La cenere basaltica: micro-tessiture e dinamiche eruttive

Abstract

This work is focused on micro-scale textural observations on ash particles, erupted from ordinary activity at Stromboli volcano (Aeolian Islands, Italy). Ash samples were collected during three sampling campaigns: 1-5 September 2008, 20 and 22 September 2008, and 18 June 2009, during which two sampling methods have been adopted: collection on a clean surface downwind and remotely controlled, electrically propelled aeromodels, penetrating directly in the eruptive plume and collecting ash samples that are less affected by transport-related sorting processes. Ash from individual explosions have been characterized in terms of quantitative proportion of principal ash components, textural, morphological and chemical features. A micro-textural approach has been conducted, working at step: from preliminary observation under binocular microscope on a medium number of 700 particles, to thin section observations, to sub-micron investigations, using high resolution instruments. Micro-texture and morphology of ash particles were observed under FE-SEM, at high magnification (30.000/100.000 X). On each particle we measured the main shape parameters (area, perimeter, max length, compactness, Feret’s diameter, Heywood’s diameter and elongation) and chemical-surface features. Ash is composed of two main type of fragments: one opaque, black coloured, micro- to crypto-crystalline, poorly or non-vesiculated with regular shapes, called “Type a” fragments, and the other one, transparent, lightly brown to yellow, red to brown; sub-aphyric to porphyric, showing large micro-phenocrystals of plagioclase and pyroxene; highly vesiculated, with irregular shapes, sometimes fluidal and showing shards morphology, called “Type b”. Rarely porphyric to vitric fragments (“Type c”), with reddish colors and vesicles inside are present and also a few single crystals of plagioclase, pyroxene and olivine (“Type d”). At persistent activity volcanoes is not easy to distinguish juvenile from lithics so in this work we decided to adopt a new nomenclature of ash particles without a priori interpretation. “Type a” show great similarities with the well known tachylite particles, while “type b” fragments are very similar to the sideromelane ones, the two types of fragments representing end-members of a continuous spectrum. Abundant “type a” fragments for example, show fluidal shapes, reddish colors and micro-phenocrystals of plagioclase and pyroxene; while “type b” fragments show brownish colors and porphyric structures. These fragments are so called “type a1” and “type b1”, respectively. A great heterogeneity does exists among ash constituents erupted from three different active group of vents. Component analysis of principal constituents well discriminate between ash erupted from three group of vents sampled in September 2008 and June 2009 and also ash from May and September 2002, used here as a comparison for the componentry of ash erupted during the ordinary activity. Morphological and chemical analysis showed that the fragments have slightly different morphologies and are all subject to alteration phenomena and precipitation of minerals of neo-formation on their surfaces. The alteration is dependent from the atmospheric conditions around the vent and not from the type of fragments. Ash erupted from individual explosions, during the periods of sampling doesn’t show any differences in morphology and chemistry of surface features, but only in terms of proportion of components. “Type b” fragments (sideromelane) predominate in the ash erupted from typical strombolian activity. During the ordinary activity at Stromboli is confirmed that in the shallow conduit of Stromboli, a magma with two different texture features does exist.