HIGHLY VARIABLE HELIUM ISOTOPE RATIOS IN THE VANUATU VOLCANIC ARC

The Vanuatu (former New Hebrides) island arc results from the eastward subduction of the Australian plate beneath the North Fiji back-arc basin. Whereas its southern part is characterized by a single chain of volcanic islands parallel to a well-defined trench, the structure of its central part is strongly modified by the collision-subduction of the d’Entrecasteaux Zone, lacking a deep trench and consisting of three N-S chains of volcanic islands. In order to get further insight into the source of volcanism in this complex tectonic setting, we have analysed helium isotopes in groundwaters and hot springs (37°-90°C) from both the southern (Tanna and Efate) and central (Ambrym, Ambae and Pentecost) volcanic islands. Additional data were also acquired on bubbling gases and olivine phenocrysts from the islands of Ambrym and Ambae, respectively. Most waters show substantial helium enrichment (between 1.3 and 24.3 times the atmospheric solubility). 3He/4He ratios vary over a wide range, going from low (crustal He-influenced) 4.4 - 4.6 Ra and 4.8 -5.6 Ra in Efate and Pentecost respectively, to 6.3 - 7.0 Ra in Tanna, 6.8 - 8.2 Ra in Ambae, and up to 10.05 Ra in Ambrym (measured in both waters and bubbling gases). This latter value is similar to those measured in the nearby north Fiji basin (9-10 Ra - Ishibashi et al., EPSL, 1994) and approaches those, further north, of Manus basin glasses (12.2±1 Ra - Macpherson et al., Geology, 1998), suggesting a possible plume (or hotspot) influence. In Ambrym, this influence is confined to the north of the central rift zone - coincident with a major volcano-tectonic fault structure, since fluids in the southwest part of the island (a few kilometres apart) display typical arc-type ratios of 7 Ra. Interestingly, the analysis of tremor activity from the two large active cones located at the centre of the island, the Marum and Benbow volcanos, also suggest two distinct magmatic sources beneath Ambrym (Legrand et al., GRL 2005). Clearly, further investigations on yet unsampled islands of the Vanuatu arc are needed to better elucidate the magmatic and geodynamic significance of this high spatial variability of helium isotope ratios highligted by our first results.