Melting versus contamination effects on 238 U– 230 Th– 226 Ra and 235 U– 231 Pa disequilibria in lavas from São Miguel, Azores Julie Prytulak a,b, ⁎, Riccardo Avanzinelli a,c , Govert Koetsier d , Katharina Kreissig b , Christoph Beier e , Tim Elliott a,d a Bristol Isotope Group, Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK b Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK c Dipartimanto di Scienze della Terra, Universita' degli Studi di Firenze, Via La Pira 4, 50121 Firenze, Italy d Faculteit der Aardwetenschappen, Vrije Universiteit, de Boelelaan 1085, 1081 HV Amsterdam, The Netherlands e GeoZentrum Nordbayern, Friedrich-Alexander-Universität Erlangen-Nürnberg, Schloβgarten 5, D-91054 Erlangen, Germany abstract article info Article history: Received 14 November 2013 Received in revised form 28 April 2014 Accepted 29 April 2014 Available online 21 May 2014 Editor: L. Reisberg Keywords: São Miguel U-series Contamination Pyroxenite/eclogite Dynamic melting The island of São Miguel, Azores piques geochemical interest due to a strikingly large range in the long-lived radiogenic isotope ratios of its lavas. The ‘enriched’ signatures (e.g., radiogenic 87 Sr/ 86 Sr and unradiogenic 143 Nd/ 144 Nd) observed in lavas found in the east of the island have been proposed to originate from recycled crustal components in their mantle sources. Such fertile lithologies should have higher melt productivities (amount of melt generated per decrement in pressure) than peridotitic mantle and this heritage ought to be evident in the U-series signatures of melts derived from such materials. Specifically, combined 238 U– 230 Th and 235 U– 231 Pa disequilibria in erupted lavas should identify significant differences in the melting rate and/or the mineralogical makeup of their sources (e.g., the amount of garnet). To this end, new 238 U– 230 Th– 226 Ra and 235 U– 231 Pa disequilibria are presented for sixteen mafic lavas (b 50 wt.% SiO 2 ) that encompass the full range of São Miguel's long-lived radiogenic isotope variability. However, primary U-series signatures of mafic lavas can be influenced by assimilation of more differentiated products. Notably, syenite nodules and cumulate alkali feld- spar xenocrysts have been identified in São Miguel basalts. We argue that secondary contamination of basalts is most evident in the hugely variable 230 Th– 226 Ra disequilibria (~18% deficits to N 300% excesses of 226 Ra) that strongly correlate with Ba/Th ratios. The longer-lived U-series nuclides are only slightly perturbed in a number of the more enriched lavas, resulting in increased noise in the dataset. In apparently uncontaminated samples, isotopically depleted lavas from the west of São Miguel display slightly higher 230 Th and 231 Pa excesses than the enriched samples from the east. The U-series signature of the enriched lavas can be modeled with a combi- nation of higher absolute values of D UU and D Th , coupled with higher melting rates compared to the depleted western source. These parameters are consistent with a peridotite source enriched with contributions from more fusible lithologies such as eclogite and/or pyroxenite in the eastern source, but not with direct derivation from such fusible lithologies. The only subtle variations in U-series isotopes coupled with large, systematic variations in radiogenic isotope compositions suggest that enriched components and/or their melts are well- mixed with mantle peridotite, thus masking their distinctive melting behavior. Finally, basalts from Sete Cidades volcano in the west of São Miguel have very similar long-lived isotope compositions and U-series disequilibria to basalts from the island of Pico, Azores. The geochemical similarity of lavas from islands at the center and periphery of the archipelago argue against systematic differences in melting behavior related to sampling different portions of an underlying plume structure. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Uranium series isotopes are short-lived, highly incompatible nu- clides that are unique tracers of the physical parameters of melting. The utility of U-series in the study of igneous rocks lies in their sensitivity to the timescales of magmatic processes (see reviews in Bourdon et al., 2003). Uranium series nuclides are formed by the decay of long-lived 238 U and 235 U. Short-lived, intermediate nuclides evolve to a state of secular equilibrium with their parents, such that their activities (rates of atomic disintegrations; denoted by round brackets) are equal and thus their activity ratios = 1. The longer- lived of these nuclides are valuable for studying magmatic processes (e.g., 230 Th: t 1/2 = ~ 75,000 yr, 231 Pa: t 1/2 = ~32,800 yr and 226 Ra: t 1/2 = ~1600 yr). Melting can fractionate daughter nuclides from their parents Chemical Geology 381 (2014) 94–109 ⁎ Corresponding author at: Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK. Tel.: +44 207 594 6474. E-mail address: j.prytulak@imperial.ac.uk (J. Prytulak). http://dx.doi.org/10.1016/j.chemgeo.2014.04.030 0009-2541/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo