Strain localisation in bimineralic rocks: Experimental deformation of synthetic calcite–anhydrite aggregates

Deformation of synthetic calcite–anhydrite aggregates to large shear strains (up to γ = 12.4 at 600 °C, 300 MPa confining pressure and a constant angular displacement rate corresponding to a shear strain rate of 10^− 3 s^− 1) resulted in the first experimental observation of strain localisation from initially homogeneous rocks. In contrast to experiments on pure calcite and anhydrite, which deformed homogeneously to large strains (γ ≥ 5), all experiments on calcite–anhydrite mixtures resulted in heterogeneous deformation at γ > 1 and the formation of narrow localised bands in the microstructures at γ > 4. In these bands, the amount of strain is at least twice as large as in the rest of the sample and individual grains of the same phase cluster and align, thereby forming microstructural layering similar to planar fabrics in natural mylonites. A switch in deformation mechanism in anhydrite from dislocation creep to diffusion creep and/or grain boundary sliding occurs simultaneously with strain localisation. It is concluded that deformation-induced heterogeneous phase distributions cause local strength differences initiating strain localisation in the calcite–anhydrite mixtures. The study suggests that the presence of two phases in combination with a change in deformation mechanism may be responsible for strain localisation in natural poly-mineralic mylonites.     

A multi-disciplinary study of the 2002–03 Etna eruption: insights into a complex plumbing system

The 2002–03 Mt Etna flank eruption began on 26 October 2002 and finished on 28 January 2003, after three months of continuous explosive activity and discontinuous lava flow output. The eruption involved the opening of eruptive fissures on the NE and S flanks of the volcano, with lava flow output and fire fountaining until 5 November. After this date, the eruption continued exclusively on the S flank, with continuous explosive activity and lava flows active between 13 November and 28 January 2003. Multi-disciplinary data collected during the eruption (petrology, analyses of ash components, gas geochemistry, field surveys, thermal mapping and structural surveys) allowed us to analyse the dynamics of the eruption. The eruption was triggered either by (i) accumulation and eventual ascent of magma from depth or (ii) depressurisation of the edifice due to spreading of the eastern flank of the volcano. The extraordinary explosivity makes the 2002–03 eruption a unique event in the last 300 years, comparable only with La Montagnola 1763 and the 2001 Lower Vents eruptions. A notable feature of the eruption was also the simultaneous effusion of lavas with different composition and emplacement features. Magma erupted from the NE fissure represented the partially degassed magma fraction normally residing within the central conduits and the shallow plumbing system. The magma that erupted from the S fissure was the relatively undegassed, volatile-rich, buoyant fraction which drained the deep feeding system, bypassing the central conduits. This is typical of most Etnean eccentric eruptions. We believe that there is a high probability that Mount Etna has entered a new eruptive phase, with magma being supplied to a deep reservoir independent from the central conduit, that could periodically produce sufficient overpressure to propagate a dyke to the surface and generate further flank eruptions.Editorial responsibility: J. Donnelly-Nolan     

Implications of a combined biostratigraphic and palaeomagnetic study of the Umbrian Maiolica Formation

The study of a 275.5 m thick section of white, pelagic limestones occupying the valley of the Fonte del Giordano river on the southern slope of Mt. Montagnola has yielded a biostratigraphically controlled clear magnetic reversal pattern after thermal cleaning. The magnetic stratigraphy of the lower 131 m of the section (Calpionellid zones) is correlatable with the M-sequence of oceanic magnetic anomalies between M-19 and M-14. The reversal stratigraphy of the upper 81.5 m of the section (Radiolaria zone) has also been tied to the oceanic polarity time scale by making linear interpolation for a missing 63 m thickness underneath it.Besides the Fonte del Giordano section two Berriasian outcrops each with a different bedding attitude were studied at Gubbio and near Cagli for tectonic tilt test giving positive results. The mean palaeomagnetic pole position for the Late Jurassic/Early Cretaceous after bedding correction is: Φ = 19.1°, Λ = 288.2°, k = 148.7, α₉₅ = 10.2° (N = 3), confirming the presence of a large swing in the polar path, a common behaviour of apparent polar wandering for the peri-Adriatic region during this time.     

