Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts

The oxygen isotope ratios of Phanerozoic zircons from kimberlite pipes in the Kaapvaal Craton of southern Africa and the Siberian Platform vary from 4.7 to 5.9‰ VSMOW. High precision, accurate analyses by laser reveal subtle pipe-to-pipe differences not previously suspected. These zircons have distinctive chemical and physical characteristics identifying them as mantle-derived megacrysts similar to zircons found associated with diamond, coesite, MARID xenoliths, Cr-diopside, K-richterite, or Mg-rich ilmenite. Several lines of evidence indicate that these ¹⁸O values are unaltered by kimberlite magmas during eruption and represent compositions preserved since crystallization in the mantle, including: U/Pb age, large crystal size, and the slow rate of oxygen exchange in non-metamict zircon. The average ¹⁸O of mantle zircons is 5.3‰, ∼0.1 higher and in equilibrium with values for olivine in peridotite xenoliths and oceanic basalts. Zircon megacrysts from within 250 km of Kimberley, South Africa have average ¹⁸O=5.32±0.17 (n=28). Small, but significant, differences among other kimberlite pipes or groups of pipes may indicate isotopically distinct reservoirs in the sub-continental lithosphere or asthenosphere, some of which are anomalous with respect to normal mantle values of 5.3±0.3. Precambrian zircons (2.1–2.7 Ga) from Jwaneng, Botswana have the lowest values yet measured in a mantle zircon, ¹⁸O=3.4 to 4.7‰. These zircon megacrysts originally crystallized in mafic or ultramafic rocks either through melting and metasomatism associated with kimberlite magmatism or during metamorphism. The low ¹⁸O zircons are best explained by subduction of late Archean ocean crust that exchanged with heated seawater prior to underplating as eclogite and to associated metasomatism of the mantle wedge. Smaller differences among other pipes and districts may result from variable temperatures of equilibration, mafic versus ultramafic hosts, or variable underplating. The narrow range in zircon compositions found in most pipes suggests magmatic homogenization. If this is correct, these zircons document the existence of significant quantities of magma in the sub-continental mantle that was regionally variable in ¹⁸O and this information restricts theories about the nature of ancient subduction. Received: 8 August 1997 / Accepted: 6 May 1998     

Optical second harmonic signals from clay minerals

Using a neodymium glass laser and time-synchronized pulse detection, second harmonic signals were observed from a number of clay mineral powders. The second harmonic generation (SHG) experiments provide the first physical evidence (other than diffraction patterns) for the polar nature of the kaolinite layer and its stacking sequences. Well-crystallized nacrite and dickite specimens gave signals comparable to those of quartz, but the SHG intensities from the smaller kaolinite and halloysite crystallites were noticeably weaker. Based on results from eight specimens, there appears to be a direct correlation between SHG intensity and particle size, similar to that reported previously for quartz.     

Ein Computer-Programm zur quantitativen Bestimmung der Keimzahl mittels der Most-Probable-Number (MPN)-Methode

There are explained the statistical fundamentals of the estimation of the number of bacteria by means of the most-probable-number method (MPN method) inclusive of the confidence intervals and the probability of the sample results. On this basis, a program in Turbo-Pascal is prepared for the operating systems C/PM or MS-DOS for the computation of both single MPN estimations and complete tables inclusive of probability and confidence limits, their listing being appended.     

Influence of finite injections and of interplanetary propagation on time-intensity and time-anisotropy profiles of solar cosmic rays

For an observer in space the intensities and anisotropies of solar cosmic-ray events are governed by the duration and the functional shape of the injection processes near the Sun and by the propagation along the interplanetary magnetic field from the Sun to the observer. We study the influence of four different types of solar injections (Gaussian, exponential, step-function and coronal diffusion), and of a purely diffusive interplanetary propagation, where the diffusion coefficient has a power law dependence on the radial distance from the Sun, =Mr on both the time-intensity and the time-anisotropy profiles at 1 AU. The main results are as follows: A slow quasi-exponential decay of the intensity can be modelled in some cases; all finite injections produce high anisotropies during the main phase of an event; an effective solar injection length can be determined from simultaneous inspection of the intensities and anisotropies; the intensities and anisotropies do to first order not depend on the analytic shape of the various injection profiles. The model is applied to the November 18, 1968 solar event as observed by Pioneer 9 in the 7.5–21.5 MeV and 21.5–60 MeV energy channels. We obtain local diffusion coefficients in the range M= (2.5–5) × 10²¹ cm² s^–1 and injection periods of the order of 10–20 hr. Closer inspection reveals the change of interplanetary propagation conditions during the event.     

Über den Transmissionsfaktor der Globalstrahlung und seine Abhängigkeit vom Aerosol

Zusammenfassung Es wird der Transmissionsfaktor der Globalstrahlung alsm ^te-Wurzel der relativen Bestrahlung (gemessene Globalstrahlung/I _o sinh) definiert. Die Auswertung von Meßreihen gemäß dieser Definition führt zu Transmissionsfaktoren zwischen 0.8 bis 0.9 Eine theoretische Überprüfung führt zu dem gleichen, Ergebnis.

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Summary The transmission factor of the total radiation is defined likem ^th root of relative irradiation (measured total radiation/I _o sinh). The evaluation of measuring series according to the definition yields transmission factors between 0.8–0.9. A theoretical trial yields the same results.

     

Microseismicity induced during fluid-injection: A case study from the geothermal site at Groß Schönebeck,North German Basin

The technical feasibility of geothermal power production in a low enthalpy environment will be investigated in the geothermal site at Groß Schönebeck, North German Basin, where a borehole doublet was completed in 2007. In order to complete the Enhanced Geothermal System, three massive hydraulic stimulations were performed. A seismic network was deployed including a single 3-component downhole seismic sensor at only 500 m distance to the injection point. Injection rates reached up to 9 m³/min and the maximum injection well-head pressure was as high as ～60 MPa. A total of 80 very small (?1.8 < M _W < ?1.0) induced seismic events were detected. The hypocenters were determined for 29 events. The events show a strong spatial and temporal clustering and a maximum seismicity rate of 22 events per day. Spectral parameters were estimated from the downhole seismometer and related to those from other types of induced seismicity. The majority of events occurred towards the end of stimulation phases indicating a similar behavior as observed at similar treatments in crystalline environments but in our case at a smaller level of seismic activity and at lower magnitudes.     

Bemerkung zu: Ambach,Beschorner und Hoinkes, “Über die Eichung des Strahlungsbilanzmessers nach R. Schulze (Lupolengerät)”

Ohne Zusammenfassung     

A high-velocity layer in the lower crust of the North German Basin

A network of deep seismic refraction profiles in Northern Germany consisting of parts of the European Geotraverse (EGT) and additional new Unes is interpreted. The most striking result is the proof of an approximately 10 km thick high-velocity layer in the lower crust. Its P-wave velocity of 6.9-7.5 km s⁻¹ is typical for shield crusts or lower crust in extensional environments intruded by mafic magma. The layer is observed in an area of roughly 150 × 180 km north of the Elbe river and seems to continue north-east, at least up to the Caledonian deformation front at the southern edge of the Ringkøbing-Fyn High. It correlates spatially with an area of high positive gravity anomalies. Here, a Moho topography of several kilometres, which had already been postulated on the basis of gravity inversions and sporadic near-vertical P_MP reflections, could be confirmed by the interpretation of seismic wide-angle records. The termination of the high-velocity lower crust at the Lower Elbe Lineament, which strikes parallel to the Teisseyre-Tornquist Zone, contributes to its definition as a major lineament in the context of central European tectonics.