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 H2O-rich bubble or H2O-dominated fluids, indicating H2O 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 Al2O3 and SiO2 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 H2O, ≥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 H2O-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.