Unusual lapilli tuff ejecta erupted at Stromboli during the 15 March 2007 explosion shed light on the nature and thermal state of rocks forming the crater system of the volcano Stefano Del Moro a, ⁎, Alberto Renzulli a , Patrizia Landi b , Sonia La Felice b , Mauro Rosi c a Dipartimento di Scienze della Terra della Vita e dell'Ambiente, Università degli Studi di Urbino “Carlo Bo”, Campus Scientifico “Enrico Mattei”, I-61029, Urbino, Italy b Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Via della Faggiola, 32, I-56126, Pisa, Italy c Dipartimento di Scienze della Terra, Università degli Studi di Pisa, Via S. Maria, 53, I-50126, Pisa, Italy abstract article info Article history: Received 10 May 2012 Accepted 21 December 2012 Available online 3 January 2013 Keywords: Basalt Pyroclast Subsolidus reaction Hydrothermal alteration Pyrometamorphism Stromboli Textural and mineralogical study of high-temperature, angular blocks erupted during the Stromboli explo- sion of 15 March 2007 was used to make inferences on the nature and thermal state of rocks forming the sub- surface of the volcano' summit crater terrace. The studied ejecta consist of lapilli tuff that formed as a result of the transformation and high temperature induration (sintering) of the basaltic scoriae, lapilli and ash origi- nally accumulated as loose tephra during the current activity of the volcano. The main processes leading to the tephra transformation were investigated through microstructural observations, mineral and glass analy- ses (SEM-EDS and EMP analyses). Investigations revealed that subsolidus reactions and partial melting of the tephra occurred, at temperatures higher than 600 °C and under variable fO 2 conditions from QFM to HM buff- ering curves. In some blocks, evidence of high-T reheating and partial melting at the expense of secondary hydrothermal minerals was also observed. In order to track the subsolidus reheating history of the basaltic pyroclasts, a detailed study of the pseudomorphic phases and reactions after olivine, driven by iron oxidation under high-T conditions, was performed. The observed mineralogical transformation suggests that the lapilli tuff material, originating from the burial of tephra routinely accumulated by persistent Strombolian explo- sions within the crater terrace, were in some cases altered by the circulation of acidic fluids and were in any case reheated due to isotherm rise forced by high heat flux and gas streaming delivered by the underly- ing magma system. It is worth noting that the ejection of these unusual volcanic lithotypes was possible be- cause a few days before the 15 March 2007 event, the craters were clogged with lapilli tuff material that slid into the crater bottom between 7 and 9 March. Findings of this study suggest that the scattered permanently active vents and shallow conduits of Stromboli are surrounded sideways and underneath the crater terrace, by a fairly large volume of high temperature rocks with variable degree of compaction, sintering up to par- tially melted. Such a spectrum of rock types is in good agreement with the conceptual model of prominent thermal zoning all around (sideway and upwards) the active magmatic system. We speculate that continuous migration upwards of isotherms led to transformation and partial melting of the normal Strombolian tephra. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Basaltic Strombolian activity commonly build up a head of debris, whose form and dimension strongly depend on the intensity of the activity, fragmentation degree of the magma and duration of the erup- tion. A scoria cone is by far the most typical volcano produced by Strombolian activity. It may be built very rapidly or may be produced by a series of discrete events that occur over time, and are mostly lo- cated on the site of a larger volcano or in the centre of a large crater or caldera. Ejecta which pile up to form the cone usually preserve their textural and chemical characteristics over time, not including post depositional, weathering transformations. However, pyroclastic rocks accumulated near the crater area of a persistently active basal- tic volcano may undergo significant transformations as a result of reheating and exposure to fumarolic activity related to the uppermost magmatic feeding system. Circulation of acidic hydrothermal fluids in- duces leaching of the rocks, and changes in their chemical, mineralog- ical, structural and textural features, leading to the formation of argillic to silicic alteration facies (Wohletz and Heiken, 1992; Fulignati et al., 1998; Del Moro et al., 2011). The high-T and low-P con- ditions, associated with fluids circulation, induce mineral and textural variations of volcanic rocks, partial melting of minerals and glass, for- mation of mineral aggregates and the nucleation of microcrystal- line phases from the melt. Minerals and textures of basaltic products Journal of Volcanology and Geothermal Research 254 (2013) 37–52 ⁎ Corresponding author. Tel.: +39 0722304247. E-mail addresses: geodelmo@gmail.com (S. Del Moro), alberto.renzulli@uniurb.it (A. Renzulli), landi@pi.ingv.it (P. Landi), rosi@dst.unipi.it (M. Rosi). 0377-0273/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jvolgeores.2012.12.017 Contents lists available at SciVerse ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores