The nature of melt inclusions inside minerals in an ultramafic cumulate from Adak volcanic center, aleutian arc: implications for the origin of high-Al basalts Pierre Schiano a, * , Robert Clocchiatti b , Pierre Boivin a , Etienne Medard a a Laboratoire Magmas et Volcans, OPGC-Universite ´ Blaise Pascal-CNRS, 5 rue Kessler, 63038, Clermont-Ferrand, France b Laboratoire Pierre Su ¨e, CEN Saclay, CEA-CNRS, 91191 Gif-sur-Yvette, France Received 6 February 2003; accepted 3 October 2003 Abstract In order to characterise the parental melts of crustal ultramafic cumulates from arc environments, we have undertaken a study of melt and fluid inclusions in olivine and clinopyroxene crystals in a typical cumulate xenolith from Adak Island, Aleutian Island Arc. The crystals contain inclusions either of silicate melts plus a H 2 O-rich bubble or H 2 O-dominated fluids, indicating H 2 O saturation of the trapped melt during the entire course of its crystallisation. Homogenisation experiments of the silicate melt inclusions give entrapment temperatures ranging between 940 and 1010 jC. After homogenisation, the melt inclusions range in composition from basalt to dacite ; the Al 2 O 3 and SiO 2 contents increase from 18.5 to 26.3 wt.% and 47.1 to 56.4 wt.%, respectively, as MgO and FeO decrease from 6.5 to f 0.1 wt.% and 6.5 to 0.3 wt.%. The melt inclusions also have high levels of H 2 O, z 6 wt.%. Comparisons of the compositional trends in the melt inclusion suite with those in experimental multiply-saturated liquids of basalts indicate that the compositional variations in the melt inclusions reflect progressive crystallisation of an olivine + clinopyroxene assemblage similar to the host cumulate xenolith, at pressures z 3.0 kbar and H 2 O- saturated conditions. A comparison between lavas from Adak Island and the melt inclusion compositions supports the hypothesis that fractional crystallisation at moderate pressures of hydrous mafic basalts generates high-alumina basalt compositions and leaves large volumes of ultramafic cumulate rocks in the crust beneath arc sections. D 2003 Elsevier B.V. All rights reserved. Keywords: Melt inclusions; High-alumina basalts; Arc volcanism; Adak Island 1. Introduction Exposed arc crustal sections, zoned ultramafic complexes and xenoliths brought up in magmas provide direct evidence for the presence of mafic and ultramafic rocks in the lower crust beneath arcs (see Wyllie, 1967; Himmelberg and Loney, 1995, and references therein). A large part of these ultramafic rocks are cumulate assemblages of mafic minerals, which as potential complements to arc magmas may give fundamental constraints regarding models for island arc magma evolution. In particular, they pre- serve information on the earlier stages of magmatic processes, whereas equilibrium assemblages in lavas 0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2003.10.001 * Corresponding author. Fax: +33-4-7334-6744. E-mail address: schiano@opgc.univ-bpclermont.fr (P. Schiano). www.elsevier.com/locate/chemgeo Chemical Geology 203 (2004) 169 – 179