Low- and high-alumina komatiites from a Late Archaean sequence,Newton Township,Ontario

Late Archaean komatiitic lavas from Newton Township, Ontario, consist of 6 chemically distinct magma types: 3 komatiites and 3 komatiitic basalts. The succession is unusual in containing both Al- and HREE-depleted komatiites and Al- and HREE-undepleted komatiites. The two types form distinct stratigraphic units separated by komatiitic basalts. Two komatiite types are strongly LREE depleted, whilst the third and the associated komatiitic basalts range from mildly depleted to enriched. Of the six magma types, only the two strongly LREE depleted komatiites represent primary mantle melts. The other komatiite type and the komatiitic basalts were derived from the primary komatiite magmas by combinations of olivine (+chromite) fractionation, assimilation of continental crust, and magma mixing. The two primary magmas may have been derived from similar sources, their contrasting chemistry being due to differing degrees of garnet segregation during melting. A generally applicable conclusion is that a wide range of komatiitic magma types can be generated from a relatively homogeneous depleted mantle, under conditions likely to prevail during the eruption of late Archean greenstone belt sequences.     

Cretaceous black shales as active bioreactors: A biogeochemical model for the deep biosphere encountered during ODP Leg 207 (Demerara Rise)

A transport-reaction model was designed to identify the combination and importance of biogeochemical processes operating in four sites drilled during ODP Leg 207 (Demerara Rise, Equatorial Atlantic). Almost 100 Ma after their deposition, deeply buried Cretaceous black shales still act as active bioreactors in great sediment depths and control the biogeochemical reaction network of the whole sediment column. According to a model calibrated at the four drill sites through inverse modeling techniques, methanogenesis could be identified as a key process that dominates not only organic matter degradation but also sulfate availability through the anaerobic oxidation of methane above the black shales. A complete depletion of sulfate within the black shale sequences promotes the remobilization of biogenic barium that reprecipitates as authigenic barite at the top of the sulfate depletion zone. Temporal dynamics of degradation processes caused continuous shifts of the barite precipitation zone during burial, thus inhibiting the formation of an authigenic barite front or causing the dissolution of earlier formed fronts. Major deviations of pore water sulfate profiles from a linear gradient coincide with depths of decelerated or accelerated transport caused by local porosity minima or maxima. Model-determined reaction rates are by far lower than those found in shallower sediments due to the low metabolic activities that are characteristic for the Deep Biosphere. But even after almost 100 Ma, changing organic matter quality still influences the degradation within the black shale sequences, as it is indicated by model results.     

Modeling and Simulation of Supercavity with Inertial Force in the Horizontal Curvilinear Motion

To make a curvilinear motion in the horizontal plane is one of the most contents for realizing the maneuverability of the supercavitating vehicle. It is significant to achieve the controllability and maneuverability of the vehicle in three dimensions both theoretically and practically on research. Models of angle of attack, gravity and inertial force effects on the supercavity in the horizontal curvilinear motion are established, respectively. The supercavity is simulated based on these models in combination with Logvinovich model and the unsteady gas-leakage rate model at the given ventilation rate, and the effect of the inertial force on it is analyzed numerically. Results show that the maximum deviation of the center line of the cross section of supercavity towards the outward normal direction of its trajectory increases as the cavitation number or curvature radius decrease and always occur in the tail because of the increase of inertial effects along the axis of supercavity from the cavitator when other models and flow parameters are constant for the given trajectory curvature. For the variable curvature, the supercavity sheds due to its instability caused by the time-varying angle of attack. The deviation increases along the length of supercavity if the curvature remains the same sign.     

