99 research outputs found
Transition from mildly-tholeiitic to calc-alkaline suite: the case of Chichontepec volcanic centre, El Salvador, Central America
No full textThe Chichontepec volcano is a Plio-Pleistocene composite volcano that erupted lavas ranging from high-alumina basalts to dacites. It experienced a caldera-forming paroxysmal eruption during the early Pleistocene. Pre-caldera lavas are mildly tholeiitic and they evolved mainly by low pressure crystal fractionation, notwithstanding the fact that most mafic lavas (low-MgO high-alumina basalts) retain traces of polybaric evolution. Conversely, post-caldera lavas, which are mainly pyroxene andesites, are clearly calc-alkaline, having evolved by open-system crystal fractionation. Sr-Nd isotopic data and trace elements characteristics indicate that the same mantle source was involved in the petrogenesis of these series. Modelling the AFC process showed that it did not play any role in the petrogenesis of these rocks; a crystal fractionation model is considered to be more relevant. A slight variation in the fractionating assemblage could have caused the transition from an early mildly tholeiitic trend to a late calc-alkaline one. Mineralogical evidence, mass-balance calculations and elemental chemistry support this hypothesis, assuming that the greater amount of pyroxene on the liquidus is at the expense of plagioclase; this would have prevented the trend in iron enrichment. (C) 1998 Elsevier Science B.V. All rights reserved
Profili di responsabilità medica alla 'luce' della medicina narrativa
No full textTraendo spunto da alcuni riferimenti letterari, il lavoro mira a introdurre gli aspetti più significativi della medicina narrativa, quale sviluppo del filone culturale che suggerisce un confronto costruttivo tra medicina e letteratura. In particolare, si delineano le differenze di approccio e i punti di incontro tra la Narrative Based e la Evidence Based Medicine.
In questa prospettiva il saggio si propone di illustrare quale apporto la medicina narrativa possa offrire al dibattito attorno ad alcune questioni centrali in tema di responsabilità medica: l’ossequio ai contenuti delle linee guida e la rilevanza del consenso informato del paziente.
L’approccio narrativo alla pratica medica, da un lato, sembra poter innalzare gli standard qualitativi della prestazione sanitaria, dall’altro, anche favorendo la realizzazione di una piena alleanza terapeutica tra il medico e il paziente, garantisce un freno alla diffusione di pratiche difensivistiche.Inspired by some literary references, the essay aims at outlining the main features of narrative medicine, such as the development of the cultural trend that suggests a constructive dialogue between medicine and literature. Specifically, the author attempts to define the differences in approach and the overlaps between Narrative Based and Evidence Based Medicine.
Under this perspective, the work seeks to illustrate what kind of contribution narrative medicine may offer to the debate concerning some key aspects of the medical liability field, in particular with regards to the contents of medical guidelines and patients’ informed consent.
Thus, the paper explains how the narrative approach to medical practice could raise the quality standards of the health care service by means of driving toward a full therapeutic alliance between physicians and patients. Among the positive aspects of such a trend, the author highlights how the narrative method put a brake on the spread of medical practitioners’ defensive practices
Evolution of the Magma System of Pantelleria (Italy) from 190 ka to Present.
No full textThe eruptive history of Pantelleria has been marked by the eruption of nine peralkaline ignimbrites, with inter-ignimbrite episodes from small, local volcanic centres. New whole-rock geochemical data are presented for seven ignimbrites and used with published data for younger units to track compositional changes with time. From ∼190 ka, silicic magmatism was dominated by comenditic trachyte to comendite compositions, evolving along generally similar liquid lines of descent (LLOD). The final ignimbrite, the Green Tuff (∼46 ka), was tapped from a compositionally zoned pantelleritic upper reservoir to a trachytic mush zone. Younger (20–7 ka) silicic magmatism has been relatively small scale, with compositions similar to the earliest pre-Green Tuff pantelleritic ignimbrite (Zinedi). These data suggest that the comenditic reservoirs may have been emplaced at deeper levels than the pantelleritic reservoirs. While both types of series evolved along similar LLOD dominated by fractionation of alkali feldspar, it is the fractionation of iron that determines whether comendite or pantellerite is produced. The deeper reservoirs were more oxidizing and wetter, thus leading to the crystallization of magnetite and therefore the fractionation of iron
A window into the high velocity body (HVB) beneath Mt. Etna:a cntribution from the study of cognate xenoliths in historical lavas
No full textMelt inclusion study on the pantelleritic plinian eruption of the Green Tuff, Pantelleria Island.
No full textPantelleria Island is the type locality for the peralkaline rhyolitic rocks called pantellerites. In the last 50 ka, after the Plinian, caldera-forming, Green Tuff eruption, volcanic activity at Pantelleria consisted of effusive and mildly explosive eruptions which mostly vented inside and along the rim of the caldera producing silicic lava flows, lava domes and poorly dispersed pantelleritic pumice fall deposits.
