Petrogenesis of plagioclase phenocrysts of Mount Etna, Sicily, with particular reference to the 1983 eruption: contribution from cathodoluminescence petrography

The cathodoluminescence (CL) characteristics of plagioclase phenocrysts in water-quenched lavas from the 1983 eruption of Etna have been investigated to examine the application of plagioclase CL to the study of magmatic proce sses. The phenocrysts have a green luminescent inner zone that is sharply b ounded by a blue luminescent outer zone, with the boundary often coinciding with a concentric zone of glass inclusions. Strong compositional differences between the green (An(70)-An(75)) and blue (An(50)-An(60)) luminescent areas are interpreted as the result of two pha ses of growth under differing conditions. The green luminescent cores are c onsidered to be anorthite-rich cumulate crystals from a basic magma which h ave been disrupted by the injection of a more evolved melt, resulting in he terogeneous nucleation and crystallization of the outer blue luminescent zo ne. The relationship between the CL and the trace element contents of the p lagioclase crystals is discussed.