39Ar−40Ar dating of multiply zoned amphibole generations (Malenco, Italian Alps)

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 Na^M4, 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-Na^M4 amphiboles have higher isotopic ages than low-Na^M4 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. Received: 1 March 1999 / Accepted: 18 August 2000     

Shock wave phenomena in collisions between a current loop and a plasmoid

We present the results of 3-D MHD simulations of collisions between an equilibrium current and a plasmoid. Three typical equilibrium configurations were analyzed. Our simulation results show that when a plasmoid approaches a current loop, an active region is created in front of the plasmoid bounded on the front side by a bow shock wave and on the back side by a reverse shock wave. The collision process modifies the current system and a strong electric field is also induced in the active region. An additional magnetic field generated by the induced current upsets the initial equilibrium condition. As a result, the whole loop is compressed and heated. We found that when the plasmoid approaches the loop, before reaching it the induced electric field amounts to its maximum value. The current loop is curved under the collision. The core of the plasmoid can not drive into the loop, it is sprung back by a magnetic counterpressure. This collision process between a plasmoid and a current loop may be responsible for the triggering of a solar flare observed by Yohkoh.     

Geochronology and excess Ar geochemistry of the Lhotse Nup leucogranite, Nepal Himalaya

⁴⁰Ar/³⁹Ar dating experiments on several coexisting minerals from two close-by leucogranite outcrops near Lhotse Nup glacier (Nepal Himalaya) reveal a complex behaviour. Four biotite and muscovite ages cluster around 15.5 Ma, a lower value than literature Rb/Sr ages on splits of the same four micas, suggesting a discrepancy with the ideal cooling age sequence observed in the Alps.A strongly discordant Ar-Ar spectrum on tourmaline does not allow a chronological interpretation.A potassium feldspar shows a slow-cooling staircase spectrum with a superimposed saddle diagnostic of excess Ar. HF leaching removed excess Ar but caused great perturbations to the minimum step ages, isochron plots, and the release of reactor-produced Ar isotopes. The present data require that the currently fashionable interpretations of feldspar systematics be radically changed.The main chronological conclusions rely on the eight mica ages. Their decrease from 18.2 to 15.3 Ma dates the cooling of the Lhotse Nup leucogranite.     

Differential gene expression in oyster exposed to sewage

In order to investigate the influence of domestic sewage on the gene expression of Pacific oysters Crassostrea gigas, suppression subtractive hybridization (SSH) method was employed. Oysters were sampled at a farming area and, after 10 days of acclimation in the laboratory, were exposed to untreated domestic sewage diluted 33% for 48 h. Gills of male oysters were excised for total RNA extraction. mRNA was purified and the differential gene expression was analyzed by SSH. We obtained 61 cDNA sequences but only 15 were identified, which includes fatty acid binding protein, multidrug resistance protein, omega glutathione S-transferase, cytochrome P450 isoform CYP356A1, among others. The identified genes are associated with different metabolic functions like biotransformation, membrane transport, aerobic metabolism and translational machinery, evidencing the potential toxic effects elicited by these effluents.     

Attenuation coefficients of Rayleigh and <Emphasis Type="Italic">Lg</Emphasis> waves

Analysis of the frequency dependence of the attenuation coefficient leads to significant changes in interpretation of seismic attenuation data. Here, several published surface-wave attenuation studies are revisited from a uniform viewpoint of the temporal attenuation coefficient, denoted by χ. Theoretically, χ( f) is expected to be linear in frequency, with a generally non-zero intercept γ?=?χ(0) related to the variations of geometrical spreading, and slope dχ/df = π/Q _e caused by the effective attenuation of the medium. This phenomenological model allows a simple classification of χ( f) dependences as combinations of linear segments within several frequency bands. Such linear patterns are indeed observed for Rayleigh waves at 500–100-s and 100–10-s periods, and also for Lg from ~2 s to ~1.5 Hz. The Lg χ( f) branch overlaps with similar linear branches of body, Pn, and coda waves, which were described earlier and extend to ~100 Hz. For surface waves shorter than ~100 s, γ values recorded in areas of stable and active tectonics are separated by the levels of \(\gamma _{D} \approx 0.2 \times 10^{-3}\) s^???1 (for Rayleigh waves) and 8 ×10^???3 s^???1 (for Lg). The recently recognized discrepancy between the values of Q measured from long-period surface waves and normal-mode oscillations could also be explained by a slight positive bias in the geometrical spreading of surface waves. Similarly to the apparent χ, the corresponding linear variation with frequency is inferred for the intrinsic attenuation coefficient, χ _i , which combines the effects of geometrical spreading and dissipation within the medium. Frequency-dependent rheological or scattering Q is not required for explaining any of the attenuation observations considered in this study. The often-interpreted increase of Q with frequency may be apparent and caused by using the Q-based model of attenuation and following preferred Q( f) dependences while ignoring the true χ( f) trends within the individual frequency bands.     

