Amiata Volcano Research Articles

Detrital orthopyroxene as a tracer of geodynamic setting:: A Raman and SEM-EDS provenance study

Raman spectroscopy is a powerful and efficient tool that is being used more and more frequently in high-resolution provenance analysis. This study is dedicated to Ca-poor orthorhombic pyroxene, which for its low durability during diagenesis can be considered as a diagnostic first-cycle phase. We focused on 15 samples of modern sands representing a full range of rock units exposed at different structural levels of continental, arc, or oceanic lithosphere and generated in diverse geodynamic settings. By coupling optical observations, SEM-EDS mineral-chemistry and Raman spectroscopy analyses on the same grains we show how orthopyroxene from mantle, deep crustal, and volcanic rocks ranging in composition from andesite to dacite and felsic differentiates can be robustly distinguished. The Mg# [Mg/(Mg + Fe)] content of orthopyroxene grains can be accurately estimated by recording the characteristic position of their six main characteristic peaks (ν1 to ν6, vibrational modes) even during routine point-counting under the Raman spectroscope. In particular, the position of the strong and narrow ν3 peak (~655 cm−1) also allows estimation of the Ca content when compared to the position of other peaks (especially ν1, ~330 cm−1). High-Mg orthopyroxene is exclusively derived from mantle rocks, whereas metagabbros of the lower continental crust and gabbroic rocks of arc crust are distinguished by their lower Ca. The lowest Mg# was recorded in detrital orthopyroxene derived from the silicic Amiata Volcano in central Italy, whereas trachytes in rift-related settings did not show unique characteristics apart from their slightly higher Ca content.

ARTIST’S IRON-BASED NATURAL EARTH PIGMENTS OF TUSCANY (MONTE AMIATA VOLCANO, ITALY)

ABSTRACT Among the artist’s iron-based natural earth pigments, the so-called terra di Siena (raw sienna), terra di Siena bruciata (burnt sienna) and terra d’ombra (umber) have been among the yellow-brown and reddish-brown earth pigments most widely used by Italian and European painters since the Renaissance. We present the history of discovery, designation, and production of these famous pigments, their geological, lithological, and geochemical characterization, and the recognition of their genesis and places of origin, based on new geological field surveys, and on the critical analysis of textual documents and rock sample collections assembled during the eighteenth and nineteenth centuries. In particular, the terra di Siena earth pigment exclusively originated at, and was extracted from, late Pleistocene paleo-lake basins surrounding the Monte Amiata volcano. This earth pigment consists of primarily lacustrine sediments composed of hydrated iron oxide (limonite/goethite) produced by biochemical authigenic precipitation from fresh waters rich in metal solutes.

The paleo-lacustrine diatomaceous deposits of Monte Amiata volcano (Tuscany, Italy) and the Ezio Tongiorgi paleontological collection in the Museum of Natural History of the University of Pisa

At the foothillof Monte Amiata volcano (southern Tuscany, Italy), small extinct lake basins of late Pleistocene age are documented. These lake basins were characterized by the deposition of two very different types of sediment: a) derived from the authigenic precipitation of iron oxides (goethite) and exploited as earth pigments; b) biogenic siliceous sediment composed of fossil diatoms and named diatomaceous earth or diatomite. The lacustrine sediments of Mount Amiata volcano were widely exploited for various applications since ancient times. Literary documents begin in the 16th century, with the descriptions of Cesalpino, Gesner, Agricola, and Imperato. Specific references to the diatomites of Monte Amiata are quoted in the 17th century by Boccone and Bonanno. The quarrying activity was described by Micheli in 1733. During the 18th and 19th centuries, the diatomaceous earths of Monte Amiata are part of the important geological collections of Micheli, Targioni Tozzetti, Baldassarri, Campani, and Tommi. A particular significance has the collection of botanic and ichthyologic fossils collected by Ezio Tongiorgi, and now preserved in the Museum of Natural History of the University of Pisa sited at the Charterhouse of Pisa in the Calci village. These paleontological samples preserve the biological and physical testimonies of the environmental and climatic changes of the late Pleistocene and are now particularly valuable because they are the only remaining evidence of the diatomaceous lacustrine deposits of the paleo-lakes of Monte Amiata. For these reasons, they represent geological materials with a fundamental cultural value.