During the last two decades, a wealth of studies focused on melt inclusions in pantellerite magmas, all converging in underlying the H2O-rich character of these melts together with high contents of halogens. Recent study on the volatile content of pantellerites from Pantelleria yielded a pre-eruptive H2O content ≤ 4.9 wt % and P ≤ 1.5 kb [Gioncada e Landi, 2010; Neave et al., 2012] and chlorine up to 0.9 wt% for magmas erupted during the most recent effusive and strombolian activity at Pantelleria. Recent experimental data confirmed the H2O-rich character of these magmas [Di Carlo et al., 2010]. However data on pantelleritic magmas emitted during large explosive eruptions associated with caldera collapses, are up to date sporadic and not exhaustive. Our study is aimed to determine the pre-eruptive volatile contents of the pantellerite magmas erupted in the Green Tuff eruption, the last caldera forming event at Pantelleria.
All melt inclusions analyzed have rhyolitic composition (SiO2 70-72 wt.%, Na2O+K2O 10-11 wt) and show extreme peralkalinity, with an (Na+K)/Al clustering at 2-2.2. Fluorine in MI range between 0.13 and 0.3 wt%, while S (< 350 ppm) is close to, or lower than, the detection limit (200-300 ppm) of the microprobe. Glasses of GT, both MI and matrix glass, are characterized by a chlorine content as high as 1.1 wt%, which remains basically unchanged in the all glass analyzed (between 1 and 1.1 wt%). This value is higher than that found in the younger pantelleritic eruptions of about 0.2-0.3 wt%.
Dissolved H2O contents in MI (measured by FT-IR) cover a range from 1.4 to 4.2 wt.%, which is comparable with the H2O measured in MI from pantelleritic pumices erupted during younger effusive, or mildly explosive, eruptions. The maximum values are slightly lower than the highest content of water found in pantelleritic melts at Pantelleria (4.9 wt%) [Neave at al., 2012]. The CO2 content was always below the detection limit of FT-IR method (50 ppm).
Chemical composition and volatile contents in the pantelleritic melt inclusions of the Green Tuff contributed to:
- derive the entrapment pressure/depth of magma accumulation, using appropriate solubility models;
- compare data derived for the Green Tuff Plinian eruption with data available for effusive and mildly explosive eruption (strombolian), at Pantelleria.
- track volatile build-up with magma evolution, i.e. pre- and syn-eruptive degassing
TEPHROCRONOLOGY WITH 40AR–39AR DATING OF RECENT SILICIC VOLCANISM AT PANTELLERIA ISLAND: IMPLICATIONS ON MAGMATIC SYSTEM.
No full textHigh-resolution 40Ar/39Ar chronostratigraphy of the post-caldera (<20 ka) volcanic activity at Pantelleria, Sicily Strait
No full textThe island of Pantelleria (Sicily Strait), the type locality for pantellerite, has been the locus of major calderaforming
eruptions that culminated, ca. 50 ka ago, in the formation of the Cinque Denti caldera produced by the
Green Tuff eruption. The post-caldera silicic activity since that time has been mostly confined inside the caldera
and consists of smaller-energy eruptions represented by more than twenty coalescing pantelleritic centers
structurally controlled by resurgence and trapdoor faulting of the caldera floor. A high-resolution 40Ar/39Ar
study was conducted on key units spanning the recent (post-20 ka) intracaldera activity to better characterize
the present-day status (and forecast the short-term behavior of) the system based on the temporal evolution of
the latest eruptions. The new 40Ar/39Ar data capture a long-term (N15 ka) decline in eruption frequency with a
shift in eruptive pace from 3.5 ka−1 to 0.8 ka−1 associated with a prominent paleosol horizon marking the only
recognizable volcanic stasis around 12–14 ka. This shift in extraction frequency occurswithoutmajor changes in
eruptive style, and is paralleled by a subtle trend of decreasingmelt differentiation index. We speculate that this
decline probably occurred (i) without short-term variations in melt production/differentiation rate in a steadystate
compositionally-zoned silicic reservoir progressively tapped deeper through the sequence, and (ii) that it
was possibly modulated by outboard eustatic forcing due to the 140 m sea level rise over the past 21 ka. The
intracaldera system is experiencing a protracted stasis since 7 ka. Coupled with recent geodetic evidence of
deflation and subsidence of the caldera floor, the system appears today to be on a wane with no temporal
evidence for a short-term silicic eruption
Petrologic constraints on melting conditions in the Strait of Sicily Rift Zone.
