Dipartimento di Biologia, Ecologia e Scienze della Terra

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Geochemical modeling of water-rock interaction in the ophiolitic aquifers of Northern Calabria
(2013-11-04) Critelli, Teresa; Pantano, Pietro; Apollaro, Carmine; Marini, Luigi
This study was developed in two distinct parts with the ultimate aim to investigate the weathering processes of the metabasalts and serpentinites of Northern Calabria, taking into account the dissolution kinetics of relevant mineral phases. In the first part, a laboratory experimental work has been carried out on two suitably characterized rock samples, a metabasalt and a serpentinite, to determine the whole-rock dissolution rates and, consequently, the kinetic parameters of each constituting mineral in these specific rocks, at 25 °C and different pH values. Mixed-flow reactors were used to perform this experimental work. Incidentally, it must be recalled that: (i) although there are many laboratory studies aimed at the determination of the dissolution rates of separate (single) minerals, less is known about the dissolution rates of individual minerals in multimineralic rocks and (ii) in geochemical modeling, the dissolution rates measured on separate mineral are customarily utilized to estimate the dissolution path of the overall rock, assuming that the dissolution rate of a separate mineral is equal to that of the same mineral in a given rock. However, the experimental results of in this study contradict this hypothesis. Indeed, it turned out that: (i) dissolution rates of individual minerals obtained from the dissolution experiments of whole rocks are significantly different from those of separate minerals and (ii) the dissolution rates of individual minerals exhibit minor differences to each other and appear to be close to the whole-rock rate. This behavior is probably constrained by the sufficiently abundant mineral(s) of lowest dissolution rate, preventing the dissolution of other faster-dissolving mineral grains as long as these do not come in contact with the aqueous phase. The second part of the study has been devoted to simulate the rock-to-water release of elements and their fate in the groundwaters interacting with metabasalts and serpentinites by means of two different reaction-path-modeling approaches. In the first approach, kinetic parameters of relevant minerals were taken from the geochemical literature and the progressive dissolution of metabasalts and serpentinites cropping out in Northern Calabria (Italy) was simulated by means of the EQ3/6 software package, version 8.0, adopting the Double Solid Reactant Method (DSRM). In the second approach, the whole-rock kinetic parameters, retrieved from the dissolution experiments on the metabasalt were used in the modeling exercise. The results of the two reaction-path-modeling approaches are in agreement with analytical data for natural waters but the second approach appears to be more accurate than the first one. The main lesson learned from this study is that the dissolution rates of individual minerals retrieved from the dissolution experiments of whole rocks are significantly different from those of separate minerals. This invalidate the assumption on the equality of these rates which is commonly adopted in geochemical modeling. These findings have important consequences on the understanding of the rock-to-water release of chemical elements and their fate in natural waters.
La cenere basaltica: micro-tessiture e dinamiche eruttive
(2014-04-04) Cannata, Chiara Benedetta; Russo, F.; Donato, P.
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.

Dipartimento di Biologia, Ecologia e Scienze della Terra - Tesi di dottorato

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