Reappraisal of fluid and sediment contributions to Lesser Antilles magmas S. Andy DuFrane a , Simon Turner b, ⁎, Anthony Dosseto a , Matthijs van Soest c a School of Earth and Environmental Sciences, Washington State University, Pullman, WA 99164, USA b GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney NSW 2109, Australia c School of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ 85287-1404, USA abstract article info Article history: Accepted 27 March 2009 Keywords: Lesser Antilles U-series isotopes Fluid Sediment Sediment melt Timescales We present new U-series disequilibrium and radiogenic isotope data for 7 mafic lavas from the Lesser Antilles arc. These are combined with published data in an internally consistent model that quantitatively estimates the amount of sediment and fluid added to the source of the Lesser Antilles arc system. Some lavas form an array consistent with bulk sediment addition (0.2–2%) whereas others appear to require addition of 0.4–2% sediment melt, particularly in the south of the arc. Evidence for both bulk sediment and sediment melt addition can be found within both the northern and central sections of the arc suggesting a thermal structure whereby the upper portions of the subducted sediment pile lie close to their solidus beneath much of the arc. Addition of up to 5% fluid derived from altered oceanic crust to these sediment enriched mantle wedge source regions can simulate the majority of the lavas on a plot of 207 Pb/ 204 Pb versus Ce/Pb. By taking into account the range in calculated wedge compositions and allowing for some mobility of Th in the fluid, the same model can also account for much of the observed range of U–Th–Ra disequilibria, especially if the eclogitic residue contains trace amounts of rutile. The implication of this more complex model is that the time scales for fluid addition and differentiation could be significantly shorter than those estimated in some previous studies. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Island arcs are important sites of crustal recycling and the majority of recent studies have identified contributions from both the subducting altered oceanic crust and the sediments carried down upon it (see Eiler, 2003, for a review). The Lesser Antilles arc (Fig. 1) has long been noted for its pronounced along-arc gradient in radiogenic isotope ratios which have been used to infer a southward increase in the relative sediment contribution (e.g. Macdonald et al., 2000, and references therein). However, estimates of the extent of this contribution have varied (e.g. White and Dupré, 1986; Ben Othman et al., 1989; Carpentier et al., 2008) and there has been debate as to what extent some of this signal may instead originate via shallow- level crustal contamination (e.g. Davidson, 1987; Thirlwall et al., 1996; Van Soest et al., 2002). This is also important for the ability of U-series isotopes to be used to evaluate the time scales of element transfer because this approach is sensitive to the inferred amounts and compositions of sediment and fluid contributions (see Turner et al., 2003 for a review). The Lesser Antilles was the subject of several early U-series studies (Turner et al., 1996; Chabaux et al., 1999) but these did not reach consensus on the processes and time scales involved, most probably as a result of some of these difficulties. Here we present new U-series data for 7 new mafic lavas which fill in some geographic gaps in pre-existing datasets. By combining these with existing data we reappraise the time scales and relative contributions of fluid and sediment components beneath the Lesser Antilles arc. 2. Analytical methods The seven new mafic samples analysed as part of this study come from the islands of Saba, Montserrat, Guadeloupe, and Grenada, and have not previously been analyzed for U-series isotopes (all isotope data were obtained on total dissolutions). Major and trace element data, Sr isotopes and Nd isotope data for samples LAS1, LSM07, LAG4 and LAG2 come from Van Soest (2000). Major elements were analysed at the Vrije Universiteit on a Philips ® PW1404/10 XRF for which the average precision is estimated to be generally better than 1% (Heuman and Davies, 1997). Trace elements analyses were carried out by inductively coupled plasma mass spectrometry at Activation Labora- tories Ltd. in Canada and the results for the BHVO-1 standard ana- lysed at the same time are provided in Table 1 indicating an accuracy generally better than 7%. Sr isotopes were analysed at the Vrije Universiteit on a Finnigan Mat ® 261 thermal ionisation mass spectro- meter using the triple jump method with on-line correction for mass bias to 86 Sr/ 88 Sr=0.1194. Analyses of the NBS987 standard yielded 87 Sr/ 86 Sr = 0.710243 ± 14 (n =52). Nd was analysed at the Vrije Universiteit on a Finnigan Mat ® 262 thermal ionisation mass spectrometer with an estimated precision of 0.00002 (2σ) following Chemical Geology 265 (2009) 272–278 ⁎ Corresponding author. E-mail address: sturner@els.mq.edu.au (S. Turner). 0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.03.030 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo