High-pressure melting experiments on garnet clinopyroxenite and the alkalic to tholeiitic transition in ocean-island basalts Shantanu Keshav a, * , Gudmundur H. Gudfinnsson b,1 , Gautam Sen a,2 , Yingwei Fei b,3 a Department of Earth Sciences, Florida International University, Miami, FL 33199, USA b Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA Received 26 November 2003; received in revised form 28 February 2004; accepted 2 April 2004 Available online Abstract It has been suggested, on the basis of recent high-pressure melting experiments, that high-Mg garnet clinopyroxenite is the most important lithology controlling the major-element budget of ocean-island lavas [Geology 31 (2003) 481– 484]. To clarify these claims and further our understanding of the petrogenesis of ocean-island basaltic (OIB) lavas, we present the results of a high-pressure (2.0 – 2.5 GPa) melting study on a high-Mg garnet-clinopyroxenite mantle nodule (77SL-582) from Hawaii. Major-element compositions of the partial melts as a function of pressure ( P), temperature (T), and degree of melting ( F), mineral chemistry of the coexisting crystalline phases, and the solidus/liquidus brackets of this particular garnet clinopyroxenite are reported. The solidus of 77SL-582, which resembles a tholeiitic picrite in terms of its bulk composition, is bracketed at 1295 F 15 and 1335 F 15 jC at 2.0 and 2.5 GPa, respectively, which is f 60–70 jC lower than the solidus of mantle lherzolite at identical pressures [Geochem. Geophys. Geosystems 1 (2000) 2000GC000070]. The solidus of 77SL-582 is also lower by f 30–40 jC than reported for a slightly alkalic, high-Mg garnet clinopyroxenite [Geology 31 (2003) 481]. At both pressures, the moderate degree ( f 18 – 20%) partial melts of 77SL-582 are strongly alkalic with f 8 – 12 wt.% nepheline in the norm. Even at a degree of melting as high as f 60%, moderately alkalic basaltic liquids are produced. With further melting, however, partial melts become hypersthene-normative. In the CaO – MgO – Al 2 O 3 – SiO 2 (CMAS) system, the eclogite surface is restricted to the tholeiitic part of the basalt tetrahedron [D.C. Presnall, Effect of pressure on fractional crystallization of basaltic magma, in: Y. Fei, C. Bertka, and B. Mysen (eds.) Mantle Petrology: Field observations and High Pressure Experimentation: A Tribute to Francis (Joe) R. Boyd, The Geochemical Society Special Publication no. 6, 1999, pp. 209 – 224.]. A comparison with high-pressure melting experiments in the CMAS system at 2.0 – 3.0 GPa indicates that the alkalic to tholeiitic transition in our experiments can be explained by a rapid expansion of the eclogite ‘‘surface’’ from the tholeiitic part to well into the alkalic part of the basalt tetrahedron in natural systems. Importantly, the partial melting trends of 77SL-582 are transverse to those seen in ocean-island basalts. In addition, although the alkalic to tholeiitic basalt transition observed in ocean-island basalts is well reproduced in our experiments, there is very little overlap between the partial melts of 77SL-582 and ocean-island basalts. It appears that most of the major-element systematics of the ocean-island basalts considered here can be explained by combined 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.04.029 * Corresponding author. Now at: Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA. Tel.: +1-202- 478-8929; fax: +1-202-478-8901. E-mail addresses: s.keshav@gl.ciw.edu (S. Keshav), g.gudfinnsson@gl.ciw.edu (G.H. Gudfinnsson), seng@fiu.edu (G. Sen), fei@gl.ciw.edu (Y. Fei). 1 Tel.: +1-202-478-8931; fax: +1-202-478-8901. 2 Tel.: +1-305-348-2299; fax: +1-305-348-3877. 3 Tel.: +1-202-478-8936; fax: +1-202-478-8901. www.elsevier.com/locate/epsl Earth and Planetary Science Letters 223 (2004) 365 – 379