Characteristics and significance of intravolcanic saprolite paleoweathering and associate paleosurface in a silicic effusive volcano: The case study of Monte Amiata (middle Pleistocene, Tuscany, Italy)

A paleoweathering saprolite mantle and an associated paleosurface have been recognized into the stratigraphic volcanic succession of the silicic effusive (trachydacite) middle Pleistocene Monte Amiata volcano (Italy). This element has been used for applying UBSU (Unconformity Bounded Stratigraphic Units) subdivision to the volcano stratigraphy. All the lavas emplaced in the onset of the volcanic activity (Bágnore Synthem, BAS) have been indistinctly affected by a saprolite in-situ alteration. Saprolite facies varies from poor lithoid consistency but with conservation of the original texture, to fractured rock with spheroidal weathering, to incoherent rock consisting of a prevalent sandy fraction, up to accumulations of residual lava blocks (core-stones, mainly in coincidence with the arcuate ridges on top of the main lava flows). Unaltered lavas of the younger phase of volcanic activity (Monte Amiata Synthem, MAS) have preserved the paleosurface within its own saprolite. Where the MAS cover is absent, the saprolite mantle represents a relict paleosurface on the present landscape. Weathering of the silicic lavas was favored by the vitrophyric texture of the parent rock and by moderate topographic reliefs formed by extensive lava plateaus positioned at various elevations. The palaeowathering saprolite is indicative of warm and relatively humid temperate climatic conditions and relative tectonic and morphogenetic stability.

Monte Amiata volcano (Tuscany, Italy) in the history of volcanology: 2—its role in the definition of “ignimbrite” concepts and in the development of the “rheoignimbrite” model of Alfred Rittmann

The explosive eruptions that occurred between nineteenth and twentieth centuries produced a fundamental cultural impact on the development of Volcanology. Pyroclastic products and ignimbrites features start to be at the base of an international debate. Various descriptions of explosive eruptions, and a new terminology of their products, such as nuee ardente and ignimbrite, were presented and extensively discussed in the framework of the International Association of Volcanology conferences held in 1961 at Catania and in 1963 at Tokyo. Ignimbrite deposits are first assimilated to welded tuffs. That attention to explosive volcanism of the first half of the twentieth century was the context in which has matured the Alfred Rittmann model of rheoignimbrite as welded ignimbrite showing secondary flowage structures. This term introduced by Rittmann in 1958, and shared by Giorgio Marinelli in 1961, was intended to describe the extensive sheet of acidic vitrophyric volcanic rocks of Monte Amiata volcano, interpreted as lava flows by all previous authors. Rheomorphic ignimbrites, in the Rittmann model, have features that strongly differentiate them from normal ignimbrites and that are very similar to what shown by acidic lava flows, as fluidal structures and wrinkles. The concept of rheomorphic ignimbrite is still in use into the volcanological literature, even if not for the Monte Amiata volcanics, nowadays definitively considered to be lava flows and domes. However, the Rittmann and Marinelli authoritative assumptions inhibited, up to present times, new volcanological interpretation of Monte Amiata acidic lavas.

MONTE AMIATA VOLCANO (TUSCANY, ITALY) IN THE HISTORY OF VOLCANOLOGY, PART 1: ITS ROLE IN THE DEBATES ON EXTINCT VOLCANOES, SOURCES OF MAGMA, AND ERUPTIVE MECHANISMS (1733–1935)

A review of the main contributions to the scientific literature between the eighteenth and twentieth centuries concerning the geology and volcanology of Monte Amiata volcano (Tuscany, central Italy) is presented. Monte Amiata, and the nearby volcano of Radicofani, are of great interest for the history of volcanology because they have the primacy of being the first to be recognized of volcanic origin in a region (Tuscany) which was not volcanically active, thirty years before Guettard's studies on the Auvergne region in France. Indeed, the Florentine botanist Pier Antonio Micheli identified as extinct volcanoes Radicofani in 1722 and Monte Amiata in 1733. Moreover, the merit of Micheli's work resides in interpreting Monte Amiata as an extinct volcano despite the absence of a conventional cone-shaped volcano morphology, and in his recognizing its rocks as lavas despite their marked differences to those produced by the known active volcanoes of its times, such as the iconic Vesuvius. During the eighteenth century and until the first half of the nineteenth century, Monte Amiata was a destination for scientific journeys by Tuscan and foreign scholars (e.g. Micheli, Baldassarri, Arduino, Fortis, Ferber, Dolomieu, Santi, Repetti, Hoffmann). In addition, its rocks were part of important collections throughout Europe visited by illustrious mineralogists. Furthermore, samples from Monte Amiata were used to illustrate the general discussions on the nature and origin of rocks such as basalt and granite. In the nineteenth century, Monte Amiata was included in the lists of known volcanoes recorded in the early treatises on volcanology made by Scrope, Daubeny, and Hoffmann, and its ‘trachyte’ was the subject of early essays on microscope petrography and chemical analysis of rocks, performed by vom Rath, Rosenbusch, Williams, Lacroix, and Washington. Between the end of the nineteenth century and the first quarter of the twentieth century, the volcano geologists of the Comitato Geologico Italiano, especially Verri, Lotti, and Sabatini, carried out field-surveys on Monte Amiata resulting in geological maps and volcano-stratigraphies. Moreover, modern petrographic (Novarese, Artini, Rodolico) and geographical (Dainelli and Olinto Marinelli) scientific studies were carried out on this volcano. Nevertheless, up to the middle part of the twentieth century, the major interest in Monte Amiata was related not to its volcanological aspects but to its natural resources: drinking waters, diatomaceous earths, earth pigments, and mercury ore-minerals.

Deciphering textural and chemical zoning of K-feldspar megacrysts from Mt. Amiata Volcano (Southern Tuscany, Italy): Insights into the petrogenesis and abnormal crystal growth

This study reports the complex textural and chemical features of K-feldspar megacrysts (up to 5 cm long) hosted in trachydacitic lava flows, lava domes, and coulées from Mt. Amiata volcano (Tuscan Magmatic Province, Central Italy). Backscattering and cathodoluminescence imaging, coupled with core to rim major and trace elements patterns, reveal complex zoning, and resorption surfaces associated with sharp chemical variations (e.g., Sr and Ba). These zoning patterns originated by disequilibrium and re-equilibration events, related to the repeated influx of mafic magmas or convective motions in the trachydacitic magma reservoir. Multiple mafic magma refilling occurences are also supported by the field record of abundant microgranular magmatic enclaves in the studied products. Our results highlight that the abnormal dimension of the studied K-feldspars originates by the interplay between petrological and kinetic processes involving: (i) extensive dissolution; (ii) heterogeneous nucleation; (iii) alternation of spasmodic growth events in disequilibrium and near-equilibrium crystallization. The repeated influx of hotter magmas and reheating can determine the thermal condition to the growth of few, large K-feldspar megacrysts. Also, the strong textural and chemical similarities observed in the K-feldspar megacrysts from Mt. Amiata volcanic rocks and Mt. Capanne monzogranite (Elba Island, Central Italy) support the hypothesis of a phenocrystic origin of intrusive K-feldspar megacrysts.

Detecting CO2 anomalies in a spring on Mt. Amiata volcano (Italy)

