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

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 2O-rich bubble or H 2O-dominated fluids, indicating H 2O 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 °C. After homogenisation, the melt inclusions range in composition from basalt to dacite ; the Al 2O 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 ∼0.1 wt.% and 6.5 to 0.3 wt.%. The melt inclusions also have high levels of H 2O, ≥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 ≥3.0 kbar and H 2O-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.