ALBERT

Notes: Abstract 40Ar/39 Ar stepwise heating on one hydrothermal anhydrite and two partly hydrothermalized feldspars from a borehole in Vulcano volcano show that the initial trapped Ar does not have a constant isotopic composition. The constant 40Ar/36Ar ratio of the anhydrite, 306±3, is not a well-defined endmember for the two feldspars, which record a variety of fluid compositions. As the system is young (〈100 ka), radiogenic Ar is much less than excess Ar.

Notes: A geochronological investigation of two rocks with an eclogitic assemblage (omphacite-garnet-quartz-rutile) from the High Himalaya using the Sm/Nd, Rb/Sr, U/Pb and Ar/Ar methods is presented here. The first three methods outline a cooling history from the time of peak metamorphism at 49±6 Ma recorded by Sm/Nd in garnet-clinopyroxene to the closure of Rb/Sr in phengite at 43±1 Ma and U/Pb in rutile at 39–40 Ma. The Sm/Nd isotopic system was fully equilibrated during eclogitization and has not been disturbed since; its mineral ages may date the peak metamorphic conditions (650±50°C at 13–18 kbar: Pognante and Spencer, 1991). The Ar/Ar data reveal the presence of substantial amounts of excess 40Ar in hornblende, and yield a statistically acceptable but geologically meaningless phengite plateau age of 81.4±0.2 Ma, inconsistent with Sm/Nd, Rb/Sr and U/Pb. This questions the use of such a chronometer for the dating of high-pressure assemblages. The results imply a Late Palaeocene or Early Eocene subduction of the northern Indian plate margin in NW Himalaya. The fact that eclogites are restricted to NW Himalaya may be the result of a peculiar p-T-t path associated with a high convergence rate during the first indentation, in contrast to the later and slow subduction in Central and Eastern Himalaya.

Notes: The inception and growth of the active Carpino-Le Piane Basin Fault System (CLPBFS; central-southern Apennines, Italy) was analysed with respect to the neighbouring Isernia and Surrounding (ISFS) and Boiano Basin (BBFS) extensional Fault Systems. 39Ar–40Ar dating showed that the BBFS was already active 649 ± 21 ka bp and that the ISFS was active at least 476 ±10 ka bp, whereas the activity of the CLPBFS started certainly later than 253 ± 22 ka bp, and very probably as recently as 〈28 ka bp. These ages, combined with structural data (geometry and kinematics of the fault systems), indicate that the inception and development of the CLPBFS could be strictly related to the stress changes caused by earthquakes occurring on the BBFS.

Notes: Abstract 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.

Notes: Abstract Mafic rocks of a Permian crust to mantle section in Val Malenco (Italy) display a multi-stage evolution: pre-Alpine exhumation to the ocean floor, followed by burial and re-exhumation during Alpine convergence. Four prominent generations of amphiboles were formed during these stages. On the basis of microstructural investigations combined with electron microprobe analyses two amphibole generations can be assigned to the pre-Alpine decompression and two to the Alpine metamorphic P–T evolution. The different amphiboles have distinct NaM4, Ca, K and Cl contents according to different P–T conditions and fluid chemistry. Analysing these mixed amphiboles by the 39Ar−40Ar stepwise heating technique yielded very complex age spectra. However, by correlating amphibole compositions directly obtained from the electron microprobe with the components deduced from the release of Ar isotopes during stepwise heating, obtained ages were consistent with the geological history deduced from field and petrological studies. The two generations of pre-Alpine amphiboles gave distinguishable Triassic to Late Jurassic/Early Cretaceous ages (≈225 and 130–140 Ma respectively). High-NaM4 amphiboles have higher isotopic ages than low-NaM4 ones, in agreement with their decompressional evolution. The exhumation of the Permian crust to mantle section is represented by the former age. The latter age concerns Cl-dominated amphibole related to an Early Cretaceous oceanic stage. For the early Alpine, pressure-dominated metamorphism we obtained a Late Cretaceous age (83–91 Ma). The later, temperature-dominated overprint is significantly younger, as indicated by 39Ar−40Ar ages of 67–73 Ma. These Late Cretaceous ages favour an Adriatic origin for the Malenco unit. Our data show that 39Ar−40Ar dating combined with detailed microprobe analysis can exploit the potential to relate conditions of amphibole formation to their respective ages.