avenue de bois preau, Rueil malmaison cede, 92852, France
AU: Jeandel, E
EM: elodie.jeandel@ifp.fr
AF: IFP, 1 & 4 avenue de bois preau, Rueil malmaison cede, 92852, France
AU: Sarda, P
EM: philippe.sarda@u-psud.fr
AF: Université de Paris-SUD, Dept. Sciences de la Terre, bât.504, Orsay cedex, 91405, France
AU: Deville, E
EM: eric.deville@ifp.fr
AF: IFP, 1 & 4 avenue de bois preau, Rueil malmaison cede, 92852, France
AB: CO2 gas samples and continental carbonates (travertines) were collected in different places of the French carbogaseous province. We performed analyses of noble gases and associated major compounds (mainly CO2) with d13C(CO2) isotopic analyses. We also made d13C and d18O measurements on the travertines present in all different sampled places to determine wether a significant part of the CO2 leaking could be trapped at the Earth surface. We wanted to test if travertines could be an indicator of the history of the system leakage, as they are potentialy datable. The main purpose of this study is to determine, using natural analogues, which context is most favourable for future CO2 storage. We collected seven gas samples from natural bubbling sources and geysers near Sainte Marguerite, Massif central, France. This area is known to present an important heat flow anomaly, due to the probable existence of a mantle plume below. The site exhibits many CO2-rich water wells associated with travertines rocks. We analysed the gas which is composed mainly of CO2 with a d13C (CO2) of around -5‰ compatible with a mantle-derived origin. The noble gas results show helium concentrations in the range of 0.28 to 8.22 ppm, with 5 samples lower than the atmospheric helium concentration of 5.24 ppm. However, within these samples, the 4He/20Ne ratios range from 2.12 to 198 and are all greater than air value of 0.288; thus air contamination can be discarded. The most intriguing result is that all our samples exhibit high and relatively homogeneous values of R/Ra, around 3.5 - 4, implying a large contribution of mantle-derived helium (R/Ra = 8 for the upper mantle) ) to the total budget of this gas. The neon and argon isotopic ratios are close to the atmospheric values, suggesting a small, if any, crustal contribution and an important Air Saturated Water (ASW) contribution, in agreement with the hydrothermalism of the area. To our knowledge, it is the first time that so low helium concentrations combined with so high 3He/4He are measured in crustal fluids. Such low concentrations indicate that He was not dissolved in water at great depth, and thus was not transported by water but migrated as a free gas phase. Our tentative interpretation is that gas comes from a degasing magma at depth, and then migrates toward the surface via faults and fractures. Contact with water should only occur at low depths (aquifers) and should be fast as indicated by the slight isotopic fractionation seen in the neon isotopes together with some elemental ratios enriched in the lightest noble gas isotope. The carbon isotopic composition of the travertines in Sainte Marguerite are consistent with leakage of deep CO2, and should provide new insights to the amount of carbon trapped as carbonates during time. Therefore, travertine precipitation can be regarded as a process trapping part of leaking CO2.
DE: 1065 Major and trace element geochemistry
SC: Hydrology [H]
MN: 2007 Fall Meeting