Stable isotope study of the Archaean rocks of the Vredefort impact structure,central Kaapvaal Craton,South Africa

The Vredefort dome in the Kaapvaal Craton was formed as a result of the impact of a large meteorite at 2.02 Ga. The central core of Archaean granitic basement rocks is surrounded by a collar of uplifted and overturned strata of the Witwatersrand Supergroup, exposing a substantial depth section of the Archaean crust. Orthogneisses of the core show little variation in whole-rock δ ¹⁸O value, with the majority being between 8 and 10‰, with a mean of 9.2‰ (n = 35). Quartz and feldspar have per mil differences that are consistent with O-isotope equilibrium at high temperatures, suggesting minimal interaction with fluids during subsequent cooling. These data refute previous suggestions that the Outer Granite Gneiss (OGG) and Inlandsee Leucogranofels (ILG) of the core represent middle and lower crust, respectively. Granulite-facies greenstone remnants from the ILG have δ ¹⁸O values that are on average 1.5‰ higher than the ILG host rocks and are unlikely, therefore, to represent the residuum from the partial melting event that formed the host rock. Witwatersrand Supergroup sedimentary rocks of the collar, which were metamorphosed at greenschist-to amphibolite-facies conditions, generally have lower δ ¹⁸O values than the core rocks with a mean value for metapelites of 7.7‰ (n = 45). Overall, through an ∼20 km thick section of crust, there is a general increase in whole-rock δ ¹⁸O value with increasing depth. This is the reverse of what is normal in the crust, largely because the collar rocks have δ ¹⁸O values that are unusually low in comparison with metamorphosed sedimentary rocks worldwide. The collar rocks have δD values ranging from −35 to −115‰ (average −62‰, n = 29), which are consistent with interaction with water of meteoric origin, having a δD of about −25 to −45‰. We suggest that fluid movement through the collar rocks was enhanced by impact-induced secondary permeability in the dome structure. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The orion-IRc2 outflow observed in COJ=4-3

A CO J=4-3 map of the Orion-IRc2 region (size 80, beam 15) is presented. The outflow has a bipolar structure observed nearly pole-on. We interpret our observations in terms of an hierarchy of tubes where the fastest gas proceeds through the narrowest, innermost cone. The gas has an inhomogeneous temperature distribution and must be highly clumped. Most of the properties derived comply with the picture developed for the few known extreme high velocity outflows; however, this outflow contains exceptionally dense gas ( > 10⁶ cm^–3).     

Estimating the phase of the response of the Earth to long-period geomagnetic fluctuations

Samples collected by the deep submersible “Alvin” from four hot spring fields (T = 3–13°C) on the crest of the Galapagos spreading ridge show pronounced and varied compositional anomalies. If it is assumed that these have a general significance, that they are associated with hydrothermal reactions between seawater and basalt wherever new oceanic crust is being produced then global fluxes can be computed. These are large. For Mg and SO₄ they balance the river input. For Li and Rb they exceed it by factors of between five and ten. Calcium is supplied at a rate equivalent to that of non-carbonate Ca from the continents. The additions of K, Ba and Si are between one third and two thirds of the river load. There are large positive and negative anomalies for Cl and Na indicating that substantial amounts of Cl may be taken up by the newly formed crust and transported deep into the subduction zones.Where there are data in common, the field measurements agree with the experimental findings at low (<5) water/rock ratios.     

Investigation for geothermal energy in Sweden

Preliminary investigations of the geothermal energy potential in Sweden are being carried out in crystalline rocks of Precambrian age, as well as in the Triassic Buntsandstone. The geothermal potential of fracture zones is also being investigated. Different methods for prospecting have been tried and compared.     

The metamorphic history of rocks buried,accreted and exhumed in an accretionary prism: an example from the Otago Schist,New Zealand

Although subgreenschist facies metamorphic rocks are widespread in the upper crust, mineralogical processes affecting these rocks are poorly understood. Subgreenschist mineralogical transitions have been invoked as critical controls on the mechanical behaviour of rocks within the crustal seismogenic zone, calling for further study of very low‐grade metamorphic assemblages. In this study a multi‐technique thermobarometric study of the Chrystalls Beach Complex mélange, which is located within the Otago Schist accretion‐collision assemblage of the South Island of New Zealand, is presented. The Chrystalls Beach Complex comprises highly sheared trench‐fill sedimentary rocks and scattered pillow basalts, and is inferred to have formed during Jurassic subduction under the paleo‐Pacific Gondwana margin. Equilibrium mineral assemblages indicate peak P–T conditions in the range 400–550 MPa and 250–300 °C, which is supported by chlorite thermometry. Relatively high pressures of burial and accretion during foliation development are inferred from phengite content and b₀ spacing analyses of white mica. Rare lawsonite occurs in a post‐foliation vein, and illite ‘crystallinity’ measurements indicate a thermal overprint during exhumation. These P–T estimates and their relative chronology indicate that the mineral assemblages developed along a clockwise P–T path. Based on variability in P–T estimates from different techniques, mineral assemblages developed during burial are largely overprinted during exhumation at similar or higher‐T than experienced along the prograde path. Observed subduction‐related subgreenschist assemblages are therefore likely to indicate lower‐P than experienced during subduction, as higher‐P mineral compositions re‐equilibrate during exhumation. The P–T path inferred in this study is similar in shape to P–T paths for higher grade parts of the Otago Schist, and other exhumed accretionary prisms around the world, and is therefore probably common for rocks buried, accreted and exhumed in accretionary prisms.     

Thermische Klärschlammbehandlung

Ohne Zusammenfassung     

Character and significance of spectacular layering features developed in the thin, alkali-basaltic sills of the Ulvö Gabbro Complex, Sweden

Summary The Ulv? Gabbro Complex consists of alkali-olivine basaltic circular bodies ∼30–80 km in diameter. These intrusions were emplaced at shallow depths (∼3 km) as thin sheets (∼300 m). Among other things, the gabbroic cumulates of the complex display: modal layering, grain-size variations, trough structures, and slump structures. The crystallization sequence is olivine+plagioclase, ulv?spinel, clinopyroxene, and apatite. A nearly continuous exposure across one of these intrusions, the Norra Ulv?n gabbro, is subdivided into: a Lower Zone (LZ), a Rhythmically Layered Zone (RZ) and an Upper Zone (UZ). LZ and RZ were formed at the floor, while UZ grew from the roof downward. Major-element variations in the cores of the cumulus minerals define fractionation trends from the base of the intrusion to the RZ-UZ boundary interpreted as a “sandwich horizon”. Modeling suggests that a significant amount of crystallized interstitial liquid is required to produce the observed stratigraphic relations. Our results suggest that the small size and shallow emplacement depth of the intrusions of the Ulv? Gabbro Complex helped to preserve evidence of primary accumulation processes. However, it is also clear that despite the limited time available postcumulus processes such as diffusional homogenization and compaction of some grains were important. Correspondence: S. ?. Larson, Earth Sciences Centre, Department of Geology, G?teborg University, POB 460, SE 40530 G?teborg, Sweden