Located at the south-western slope of the Mt. Amiata volcano (Tuscany, Central Italy), the Bagnore spring has been investigated for geochemical precursors of earthquakes from 2004 to 2015. Over this period, several parameters of the spring have been monitored by discrete and continuous sampling. An automatic continuous monitoring station, equipped with sensors for the measurement of temperature, pH, redox potential, electrical conductivity, CO2 and CH4 dissolved concentration, is transmitting the registered signals to the remote server in Pisa 150 km northwest of Mt. Amiata. The Bagnore spring drains a shallow, short-circuiting aquifer hosted in the volcanic rocks of the volcano. Its emergence point is located in proximity of the intersection of two major fault systems that are supposed to provide a preferential ascent path to hydrothermal gases, mainly represented by CO2(g) and H2S(g), locally rising from depth.The most evident change occurred in the registered signals over the period is represented by the sudden increase in CO2 concentration measured starting from April 2010. Along with this increase in CO2, a slight increase in water temperature and in SO4 concentration, associated to a decrease in pH, was also recorded. This trend has been interpreted as an evidence for the augmented inflow of deep gases into the shallow aquifer. The CO2 continuous signal recorded by the Bagnore automatic station has been then processed by applying multiple statistical techniques (i.e. artificial neural network analysis and Census I method) in the search for anomalies possibly related to local seismic activity. Anomalous signals have been detected starting from April 24, 2010, and the correlation with the most energetic seismic events has been tentatively proposed.

Reply to: Barazzuoli P., Bertini G., Brogi A., Capezzuoli E., Conticelli S., Doveri M., Ellero A., Gianelli G., La Felice S., Liotta D., Marroni M., Manzella A., Meccheri M., Montanari D., Pandeli E., Principe C., Ruggieri R., Sbrana A., Vaselli V., Vezzoli L., 2015. COMMENT ON: “Borgia, A., Mazzoldi, A., Brunori, C.A., Allocca, C., Delcroix, C., Micheli, L., Vercellino, A.,

The volcanic spreading model by Borgia et al. (2014) is accurate in describing the extensional structures found on the edifice and the radial compressional structures existing all around the base of Amiata Volcano. Volcanic conduits, extensional structures, and direct contact between the volcanic rocks and the Tuscan Units, constitute the hydraulic connection between the potable fresh-water aquifer contained in the volcanites and the underlying hydrothermal system. Therefore, gaseous phases tend to flow upward (particularly through faults) carrying pollutants into the freshwater aquifer, while the freshwater recharges (also through primary permeability) the exploited geothermal fields.

Messinian-earliest Zanclean tectonic-depositional dynamics of the Cinigiano-Baccinello and Velona basins (Tuscany, Italy)

The paper reports a revision of the tectonic-depositional evolution of the continental Cinigiano-Baccinello and Velona basins, located in the Amiata Volcano region, with special emphasis on the Messinian dynamics. Integration of facies analysis, magnetostratigraphy, and structural geology allowed a comparison of the evolution of these basins and a discussion of possible local to regional implications. At a local scale, crustal shortening, accommodated by thrust faults and related anticlines delimiting the basins, determined a dynamic physiographic and hydrographic scenario during the Messinian. Uplift of the tectonically-controlled shoulders was paired with pulses of subsidence in the basins that favoured the development of palustrine-lacustrine settings or endorheic alluvial plains. Stages of quiescent tectonics favoured fluvial incision of structural thresholds and the development of a south-directed drainage system, particularly developed during the late Messinian in coincidence with the Mediterranean Messinian Salinity Crisis. The dominant fluvial and clastic depositional pattern recorded in these basins during such a regional scale event points to local tectonic activity of the Northern Apennines playing a major role than the climatic, eustatic and geodynamic factors that controlled the Mediterranean region.

New 40Ar-39Ar dating and revision of the geochronology of the Monte Amiata Volcano, Central Italy

The duration of the Mt. Amiata volcanic activity is still a matter of debate, in spite of the presence of several geochronological data in the literature. We performed new 40Ar-39Ar dating on the sanidinegroundmass pairs of the upper stratigraphic units: Dome and massive Lava flows Complex (DLC) and Olivine Latitic final Lavas (OLF). The aim was twofold: to check the reliability of sanidine ages as geochronometer in these products, questioned in the literature, and to better define the chronology of the late activity of this volcano. Ages obtained on coexisting sanidine and groundmass of the Dome and massive Lava flows Complex (DLC) samples overlap within errors, demonstrating that sanidine crystals recorded reliable emplacement ages in these rocks. The Olivine Latite final lavas (OLF) display a different scenario, where the groundmass has an age younger than that of the sanidine, which is xenocrystic and, evidently, retains inherited Ar. Preferred ages for analysed DLC samples are comprised between 301 and 294 ka, an interval of time too short to resolve the ages of the four dome samples taken into account. The Ermeta lava is about 60 ka younger. We propose that the majority of Mt. Amiata volcanic rocks were emplaced in a narrow interval of time, whilst a temporal gap, which needs more detailed constraints, exists with at least one of the Olivine Latite final lavas (OLF).

