Boninites as windows into trace element mobility in subduction zones Stephan Ko ¨nig a,b, * , Carsten Mu ¨ nker a,c , Stephan Schuth a,d , Ambre Luguet a , J. Elis Hoffmann a , Jonathan Kuduon e a Rheinische Friedrich-Wilhelms-Universita ¨ t Bonn, Steinmann-Institut fu ¨ r Geologie, Mineralogie und Pala ¨ ontologie, Poppelsdorfer Schloss, 53115 Bonn, Germany b Westfa ¨ lische Wilhelms-Universita ¨t Mu ¨ nster, Institut fu ¨ r Mineralogie, Corrensstrasse 24, 48163 Mu ¨ nster, Germany c Universita ¨t zu Ko ¨ ln, Institut fu ¨ r Geologie und Mineralogie, Albertus-Magnus-Platz, 50674 Ko ¨ ln, Germany d Universita ¨t zu Ko ¨ ln, Geographisches Institut, Albertus-Magnus-Platz, 50923 Ko ¨ ln, Germany e Department of Mining, Geological Survey Division, Volcanological Observatory Branch, Rabaul, Papua New Guinea Received 12 June 2009; accepted in revised form 2 October 2009; available online 7 October 2009 Abstract Boninites are subduction-related rocks originating from re-melting of highly depleted mantle sources left after extraction of tholeiitic melts. Due to their depleted nature, the incompatible trace element inventory of boninites is virtually entirely inher- ited from slab components without a significant contribution from the refractory mantle wedge. Thus, boninites constitute an excellent window into processes controlling trace element mobilization at the slab–mantle wedge interface. In order to con- strain the behaviour of trace elements in subduction zones with a special emphasis on high field strength elements, we analyzed low-Ca boninites and associated tholeiitic basalts from Cape Vogel, Papua New Guinea (PNG) and compare them with com- positions of high-Ca boninites and associated tholeiitic basalts from Cyprus. High-precision HFSE (Nb, Ta, Zr, Hf, W) con- centration data of the boninites and associated tholeiitic basalts were obtained by isotope dilution. Major, trace element, and Sr–Nd–Hf–Pb isotope compositions clearly document a significant contribution of slab-derived melts involved in the petro- genesis of the PNG boninites, whereas only fluid-like subduction components were involved in the petrogenesis of the PNG basalts and the Cyprus suite. Low-Ca boninites from PNG are derived from a more refractory mantle source (21% deple- tion) than the high-Ca boninites from Cyprus (11% depletion) and their respective tholeiitic precursors (<10% depletion). In agreement with the more depleted nature of their mantle source, boninites exhibit a significantly stronger overprint by slab components. High-precision HFSE data indicate that, in comparison to LILE, a somewhat lower but measurable mobiliza- tion of all investigated HFSE in both slab-derived fluids and melts is evident. Modelling calculations demonstrate that the subduction components dominate the LILE budget and also largely control LREE and HFSE abundances in the boninite sources. Notably, the increasing influence of slab-derived fluids results in a decrease of the negative Nb–Ta anomaly, most likely reflecting a similar mobility of Nb–Ta and LREE at higher pressures near the critical point of fluid-melt miscibility. Ratios of Zr/Hf and Nb/Ta in the melt-like slab components dominating in the sources of the PNG boninites were probably fractionated in equilibrium with garnet–amphibolitic mafic oceanic crust. HFSE ratios in the Cyprus boninites are best explained by dehydration of subducted pelagic sediments in the absence of Ti-rich phases such as rutile. Our results also con- firm previous assertions that the mobility of HFSE decreases in the order Sb > W–Mo > Nb–Ta > Zr–Hf. Furthermore, Mo–W systematics may provide a potential novel tracer for the amount, composition and redox state of subducted pelagic sediments that contribute to the geochemical budget of intra-oceanic arc systems. Ó 2009 Elsevier Ltd. All rights reserved. 0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.10.011 * Corresponding author. E-mail address: stephan.koenig@uni-bonn.de (S. Ko ¨ nig). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 74 (2010) 684–704