Spatial distribution of helium isotopes in volcanic gases and thermal waters along the Vanuatu (New Hebrides) volcanic arc

Abstract

We report the first helium isotope survey of volcanic gases, hot springs and some olivine phenocrysts along the Vanuatu island arc, from Tanna in the south to Vanua Lava in the north. Low CO2 content and low 3He/4He ratios in thermal fluids of Epi (4.0±0.1Ra), Efate (4.5±0.1Ra) and Pentecost (5.3±0.5Ra) islands coherently indicate reduced mantle gas leakage and crustal contamination by radiogenic helium on these extinct volcanic systems of the former (Pliocene) arc. Instead, presently active Vanuatu volcanoes display 3He/4He and C/3He ratios typical of subduction-related volcanic arcs: 3He/4He ratios range from 6.4±0.5Ra in southernmost Tanna and 7.23±0.09Ra in northernmost Vanua Lava to typical MORB values in the central islands of Gaua (7.68±0.06Ra), Ambrym (7.6±0.8Ra) and Ambae (7±2Ra in groundwaters, 7.9±1.4Ra in olivine phenocrysts, and 8.0±0.1Ra in summit fumaroles of Aoba volcano). On Ambrym, however, we discover that hydrothermal manifestations separated by only 10–15km on both sides of a major E–W transverse fault zone crossing the island are fed by two distinct helium sources, with different 3He/4He signatures: while fluids in southwest Ambrym (Baiap and Sesivi areas) have typical arc ratios (7.6±0.8Ra), fluids on the northwest coast (Buama Bay area) display both higher 3He/4He ratios (9.8±0.2Ra in waters to 10.21±0.08Ra in bubbling gases) and lower C/3He ratios that evidence a hotspot influence. We thus infer that the influx of Indian MORB mantle beneath the central Vanuatu arc, from which Ambrym magmas originate, also involves a 3He-rich hotspot component, possibly linked to a westward influx of Samoan hotspot material or another yet unknown local source. This duality in magmatic He source at Ambrym fits with the bimodal composition and geochemistry of the erupted basalts, implying two distinct magma sources and feeding systems. More broadly, the wide He isotopic variations detected along the Vanuatu arc further verify the complex tectonic and magmatic framework of this intra-oceanic island arc.