Geochemistry and Sr-Nd-Pb isotopes of Monte Amiata Volcano, Central Italy: evidence for magma mixing between high-K calc-alkaline and leucititic mantle-derived magmas

Monte Amiata is a small volcano composed by trachytic to olivine latitic lava flows and domes emplaced in a very short time between 305 and 231 ka. The main petrographic features are represented by the occurrence of i) abundant rounded magmatic enclaves increasing in dimension and quantity passing from early to late erupted Monte Amiata volcanic rocks, ii) large sanidine megacrysts, mainly confined in the second stage of activity characterised by the emplacement of exogenous domes and massive lava flows, and iii) mafic olivine latitic lava flows, with intermediate compositions between the early silica-rich volcanic rocks and the most mafic rounded magmatic enclaves hosted by the Monte Amiata volcanic rocks. The occurrence of rounded magmatic enclaves testifies fresh magma injection and stirring within a differentiated magma reservoir. This triggered the pouring out of the viscous trachydacitic resident magma. A reverse differentiation pathway is observed with time of magma emplacement, which is accompanied by the decrease of silica contents and increase of MgO and compatible elements passing from early trachydacites to final olivine-latites. The same timely reverse differentiation pathway is observed among magmatic enclaves, with the most mafic terms hosted by final olivine-latitic lava flows. Fine-grained rounded magmatic enclaves, indeed, range in composition from potassic trachybasalt (absarokite) to olivine-latite. The overall geochemical and isotopic features agree with a mixing process between a highly differentiated (i.e., high silica), and partially crystallised, high-K calc-alkaline end- member and a mafic ultrapotassic magma possibly leucite-bearing. Absence of leucite in the Amiata rocks and enclaves is due to high-silica activity of derived magmas caused by the high-silica end-member of the mixing process.

Faults strengthening and seismicity induced by geothermal exploitation on a spreading volcano, Mt. Amiata, Italia

Seismogenic structures such as faults play a primary role in geothermal system generation, recharge and output. They are also the most susceptible to release seismic energy over fluid injection/extraction operations during anthropic exploitation. We describe the microseismic activity recorded in 2000–2001 in the Piancastagnaio geothermal field, on the SE flank of Mt. Amiata volcano, southern Tuscany, Italy. From our field observations we find that a relatively high percentage (i.e. about 5%) of the recorded events are of hydro-fracturing origin and have a distinct waveform seismic signature when compared to the recorded events of tectonic shear-fracturing origin. While hydrofracturing events are mostly concentrated around the geothermal fields, the spatial distribution of hypocenters shows a deepening and a density increase of the micro-seismic activity from the volcanic axis toward the exploited geothermal reservoir, suggesting that volcanic spreading at Amiata is still active. The study of different data-sets from different time periods together with the knowledge from Terzaghi's law that production of large quantity of pore-fluid with the associated fluid pressure reduction could augment the stress normal to faults' surfaces (and thus their resistance to slip), make us argue that the process of volcanic spreading affecting the edifice of Amiata may allow augmented accumulation of stresses on faults, eventually leading to the release of higher stress drops, once ruptures occur. The Gutenberg–Richter magnitude–frequency distribution shows that the strongest events on record have a local magnitude in the 5–5.5ML range, for 100-year recurrence time. In conclusions, we infer that geothermal exploitation at Mt. Amiata should be closely monitored in order to understand how fluid injection/production is responsible for the hydrofracturing seismic activity and affects stress accumulation on and rupture of faults within and in the neighborhood of the geothermal fields. This understanding may allow a geothermal field management that will hopefully reduce the risk for inducing larger seismic events in the area.

