Magnetic anisotropy of granitic rocks of the Bergell massif (Switzerland)

Magnetite as well as ilmenohematite are contained by the Bergell granitic rocks, but the fabric of low field susceptibility is due to the magnetite only and is equivalent to the macroscopic biotite fabric. Microscopic observations and high field anisotropy measurements show that the ilmenohematite trigonal axes are aligned parallel to the directions of minimum low field susceptibility anisotropy. The deflection of the stable direction of natural remanent magnetization caused by the alignment of ilmenohematite is corrected using the susceptibility anisotropy data. The resulting virtual palaeopole position deviates from other Oligocene to Miocene pole positions of stable Europe suggesting that since the time of intrusion some 25 my ago the Bergell massif has been rotated anti-clockwise by an angle of ～30° about a vertical axis.     

Structural and mineralogical evaluation of an experimentally produced impact crater in granite

A crater 30 cm in diameter and 4.4 cm in depth was produced upon impact of an aluminium sphere with a homogeneous granite target. The volume excavated was 748 cm³, the mass ejected 1933 g. The crater geometry is compared with previous laboratory experiments. Mineralogical investigations revealed that shock induced, microscopic fracturing is lowest in the direction of uniaxial compression, followed by a 45° profile. Due to reflections of stress waves at the free surface, the horizontal profile displayed the highest fracture index. Kinking of biotite was very common in samples close to the crater walls (≈ 50 kb). However it faded out at a distance which corresponds to approximately 10 kb. This seems to be the lower pressure limit for the formation of kink bands under shock conditions.     

Aufenthalt der Fische im Bereich von Stauwehren

Ohne Zusammenfassung Der Deutschen Forschungsgemeinschaft sei für die Unterstützung der Arbeit gedankt.     

Debye-scherrer investigations of experimentally shocked silicates

Small ballistic ranges were used to perform controlled laboratory shock experiments on 12 selected silicates [quartz (30–310 kb), oligoclase (30–340 kb), andesine (40–100 kb), olivine (80–500 kb), forsterite (50–150 kb), enstatite (60–150 kb), biotite (10–90 kb), hornblende (50–150 kb, garnet (40–160 kb), kunzite (60–150 kb), beryl (60–140 kb), topaz (60–150 kb)]. At least 4 pressure points per mineral are available. Debye-Scherrer investigations of shocked materials revealed a gradual lattice breakdown of crystalline matter under shock. Individual mineral species behave selectively. Sheet silicates break down very easily, followed by tecto-silicates. Chain-, ino- and ortho-silicates are of considerably higher shock resistance. Depending on the mineral species, the first sign of shock damage is evidenced in the long range order at 20–70 kb. At intermediate pressures (100–200 kb) the long range order is essentially destroyed with the short range order heavily disturbed. At pressures exceeding 300 kb tecto-silicates are completely collapsed. The degree of internal fragmentation is strongly related to shock pressure, thus providing a sensitive tool for absolute pressure calibration of shocked materials. The internal fragmentation is structurally controlled, leading to polycrystalline aggregates of strongly preferred orientation. The grain size distribution of the fragmentation products is highly heterogeneous. The mechanisms leading to fragmentation as evidenced by the X-ray patterns are highly complex. The formation of high pressure polymorphs is discussed. Though application of the new results to naturally shocked rocks may have some limitations, the usefulness of Debye-Scherrer investigations in the study of shocked materials is demonstrated. Lunar Science Institute Contribution, No 2.     

Measurement of nitric oxide and related parameters in the equatorial mesosphere and lower thermosphere

A sounding rocket was launched in March 1982 from Thumba, India, shortly after sunrise. The measurements included the concentration of nitric oxide and ozone, the total ion density and the Lyman-α flux. Hence most parameters important for the formation of the D-region during daytime are available with the exception of solar radiation other than Lyman-α which only becomes important above 95 km. The mutual agreement is satisfactory which adds weight to the measurements.     

Experimental Study on the Solubility of Cr^2 in Olivine,Orthopyroxene and Spinel Solid Solutions

Experiments have been performed on the system MgO-SiO2-Cr-O at 0-2.88 GPa and 1100-1450℃,focusing on the stability of Cr^2 in olivine(O1),orthopyroxene(Opx) and spinel(Sp) and its partitioning between these phases.Analytical reagent grade chemicals,MgO,SiO2,Cr2O3.and Cr were used to make starting mixtures.Excess Cr(50%) was then added in these mixtures to ensure that the resultant phases were in equilibrium with the metal Cr.Flux of BaO B2O3(%) was added for facilitating experimental equilibrium and crystal growth.Cr was used as capsule material.All phases in the product were identified by X-ray and analyzed by electron microprobe,The contents of CrO in the different phases(O1,Opx and Sp)were calculated according to stoichiometry.The obtained results of calculation indicate that Cr^3 in Ol and Opx is negligible.The experimental results show;(a) with increasing temperature and decreasing pressure,Cr^2 solubility in Ol,Opx and Sp increases;(b) with in creasing temperature,the partitioning coefficient of Mg and Cr^2 between Ol and Opx decreases,that between Opx and Sp increases,and that between Ol and Sp remains almost unchanged;(c) the effect of pressure on all partitioning coefficients is negligible.     

