He, Ne, Ar, and C isotope systematics of geothermal emanations in the Lesser Antilles Islands Arc
- Freie Univ., Berlin (Germany)
The authors 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 < {sup 3}He/{sup 4}He < 8.6 Rc/Ra) and carbon ({minus}20 < {delta}{sup 13}C{sub PDB} < +0.5) are variable and require a variety of crustal and magmatic sources. The diversity of {delta}{sup 13}C{sub PDB} and {sup 3}He/CO{sub 2} ratios within individual volcanic centers suggests that crustal sources (e.g., limestone) contaminate magmatic CO{sub 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 {sup 3}He/{sup 4}He signature of summit fumaroles, thought to reflect the {sup 3}He/{sup 4}He 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 {sup 3}He/{sup 4}He signatures of summit fumaroles. On the northern islands (Nevis, Montserrat, Guadeloupe, and Dominica) summit fumaroles have the N-MORB signature ({sup 3}He/{sup 4}He = 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 centers of the Southern Islands (Martinique, St Lucia, and Grenada), and summit fumaroles on these centers match these variations by variable radiogenic He-enrichments, i.e., lower {sup 3}He/{sup 4}He ratios. This correlation suggests that radiogenic Sr and Pb enrichments of lavas and low {sup 3}He/{sup 4}He 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, the authors 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 {sup 3}He/{sup 4}He 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.
- OSTI ID:
- 355670
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 63, Issue 3-4; Other Information: PBD: Feb 1999
- Country of Publication:
- United States
- Language:
- English
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