Origin of the gases released from the Acqua Passante and Ermeta wells (Mt. Amiata, central Italy) and possible environmental implications for their closure

<div class="page" title="Page 2"><div class="layoutArea"><div class="column"><p>The Mt. Amiata volcano (Tuscany, central Italy) hosts the second largest geothermal field of Italy. Its SW and NE sectors are characterized by the presence of several CO<span><sub>2</sub></span>-rich (>95% by vol.) gas discharges. An intense Hg mining activity had taken place from the 19th century up to the end of the ’70s, particularly close to Abbadia San Salvatore, during which two drillings (Acqua Passante and Ermeta) intercepted a CO<span><sub>2</sub></span>-rich gas fertile horizon. The related gases are emitted in the atmosphere since 1938 and 1959, respectively, causing severe concerns for the local air quality. In this work the results of a geochemical and isotopic survey carried out on these gas emissions from March 2009 to January 2014 are presented. CO<span><sub>2</sub></span> fluxes from both the two wells and soil from an area of about 653,500 m<span><sup>2</sup></span> located between them were measured. The two wells are emitting up to 15,000, 92 and 8 tons y<span><sup>-1</sup></span> of CO<span><sub>2</sub></span>, CH<span><sub>4</sub></span> and H<span><sub>2</sub></span>S, respectively, while the computed soil CO<span><sub>2</sub></span> output was estimated at 4,311 ton y<span><sup>-1</sup></span>. The spatial distribution of the CO<span><sub>2</sub></span> soil flux suggests the presence of preferential patterns, indicating sites of higher permeability. Since the local municipality is evaluating the possibility to plug the Ermeta vent, a temporarily closure should first be carried out to test the possible influence of this operation on the diffuse soil degassing of deep-originated CO<span><sub>2</sub></span> in the surrounding area. This implies that diffuse soil gases should carefully be monitored before proceeding with its definitive closure.</p></div></div></div>

Volcanic spreading forcing and feedback in geothermal reservoir development, Amiata Volcano, Italia

We made a stratigraphic, structural and morphologic study of the Amiata Volcano in Italy. We find that the edifice is dissected by intersecting grabens that accommodate the collapse of the higher sectors of the volcano. In turn, a number of compressive structures and diapirs exist around the margin of the volcano. These structures create an angular drainage pattern, with stream damming and captures, and a set of lakes within and around the volcano. We interpret these structures as the result of volcanic spreading of Amiata on its weak substratum, formed by the late Triassic evaporites (Burano Anhydrites) and the Middle-Jurassic to Early-Cretaceous clayey chaotic complexes (Ligurian Complex). Regional doming created a slope in the basement facilitating the outward flow and spreading of the ductile layers forced by the volcanic load.We model the dynamics of spreading with a scaled lubrication approximation of the Navier Stokes equations, and numerically study a set of solutions. In the model we include simple functions for volcanic deposition and surface erosion that change the topography over time. Scaling indicates that spreading at Amiata could still be active. The numerical solution shows that, as the central part of the edifice sinks into the weak basement, diapiric structures of the underlying formations form around the base of the volcano. Deposition of volcanic rocks within the volcano and surface erosion away from it both enhance spreading. In addition, a sloping basement may constitute a trigger for spreading and formation of trains of adjacent diapirs. As a feedback, the hot hydrothermal fluids decrease the shear strength of the anhydrites facilitating the spreading process.Finally, we observe that volcanic spreading has created ideal heat traps that constitute todays' exploited geothermal fields at Amiata. Normal faults generated by volcanic spreading, volcanic conduits, and direct contact between volcanic rocks (which host an extensive fresh-water aquifer) and the rocks of the geothermal field, constitute ideal pathways for water recharge during vapour extraction for geothermal energy production. We think that volcanic spreading could maintain faults in a critically stressed state, facilitating the occurrence of induced and triggered seismicity.

