Plagioclase textures and compositional zoning in recent Mt. Etna lavas: physical-chemical constrains of the shallow feeding system

Textures and compositional zoning of plagioclase reflect changes in the physical and chemical conditions of the host magma, thus representing an useful tool to investigate the processes occurring in the feeding system. An open-conduit volcano such as Mt. Etna is the perfect site to investigate interactions between basaltic melts with different volatiles content and the effect of degassing on mineral phase nucleation and growth. In this context, an accurate textural study on more than 130 samples from recent Mt. Etna eruptions (2001, 2002-2003, 2004-2005 and 2006) was carried out. A classification scheme was developed, taking into account the different portions of the crystals constituted by euhedral or rounded (clear, patchy, sieved and dusty) cores and dusty or melt inclusion alignment rims, divided by oscillatory-zoned overgrowth. In this way, each crystal portion could be attributed to a phase of growth or dissolution that occurs at specific P-T-fO2 and H2O content. Thermo-barometric and crystal-melt equilibrium equations were in fact applied to calculate the intensive parameters such as P, T, fO2 and the amount of dissolved water content for each portion of a single crystal [1, 2, 3]. Equilibrium conditions were checked assuming primary melt composition for plagioclase cores and whole rock for plagioclase rims. The obtained data were compared with MELTs modeling aiming at constraining plagioclase composition and theoretical stability field. Results indicate that Mt. Etna plumbing system is rather continuous, excluding the presence of relevant well-defined magma chambers. This feeding system is persistently filled by magmas with similar composition in terms of major and trace element but different dissolved H2O, which constrain the depth and composition of plagioclase cores. A small number of these cores (patchy) nucleate at 14 km of depth, whereas most of them (clear, sieved and dusty) nucleate between 7 and 4 km below the summit. At this depth, mixing probably occurs between variably H2O-enriched magmas causing dissolution of the preexisting An-poor crystals, as suggested by the rounded cores. In many cases, quite high H2O content (> 3wt%) in magma forces nucleation of An-rich, euhedral crystals at shallow depth (∼100 Mpa or 3.7 km below the summit) with growth caused by volatile loss prior and during eruption.