Mantle sources and crustal input as recorded in high-Mg Deccan Traps basalts of Gujarat (India)

Near-primitive picritic basalts in the northwestern Deccan Traps have MgO > 10 wt.% and consist of two groups (low-Ti and high-Ti) with markedly different incompatible element and Nd–Sr–Pb isotope characteristics. Many elemental characteristics of the low-Ti picritic basalts are similar to those of transitional or normal ocean ridge basalts. However, values of ratios like Ba/Nb (13–30) and Ce/Pb (4–11), and isotopic ratios (e.g., ε_Nd(t) + 0.3 to − 6.3, (²⁰⁷Pb/²⁰⁴Pb)_t 15.63–15.75 at (²⁰⁶Pb/²⁰⁴Pb)_t 18.19–18.84, δ¹⁸O_olivine as high as + 6.2‰) are far-removed from ocean-ridge-type values, indicating a significant contribution from continental crust. The crustal signature could represent crustal contamination of ascending magmas; alternatively, it could represent a minor component within the Indian lithospheric mantle of anciently subducted sedimentary material or fluids derived from subducted material. In contrast, the high-Ti picritic basalts are chemically and isotopically rather similar to recent shield lavas of the Réunion hotspot (e.g., ε_Nd(t) + 2 to + 4) and to volcanic rocks along the postulated pre-Deccan track of this hotspot in Pakistan. Neither type of picritic basalt is parental to the voluminous flows comprising the bulk of the Deccan Traps. However, many of the Deccan primary magmas could have been derived from mixtures of a high-Ti-type, Réunion-like source component and a component more similar to, or even more incompatible-element-depleted than, average ocean-ridge mantle.     

Records of environmental and climatic changes during the late Holocene from Svalbard: palaeolimnology of Kongressvatnet

A multi-core, multidisciplinary palaeolimnological study of the partially varved sediment of a deep, meromictic, arctic lake, Kongressvatnet (Svalbard, Western Spitsbergen), provides a record of environmental and climatic changes during last ca. 1800 years. The chronology of sedimentation was established using several dating techniques (¹³⁷Cs, ²¹⁰Pb, varve counts, palaeomagnetic correlation). A multiproxy record of palaeolimnological variability was compiled based on sedimentation rates, magnetic properties, varve thickness, organic matter, geochemistry, pigments from algal and photosynthetic bacteria, mineralogy and biological assemblages (diatoms, Cladocera). The major features recognised in our master core K99-3 include a shift in sediment source and supply (magnetic measurements, geochemistry) probably caused by glaciological changes in the catchment around 38–32 cm core depth (AD 700–820). Additional environmental changes are inferred at 20–18, 8–4.5 and 3–2 cm (AD ca. 1160–1255; 1715–1880; 1940–1963, respectively). During the past ca. 120 years a prominent sedimentological change from brownish-grey, partly laminated silt-clay (varves) to black organic-rich deposits was observed. From AD 1350 to AD1880 the sediment is comprised of a continuous sequence of varves, whereas the earlier sediments are mostly homogeneous with only a few short intercalated laminated sections between AD 860 and 1350. Sedimentation and accumulation rates increased during the last 30 years (modern warming). Pigment concentrations are very low in the lower ca. 32 cm of the core (AD 820) probably because of the high turbidity high energy environment. The high sulphur content in the uppermost 32 cm of sediment has given rise to two horizontally stratified populations of sulphur anaerobic photosynthetic bacteria, as inferred from their specific carotenoids. These bacteria populations are much more abundant during the Little Ice Age (LIA) than during warmer periods (e.g., during the Medieval Warm Period and 20th century). Diatoms are lacking from the core base up to 18 cm (ca. AD 1255); at this level, species indicative of mesotrophic water are present, whereas from 17 cm to the top of the core, oligotrophic taxa such as Staurosira construens/S. pinnata complex dominate, indicating extended ice coverage and more oligotrophic waters during the LIA. The concentration of Cladocera subfossil remains (dominated by Chydorus) are relatively high in the deepest sections (54–32 cm), whereas the upper 32 cm are characterized by a very low concentration of remains, possibly because of the strongly anoxic conditions, and in this upper sediment section rotifer resting eggs become prevalent. We interpret these changes as responses to climate forcing through its impact on glacial melt water, lake ice cover duration and mainly redox conditions in deep water. The observed changes suggest that at least some of our recorded changes may parallel the Greenland Ice core, although our study added more details about the inferred climatic changes. Further aspects are discussed, such as catchment processes, glacial activity, duration of the Medieval Warm Period, the Little Ice Age, local human activity, and limnology.     

