NOMARSKI DIFFERENTIAL INTERFERENCE CONTRAST (NDIC) ANALYSIS OF MOUNT ETNA PLAGIOCLASE THROUGH HISTORICAL TIME AND ITS POSSIBLE IMPLICATIONS FOR VARIATION IN MAGMATIC PLUMBING

Textural analysis of crystal populations provides petrologists with an opportunity to understand the dynamics of a subvolcanic plumbing system. The origin of these textures directly relates to the kinetics of crystal growth and various open system processes (Streck, 2008). Seven samples were chosen from historical eruptions at Mount Etna (1646, 1651, 1669-1, 1669-2, 1852, 1971, and 1991) and were studied using NDIC techniques. The most striking feature of Mount Etna plagioclase is the intra-crystal complexity. In each of the samples two plagioclase populations were identified based on core texture. Patchy zoning is the most common core texture and is identified by rectangular to sub-rectangular inclusions and skeletal behavior of plagioclase. The other crystal population was identified by having cores that were euhedrally or anhedrally oscillatory zoned. Patchy zones and oscillatory zones also occur as peripheral crystal textures. The peripheral textures occur as either well developed bands that completely surround the core of the crystal, or as small peripheral patches. Other textures seen were peripheral dissolution zones and unzoned rims. Textural analysis showed there has been significant textural variation through historical time. The 23-year period leading up to, and including the 1669 eruption, is texturally distinct from the later eruptions. The most common texture in the 1646-1669 flows is patchy zoning with poorly developed peripheral oscillatory zones. In the 1971 and 1991 samples plagioclase have thick, euhedral, and well-developed oscillatory zones surrounding the core of the crystal. Based on textural and other petrologic data it appears that decompression due to volatile loss or ascent drives initial plagioclase crystallization and results in patchy zoned cores. Crystals with oscillatory zoned cores may represent antecrystic populations related to crystallization following earlier injections into the shallow plumbing system. The dramatic textural changes through time suggest that have been differences between the storage behaviors of these magmas in the shallow level plumbing systems.