No full textThe Strait of Sicily Rift Zone (SSRZ) is a northwest-southeast trending transtensional rift situated in the Mediterranean Sea between Sicily and north Africa. The SSRZ consists of three basins: the Pantelleria Trough, the Linosa Basin, and the Malta Trough. Volcanoes are situated in or adjacent to all except the Malta Trough, and include two islands (Pantelleria and Linosa) and several seamounts. The thickness of the crust throughout the region is 25-35 km, but thins to 16-18 km in the basins of the SSRZ. The Pantelleria Trough is characterized by high average heat flow values and a strong positive Bourger anomaly, which suggest assthenospheric upwelling to ~60 km and the presence of abundant basaltic material at depth.
Mafic lavas ranging in composition from alkaline and transitional basalt to hawaiite (~45-50 wt% SiO2) comprise the dominant volume of eruptive products in the SSRZ; evolved rocks (trachyte and pantellerite) and common only on Pantelleria, where they crop out over ~94% of the island’s surface. Mafic lavas on Pantelleria are also the most evolved with Mg# ≤ 58, Ni ≤ 98 ppm, and Cr ≤ 183 ppm; more primitive mafic lavas can be found forming the seamounts (Mg# ≤ 67, Ni ≤ 280 ppm, and Cr ≤ 391 ppm) and cropping out on the island of Linosa (Mg# ≤ 71, Ni ≤ 409 ppm, and Cr ≤ 674 ppm) . There are two distinct mafic series on Pantelleria: a generally older (>50 ka) High Ti-P series (“Paleo-Pantelleria”), characterized by TiO2 > 3.0, P2O5 > 0.9, and high REE ratios; and a generally younger (<50 ka) Low Ti-P series (“Neo-Pantelleria”), characterized by TiO2 < 3.0, P2O5 < 0.75, and low REE ratios. Linosa mafic lavas are more similar to the Neo-Pantelleria series, whereas Seamounts may be more similar to either series.
To constrain the melting conditions in the SSRZ we have attempted to estimate the ancestral magma compositions for the most primitive basalts following Hertzberg and O’Hara (2002) and Leeman et al. (2005, 2009) by which equilibrium olivine is incrementally added to basalt until the calculated ancestral basalt has a Mg# shown experimentally to have been in equilibrium with mantle peridotite. From this composition, the temperature of magmatic segregation may be calculated following Sugawara (2000), and the pressure of segretation may be calculated following Alberede (1992). For the purposes of our investigation, we assumed FeO/FeO* = 0.85, with KD for MgO-FeO between olivine and melt calculated following Hertzberg and O’Hara (2002). From our pressure estimates, we inferred depths of segregation assuming a 17 km thick crust with an average crustal density of 2.69 g/cm3 and an average mantle density of 3.20 g/cm3. Results are presented in Figure 1. High Ti-P magmas are the result of partial melting at greater depths (69-75 km), near the spinel-garnet transition zone, whereas Low Ti-P magams are the result of partial melting at shallower depths (52-62 km)
New Inferences on Magma Dynamics in Melilitite???Carbonatite Volcanoes: The Case Study of Mt. Vulture (Southern Italy)
No full textThis study provides the first micro-thermometric data of fluid inclusions (FIs) in mafic loose
(disaggregated) xenocrysts and ultramafic xenoliths in explosive products of the melilitite-carbonatite Mt.
Vulture volcano (southern Italy). Pure CO2 late stage FIs hosted in rock-forming minerals of wehrlite xenoliths
and clinopyroxene xenocrysts were trapped at the local crust-mantle boundary (32 km). In contrast, trapping
pressures within the loose olivine xenocrysts are from 3.2 to 4.5 kbar (8–13 km). Considering the ongoing
degassing of mantle-derived CO2 rich gases, together with seismic evidences of the presence of low amount of
melts at depth, and the tectonic control of the past volcanic activity, our study opens new perspective about the
hazardous nature of the “quiescent” melilitite-carbonatite volcanoe
Pantelleria island (Strait of Sicily): volcanic history and geomorphological landscape
No full textPantelleria is a volcanic island located in the Strait of Sicily, 95 km far from the Sicilian
coastline and 67 km from Cape Bon (Tunisia). The volcanological history of the island
begins approximately 324 ka BP and the last eruptive event was a submarine eruption that
occurred on 1891 A.D. Eruptive activity was characterized by seven very intense explosive
events, the latest being the Green Tuff (44 ka). They have all produced ignimbrite sheets
that covered large sectors of the island. The landscape of the island mirrors the variety of
the eruptive styles and their interplay with volcano-tectonics. The most evident
geomorphological features are represented by: (i) the mantle-like distribution of the Green
Tuff ignimbrite; (ii) the arcuate remnants of the two large caldera collapses, and (iii) the
intracalderic scoria cones, lava domes and lava fields. A very dense distribution of dry
walls, built since Roman times, perfectly integrate the volcanic landscape, preventing from
erosion and rock falls
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