1,721,065 research outputs found
Problematiche petrologiche e geologiche delle Prasiniti di Punta Bianca (La Spezia).
No full textAim of this note is to analyze geological and petrological problems concerning the P. Bianca Prasinites. They consist of a number of meta-basaltic megebreccias embedded in a Middle Triassic sedimentary sequence. The meta-basalts show geochimical affinity with continental Alkaly-Basalts. The sedimentary sequence is intensively deformed by pressure solution cleavage and isoclinal folding, developed in a westward verging reverse shear zone
The San Venanzo maar and tuff ring, Umbria, Italy: eruptive behaviour of a carbonatite-melilitite volcano.
No full textThe late Pleistocene San Venanzo maar and nearby Pian di Celle tuff ring in the San Venanzo area of Umbria, central Italy, appear to represent different aspects of an eruptive cycle accompanied by diatreme formation. Approximately 6x106 m3 of mostly lapillisized, juvenile ejecta with lesser amounts of lithics and 1x106 m3 of lava were erupted. The stratigraphy indicates intense explosive activity followed by lava flows and subvolcanic intrusions. The pyroclastic material includes lithic breccia derived from vent and diatreme wall erosion, roughly stratified lapilli tuff deposited by concentrated pyroclastic surge, chaotic scoriaceous pyroclastic flow and inverse graded grain-flow deposits. The key feature of the pyroclastics is the presence of concentric-shelled lapilli generated by accretion around the lithics during magma ascent in the diatreme conduits. The rock types range from kalsilite leucite olivine melilitite lavas and subvolcanic intrusions to carbonatite, phonolite and calcitic melilitite pyroclasts. Juvenile ejecta contain essential calcite whose composition and texture indicate a magmatic origin. Pyroclastic carbonatite activity is also indicated by the presence of carbonatite ash beds. The San Venanzo maar-forming event is believed to have been trigered by fluid-rich carbonatite-phonolite magma. The eruptive centre the moved to the Pian di Celle tuff ring, where the eruption of degassed olivine melilititic magma and late intrusions ended magmatic activity in the area. In both volcanoes the absence of phreatomagmatic features together with the presence of large amounts of primary calcite suggests carbonatite segregation and violent exsolution of CO2 which, flowing through the diatremes, produced the peculiar intrusive pyroclastic facies and triggered explosions
CO2 discharge and volcanic risk in Italy
No full textItaly offers a unique opportunity to investigate localised solid/fluid transfer of C towards the surface. Italy emits 2.5-5x1011 mol. a-1 of CO2. Additional evidence for CO2 fluxing in Italy includes diatremes and maars that contain carbonatite, melilitite and mantle nodules. 1020kg of solid C is stored in the deep Earth, while the flux of C-O-H at the surface may catastrophically change. Mantle/core outgassing is considered to have a δ13C of about -5‰. Italian carbonatites have δ13C‰ between -8 and -4, R/Ra up to 7.33 (plume-type values). Fresher Italian carbonatites have δ13C, δ18O and δ11B values of −6.7‰ (-4.8‰ intrusive), +10.2‰ (+10.3‰ intrusive) and −5.9‰, respectively (mantle values). The change from natrocarbonatite to Ca-carbonatite at Oldoinyo Lengai is accompanied by a change in δ18O from +6.5‰ to +25‰, while δ13C remains unchanged. Nyerereite inclusions from Italian carbonatites suggest that similar processes contributed to δ18O evolution. Radiogenic mantle end-members, detected in Italian carbonatites, are related to deep alkali-carbonatites and metasomatic reactions with pyrope harzburgite. A specific concourse of geological causes is required to produce the above features, which together imply geologically instant mantle decompression, concentration of volatiles in a melt phase and explosive detachment from the source (diatresis). Magmatic convoy ascent needs to be 10-30 m/sec. to prevent mantle xenoliths settling. The only realistic agent of acceleration is deep-seated concentration of volatiles in fluidisated system with a high proportion of carbon dioxide. The nature of the Italian ultra-alkaline rocks implies that the high concentration of juvenile propellant resulted in extremely violent volcanic activities and that the potential volcanic risk implied by carbonatite occurrences and CO2 explosion along the Apennine structural trends must be re-evaluated in terms of this factor. The proposed carbonatite eruption triggering mechanism may generate LP supersonic shocks in the mantle, which would be possibly recognised as a precursor of surface eruption