Distribution of gaseous Hg in the Mercury mining district of Mt. Amiata (Central Italy): A geochemical survey prior the reclamation project

The Mt. Amiata volcano is the youngest and largest volcanic edifice in Tuscany (central-northern Italy) and is characterized by a geothermal field, exploited for the production of electrical energy. In the past Mt. Amiata was also known as a world-class Hg district whose mining activity was mainly distributed in the central-eastern part of this silicic volcanic complex, and particularly in the municipality of Abbadia San Salvatore. In the present work we report a geochemical survey on Hg0 measurements related to the former mercury mine facilities prior the reclamation project. The Hg0 measurements were carried out by car for long distance regional surveys, and on foot for local scale surveys by using two LUMEX (915+ and M) devices. This study presents the very first Hg0 data obtained with this analytical technique in the Mt. Amiata area. The facilities related to the mining areas and structures where cinnabar was converted to metallic Hg are characterized by high Hg values (>50,000ngm−3), although the urban center of Abbadia San Salvatore, few hundred meters away, does not appear to be receiving significant pollution from the calcine area and former industrial edifices, all the recorded values being below the values recommended by the issuing Tuscany Region authorities (300ngm−3) and in some cases approaching the Hg background levels (3–5ngm−3) for the Mt. Amiata area.

The sagging deep-seated gravitational movements on the eastern side of Mt. Amiata (Tuscany, Italy)

The eastern side of the Mt. Amiata volcano is affected by a series of deep-seated gravitational slope deformations (DsGSDs). The San Piero and the Podere Mezzavia DsGSDs affect the lower part of the slope. The main escarpments are located on the outer edges of the lava flows, but the landslides mostly affect the pre-volcanic Ligurian Terrains. A deeper movement, possibly exceeding 100 m in thickness, is evidenced by a long trench at the base of the main escarpment that indicates a sagging type movement. This deeper movement is responsible for the activation of a series of superficial rock and mud flows that show evidence of ongoing activity. The most likely location of the sliding surface is the tectonized contact between the Santa Fiora and Argille a Palombini Fms within the Ligurian units, although the superficial landslides prevent our determining with certainty if a clear-cut sliding surface already developed connecting the upper and the lower parts of the slope. These DsGSDs were activated along the flanks of a larger movement that affects the lava flow units cropping out in the middle slope of the volcano. A long main escarpment, secondary escarpments, trenches and borehole data suggest that the thickness could locally exceed 200 m and generate another sagging type movement. Up-slope and up-movement-facing counterscarps indicate the existence of a listric elongated spoon-shaped compound embryonic sliding surface. This sagging, which hosts the towns of Abbadia San Salvatore and Piancastagnaio, appears to be in a quiescent stage, according to preliminary monitoring with a global positioning system (GPS) network. The downcutting of the river network along the softer Pliocene terrains of the Radicofani basin is enhanced by the general uplift of the Apennines and seems to be the major factor in the activation of these DsGSDs.

Transtensional shear zones controlling volcanic eruptions: the Middle Pleistocene Mt Amiata volcano (inner Northern Apennines, Italy)

Terra Nova, 00, 1–10, 2010 Abstract The Mt Amiata volcano is characterized by the SW–NE alignment of the eruption centres, as well as other Italian Pleistocene volcanoes. We carried out a structural and kinematic analysis in a broad stripe comprehending the eruption centres and the substratum, towards NE and SW. The results point out that a transtensional left-lateral brittle shear zone affects the pre-Neogene substratum, the Middle Pleistocene volcanic rocks and the Late Pleistocene travertines. This transtensional structure is active since the Early Pliocene. It controlled the location of the eruption centres through pull-apart structures, thus providing the deep structural channels for the uprising of magma to the surface. The Mt Amiata volcano represents a key study area to investigate the Pleistocene to Present relationships between tectonics and magmatism in the Tyrrhenian area.