P -T-t history of the Lower Austroalpine Nappe Complex in the “Tarntaler Berge” NW of the Tauern Window: implications for the geotectonic evolution of the central Eastern Alps

New petrologic and ⁴⁰Ar/³⁹Ar geochronologic data constrain conditions of Alpine metamorphism along the northwestern border of the Tauern Window. The P-T estimations based on phengite barometry were determined for samples from units of the Lower Austroalpine nappe complex exposed above the Southpenninic interior of the Tauern Window, and from upper parts of the Southpenninic “Bündner Schiefer” sequence. Results suggest that both Mesozoic metasedimentary nappe units (Reckner and Hippold Nappes) and an ophiolitic nappe (Reckner Complex) of the Lower Austroalpine nappe complex have been metamorphosed at pressures between 8 and 10.5 kbar and temperatures around 350 °C. The structurally highest Lower Austroalpine unit (Quartzphyllite Nappe) was not affected by high-pressure metamorphism and records maximum P-T conditions of approximately 4 kbar and 400 °C. Highest parts of the structurally underlying Southpenninic Bündner Schiefer sequence were metamorphosed at intermediate pressures (6–7 kbar). Temperatures increased in all structural units during decompression. Whole-rock ⁴⁰Ar/³⁹Ar plateau ages of silicic phyllites and cherts with abundant high-Si phengites record ages around 50 Ma in the Reckner Nappe, and 44–37 Ma in the Hippold Nappe and Southpenninic Bündner Schiefer sequence. These ages are interpreted to date closely the high-pressure metamorphism. The Lower Austroalpine-Southpenninic border area in the NW Tauern Window appears to have evolved along an indented, fragmented active continental margin where the Reckner Complex represents one of the oldest sections of the Southpenninic (Piemontais) Oceanic tract that was originally situated close to, or even within, the Lower Austroalpine continent. During closure of the Piemontais Ocean, the resultant subduction zone did not entrain components of the Reckner Complex or its cover sequences (Reckner and Hippold Nappes): therefore “Eoalpine” high-pressure metamorphism did not occur. Sequences exposed within the study area were subducted to relatively shallow depths during the last stage of consumption of oceanic crust and immediately prior to final continental collision. Received: 30 July 1996 / Accepted: 7 April 1997     

Magnetostratigraphy of the Permo-Triassic boundary section at Shangsi (Guangyuan, Sichuan Province, China)

Continous marine sedimentation characterizes many Late Permian to Early Triassic sections on the Yangtze terrane in South China. The Permo-Triassic (P/Tr) boundary section at Shangsi (Sichuan Province) consists of limestones intercalated with clays and mudstones which belong to the Wuchiapingian and Changxingian (Upper Permian) and the Griesbachian and Dienerian (Lower Triassic) stages. The P/Tr boundary is formed by a clay horizon with an unusually high iridium concentration. The intensity of natural remanent magnetization is very low with a mean of 0.15 mA m⁻¹. About 40% of the samples contain secondary or unstable magnetization components only, whereas the remaining samples carry a characteristic remanent magnetization thought to reflect the polarity of the geomagnetic field during deposition with sufficient accuracy. Normal and reversed polarity of the characteristic magnetization constitute a pattern of at least six polarity zones, the P/Tr boundary being situated very close to the transition from a reversed to a normal polarity zone. The Shangsi polarity sequence represents part of the Illawarra interval of mixed polarity, the exact beginning of which has still to be determined.     

Remanent magnetization of andesitic and dacitic pumice from the 1985 eruption of Nevado del Ruiz (Colombia) reversed due to self-reversal

The natural remanent magnetization of andesitic pumice emitted during the 1985 eruption of the Nevado del Ruiz volcano (Colombia) has a direction opposite to the present geomagnetic field. The self-reversing mechanism can be re-activated in the laboratory during cycles of heating and subsequent cooling in air and zero magnetic field. Laboratory-produced thermoremanent magnetization is dominated by the same self-reversal process in fields up to several mT. Microchemical, optical and Curie temperature analyses indicate that the ferromagnetic minerals are members of the magnetite-ulvöspinel and hematite-ilmenite series with average compositions of Fe_2.73Ti_0.27O₄ and Fe_1.38Ti_0.62O₃, respectively. In analogy with the magnetic behaviour of synthetically grown antiferromagnetic-ferromagnetic FeMn-FeNi films, the self-reversal can probably be interpreted in terms of an exchange field acting between a Ti-poor canted antiferromagnetic and a Ti-rich ferrimagnetic phase in the hemoilmenite grains.