The Sirente crater, Italy: impact versus mud volcano origins
No full textThe Sirente crater is a circular structure with a diameter of 80 m. The rim deposit is an inverse-graded, matrix-supported breccia. Sedimentological features of the rim deposit suggest that the crater is not related to an explosion or violent mechanical displacement. The structure and texture of the deposit exhibit a primary sedimentary character. The rim deposits do not contain artifacts and do not show evidence of reworking. A multistage formation is reconstructed for the rim growth and associated deposits. The geometry and sedimentology of the deposits indicate that they were produced by the extrusion and accumulation of mudflow deposits. The dominant ejection mechanism was low mud fountains and the transport medium was water. Petrographic and geochemical evidence does not indicate any physical or cryptic trace of an extraterrestrial body.The most realistic agent that explains the observed effects is a rapid local emission of mud and/or water. Geological processes capable of producing these features include piping sinkholes or, more probably, "caldera"-type mud volcanoes, which may result from underground water-table perturbation and/or decompression of deep CO2/hydrocarbon gas reservoirs due to tectonic deformation or faulting activity during a seismic event. In both cases, the name "crater" for this geological form may be maintained, but there is no compelling evidence for an impact origin.In this paper, the scientific literature on the Sirente crater is reconsidered in the light of new morphological, sedimentological, geochemical, and archaeological data. A new mechanism is proposed involving mud-fountaining
Environmental Distribution of the Radon in a Heavily Populated Area: Preliminary Hazard Evaluation and Inference on Risk Factors in Pescara, Central Italy
No full textThe presence of ionizing sources is a high-risk condition if related to a poor management of the hygiene and health of
the anthropic environment. Increased hazard derives from the addition of artificial sources to natural sources and the
consequent possible late occurrence of epidemic cancer. Therefore, the expenses for medical treatments and potential
losses of human lives are thought to be relevant. Although the role of natural exposure is still poorly assessed, it is reasonable
that it accounts for a chronic hazard, while the artificial one may constitute an acute hazard. In theory, the
medium and large-scale monitoring of the Radon is simple and can be applied in detail to sensible targets. However,
mitigation of Radon risk is particularly complex due to the intrinsic structural vulnerability of the urban environment
and the general lack of epidemiological data that constrain the extent of specific biological damage. In Italy was suggested
a limit to the exposure in working place, instead limits for other private and public facilities are not well established.
Despite legal advice, the sensitivity of the social system is low due to the elusive nature of the Radon hazard, and
the case considered in this paper account for unpreparedness of the Sanitary and Environmental Authorities when facing
to a possible crisis. A monitoring field survey revealed Radon concentrations of at least three times higher than that
expected geologically in a fairly localized area of Pescara, Central Italy. The values are about 25 - 30 times the maximum
allowed in the buildings. However, these measures are underground and average indoor values in the area were
still acceptable. The measures repeated after a year confirms an upward tendency of the previous values. However, it
was not possible to go deeper in the investigation about the nature of this underground anomaly because of the strong
opposition of some members of the Environmental and Sanitary Authorities. Some rumours filtered by one of this Institution,
suggesting a possible correlation of the anomaly with the uncontrolled disposal of radio-iridium needles used in
the nearby hospital. A further legal action instructed against the Author discouraged the publication of the data so far.
This account for a situation of increased risk. Even if hazardous natural Radon emissions can be investigated, it is difficult
to evaluate vulnerability factors related to non-natural diffusion of radio-nuclides progenitors of the Radon (i.e.
uranium and radium). Confidence on notional calculation of the hazard by means of algorithms, decreases the alert
threshold and promotes the potentially involved authorities to discourage further studies. This increases the vulnerability
of the system. Due to negligence and violation of safety norms in Italy, accidents involving ionization agent dispersion
in the environment are likely and are an instructive study case. The result of this study may promote mitigation
actions and, hopefully, a decrease of the radioactivity risk in a populated area. This paper is intended as a case history
depicting unexpected Radon distribution in a city. In these conditions, the density of population and the system unawareness
contribute greatly to raise the risk especially if a natural explanation could not find. The suspect of an artificial
source, far more hazardous than natural Radon itself, is still up for the investigated area
Alkaline and ultramafic lamprophyres in Italy: Distribution, mineral phases, and bulk rock data
No full textSeveral lamprophyre outcrops occur in Italy paralleling the European cycles. Lamprophyre occurrences from the lower Cretaceous to the Oligocene are comprised primarily of dykes and rarely lavas, generally emplaced in isolation. They are ultramafic or alkaline lamprophyres located in eight different places ranging from the eastern Alps to the areas of Tuscany, Sardinia, Abruzzi, and Puglia and exhibit similar geochemistry, suggesting that they originated from the same mantle source, despite different tectonic conditions. Lamprophyres relate to partial melting of a mantle source rich in large-ion lithophile-elements (LILEs) and C-O-H. Distribution of high-field-strength elements (HFSEs) and their ratio to LILEs depends upon the presence of specific scavenger phases, which makes the geochemistry of these and similar rocks very different with respect to that of basalts. Italian Lamprophyres preserve different specific isotopic features that can be described in term of mixing of two mantle end-members, one of which is highly radiogenic. Lamprophyre magma emplacement should occur during structural extension after major compression episodes. However, this tectonic model seems much too simple for the Italian lamprophyres and is inconsistent with the Mediterranean Tethys geodynamic evolution. On the other hand, Italian lamprophyres evolved into lamproites after the Lower Oligocene era and finally into leucitites and kamafugites (plus carbonatites), ultrapotassic rocks which could be considered anhydrous, petrologic equivalents of lamprophyres. Certainly this requires very specific source conditions, possible related to a mature stage of mantle metasomatisms triggered by repeated episodes of (alkaline) carbonatite invasion, melt extraction, and upper mantle decompression. Italian lamprophyres demonstrate, with their long-term constant geochemistry and isotopic features, that the metasomatic agent is unrelated in time and space to the relatively shorter subduction phases of the Mediterranean Sea area and better fits with the presence of a large, long-lived, pulsing deep plume
CO2 magmatism in Italy: from deep carbon to carbonatite volcanism
No full textThe importance of CO2 in the Italian mantle and magmatism is supported by the presence of Middle-Upper Pleistocene carbonatites along the Italian Apennine graben-systems. Carbonatites are co-eruptive and chemically conjugate to kamafugites (kalsilite melilitite or foidite). Immiscibility phenomena largely explain the genetic relationship between the two rock-types. Their peculiar geochemistry reflects the differing solubility of the high field strength elements in rocks which have different peralkalinities. Dolomite and calcite dominated inclusions occur in mantle nodules and plutonic rocks such as soviets and melilitolites, which have been brought up by extrusive carbonatites. Nyerereite was found in melilite and apatite inclusions although the Italian extrusive carbonatites are always calcitic. Sequestration by abundant leucite, kalsilite, haüyne and nepheline, and dispersion of alkalis in subvolcanic aqueous fluids, may explain the low alkaline character of Italian carbonatites. The high Ca, CO2, F, S, Cl content of primitive melt inclusions in high pressure crystals suggests that these elements were not assimilated by the magma from the crust during its ascension towards the surface. In addition, the high radiogenic isotopic composition and compatible element content are regarded as primary magmatic characteristics. Veined mantle nodules show pervasive reaction with carbonatitic melts as illustrated by the presence of amphibole, fassaitic cpx and phlogopite phases. Metasomatising carbonatitic melts are thought to be released from a deep plume. The geodynamic and geochemical setting of Italian carbonatites implies a very deep source for heavy Carbon and it has even been speculated that this Carbon could be originated from the Earth's core
- …
