Volcanic ash aggregation enhanced by seawater interaction: the case of the Secche di Lazzaro phreatomagmatic deposit (Stromboli)

Daniele Morgavi, Luca Valentini, Massimiliano Porreca, Azzurra Zucchini, Alessandro Di Michele, Miriam Ielpo, Antonio Costa, Stefano Rossi, Patrizia landi, Diego Perugini

Abstract


The Secche di Lazzaro formation (ca. 6.2-7 kys BP) is a phreatomagmatic deposit situated in the southwestern part of the island of Stromboli (Aeolian Archipelago, Italy). The volcanic sequence is constituted by three main units. In the lower unit accretionary lapilli are particularly abundant and are characterized by strong cementation between the particles and an uncommon resistance to breakage. To understand the processes behind the formation of the Secche di Lazzaro (SDL) accretionary lapilli a multi-analytical approach was used on the lapilli Aggregate Tuff (AT), and on single Accretionary Lapilli (AL). We carried out granulometric analysis, Field Emission – Scanning Electron Microscopy (FE-SEM), Electron Microprobe Analysis (EMPA), X-ray powder diffraction (XRPD) and 3D imaging by X-ray micro-tomography (X-mCT). The granulometric data shows that most particles in the AT have a diameter equal to phi -1 corresponding to 2 mm. The EMPA, FE-SEM and XRPD analyses reveal the presence of different phases, mainly plagioclase, K-feldspar, halite, and clinopyroxene, together with volcanic glass. From the X-mCT analysis, we constrained the particle distribution and estimated the porosity of LA and AL. The results of the FE-SEM images provided the chemical distribution within individual lapilli allowing the identification of rim and core zoning as well as the presence of halite (NaCl) located both on the border of single lapilli and on the juncture among different lapilli. Moreover, halite also occurs among different aggregates in single AL, thus acting as a binding agent, as well as within rim pores. The results of this work shed new light into the formation of accretionary lapilli in phreatomagmatic eruption at volcanic island involving marine water.


Keywords


Accretionary Lapilli, Stromboli, X-ray microtomography, 3D, Salt

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References


DOI: https://doi.org/10.4401/ag-7874
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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X