AL-SPINELS IN PRIMITIVE ARC VOLCANICS

Al-rich spinels (100Cr/(Cr + Al) < 5, Al2O3 > 50 wt%) are common in al pine peridotites, both terrestrial and lunar mafic and ultramafic cumu lates, and in certain metamorphic rocks, but they are apparently rare in terrestrial volcanic rocks. Here we describe the occurrence of Al-r ich spinel inclusions in olivine phenocrysts in island are volcanic ro cks from five new localities: Bukit Mapas (Sumatra) and eastern Ball i n the Sunda are, and Epi, Merelava, and Ambrym islands in the Vanuatu are. More commonly, relatively Cr-rich spinels also occur as inclusion s in the same olivine phenocrysts, and it appears that the Cr-poor alu minous spinels must be in disequilibrium with the host basaltic melts. In the rocks studied, Al-rich spinels also coexist with trapped silic ate glasses and highly aluminous clinopyroxene in melt inclusions in o livine. This paragenesis suggests an origin involving contamination by localised Al-rich melt pockets as opposed to a xenocrystic origin. Tw o mechanisms to produce this high-Al melt in basaltic magma chambers a re suggested: (1) localized high-Al melt production by complete breakd own of assimilated lower crustal gabbroic rocks. In this model the hig h-Al melt may crystallise Al-rich spinels which are subsequently trapp ed as solid inclusions by phenocryst phases of the host basaltic melt or may be trapped as melt inclusions in which Al-rich spinels and Al-r ich clinopyroxene crystallise as daughter phases, and (2) incongruent breakdown of amphibole in amphibole-rich cumulates in sub-are, or sub- GIB volcano magma chambers. The latter reaction produces a melt with s imilar to 20-22% of Al2O3, aluminous clinopyroxene, Al-rich spinel and olivine. Mixing between these amphibole breakdown products and host b asaltic melt may occur throughout the evolution of a magmatic system, but particularly during recharge with hot magnesian basalt batches. Al uminous spinels and aluminous clinopyroxene produced during amphibole breakdown, or perhaps crystallised from aluminous melt produced in the same reaction, are incorporated into the magma during recharge, and s ubsequently trapped, together with the coexisting Cr-spinels, by cryst allising olivine and clinopyroxene.