Plagioclase as Archive of Magma Ascent Dynamics on "Open Conduit" Volcanoes: the 2001-2006 Eruptive Period at Mount Etna

In the last decade Mount Etna activity was characterized by different eruptive styles, from effusive to highly explosive with subterminal and flank eruptions, not accompanied by significant variation in lava compositions. The most important parameters in determining the eruptive styles are therefore to be searched in the volatile contents of magmas and/or its dynamic during uprising. Plagioclase stability is strongly dependent on physico-chemical variations of the melt and it can be used as a tool to record the processes occurring within the feeding system. Textural and compositional study of plagioclase has been performed on the products emitted during the 2001, 2002-2003, 2004-2005 and 2006 eruptions and compared with theoretical models that constrain its stability field. Textural and compositional features of plagioclase have been studied in detail, and related to growth or resorption events occurring at specific P-T-fO2 conditions and water contents. Crystals present simple oscillatory zoning or complex textures at core or at rim. Four distinct complex textures at the cores have been recognized: i) clear and rounded, ii) dusty and rounded, iii) sieved and iv) patchy. Two distinct textures are common close to the crystal rim: i) dusty or ii) with melt inclusion alignments. All plagioclases present a thin (10-20 μm) outermost less calcic rim. The intensive variables of the magmatic system (P-T-fO2) have been determined using geothermobarometers and crystal-melt equilibria. fO2 has then been used to estimate the Fe3+/Fe2+ ratio and to reconstruct the primitive magma composition by adding a olivine-clinopyroxenitic assemblage to the least evolved erupted lava. The plagioclase stability field was determined by MELTS calculations at different pressures (400-0 Mpa) and H2O contents (3.5-0 wt%). Water contents in magma have been estimated using plagioclase-cpx-melt hygrometer. Plagioclase stability models indicate: i) H2O strongly influences the plagioclase stability allowing the crystallization of more calcic composition only at shallow level; ii) patchy cores form at the highest pressure (up to 350 MPa) and low water content (lower than 1.7% wt); ii) clear dissolved cores form at lower pressure (150 MPa) and higher water content (1.5-2.8% wt); iv) dusty rims form at even lower pressure straddling the H2O-saturation and v) melt alignments form during degassing. According to experimental works each of these texture can be related to different process within the feeding system, such as multiple magma inputs (patchy core), volatile addition or increase in T (clear core), mixing (dusty rims) and rapid decompression and degassing (melt alignment rims). These inferences were successfully compared with eruptive evolution of each event as deduced from direct observation, geophysical data and other petrological studies. The overall picture evidences that plagioclase crystallizes under polybaric conditions in a vertically extended and continuous feeding system in which at least two magma ponding zones at crustal level have been identified. Plagioclase stability also indicates that a large variability in water content characterizes the magma column and that its concentration tend to increase upwards.