Dynamics of magma supply at Mt. Etna volcano (Southern Italy) as revealed by textural and compositional features of plagioclase phenocrysts Marco Viccaro ⁎, Pier Paolo Giacomoni, Carmelo Ferlito, Renato Cristofolini Università di Catania, Dipartimento di Scienze Geologiche, Corso Italia 57, I-95129, Catania, Italy abstract article info Article history: Received 30 January 2009 Accepted 31 December 2009 Available online 11 January 2010 Keywords: Etna Plagioclase Texture Crystallization Oscillatory zoning Dissolution Resorption A systematic study of textural and compositional zoning (An% and FeO variation) in plagioclase phenocrysts of historic (pre-1971) and recent (post-1971) lavas at Mount Etna was made through back-scattered electron (BSE) images and electron microprobe analyses (EMP). The textures considered include oscillatory zoning and several types of dissolution, resorption and growth textures at the phenocryst cores and/or rims. Two patterns of oscillation were recognized from the combined An–FeO variation: 1) Low Amplitude–High Frequency (LAHF) and 2) High Amplitude–Low Frequency (HALF). The first pattern is interpreted here as due to kinetic effects at the plagioclase/melt interface which developed during crystallization in closed reservoirs. The second, which sometimes involves thin dissolution surfaces marked by irregular edges, angular unconformities and complex dissolution–regrowth patterns, might imply crystallization in a more dynamic regime, probably driven by chemical and physical gradients of the system (e.g., convection in a steadily degassing open-conduit). Dissolution and resorption textures at the core vary from patchy (exclusive to plagioclases within pre-1971 lavas) to strongly sieved, and can be related to increasing rates of decompression under H 2 O-undersaturated conditions. Thick sieve-textured envelopes at the phenocryst rims, generally coupled with marked An–FeO increase, result from mixing with more primitive and volatile- rich magmas. In the same crystals from recent activity, An and, to a lesser extent, FeO increase, consistent with the mixing of H 2 O-rich magmas similar in their mafic character to the resident magma (cryptic mixing). Two types of growth textures were also recognized at the crystal rims: 1) stripes of regularly-shaped melt inclusions and 2) swallow-tailed, skeletal crystals. In the first instance, the concordant An–FeO decrease suggests crystallization caused by fast ascent-related decompression accompanied by volatile loss. In the second, An decrease at effectively constant FeO contents may indicate crystallization at a high level of undercooling from already degassed magma, followed by rapid quenching; such a feature might be acquired during syn- or post-eruptive conditions. Although textures found in historic lavas are rather similar to those in the recent ones, some differences occur, such as lack of crystals with patchy cores in recent products and lower An contents in crystals of historic ones. The available data allowed us to obtain information on the dynamics of the feeding system, highlighting their possible modifications over time. In particular, historically erupted magmas, generally acknowledged to be volatile-poor, may have ascended through the deep portions of the plumbing system under H 2 O-undersaturated conditions at lower rates than the recent ones, recognized as more volatile-rich. Eruption triggering mechanisms from closed reservoirs in the shallow portions of the feeding system are similar for both historic and recent events, and may be generally favoured by a recharging phase of more primitive, undegassed magma or by a few episodes of important fracture opening (e.g., in response to an earthquake swarm). © 2010 Elsevier B.V. All rights reserved. 1. Introduction The complex volcanic structure of Mount Etna allows magmas to interact with the crust and to differentiate at several levels of the feeding system. Dealing with data on the resulting rocks, it is therefore difficult to assess the relative roles of fractional crystalliza- tion or mixing processes between variously evolved magmas. Whole rock compositions, potentially due to a superimposition of several differentiation processes, may buffer any possible variation, so that tracking processes of magma evolution can be difficult. On the other hand, in situ investigations on phenocryst zoning patterns in volcanic rocks have been found to be a powerful tool for understanding differentiation processes within magma chambers (e.g., Davidson et al., 1991; Davidson and Tepley, 1997; Davidson et al., 2007; Ginibre Lithos 116 (2010) 77–91 ⁎ Corresponding author. Tel.: +39 095 7195741; fax: +39 095 7195760. E-mail addresses: m.viccaro@unict.it (M. Viccaro), pp.giacomoni@unict.it (P.P. Giacomoni), cferlito@unict.it (C. Ferlito), rcristof@unict.it (R. Cristofolini). 0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.12.012 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos