He, Ne, Ar, and C isotope systematics of geothermal emanations in the Lesser Antilles Islands Arc

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We present He, Ne, Ar, and C isotope analyses of hydrothermal brines and gases from fumaroles, hot springs, mofettes and hydrothermal exploration drillings on the major islands of the Lesser Antilles Arc. The origin of hydrothermal brines, which have been analyzed also for O and H isotopes, is essentially meteoric-hydrothermal. Air-corrected isotope compositions of helium (2.2 Rc/Ra < 3He/ 4He < 8.6 Rc/Ra) and carbon (−20 < δ 13C PDB < +0.5) are variable and require a variety of crustal and magmatic sources. The diversity of δ 13C PDB and 3He/CO 2 ratios within individual volcanic centres suggests that crustal sources (e.g., limestone) contaminate magmatic CO 2 en route from high-level magma reservoirs (depth < 15 km) to the surface. A similar contamination may be found for magmatic helium on distal springs. The 3He/ 4He signature of summit fumaroles, thought to reflect the 3He/ 4He signature of high-level magmas, shows a remarkable systematic variation along the arc. In addition, there is a correlation throughout the arc between published Sr, Pb, and Nd isotope signatures of lavas and the 3He/ 4He signatures of summit fumaroles. On the northern islands (Nevis, Montserrat, Guadeloupe, and Dominica) summit fumaroles have the N-MORB signature ( 3He/ 4He = 8 ± 1 R/Ra), and the isotope signature of lavas is not dissimilar from comparable intra-oceanic arc tholeiites elsewhere. Variable enrichments in radiogenic Sr and Pb have been reported for lavas of individual volcanic centres of the Southern Islands (Martinique, St.Lucia, and Grenada), and summit fumaroles on these centres match these variations by variable radiogenic He-enrichments, i.e., lower 3He/ 4He ratios. This correlation suggests that radiogenic Sr and Pb enrichments of lavas and low 3He/ 4He signatures on summit fumaroles have a common origin, i.e., a terrigenous contaminant derived from the Orinoco depositionary fan. Crustal assimilation is thought to decouple the He isotope system from any other radiogenic isotope system and, therefore, we argue that the observed correlation of He, Sr, Pb, and Nd isotope systems is related to a terrigenous contaminant derived from subducted sediments. Support for this scenario also comes from the matching of low 3He/ 4He ratios and tectonic features of the forearc thought to favor the subduction of forearc sediments. The present study offers a first clue that, under suitable conditions, crustal helium from oceanic sediments might be subducted to the depth of arc magma sources and, possibly, even recycled into the deeper mantle.