U-series isotope and geodynamic constraints on mantle melting processes beneath the Newer Volcanic Province in South Australia Zoe Demidjuk a,1 , Simon Turner a, ⁎ , Mike Sandiford b , Rhiannon George a , John Foden c , Mike Etheridge a a GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia b School of Earth Sciences, University of Melbourne, VIC 3010, Australia c School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005, Australia Received 25 July 2006; received in revised form 13 February 2007; accepted 10 July 2007 Editor: C.P. Jaupart Available online 17 July 2007 Abstract Young (b 5 kyr) olivine- and clinopyroxene-phyric ne-hawaiites from Mounts Gambier and Schank in the Newer Volcanic Province in South Australia have been analysed for major and trace elements as well as for Sr and Nd isotopes and 238 U– 230 Th disequilibria in order to constrain the mantle melting processes responsible for their origin. The rocks are relatively primitive (6.9– 9.1% MgO), incompatible trace element-enriched alkali basalts with 87 Sr/ 86 Sr = 0.70398–0.70415 and 143 Nd/ 144 Nd = 0.51280– 0.51271. Trace element modelling suggests that they reflect 3–6% partial melting in the presence of 2–8% residual garnet. Trends towards low K/K ⁎ are accompanied by decreasing 87 Sr/ 86 Sr and provide evidence for the involvement of hydrous phases during melting. 230 Th excesses of 12–57% cannot be simulated by batch melting of the lithosphere and instead require dynamic melting models. It is argued that the distinction between continental basalts bearing significant U–Th disequilibria and those in secular equilibrium reflects dynamic melting in upwelling asthenosphere, rather than static batch melting within the lithosphere or the presence or absence of residual garnet. Upwelling rates are estimated at ∼ 1.5 cm/yr. A subdued, localised topographic uplift associated with the magmatism suggests that any upwelling is more likely associated with a secondary mode localised to the upper mantle, rather than a broad zone of deeply-sourced (plume) upwelling. Upper mantle, ‘edge-driven’ convection is consistent with seismic tomographic and anisotropy studies that imply rapid differential motion of variable thickness Australian lithosphere and the underlying asthenosphere. In this scenario, melting is linked to a significant contribution from hydrous mantle that is envisaged as resulting either from convective entrainment of lithosphere along the trailing edge of a lithospheric keel, or inherited variability in the asthenosphere. © 2007 Elsevier B.V. All rights reserved. Keywords: alkali basalt; geochemistry; U–Th isotopes; intraplate magmatism; edge-driven convection; South Australia 1. Introduction Spatially and often volumetrically extensive, basaltic, intraplate magmatism is frequently attributed to partial melting within upwelling mantle plumes although the details of the melting processes are not always well Earth and Planetary Science Letters 261 (2007) 517 – 533 www.elsevier.com/locate/epsl ⁎ Corresponding author. E-mail address: sturner@els.mq.edu.au (S. Turner). 1 Present address: SRK Consulting Ltd. Level 6, 44 Market Street, Sydney NSW 2000, Australia. 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.07.006