Goldschmidt Conference Abstracts 2007 A115

226Ra activities and 226Ra/Ba ratios on Geochemistry of primary magmas of the Kerguelen plateau, Southern St Vincent (Lesser Antilles Arc) Ocean (KEOPS project) tracked by melt inclusions 1 1 2 M. BOURQUIN , P. VAN BEEK , J.-L. REYSS , A.-S. BOUVIER1, N. MÉTRICH2 AND E. DELOULE1 1 3 3 M. SOUHAUT , S. JACQUET , F. DEHAIRS , 1 4 1 CRPG-CNRS, Nancy, France ([email protected]; M. CHARETTE AND C. JEANDEL [email protected]) 1LEGOS, Observatoire Midi Pyrenees, av. Edouard Belin, 2LPS, CNRS-CEA, Saclay, France ([email protected]) 31400 Toulouse, France ([email protected]) The Lesser Antilles arc volcanism results from the 2Laboratoire des Sciences du Climat et de l’Environnement, subduction of the Atlantic plate beneath the Caribbean plate. 91198 Gif-sur-Yvette, France ([email protected]) Lavas are dominantly andesitic, but MgO-rich basalts are 3Vrije Universiteit Brussel - Analytical and Environmental found in the south part of the arc (St-Vincent, Grenade Chemistry Department –Pleinlaan 2, B-1050 Brussels, islands). Major, trace elements and isotope geochemistry on Belgium ([email protected]) whole rocks, as experimental works, suggest that magmas are 4Woods Hole Oceanographic Institution, Woods Hole generated by the melting of a N-MORB type mantle source MA02543, USA ([email protected]) contaminated by at least two components, involving fluids derived from subducted oceanic crust and sediments. High biological productivity takes place on the Kerguelen However, the nature and the relative influence of each Plateau in the Indian sector of the Southern Ocean known to components and the importance of the crustale contamination be a HNLC region. Natural iron fertilization is suspected in are still debated (Macdonald et al., 2000). In order to that area. One goal of the KEOPS project is to understand the discriminate the importance of these different contributions, mechanisms controlling iron fertilization. Ra isotopes that are we undertook the analysis of melt inclusions trapped in produced in the sediment and diffuse in the water column may olivines of basaltic scoriae (MgO> 12.5 wt%) of St-Vincent. be a good analogue for tracing the input of sedimentary iron Major elements of more than 200 melt inclusions are and its fate on the Kerguelen Plateau. We measured all four measured by electron microprobe SX50, and H2O, Li, B, Cl, 7 11 18 radium isotopes during KEOPS (224Ra, 223Ra, 228Ra and 226Ra) F, δ Li, δ B, δ O of 50 melt inclusions are determined by in seawater in order to provide information on the water mass SIMS 1270. Chemical and isotopic characterisations point out pathways on the Kerguelen plateau. We also measured a broad variability for all the analysed elements. Major dissolved Ba concentrations, Ba being a chemical analogue of elements indicate a differentiation trend, and a second trend Ra. Here we report 226Ra activities and 226Ra/Ba ratios with lower SiO2 and higher CaO than whole rocks. B, Cl and 7 obtained during repeated visits of station A3 located in the H2O are variably enriched compared to MORB. δ Li values 11 18 middle of the bloom area on the Kerguelen Plateau. We range from -10 to +7‰, δ B from -16 to +12‰, and δ O compare these data to those obtained outside the plateau, in from +3 to +10‰. These variations reflect addition of the HNLC area. We observe a pronounced temporal variability contaminants. Melt inclusions provide new data which in the 226Ra activities and 226Ra/Ba ratios found at station A3 confirm that primary magmas of St-Vincent are generated by suggesting that the dynamic system has changed during the melting of a N-MORB type mantle source weakly affected by repeated visits or that biological activity significantly impacts the addition of fluids derived from the dehydration of AOC 226Ra activities and 226Ra/Ba ratios. and the melting of sediments.

References Macdonald, R., Hawkesworth, C.J.; Heath E., 2000, Earth- Science Reviews, 49, 1-76.