Temporal magma source changes at Gaua volcano, Vanuatu island arc

Type Article
Date 2016-08
Language English
Author(s) Beaumais Aurelien1, 2, Bertrand Herve3, 4, Chazot Gilles1, Dosso Laure5, Robin Claude6
Affiliation(s) 1 : Univ Bretagne Occidentale, Inst Univ Europeen Mer, Domaines Ocean, CNRS,UMR6538, Pl Copern, F-29280 Plouzane, France.
2 : Univ Southampton, Ocean & Earth Sci, Natl Oceanog Ctr Southampton, European Way, Southampton SO14 3ZH, Hants, England.
3 : Univ Claude Bernard, Lab Geol Lyon, CNRS, UMR 5276, F-69364 Lyon, France.
4 : ENS Lyon, F-69364 Lyon, France.
5 : CNRS, UMR6538, IFREMER, Dept Geosci Marines, F-29280 Plouzane, France.
6 : Univ Clermont Ferrand, Lab Magmas & Volcans, IRD Unite R163, CNRS,UMR 6524, Clermont Ferrand, France.
Source Journal Of Volcanology And Geothermal Research (0377-0273) (Elsevier Science Bv), 2016-08 , Vol. 322 , P. 30-47
DOI 10.1016/j.jvolgeores.2016.02.026
WOS© Times Cited 10
Note Understanding volcanoes in the Vanuatu arc
Keyword(s) Gaua, Vanuatu, Geochemistry, Isotopes, Subduction, Mantle source
Abstract Gaua Island (also called Santa Maria), from the central part of the Vanuatu arc, consists of a large volcano marked by a caldera that hosts the active Mount Garet summit cone. In this paper, a geochemical study including Sr, Nd, Pb and Hf isotopic compositions of 25 lavas emitted since 1.8 Ma is presented, with a focus on the volcanic products that preceded (old volcanics, main cone and pyroclastic series) and followed (Mount Garet) the caldera forming event.

All lavas show an island arc signature with enrichment in LILE and depletion in HFSE. Post-caldera lavas define a medium-K calc-alkaline trend, whereas lavas from the former main cone have high-K calc-alkaline compositions. Compared to the pre-caldera volcanic suite, the Mount Garet lavas have similar Th/Nb (~ 1.5), 143Nd/144Nd (~ 0.51295) and 176Hf/177Hf (~ 0.28316) ratios, but higher Ba/La (~ 42 vs. ~ 27) and 87Sr/86Sr (0.70417 vs. 0.70405) ratios and lower Ce/Pb (~ 2.7 vs. ~ 4.6), La/Sm (~ 2.5 vs. ~ 4.0) and 206Pb/204Pb (18.105 vs. 18.176) ratios. High Th/Nb and low Nd and Hf isotopic ratios compared to N-MORB suggest the contribution of ~ 2% of subducted sediment melt to the mantle source of Gaua magmas. Most of the observed differences between pre- and post-caldera lavas can be accounted for by the involvement of at least two portions of the mantle wedge, metasomatized by different slab-derived aqueous fluids. In addition, the lower La/Sm (at a given 143Nd/144Nd) ratios of Mount Garet lavas suggest a higher degree of partial melting (~ 10–15%) compared to the pre-caldera lavas (~ 5%). The Santa Maria Pyroclastic Series (SMPS) eruption probably triggered the caldera collapse, in response to emptying of the magmatic chamber. This event may have allowed new access to the surface for a geochemically distinct batch of magma issued from a separate magma chamber, resulting in the birth and construction of Mount Garet within the caldera. As both magmatic suites were emitted over a very short time, the storage of their parental magmas beneath the volcano is still possible
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