Diaplektische Gläser in den Breccien des Ries von Nördlingen als Anzeichen für Stoßwellenmetamorphose

Two kinds of glasses are to be found in the breccias of the Ries basin both which have been generated by shock wave action on the rocks of the crystalline basement: a) Normal glasses, containing flow structures and vesicles; they are formed by shock waves of high energy which after unloading, leave behind material, the temperature of which is above the melting point. b) Diaplectic glasses without vesicles and flow structures, preserving the grain boundaries, cleavages and twin lamellae of the primary minerals; they are formed by shock waves of lower energy which destroy the crystal lattice but after unloading, leave behind material, the temperature of which is below the melting point. — Measurements have been taken of the density and index of refraction of diaplectic quartz- and plagioclase-glasses. The values obtained proved to be higher than those of the normal glasses and lower than those of the crystalline phases. Diaplectic glasses are distinguishable from normal glasses by their physical properties. They represent intermediate stages of structural order between the crystalline and normal glass phases.     

Petrogenesis of picrites from the Caribbean Plateau and the North Atlantic magmatic province

We studied the petrography, mineralogy and geochemistry of picrites from three different regions: the island of Curaçao which forms part of a Cretaceous oceanic plateau; Iceland, an active hot spot on the mid-Atlantic ridge; and the early Tertiary volcanic margin off the coast of Greenland, which formed during the rifting that created the Atlantic ocean. Using the compositions of olivine phenocrysts and relations between MgO and FeO, Al₂O₃ and Ni, we estimated compositions of parental liquids and the proportion of accumulated olivine in each rock. Picrites from Curaçao formed mainly from a liquid with 12 wt.% MgO and they contain up to 55 wt.% excess olivine in the form of phenocrysts. A small proportion of more forsterite-rich olivine grains are xenocrysts from a more magnesian source. Picrites from Iceland formed from a slightly less magnesian liquid but one with also about 12 wt.% MgO and they contain both olivine and plagioclase in the accumulated assemblage. Picrites from the Greenland volcanic margin formed from a liquid that was distinctly more magnesian, with up to 20 wt.% MgO. In some of these rocks the proportion of accumulated olivine was minimal and in these the whole-rock composition is roughly equivalent to the liquid composition. The picrites from the three areas formed under different conditions and through contrasting melting processes. The Curaçao picrites derive from pooled liquids formed through moderate degrees of melting at moderate depths beneath a relatively old and thick oceanic lithosphere. Iceland picrites, on the other hand, formed through advanced fractional melting of mantle that ascended almost to the base of the crust at the mid-ocean spreading center. An unusual combination of relatively high concentrations of incompatible trace elements and high MgO indicates that the Greenland picrites formed by relatively low degrees of melting at greater depths in the mantle.     

Pb and Nd isotopic study of two archean komatiitic flows from Alexo,Ontario

Isotopic analysis of two Archean komatiitic flows from Alexo, Ontario, gives a Pb-Pb isochron age of 2690 ± 15 Ma and a Sm-Nd isochron age of 2752 ± 87 Ma. These ages agree well with U-Pb zircon ages from underlying and overlying volcanics. The variations in element ratios that define the isochrons were not produced during crystallization of the lavas. The spread in U/Pb was caused by submarine alteration soon after eruption, and the spread in Sm/Nd resulted from (a) differences in the composition of the residue of melting, and (b) contamination of the upper komatiite flow through thermal erosion of the lower flow.The ¹⁴⁷Sm/¹⁴⁴Nd ratio of uncontaminated komatiite is 0.25 which reflects the depleted nature of its mantle source. The Th/U ratio of about 3.4 is probably also representative of depleted mantle. The initial ?_Nd of +2.44 ± 0.51 indicates that the mantle depletion took place long before magma formation.     

