Al-Spinelle in primitiven Inselbogen-Vulkaniten

Al-rich spinels (100Cr/(Cr + Al) < 5, Al2O3 > 50 wt%) are common in alpine peridotites, both terrestrial and lunar mafic and ultramafic cumulates, and in certain metamorphic 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 Ball in the Sunda are, and Epi, Merelava, and Ambrym islands in the Vanuatu are. More commonly, relatively Cr-rich spinels also occur as inclusions in the same olivine phenocrysts, and it appears that the Cr-poor aluminous spinels 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 paragenesis suggests an origin involving contamination by localised Al-rich melt pockets as opposed to a xenocrystic origin. Two mechanisms to produce this high-Al melt in basaltic magma chambers are suggested: (1) localized high-Al melt production by complete breakdown of assimilated lower crustal gabbroic rocks. In this model the high-Al melt may crystallise Al-rich spinels which are subsequently trapped 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-rich 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 similar to 20-22% of Al2O3, aluminous clinopyroxene, Al-rich spinel and olivine. Mixing between these amphibole breakdown products and host basaltic melt may occur throughout the evolution of a magmatic system, but particularly during recharge with hot magnesian basalt batches. Aluminous 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 subsequently trapped, together with the coexisting Cr-spinels, by crystallising olivine and clinopyroxene.