Geochronology of the Plio-Pleistocene Hyblean volcanism (SE Sicily): New 40Ar/39Ar data

The first results of an ongoing geochronological study on the Plio-Pleistocene Hyblean volcanic rocks are presented and discussed. 40Ar/39Ar age determinations are reported for separated groundmass and plagioclase fractions from alkaline and tholeiitic products. 40739Ar age spectra on analysed groundmasses vary from almost ideal to slightly discordant. Obtained ages, even if do not give an exhaustive geochronological picture, reveal that the Plio-Pleistocene volcanic rocks within the Hyblean Plateau are younger going from south (2.4 Ma, Torrente Risicone) to north (1.4 Ma, Vallone del Loddiero). These chronological data, in agreement with the geological observations, let us exclude a possible link of the Hyblean volcanism with a “mantle plume”. The Hyblean Plateau is a stable segment of the African plate; taking into account the NW movement of this plate, a mantle plume would produce a younger volcanism from north to south in this area. It is more likely that lithospheric faults, set out on the Hyblean Plateau following the Africa-Eurasia interaction, have favoured and controlled the volcanism in this Mediterranean sector since Late Cretaceous

Multi-method approach to dating glass: the case of Basiluzzo Islet (Aeolian archipelago, Italy)

A rhyolitic lava flow from Basiluzzo islet (Aeolian Islands), has been analysed with the Fission tracks (FT) and 40Ar–39Ar methods on glass, and with the U/Th method on whole rock to constrain its age and to compare the behaviour of different dating methodologies on glass samples late Quaternary in age. Laser 40Ar–39Ar total fusion analyses were performed on populations of grains. Due to the low yields of radiogenic 40Ar the age data are characterised by very high errors. The weighted average of the ages of the whole population is 55.7 ± 8.7 ka (MSWD = 0.7). The isochron age calculated on all points is 40.6 ± 11.4 ka (MSWD = 0.6), with an initial 40Ar/36Ar ratio of 297.8 ± 1.8; the isochron is characterised by very little spread among points. The procedure named ‘point-counting technique’ was adopted in FT dating. Spontaneous track mean size resulted reduced by around 20% compared to induced tracks, which indicates that the determined FT age, 28.6 ± 3.6 ka, is a reduced age, due to a certain amount of track annealing. For this reason the plateau technique for correcting thermally lowered ages was applied. We determined a plateau age (commonly assumed as a reliable estimate of the glass formation age) of 43.4 ± 7.1 ka. Four sub-samples of whole rock from Basiluzzo lava flow have been analysed using U/Th isochron method. The 238U/232Th and 230Th/232Th activity ratios of sub-samples have been determined by alpha counting and plotted on an isochron diagram. The resulting age is 46 ± 8 ka and the 234U/238U activity ratios are always close to one, demonstrating that no significant processes of alteration have occurred. The relatively high error associated with the age is due to a low fractionation of U/Th ratio in the analysed whole rocks. The ages obtained with different methods, 43.4 ± 7.1 ka (FT plateau age), 40.6 ± 11.4 ka (40Ar–39Ar isochron age of all grains), and 46 ± 8 ka (U/Th isochron) agree at the 1σ level, excluding a Holocene age for this sample. This could be valuable information for the Department of Civil Protection because it seems to mitigate the potential risk for present volcanic activity in the area. All ages are affected by very high analytical errors, which are due to the characteristics of the material analysed. Young ages result in low tracks numbers (FT dating) and barely detectable amounts of radiogenic 40Ar in the presence of high atmospheric contamination (40Ar–39Ar dating). Stratigraphic successions without strict chronologic constraints might however benefit even from age data with low precision

Biological aspects of HIV infection.

In the present paper the authors review the biological properties of the HIV infection. The preferential target of HIV infection is the lymphocyte that expresses the CD4 phenotype. Infection of CD4 subsets leads to a remarkable cytopathic effect that can be blocked by antibodies anti CD4 epitope. Actually the HIV envelope proteins recognize an epitope of the CD4 surface molecules; as consequence the antibodies anti idiotypic directed to CD4 indirectly present the binding of envelope proteins to CD4. The relationship between this retrovirus and the immunological pathogenesis of the disease are also discussed. HIV infection is facilitated in groups affected by alterations of the immune system as drug abusers, haemophiliacs and homosexuals. Secondary immunological abnormalities are consistently pronounced in people seropositive for the virus and affected by a progressive generalized lymphoadenopathy (PGL). The degeneration of primary lymphatic organs causes a reduction of the T-cell response against any antigenic challenge. After the PGL phase the disease shifts on AIDS, that is an irreversible stadium accompanied by the deterioration of the clinical status and by an increase of immunosuppression that favours the arise of tumours. In addiction HIV infection may cause severe central nervous system (CNS) dysfunctions, the HIV associated neurological syndromes include myelopathy, meningitis, encephalitis and peripheral neuropathy. HIV has also been isolated from brain tissue and from cerebral spinal fluid

Biological aspects of HIV infection.

In the present paper the authors review the biological properties of the HIV infection. The preferential target of HIV infection is the lymphocyte that expresses the CD4 phenotype. Infection of CD4 subsets leads to a remarkable cytopathic effect that can be blocked by antibodies anti CD4 epitope. Actually the HIV envelope proteins recognize an epitope of the CD4 surface molecules; as consequence the antibodies anti idiotypic directed to CD4 indirectly present the binding of envelope proteins to CD4. The relationship between this retrovirus and the immunological pathogenesis of the disease are also discussed. HIV infection is facilitated in groups affected by alterations of the immune system as drug abusers, haemophiliacs and homosexuals. Secondary immunological abnormalities are consistently pronounced in people seropositive for the virus and affected by a progressive generalized lymphoadenopathy (PGL). The degeneration of primary lymphatic organs causes a reduction of the T-cell response against any antigenic challenge. After the PGL phase the disease shifts on AIDS, that is an irreversible stadium accompanied by the deterioration of the clinical status and by an increase of immunosuppression that favours the arise of tumours. In addiction HIV infection may cause severe central nervous system (CNS) dysfunctions, the HIV associated neurological syndromes include myelopathy, meningitis, encephalitis and peripheral neuropathy. HIV has also been isolated from brain tissue and from cerebral spinal fluid

Studio petrografico e cronologico di un livello tufitico intercalato nella successione continentale pre-evaporitica dell'Area di Sassa (provincia di Livorno).

A crystal tuff layer about 5 cm thick, was found at Faltona (comune di Bibbona, Livorno) interlayered in clay sediments belonging to the Miocene pre-evaporitic fluvio-Iacustrine succession of the Volterra - Radicondoli - Chiusdino Basin. The crystal tuff is fairly well lithified and is made up of predominant arenitic-grained plagioclase and biotite crystals. The biotite K/Ar dating yelded an apparent age of 8.07 ± 0.11 Ma. According to the most recent literature, no magmatic activity was present at this time in the Tuscan area; nevertheless, the crystal tuff mineral assemblage and its grain size suggest a parentage with the volcanic activity of the nearby Island of Capraia. The finding of a vo1canic deposit in the Tortonian fluvio-Iacustrine succession outcropping west of the "Dorsale Medio-toscana" permitted to draw some considerations about the age of the whole succession and its sedimentation rate