Lithium isotopes in island arc geothermal systems: Guadeloupe, Martinique (French West Indies) and experimental approach

Abstract

. Lithium isotopes in island arc geothermal systems: Guadeloupe, Martinique (French West Indies) and experimental approach. Geochimica et Cosmochimica Acta, Elsevier, 2010, 74 (6) AbstractWe report Li isotopic measurements in seawater derived waters discharged from geothermal wells, thermal and submarine springs located in volcanic island arc areas in Guadeloupe (the Bouillante geothermal field) and Martinique (Lamentin plain and the Diamant areas). The lithium isotopic signatures of the geothermal fluids collected from deep reservoirs are homogeneous for a given site.However, the δ 7 Li signatures of each of these reservoirs are significantly different. We provide the first low temperature (25-250°C) experiments of Li isotope exchange during seawater/basalt interaction), which confirm that Li isotopic exchange is strongly temperature dependent, as previously inferred from natural studies. Li isotopic fractionation ranges from +19.4‰ (Δ solution -solid ) at 25°C to +6.7‰ at 250°C.The experiments also evidence the importance of Li isotopic fractionation during formation of Libearing secondary minerals by the uptake of lithium into the alteration minerals. Application of experimental results to the Bouillante area suggests that the geothermal water is in equilibrium at 250-260°C with a deep and large reservoir located in the transition zone possibly at the contact between volcanic flows and basaltic dikes. For the Lamentin and Diamant areas, the geothermal fluid appear to be partially in equilibrium at 90-120°C and 180°C, respectively, with reservoir sedimentary rocks. Our study highlights that lithium isotopic systematics is a powerful tool for the characterization of the origin of geothermal waters as well as the nature of their reservoir rocks.

Lithium isotopes as tracers of groundwater circulation in a peat land

“…As Li is 59 3 a recently developed isotopic tracer, not all the processes that could induce and control 60 isotopic fractionation are as yet well constrained. However, in the context of water/rock 61 interactions, numerous studies (Huh et al, 1998(Huh et al, , 2001Pistiner et al, 2003; Kisakurek et al, 62 2004 Kisakurek et al, 62 , 2005 Pogge von Strandmann et al, 2006;Millot et al, 2007Millot et al, , 2010aMillot et al, , 2010b have 63 clearly shown that isotopic fractionation supports the enrichment of the heavy isotope ( 7 Li) in 64 solution, the light isotope ( 6 Li) being preferentially retained in secondary weathering 65 minerals. Silicate rocks display Li isotope compositions ranging from -2 to +10‰ (Coplen et 66 al., 2002;Teng et al 2004), that of seawater is ~ +31‰ (Millot et al, 2004), river water has 67 intermediate isotopic compositions (+6 to +23‰, Huh et al, 1998), and saline (thermo-68 mineral) water generally has isotopic compositions in the range of 0 to +15‰ 69 Négrel, 2007, Millot et al, 2010b) although Falkner et al (1997) found Li isotopic 70 compositions in the range of +17 to +35‰ in hot spring water from Lake Baikal.…”

Weathering regime associated with subsurface circulation on volcanic islands

“…We have already shown the use of Li as a tracer of hydrothermal activities, which we have verified by our samples-for example, well 6 with a water temperature of 160°C (260°C in situ) has the highest Li concentration. It is also verified that the presence of Li varies mainly with temperature in volcanic areas (Millot et al 2010;Vigier et al 2009). Consequently, we can use Li as a tracer of hydrothermal waters due to its presence in very high amounts in high-temperature waters.…”

“…Previous work has used the Li/Na or Li/B ratios to constrain the hydrothermal signature (Millot and Negrel 2007;Millot et al 2010). Each river, hydrothermal spring or well water in our study can be characterized by a number of elemental molar ratios.…”

“…The rivers present scattered variations of Li/Na of between 5 9 10 -5 and 5 9 10 -3 and SO 4 /Na between 0.2 and 4.5. The high Li concentration in the wells and thermal springs are due to high-temperature weathering of the rocks and may also have a magmatic origin (input of hot acid fluids into aquifers) as illustrated in the study by Millot et al (2010) for Guadeloupe and Martinique. It is a good geochemical indicator of the alteration temperature.…”

Geofluid research using lithium isotopic tool advances understanding of whole crustal activity

“…This figure was modified from the original drawn by deceased Prof. Chan et al, 1999;Elliott et al, 2004;Tomascak, 2004;Nishio et al, 2005;Elliott et al, 2006;Nishio et al, 2007Li Huh et al, 1998Huh et al, 2001;Kisakurek et al, 2005;Vigier et al, 2009;Millot et al, 2010Li Chan et al, 1992Chan et al, 1994;James et al, 1995 Millot andNegrel, 2007;Millot et al, 2010 Li Figs. 6a 6b …”