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Seeing through the effects of crustal assimilation to assess the source composition beneath the southern Lesser Antilles arc.

Bezard, R. and Turner, S. and Davidson, J.P. and Macpherson, C.G. and Lindsay, J.M. (2015) 'Seeing through the effects of crustal assimilation to assess the source composition beneath the southern Lesser Antilles arc.', Journal of petrology., 56 (4). pp. 815-844.

Abstract

Assessing the impact of crustal assimilation on the composition of oceanic arc lavas is important if source composition is to be correctly interpreted. This is particularly the case in the Lesser Antilles where lavas encompass a very large range in radiogenic isotope compositions. Here we present new 176Hf/177Hf and trace element data for a suite of samples from St Lucia in the southern Lesser Antilles arc where assimilation of sediments located within the arc crust has been shown to influence significantly Sr–Nd–Pb isotope compositions. We show that a high rate of assimilation (r = 0·8) of sediment is responsible for the co-variation of Th/Th*, La/Sm, 87Sr/86Sr, 206/207/208Pb/204Pb, 143Nd/144Nd and 176Hf/177Hf towards extreme ‘continental’ compositions. Lavas that escaped sediment assimilation have a typical oceanic arc signature and provide the best indication of mantle source characteristics beneath St Lucia. They display similar Ba/Th, La/Sm and Nd isotopic compositions to lavas further north in the arc, but with slightly more radiogenic Sr and Pb. Addition of less than 2% of local bulk subducting sediment, or less than 3·5% of sediment partial melt or fluid, to the mantle wedge can explain these compositions; these estimates are similar to those previously proposed for the northern arc. After correction for the effects of sediment assimilation, the St Lucia lavas have only slightly more radiogenic Pb and Sr isotope signatures compared with the northern islands; this can be attributed to differences in the isotopic composition of the local subducting sediment rather than to greater sediment input, as has been previously proposed. Comparison of St Lucia with the other southern Lesser Antilles islands suggests that similar mantle source compositions exist beneath Martinique, St Vincent and perhaps Bequia, whereas a more ‘continental’ source might characterize Ile de Caille, Kick ’em Jenny and Grenada.

Item Type:Article
Keywords:Lesser Antilles, Assimilation, Oceanic arc, Sediment, St Lucia.
Full text:(AM) Accepted Manuscript
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Status:Peer-reviewed
Publisher Web site:http://dx.doi.org/10.1093/petrology/egv018
Publisher statement:This is a pre-copyedited, author-produced PDF of an article accepted for publication in Journal of Petrology following peer review. The version of record Bezard, R., Turner, S., Davidson, J.P., Macpherson, C.G. and Lindsay, J.M. (2015) 'Seeing through the effects of crustal assimilation to assess the source composition beneath the southern Lesser Antilles arc.', Journal of petrology., 56 (4). pp. 815-844. is available online at: http://dx.doi.org/10.1093/petrology/egv018
Date accepted:31 March 2015
Date deposited:27 July 2015
Date of first online publication:13 May 2015
Date first made open access:01 April 2016

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