Seyfert galaxies and the cosmic γ-ray diffuse background

Three active galaxies, generally classified as Seyferts, have been discovered recently to be powerful, low energy -ray sources. The similarity between their spectral characteristics and those of the cosmic background at -ray energies suggests that these objects could make a significant contribution to this diffuse flux. This contribution has been assessed using two different number densities of -ray-emitting Seyfert galaxies based on optical and X-ray data. The comparison of the estimated and measured diffuse -ray background intensities is used to gain a deeper understanding of the metabolism of Seyfert galaxies.     

Geochronology of the Larderello geothermal field: new data and the “closure temperature” issue

The Larderello geothermal field is generally accepted to have been produced by a granite intrusion at 4–9 km depth. Hydrothermal parageneses and fluid inclusions always formed at temperatures greater than or equal to the current ones, which implies that the field has always undergone a roughly monotonic cooling history (fluctuations < 40 K) since intrusion of the granite at 4 Ma. The heat required to maintain the thermal anomaly over such a long period is supplied by a seismically anomalous body of 32000 km³ rooted in the mantle. Borehole minerals from Larderello are thus a unique well-calibrated natural example of thermally induced Ar and Sr loss under geological conditions and time spans. The observations (biotites retain Ar above 450°C) agree well with other, albeit less precise, geological determinations, but contrast with laboratory determinations of diffusivity from the literature. We therefore performed a hydrothermal experiment on two Larderello biotites and derived a diffusivity D _Lab(370°C)=5.3·10^-18 cm²s^-1, in agreement with published estimates of diffusivity in annite. From D _Lab and the rejuvenation of the K/Ar ages we calculate maximum survival times at the present in-hole temperatures. They trend smoothly over almost two orders of magnitude from 352 ka to 5.3 ka, anticorrelating with depth: laboratory diffusivities are inconsistent not only with geological facts, but also among themselves. From the geologically constrained lifetime of the thermal anomaly we derive a diffusivity D _G(370°C)=3.81·10²¹ cm²s^-1, 3±1 orders of magnitude lower than D _Lab. The cause of these discrepancies must be sought among various laboratory artefacts: overstepping of a critical temperature T ^*; enhanced diffusivities in wet experiments; presence of fast pathway (dislocation and pipe) diffusion, and of dissolution/reprecipitation reactions, which we imaged by scanning electron microscopy. These phenomena are minor in geological settings: in the absence of mineral transformation reactions, complete or near-complete resetting is achieved only by volume diffusion. Therefore, laboratory determinations will necessarily result in apparent diffusivities that are too high compared to those actually effecting the resetting of natural geochronometers.This word is dedicated to the memory of Aldo Valori (1958–1991)     

Time calibration of a PT-path from the Western Tauern Window,Eastern Alps: the problem of closure temperatures

New Hornblende K-Ar and ³⁹Ar-⁴⁰Ar and mica Rb-Sr and K-Ar ages are used to place specific timemarks on a well-constrained pressure-temperature path for the late Alpine metamorphism in the Western Tauern Window. After identification of excess ⁴⁰Ar, the closure behavior of Ar in hornblende is compared with that of Sr and Ar in phengite and biotite. Samples were collected in three locations, whose maximum temperatures were 570° C (Zemmgrund), 550° C (Pfitscher Joch), and 500–540° C (Landshuter Hütte).The average undisturbed age sequence found is: Phengite Rb-Sr (20 Ma)>hornblende K-Ar (18 Ma)>phengite K-Ar (15 Ma)>biotite Rb-Sr, K-Ar (13.3 Ma)>apatite FT (7 Ma). Except for the phengite Rb-Sr age, the significance of which is debatable, all ages are cooling ages. No compositional effects are seen for closure in biotite. Additionally, Rb-Sr phengite ages from shearzones possibly indicate continuous shearing from 20 to 15 Ma, with reservations regarding the validity of the initial Sr correction and possible variations of the closure temperatures. The obviously lower closure temperature (T _c) for Ar in these hornblendes than for Sr in the unsheared phengites indicates that the T _c sequence in the Western Tauern Window is different from those observed in other terrains. In spite of this discrepancy, valuable geological conclusions can be drawn if the application of closure temperatures is limited to this restricted area with similar T, P and : (1) All ages of samples located on equal metamorphic isotherms decrease from east to west by about 1 Ma which is the result of a westward tilting of the Tauern Window during uplift. (2) In a PT-path, the undisturbed cooling ages yield constantly decreasing uplift rates from 3.6 mm/a to 0.1 mm/a. (3) Use of recently published diffusion data for Ar in hornblende (T _c=520° C) and biotite (T _c=320° C) suggests an extrapolated phengite closure temperature for Sr at 550° C. This suggests that the prograde thermal metamorphism at this tectonic level of the Tauern Window lasted until some 20 Ma ago.     

A new source of isotopically heavy sulphur in lode-gold deposits of NW Spain: a batch volatilization process in the conversion of pyrite to pyrrhotite by contact metamorphism

The conversion of pyrite to pyrrhotite is identified in this study as a new source of isotopically heavy sulphur in thermal domes. Contact metamorphism of a sequence of pyrite-rich shales by the Penedela blind intrusion (NW Spain) causes the conversion of diagenetic pyrite to pyrrhotite at a temperature of 150 °C. This process occurred in a closed system with total consumption of the diagenetic pyrite and production of H₂S-rich fluids. The H₂S liberated in this unidirectional reaction shows a relatively heavy δ³⁴S, in agreement with a batch volatilization process. The mixture of this H₂S-rich fluids with derived sulphur from Lower Cambrian carbonates produced gold-bearing pyrite at a temperature of 340 °C. The contribution from these two different sulphur sources was 86% and 14%, respectively. The metamorphic fluids migrated through permeable zones such as fractures and shear zones, producing a lode-gold deposit.     

Chronostratigraphy of Monte Vulture volcano (southern Italy): secondary mineral microtextures and <Superscript>39</Superscript>Ar-<Superscript>40</Superscript>Ar systematics

The eruptive history of Monte Vulture has been the subject of several geochronological investigations during the past decades, which reliably dated only a small number of eruptions. Understanding the causes of sub-optimum data yield in the past requires an interdisciplinary approach. We re-analyzed samples from previous works and present new data on samples from the main volcano-stratigraphic units of Monte Vulture, so as to provide an improved, consistent chronostratigraphic database. Imaging of minerals by cathodoluminescence and backscattered electrons reveals that heterochemical, high-temperature deuteric reaction textures are ubiquitous. Such observations are common in metamorphic rocks but had not frequently been reported from volcanic rocks. In view of the mineralogical complexity, we base our chronological interpretation on isochemical steps, defined as steps for which the Cl/K and/or the Ca/K ratios are constant. Isochemical steps carry the isotopic signature of chemically homogeneous mineral phases and therefore allow a well-constrained age interpretation. Comparison of old and new ³⁹Ar-⁴⁰Ar data proves the reproducibility of age spectra and their shapes. This quantifies the analytical reliability of the irradiation and mass-spectrometric analyses. Anomalous age spectra are a reproducible property of some specific samples and correlate with mineralogical anomalies. The present data allow us to fine-tune the age of the volcanostratigraphic units of Monte Vulture during the known interval of main volcanic activity from ca. 740 to 610 ka. After a very long stasis, the volcanic activity in the Monte Vulture area resumed with diatremic eruptions, one of which (Lago Piccolo di Monticchio, the site of a palynological-paleoclimatological drilling) was dated at ca. 140 ka.     

The gas scintillation proportional counter in the spacelab environment: In-flight performance and post-flight calibration

We describe the in-orbit performance of the gas scintillation proportional counter which formed part of the Spacelab-1 payload. Discontinuities in the instrument gain are observed (similar to those of the xenon-filled GSPC's on the EXOSAT and TENMA satellites). A post-flight recalibration of the instrument was performed using synchrotron radiation, which found the discontinuities to be coincident with the xenonL edges.     

The evolution of latera caldera (central Italy) in the light of subsurface data

Subsurface geothermal exploration has considerably added to our knowledge of the Latera volcanic complex. A syenitic body is located about 2 km below the present-day surface; K-Ar data point a 0.9 Ma age. The primary magma was a silica-saturated trachyte; undersaturated, hauyne-bearing products are found near the carbonatic wall-rocks and have been interpreted as reaction products. Subsurface data from deep drilling and geophysical surveys suggest that the Latera caldera resulted from three main successive collapse phases: (i) formation of an old caldera, now buried, related to the eruption of ignimbrites from the syenitic magma chamber; (ii) sinking of the eastern sector as a consequence of the formation of the nearby Bolsena caldera (0.3 Ma); (iii) multistage formation of the present Latera caldera (0.16 Ma).