Hard X-ray bremsstrahlung produced by electrons escaping a high-temperature thermal source in a solar flare

The problem of production of flare hard X-rays by bremsstrahlung from hot thermal escaping electrons (Skrynnikov and Somov, 1982) in a chromospheric plasma is studied.The Landau kinetic equation is solved near the thermal source of energized electrons in a homogeneous magnetic tube to compute the anisotropic inhomogeneous distribution of the thermal escaping electrons.The intensity and polarization of hard X-rays is also computed and a comparison of theoretical results with observational data is made.On leave from: Istituto di Astronomia, Largo E, Fermi 5, I-50125 Firenze, Italy.     

H2O contents and D/H ratios of nominally anhydrous minerals from ultrahigh-pressure eclogites of the Dabie orogen, eastern China

Garnet and omphacite from ultrahigh-pressure eclogites from the Dabie orogen, eastern China were investigated by Micro-FTIR. The results show that all garnet and omphacite grains contain structural water occurring as hydroxyl (OH), with H₂O contents varying from 14 to 1915 ppm (H₂O wt) and from 105 to 695 ppm, respectively. Within the same sample, the water contents are either homogeneous at the grain scale or vary systematically from higher in the core to lower in the rim. Low water contents at crystal rims possibly result from hydroxyl exsolution after pressure decrease upon rock exhumation. The δD values of omphacites are between −108.4‰ and −114.2‰, and independent of water contents. Heterogeneous water contents of garnet occur at the centimeter scale and fluid mobility during UHP metamorphism was very limited. The estimated whole-rock water content based on mineral H₂O contents is between 260 and 750 ppm, thereby implying that eclogitic rocks formed during continental subduction have the potential to recycle (at least) several hundreds ppm water to mantle depths. The preserved chemical differences likely indicate that the eclogitic rocks resided at mantle conditions for a limited time span, and imply that they were exhumed shortly after subduction. The water released during decompression might represent the early-stage retrograde fluid.     

A simple and efficient procedure for the numerical simulation of wind fields in complex terrain

In spite of recent progress in the prognostic numerical simulation of the atmospheric boundary layer, the explicit simulation of turbulent flows in actual complex terrain is generally still very complicated and time consuming for many environmental applications. In an attempt to develop simpler and more efficient application oriented techniques, although less refined, we propose a multi-step procedure for simulating wind fields. Once obtained the necessary meteorological input, the mass-consistent modelling technique is used to perform high-resolution mean wind flow simulations taking into account recent developments in the atmospheric boundary-layer theory. Besides, a procedure based on a generalisation of the local logarithmic law-of-the-wall over complex terrain is used to estimate the effective parameters characterising the simulated wind profiles. Turbulence intensities and spectral properties are then calculated through the estimated effective parameters, in particular through the effective friction velocity parameter. Finally, time series of the instantaneous velocity field are simulated by the Monte Carlo technique. Two applications of the proposed approach are discussed briefly: the first one is related to a coastal area in southern Italy (the Messina Straits), where the construction of the world’s longest central span bridge is being planned; the second one corresponds to the flow in a mountainous area in northern Italy (the Albenga Airport).