Spinel in the allivalites of Malyi Semyachik Volcano, Kamchatka

A microprobe study of Ol-An nodules (allivalites) as sampled from the basaltic andesites of Malyi Semyachik Volcano in Kamchatka revealed small inclusions of a mafic mineral in crystals of olivine and anorthite and in the intergranular mass. The mineral was later identified as spinel. A microprobe analysis of the spinel showed that the grains that were found in minerals are not zonal, with spinel inclusions of different chemical compositions occurring in a single crystal. In contrast to this situation, the spinel crystals in the intergranular basis are zonal and exhibit a reaction rim. The spinel compositions form a long and well-pronounced trend that occurs in the solvus zone of solid solutions that has not been adequately described in the literature. The existence of the resulting spinel trend in the solvus zone can be explained by an early capture of spinel grains by growing silicate crystals and by their rapid cooling after the eruption, which chilled the metastable solution. Spinel grains occasionally exhibited initial phases in the decomposition of a solid solution owing to their long residence in a cooling lava flow at temperatures below the solvus. It is supposed that these spinels were formed somewhat earlier or simultaneously with the crystallization of the silicate phase. This diversity in the compositions of non-zonal spinel can be explained by thermo diffusion that led to homogeneous compositions in the initially zonal spinel crystals after they had been included in the host mineral crystals.