Geophysical investigations of large landslides in the Carnic Region of southern Austria

The area under investigation for the past two decades is in the vicinity of the Gailtal lineament, which is the most dominant tectonic feature of the eastern Alps of southern Austria. An area of about 8 km² is in a state of constant instability, as documented by movement of road tracks of several centimetres per year. Geotechnical and surveying techniques have been used to measure these movements in the past but without solving the problem of the mechanism of these failure processes. Geophysical methods (seismic refraction, geoelectrics, and electromagnetics) were applied in order to determine the validity of one of the discussed movement models. In-situ velocity measurements were used to identify different lithologies beneath surficial talus deposits. The thickness of these talus deposits, of about 4–30 m, found by seismic refraction clearly demonstrates that huge ‘blocks’ (i.e. more or less undisturbed lithologic units) within the talus/debris are in close contact with the basement. This basement, which shows lower seismic velocities in different parts combined with low electric resistivities, is obviously strongly disturbed by different failure surfaces. The different gliding velocity of the blocks and the talus/debris deposits leads to a geological model in which huge rock blocks move slowly in relation to the disintegrating basement, whereas the talus/debris deposits move over the surface of these blocks at a higher velocity. The interpretation of these landslide studies is not a straightforward analysis. It is a complex problem with a complex solution, including all information from geotechnical, geophysical, and surveying investigations.     

Geochronology and geochemistry of the Late Triassic Longtan pluton in South China: termination of the crustal melting and Indosinian orogenesis

The Indosinian orogeny is recorded by Triassic angular unconformities in Vietnam and South China and by widely occurring granitoids in the Yunkai-Nanling and the Xuefengshan belts of South China. The Longtan pluton in the northwestern part of the Xuefengshan belt is a typical high-K, calc-alkaline, I-type granitoid, which can shed light on the relationship between the Indosinian tectonic and magmatic activity in the region. Three precise zircon U–Pb ages yielded a mean of 218 ± 0.8 Ma, which is taken as the age of crystallization. The pluton consists of both granodiorite (64.59–68.01 % SiO₂ and 3.25–4.22 % K₂O) and granite (70.49–71.80 % SiO₂ and 4.07–4.70 % K₂O). The granodiorites are characterized by relatively high Mg# (54–57), low contents of Na₂O (3.2–4.3 wt%), low abundances of incompatible elements (LILE, Nb and P), high initial ⁸⁷Sr/⁸⁶Sr (0.7175–0.7184) and negative ε_Nd(t) (?9.98 to ?9.72). REE patterns show moderate fractionation ((La/Yb)_cn = 8.07–18.80) with negative Eu anomalies (Eu/Eu* = 0.62–0.86). Compared with the granodiorite, the granite has a wider range of Mg# (49–59), lower contents of Na₂O (2.8–4.2 wt%), higher initial ⁸⁷Sr/⁸⁶Sr (0.7232–0.7243) and more negative ε_Nd(t) (?12.07 to ?11.24) values. REE patterns are relatively flat ((La/Yb)_cn = 14.73–29.37) with smaller negative Eu anomalies (Eu/Eu* = 0.48–0.63). The granodiorite has lower K₂O/Na₂O and Al₂O₃/(MgO + FeO_Tot) values than the granite. Based on major and trace element geochemistry and Sr–Nd isotopes, we interpret the Longtan granodioritic magma to have been derived by partial melting of interlayered Proterozoic metabasaltic to metatonalitic source rocks, whereas the granite was probably derived from a mixture of Proterozoic metagraywackes and metaigneous rocks. Field, petrographic and geochemical evidence indicate that partial melting and fractional crystallization were the dominant mechanism in the evolution of the pluton. The Longtan granodiorites and granites are petrologically and geochemically similar to typical Indosinian varieties and are considered to have been produced in a similar manner. The Indosinian granitoids in the region show a magmatic peak age of ~238 Ma from the Yunkai-Nanling belt in the southeast and a magmatic peak age of ~218 Ma of the Xuefengshan belt to the northwest. These early and late magmatic episodes of the Indosinian granitoids also display slight variations of regular compositions, ε_Nd(t) values and T _DM ages. Thus, we propose a syncollisional extension model that Indosinian granitoids were generated by decompressional partial melting of crustal materials triggered by two extensions during collision of the Indochina and South China blocks. The Longtan pluton in the northwesternmost part of the orogenic belt marks the termination of the Indosinian magmatism and orogenesis.