Printed in Austria Al-spinels in primitive arc volcanics

Al-rich spinels (100Cr/(Cr + A1) < 5, A1203> 50 wt%) are common in alpine perido-tites, both terrestrial nd lunar marie and ultramafic umulates, and in certain metamor-phic rocks, but they are apparently rare in terrestrial volcanic rocks. Here we describe the occurrence of Al-rich spinel inclusions in olivine phenocrysts in island are volcanic rocks from five new localities: Bukit Mapas (Sumatra) and eastern Bali in the Sunda arc, and Epi, Merelava, and Ambrym islands in the Vanuatu arc. More commonly, relatively Cr-rich spinels also occur as inclusions in the same olivine phenocrysts, and it appears that the Cr-poor aluminous pinels must be in disequilibrium with the host basaltic melts. In the rocks studied, Al-rich spinels also coexist with trapped silicate glasses and highly aluminous clinopyroxene in melt inclusions in olivine. This para-genesis uggests an origin involving contamination by localised Al-rich melt pockets as opposed to a xenocrystic origin. Two mechanisms to produce this high-A1 melt in basaltic magma chambers are suggested: (1) localized high-A1 melt production by complete breakdown of assimilated lower crustal gabbroic rocks. In this model the high-A1 melt may crystallise Al-rich spinels which are subsequently trapped as soli