A Geochemical Multi-Methodological Approach in Hazard Assessment of CO2-Rich Gas Emissions at Mt. Amiata Volcano (Tuscany, Central Italy)

The present work aims to assess the hazard for human health related to CO2 anomalous concentrations in air emitted from dry gas vents located in the NE area of Mt. Amiata volcano (Tuscany, central Italy). A geochemical multi-methodological approach is adopted to determine the composition and the flux rate of the gas discharges in order to establish (1) the origin of the gas vents and (2) the behaviour of the discharged gases in the areas surrounding the emission sites. The gas vents are hosted within sub-circular morphological depressions (∅ ∼ 10–30 m), which likely originated by the collapse of cavities formed at shallow depth in the ground by dissolution of Triassic anhydrite formations and recent travertine deposits. CaCO3 and CaSO4 dissolution is mainly related to the underground circulation of CO2-rich fluids whose hydrological pattern is regulated by local and regional tectonics. The CO2-rich (up to 996,070 μmol/mol) gases tend to accumulate within the topographic lows, thus creating a sort of CO2 ponds, and the knowledge of their evolution in time and space is important to evaluate the related hazard. Consequently, a conceptual model of CO2 diffusion in air is developed to understand the dynamic of the CO2 accumulation/dispersion process based on (1) a 24-h continuous measurement of the CO2 flux from one of the main emission sites and (2) the recording of the main meteoric parameters, i.e. air temperature, wind direction and speed to check their influence. The results indicate that the threshold of CO2 concentrations considered dangerous for the human health is frequently overcome. Moreover, when meteoric conditions, i.e. low wind and cloudy weather, did not allow a rapid dispersion of the gas phase emitted from the dry vents, CO2-rich clouds periodically overflowed the morphological depressions for several tens of meters without any significant mixing with air. On the basis of these considerations, the monitoring of the output rate from the main gas emissions, combined with the continuous control of the local meteorological parameters, may be considered an efficient procedure to mitigate the CO2 hazard deriving from dry gas vents. An improvement of the protocol can be achieved in case of installations of CO2 sensors located in the most sensitive areas and connected to a telemetry system able to transmit the data in real time to the closest Civil Defence centre. The CO2 degassing sites can also represent a tourist attraction after the installation of suitable metallic fences and a proper campaign of information about these natural phenomena.

Hybrid character and pre-eruptive events of Mt Amiata volcano (Italy) inferred from geochronological, petro-geochemical and isotopic data

A new geochronological and geochemical study was carried out to better constrain the petrogenesis and eruptive history of Monte Amiata, a large Pleistocene trachydacitic volcano of Southern Tuscany. Previous studies suggested a magma mixing origin between calc-alkaline silicic melts from the Tuscan Magmatic Province (TMP) and potassic mafic melts like those found in the Roman Magmatic Province (RMP). Two eruptive episodes–the first at ca. 300 kyr, the second at ca. 200 kyr–were distinguished from the few available ages. However, both the involvement of a RMP-like melt as mafic end-member and the timing of volcanic activity remained to be ascertained. The K–Ar ages obtained on plagioclase, sanidine and glass separated from Mt Amiata volcanic rocks demonstrate the sanidine is the most suitable phase for K–Ar dating. Sanidine yields ages of 304–293 kyr for the basal trachydacitic unit (BTC), 298–280 kyr in the domes unit (DLC) and unexpected older ages of 312–308 kyr for the more mafic summit lava unit (OLL). A careful re-examination of the literature ages together with those obtained in this study shows that they tend to a common age of ca. 300 kyr whatever the volcanic unit. We interpret this as a reset of the K–Ar chronometer in response to a consequent recharge of the silicic magma reservoir by hot mafic melts. This recharge most probably triggered the first volcanic eruption of Mt Amiata magmas. In our model, we suppose an initially chemically-stratified magma chamber; the input of deep hot mafic melts reset the crystals clock and probably allowed the eruption of the huge amount of trachydacitic crystal mush. We propose that the controversial BTC unit could have emplaced during a non-explosive eruption if we consider either pre-eruption passive degassing or extrusion of the trachydacites as magmatic foam. First Pb isotopic data of mafic enclaves from the trachydacitic units, together with major and trace elements and new Sr and Nd data support the magma mixing as the dominant process at the origin of the Mt Amiata volcanic rocks. The similar LILE/HFSE ratios evidenced in this contribution between the magmatic enclaves of Mt Amiata and RMP volcanic rocks, together with their comparable Sr, Nd and Pb isotopic compositions, definitively argue for the involvement of a RMP-like melt in the mixing. The Mt Amiata is thus indisputably a hybrid volcano between TMP and RMP in terms of petrogenesis and ages.