Temporal magma source changes at Gaua volcano, Vanuatu island arc

2016

Article référencé dans le Web of Science WOS:000387196200004

Beaumais A., Bertrand H., Chazot G., Dosso L., Robin Claude

Vergniolle S. (ed.), Métrich N. (ed.), Understanding volcanoes in the Vanuatu arc

Journal of Volcanology and Geothermal Research, 2016, 322 (No Spécial), p. 30-47 ISSN 0377-0273

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 (similar to 1.5), Nd-143/Nd-144 (similar to 0.51295) and Hf-176/Hf-177 (similar to 0.28316) ratios, but higher Ba/La (similar to 42 vs. similar to 27) and Sr-87/Sr-86 (0.70417 vs. 0.70405) ratios and lower Ce/Pb (similar to 2.7 vs. similar to 4.6), La/Sm (similar to 2.5 vs. similar to 4.0) and Pb-206/Pb-204 (18.105 vs. 18.176) ratios. High Th/Nb and low Nd and Hf isotopic ratios compared to N-MORB suggest the contribution of similar to 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 Nd-143/Nd-144) ratios of Mount Garet lavas suggest a higher degree of partial melting (similar to 10-15%) compared to the pre-caldera lavas (similar to 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.

Géologie et formations superficielles [064] ; Géophysique interne [066]

VANUATU ; GAUA VOLCAN

Fonds IRD [F B010068348]

fdi:010068348