Reflections on frequency dependence in earthquake-source inversions

The corner frequency of the spectrum of particle acceleration in a seismic wave radiated by an earthquake source marks the transition between the low-frequency band, in which the spectrum rises as frequency squared, and the high-frequency band, in which the spectrum is flat. These two distinct bands are controlled by different characteristics of faulting. The low-frequency range is governed by the value of slip alone, while the high-frequency range is controlled by both the total slip and slip velocity. This distinction explains why inversions of geodetic and shorter-period seismic data sense different characteristics of source process and are not generally comparable. Neglect of the sensitivity of seismic data to both slip and its rate may lead to false images of the inverted slip on the fault, as these characteristics trade off with each other. A recent example of the 2011 M _w 9.0 Tohoku-oki, Japan, earthquake demonstrates that the observed “frequency-dependent” variations in the rupture process over the fault plane should not be considered unique to this particular event but rather a natural consequence of the frequency-dependent inversion.     

Non-Equilibrium Distribution Functions in the Heliospheric Interface and Their Relaxation by Local Wave–Particle Interactions

The partially ionized local interstellar medium, before interacting with the heliospheric plasma on the upwind side, most probably undergoes an outer bow shock. After conversion into a sub-magnetosonic plasma flow, it then passes around the heliopause. While the ionized component at the bow shock undergoes abrupt changes of its dynamical properties, the neutral component first continues to flow downstream of the shock with its unperturbed properties. Consequently, the two fluids immediately after the bow shock passage are out of dynamical and thermodynamical equilibrium. Neutral atoms move with a higher bulk velocity and are cooler than the ions. Due to intensive local charge-exchange couplings between neutral atoms and protons these different properties tend to mix each other via momentum and energy exchanges. It turns out that the charge exchange period is shorter than the relaxation period. Hence the distribution functions cannot relax rapidly enough to their highest-entropy forms, i.e. shifted Maxwellians. Here we study the transport processes of newly injected ions in velocity space considering their quasi-linear and non-linear interactions with the ambient MHD turbulence in the plasma interface region. For that purpose we study the turbulence levels in the helio-sheath plasma region. We calculate the expected deviations from equilibrium distributions of ionic and atomic species in the outer heliospheric interface. It clearly turns out from these studies that non-relaxated non-equilibrium distribution functions have to be expected both for O-/H-ions and atoms in this region. This has inherent implications for the diagnostics of interstellar parameters, deduced from observations made further inwards from the interface region.     

Comparison of the antioxidant defense system in Crassostrea rhizophorae and Crassostrea gigas exposed to domestic sewage discharges

Oysters Crassostrea rhizophorae and Crassostrea gigas were kept for fourteen days at four sites in S?o José, SC, Brazil, chosen according to a sewage discharge contamination gradient. Enzymatic activities of CAT, GST, G6PDH and GR were evaluated in gills and digestive glands. Higher CAT activity was observed in tissues of C. rhizophorae in response to sewage contamination possibly indicating peroxisome proliferation induction. C. gigas showed elevated G6PDH activity in digestive gland, and GR in gills, after domestic sewage exposure. In conclusion, C. rhizophorae and C. gigas showed different biochemical responses after in situ exposure to domestic sewage. However, C. rhizophorae showed more significant changes in CAT suggesting that this organism could be a better monitor to this kind of effluent.     

Strontium-90 in surface and groundwater of the reclamation systems at the Chernobyl exclusion zone

The left bank of the Pripiat' river, opposite of the Chernobyl Nuclear Power Plant (ChNPP) is covered with a developed network of drainage canals, built for the reclamation of the swampy floodplain. This area is highly contaminated after the disaster in 1986. Concentrations of the most mobile radionuclide, ⁹⁰Sr, are comparable with ¹³⁷Cs, and reach many MBq m^-2. Their ratio in the upper 10 cm of soil is about 1. The high surface contamination as well as precipitation and flooding have been responsible for the water pollution in this area. Some specifics of ⁹⁰Sr behaviour in surface and groundwater, controlled by the hydrological regime, are described.