Source depletion and extent of melting in the Tongan sub-arc mantle J.T. Caulfield ⁎, S.P. Turner, A. Dosseto, N.J. Pearson, C. Beier GEMOC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney NSW 2109, Australia ABSTRACT ARTICLE INFO Article history: Received 7 March 2008 Received in revised form 21 June 2008 Accepted 23 June 2008 Available online 4 July 2008 Editor: R.W. Carlson Keywords: HFSE depletion sub-arc back-arc hydrous melting The fluid immobile High Field Strength Elements (HFSE) Nb and Ta can be used to distinguish between the effects of variable extents of melting and prior source depletion of the Tongan sub-arc mantle. Melting of spinel lherzolite beneath the Lau Basin back-arc spreading centres has the ability to fractionate Nb from Ta due to the greater compatibility of the latter in clinopyroxene. The identified spatial variation in plate velocities and separation of melt extraction zones, combined with extremely depleted lavas make Tonga an ideal setting in which to test models for arc melt generation and the role of back-arc magmatism. We present new data acquired by laser ablation-ICPMS of fused sample glasses produced without the use of a melt fluxing agent. The results show an arc trend towards strongly sub-chondritic Nb/Ta (b 17) with values as low as 7.2. Melting models show that large degree melts of depleted MORB mantle fail to reproduce the observed Nb/Ta. Alternatively, incorporation of residual back-arc mantle that has undergone less than 1% melting into the sub-arc melting regime reproduces arc values. However, the extent of partial melting required to produce the composition of the Lau Basin back-arc basalts averages 7%. This apparent discrepancy can be explained if only the lowermost 4 km of the residua from the mantle melt column beneath the back- arc is added to the source of arc magmas. We have identified that the degree of arc/back-arc coupling displayed in the rock record provides an index of the depth of hydrous melting beneath the arc. In this case, this would imply a depth of ~75 km for generation of arc magmas, indicating that hydrous melting in the mantle wedge is triggered by the breakdown of hydrous phases in the subducting slab. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Volcanic arcs represent the surface expression of large-scale melt extraction from the upper mantle at subduction zones. Models for melt generation and transport have been constructed based on both geophysical (Conder and Wiens, 2007; Hall and Kincaid, 2001; Spiegelman and McKenzie, 1987) and geochemical observations (McCulloch and Gamble, 1991; Turner and Hawkesworth, 1998; Woodhead et al., 1993). A major geochemical characteristic of arc rocks is their depletion in High Field Strength Elements (HFSE) relative to the light rare earth elements (LREE) (Gill, 1981). Such a signal is not observed in Mid-ocean Ridge Basalts (MORB) or Ocean Island Basalts (OIB). Part of this signature is probably derived from the contribution of subducted sediments in the arc magma source and inherited from the negative HFSE anomalies that characterise them (e.g. Ewart et al., 1977; Plank, 2005; Turner et al., 1997). However, low concentrations of HFSE in subduction related rocks are also widely believed to be a function of their highly fluid immobile character (Münker et al., 2004; Zack and Timm, 2007). The higher ionic potential (charge to size ratio) of niobium (Nb) and tantalum (Ta) relative to the other HFSE make them least sensitive to addition of the slab derived fluid flux. Fractionation of Nb from Ta is often inferred to reflect depletion of the mantle wedge source region by prior melt extraction in the back-arc (Woodhead et al., 1993). Similarly, the deviation of Nb/Ta from chondritic MORB values (~ 17) in arc lavas has been interpreted by Stolz et al. (1996) to record variations in source composition due to depletion during prior melt extraction. Re-melting of wedge material that has passed through a back-arc melting regime has implications for models of arc/back-arc coupling and mantle flow paths beneath arcs. In contrast, Eiler et al. (2000) have suggested that low HFSE concentrations result from greater extents of partial melting beneath the arc itself. Here we present results of a study of Tongan lavas and tephras designed to differentiate between these two modes of melt production by developing models for melt generation in the mantle underlying an arc/back-arc system. 1.1. The Tonga arc Numerous studies have documented the formation and evolu- tion of the Tonga–Lau region resulting in a tightly constrained arc/ back-arc system (Bevis et al., 1995; Cole et al., 1990; Parson and Hawkins, 1994; Zellmer and Taylor, 2001). Initiation of extension around 6 Ma resulted in splitting of the Lau Ridge to form the Lau Basin, bound to the west by the remnant Lau Ridge, to the east by the Tonga arc (Parson and Hawkins, 1994). The intra-oceanic Tonga Earth and Planetary Science Letters 273 (2008) 279–288 ⁎ Corresponding author. Tel.: +61 2 9850 4406; fax +61 2 9850 8943. E-mail address: jcaulfield@els.mq.edu.au (J.T. Caulfield). 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.06.040 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl