Textures exhibited by feldspars in the Giant Mottled Anorthosite (GMA) of the bastard unit in the Upper Critical Zone,Western Bushveld Complex

Summary The Bastard Unit, overlying the Merensky Unit, is the uppermost and most complete of the cyclic units in the Upper Critical Zone and its upper contact is taken as the boundary between the Critical and Main Zones (as recommended by SACS, 1980). The Unit is different from underlying units for three reasons: it does not have a well-defined top contact (with an overlying unit); it is much thicker than the Merensky and Footwall Units; and it has an unusually thick sequence of mottled anorthosite at its top. This distinctive composite anorthositic sequence is termed the Giant Mottled Anorthosite (GMA) which is predominantly made up of poikilitic anorthosite. The GMA can broadly be subdivided into three distinct parts; the Lower Giant Mottled Anorthosite (LGMA), Giant Mottled Middling (GMM) and the Upper Giant Mottled Anorthosite (UGMA).Petrographic examination of 82 samples taken in the GMA from 11 borehole profiles around the western limb of the Complex revealed that the LGMA and GMM are essentially adcumulates while the UGMA is orthocumulate in character. By its very nature the dominant phase throughout the GMA is cumulus plagioclase feldspar ( 85%) with the balance being made up of intercumulus orthopyroxene and clinopyroxene and minor biofite — all of which constitute the darker mottles. Intercumulus inverted pigeonite occurs in the upper part of the GMM, throughout the UGMA and for c. 30 m into the Main Zone. K-feldspar also joins the paragenesis within the UGMA and occurs both as intercumulus orthoclase and antiperthite. Cumulus plagioclase grains, showing varying degrees of complex oscillatory-zonal growth are common in the upper part of the GMA. These grains are interpreted as records of growth episodes during earlier influxes of magma which were responsible for the development of the underlying cyclic units, but escaped incorporation into the cumulate pile.

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Feldspat-Texturen im Giant Mottled Anorthosite der Bastard-Einheit in der oberen kritischen Zone des westlichen Bushveld-Komplexes

Zusammenfassung Die Bastard-Einheit, die die Merensky-Einheit überlagert, ist die oberste und am besten entwickelte der zyklischen Einheiten in der oberen kritischen Zone; ihr oberer Kontakt gilt als die Grenze zwischen der Kritischen und der Main-Zone (entsprechend den Empfehlungen von SACS, 1980). Die Einheit unterscheidet sich aus drei Gründen von den Einheiten im liegenden: sie hat keinen gut definierten oberen Kontakt (mit einer darüberliegenden Einheit), sie ist wesentlich mächtiger als die Merensky und die Footwall-Einheiten, und sie hat eine ungewöhnliche mächtige Abfolge von Mottled Anorthosite als obersten Teil der Abfolge. Diese wohldefinierte anorthositische Abfolge wird als Giant Mottled Anorthosite (GMA) bezeichnet, und besteht hauptsächlich aus poikilitischem Anorthosit. Die GMA kann in 3 Teile, den unteren (LGMA), den mittleren (GMM) un den oberen (UGMA) unterteilt werden.Die petrographische Untersuchung von 82 Proben in der GMA aus 11 Bohrlöchern im Westteil des Bushveld-Komplexes zeigt, daß LGMA und GMM im wesentlichen Adkumulate sind, während die UGMA ein Orthokumulat darstellt. Die Hauptphase in der gesamten GMA ist Kumulus-Plagioklas ( 85 %); der Rest besteht aus Intercumulus-Orthopyroxen, Klinopyroxen, und geringen Mengen von Biotit. Diese sind die Komponenten der dunkleren mottles. Intercumulus invertierter Pigeonit kommt im oberen Teil der GMM, in der gesamten UGMA und in den unteren 30 Metern der Main-Zone vor. Innerhalb der UGMA tritt K-Feldspat hinzu, dieser kommt als Intercumulus Orthoklas und Antiperthit vor. Körner von Cumulusplagioklas mit verschiedenen Intensitäten komplexer Zonierung sind im oberen Teil der GMA verbreitet. Diese Körner werden als Hinweise auf Wachstumsepisoden während früherer Magmenzufuhr-Phasen interpretiert, die für die Entwicklung der zyklischen Einheiten im Liegenden verantwortlich waren, aber selbst nicht Teil der Cumulatabfolge wurden.

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With 5 Figures     

Fracture initiation, growth and effect on stress field: a numerical investigation

A numerical investigation was made of the relationships between fracture initiation, growth, stress field and boundary conditions. Two-dimensional plane strain continuum models were used in which fractures appeared as zones of strain localization developed through application of a strain softening Mohr–Coulomb constitutive model. R and R′ fractures developed first, followed by Y fractures at larger strains. The models showed that equal development of conjugate R and R′ fractures is easily changed to favor one or the other set by minor variations in model initial conditions. Strength loss in fractures caused stress field rotations in regions bounded by fractures, altering the orientation of subsequent fractures. The amount and sense of stress field rotation is dependent on the strength loss during displacement on the fractures, the orientation of fractures, and on the boundary conditions. Y oriented fractures could be explained on the basis of a Mohr–Coulomb failure criterion provided that stress field rotation is accounted for. Monitoring of fracture slip activity showed that, under conditions of constant boundary velocity, slip was discontinuous in time, alternating on fractures throughout the model.     

The Palaeoproterozoic Woodlands Formation of eastern Botswana-northwestern South Africa: lithostratigraphy and relationship with Transvaal Basin inversion structures

The Woodlands Formation (uppermost Pretoria Group) of eastern Botswana overlies thick quartzites of the Sengoma Formation (Magaliesberg Formation) and comprises a lower unit of interbedded mudrocks and fine-grained recrystallised quartzitic sandstones, succeeded by chaotic and very coarse-grained inferred slump deposits. Within the adjacent western region of South Africa, interbedded mudrocks and quartzitic sandstones stratigraphically overlying the Magaliesberg Formation are now assigned to the lower Woodlands Formation. Within the entire region, interference folding produced by northeast-southwest (F₁ and F₃) and northwest-southeast (F₂) compression, and concomitant faulting characterised inversion of the Pretoria Group basin. This deformation is of pre-Bushveld age and affected all units in the Pretoria Group, including the uppermost Silverton, Magaliesberg and Woodlands Formations, and intrusive Marico Hypabyssal Suite (pre-Bushveld) mafic sills. The Nietverdiend lobe of the Bushveld Complex, intrusive into this succession, was not similarly deformed. Movement along the major Mannyelanong Fault in the northwest of the study area post-dated Transvaal Basin inversion, after which the “upper Woodlands” chaotic slump deposits were formed. The latter must thus belong to a younger stratigraphical unit and is possibly analogous to apparently syntectonic sedimentary rocks (Otse Group) in the Otse Basin of eastern Botswana.     

The chemistry of orthophosphate uptake from seawater on to calcite and aragonite

The chemistry of orthophosphate uptake from synthetic seawater onto the surfaces of synthetic calcite, aragonite and low-magnesium biogenic calcite has been studied, in order to elucidate the kinetics of the process (generally believed to be the major control of dissolved reactive phosphate in carbonate-rich marine sediments). Our results differ from those obtained by others, who have studied orthophosphate uptake in low ionic strength solutions and at much higher supersaturations relative to apatite.In both ‘free drift’ and chemostat experiments, Mg and F have only a minor effect on the reaction rate. Even at constant solution composition the rate of orthophosphate uptake was found to decrease by 10⁶ over a two week period. The data from the ‘free drift’ experiments can be fitted to the Elovich equation. This indicates that the kinetics observed for this reaction can be explained by an exponential decrease in available surface reaction sites and/or a linear increase in the activation energy associated with chemisorption as the reaction proceeds.     

Carbon sources in arc volcanism, with implications for the carbon cycle

New CO₂/³ He data from the East Sunda Arc (Indonesia) confirm earlier observations that arc volcanic gases have higher CO₂/³ He ratios than MOR environments.On average, > 80% of arc volcanic CO₂ is recycled, exogene carbon. Addition of a few percent of carbonate-bearing sediments to the mantle wedge explains much of the carbon abundance andcarbon isotopic data of arc gases, but can not explain the He isotope observations. The CO₂/³He in arc volcanoes is not strongly dependent on the composition of modem trough sediments (e.g. deep sea clays vs carbonate-rich sequences), and calcite veins in the hydrothermally altered subducted slab may provide a contribution to the recycled carbon flux of, arcs. The sum of globally deep-subducted sediment and slab carbon exceeds the estimated arc CO₂ flux, and approximately 3.5 teramole of carbon may return annually to the mantle in convergent zones. The modem combined processes of MOR volcanism, slab alteration, and subduction volcanism do not produce a substantial carbon flux into the exosphere, and rate-changes in ocean floor spreading are unlikely to cause major changes in atmospheric CO₂ as a result of changes in the volcanic CO₂ fluxes. Intense pulses of flood basalt volcanism, however, may alter the CO₂ contents of the atmosphere over the course of a millenium or so, and influence global climate.     

Solubility of manganotantalite, zircon and hafnon in highly fluxed peralkaline to peraluminous pegmatitic melts

The behavior of tantalum and zirconium in pegmatitic systems has been investigated through the determination of Ta and Zr solubilities at manganotantalite and zircon saturation from dissolution and crystallization experiments in hydrous, Li-, F-, P- and B-bearing pegmatitic melts. The pegmatitic melts are synthetic and enriched in flux elements: 0.7–1.3 wt% Li₂O, 2–5.5 wt% F, 2.8–4 wt% P₂O₅ and 0–2.8 wt% B₂O₃, and their aluminum saturation index ranges from peralkaline to peraluminous (ASI_Li = Al/[Na + K + Li] = 0.8 to 1.3) with various K/Na ratios. Dissolution and crystallization experiments were conducted at temperatures varying between 700 and 1,150°C, at 200 MPa and nearly water-saturated conditions. For dissolution experiments, pure synthetic, end member manganotantalite and zircon were used in order to avoid problems with slow solid-state kinetics, but additional experiments using natural manganotantalite and zircon of relatively pure composition (i.e., close to end member composition) displayed similar solubility results. Zircon and manganotantalite solubilities considerably increase from peraluminous to peralkaline compositions, and are more sensitive to changes in temperature or ASI of the melt than to flux content. A model relating the enthalpy of dissolution of manganotantalite to the ASI_Li of the melt is proposed: ∆H _diss (kJ/mol) = 304 × ASI_Li − 176 in the peralkaline field, and ∆H _diss (kJ/mol) = −111 × ASI_Li + 245 in the peraluminous field. The solubility data reveal a small but detectable competitivity between Zr and Ta in the melt, i.e., lower amounts of Zr are incorporated in a Ta-bearing melt compared to a Ta-free melt under the same conditions. A similar behavior is observed for Hf and Ta. The competitivity between Zr (or Hf) and Ta increases from peraluminous to peralkaline compositions, and suggests that Ta is preferentially bonded to non-bridging oxygens (NBOs) with Al as first-neighbors, whereas Zr is preferentially bonded to NBOs formed by excess alkalies. As a consequence Zr/Ta ratios, when buffered by zircon and manganotantalite simultaneously, are higher in peralkaline melts than in peraluminous melts.     

Determination of lower and upper bounds of predicted production from history-matched models

We present a method to determine lower and upper bounds to the predicted production or any other economic objective from history-matched reservoir models. The method consists of two steps: 1) performing a traditional computer-assisted history match of a reservoir model with the objective to minimize the mismatch between predicted and observed production data through adjusting the grid block permeability values of the model. 2) performing two optimization exercises to minimize and maximize an economic objective over the remaining field life, for a fixed production strategy, by manipulating the same grid block permeabilities, however without significantly changing the mismatch obtained under step 1. This is accomplished through a hierarchical optimization procedure that limits the solution space of a secondary optimization problem to the (approximate) null space of the primary optimization problem. We applied this procedure to two different reservoir models. We performed a history match based on synthetic data, starting from a uniform prior and using a gradient-based minimization procedure. After history matching, minimization and maximization of the net present value (NPV), using a fixed control strategy, were executed as secondary optimization problems by changing the model parameters while staying close to the null space of the primary optimization problem. In other words, we optimized the secondary objective functions, while requiring that optimality of the primary objective (a good history match) was preserved. This method therefore provides a way to quantify the economic consequences of the well-known problem that history matching is a strongly ill-posed problem. We also investigated how this method can be used as a means to assess the cost-effectiveness of acquiring different data types to reduce the uncertainty in the expected NPV.     

Settling and resuspended particles: A source or a sink of phosphate in two contrasting oligotrophic high mountain lakes?

This study focuses on two Mediterranean oligotrophic high mountain lakes located in the Sierra Nevada National Park (southern Spain): Río Seco (RS) and La Caldera (LC). A combination of field measurements and laboratory experiments is used: (i) to quantify in situ settling fluxes; (ii) to study the soluble reactive phosphorus (SRP) release or uptake by settling and resuspended particles; and (iii) to discriminate between the biotic and abiotic contribution for such patterns. In general, all suspensions (lake water untreated and lake water enriched with settling and with resuspended matter) in both study lakes release significantly more SRP to the solution when biological activity was suppressed. Biological uptake from settling and resuspended matter is likely to be limited by the bacterial consumption of P. Despite of these similarities, this study has revealed notable differences in the effect of sediment resuspension on SRP dynamics in both study lakes, when simulating natural conditions (biotic and abiotic processes). While in LC, the enrichment of lake water with settling and with resuspended matter did not cause an increase in SRP concentrations in lake water, SRP concentrations in RS at the end of the experiment were significantly higher (probability P < 0.05) in lake water enriched with resuspended matter (3.2 μg/l) than in natural lake water (lower than the detection limit). Accordingly, it is reasonable to expect that sediment resuspension, which occurs more frequently in RS compared with LC, affects drastically the SRP availability in the water column in RS.     

Submarine channel levee shape and sediment waves from physical experiments

Submarine channel levees aggrade through repeated overspill events from the channel axis. The shape of the levees may therefore reflect some characteristic(s) of the overspilling flow. It has been noted that basin floor levees typically have a relatively low-relief and taper exponentially to their termination; in contrast slope channel levees may be much steeper close to the channel. A simple physical experiment was performed where a surge-like sediment-laden current flowed through a curved channel. Significant overspill occurred and generated a deposit flanking the channel on either side. The experiment was repeated 25 times to build up low-relief channel-levees. It was found that in proximal areas, levees were steep and characterised by power-law decays, a transitional zone of logarithmically thinning levee was found a little further down-channel, followed by exponential decays in medial to distal areas. The style of levee decay is a function of spatial variation in overbank sedimentation rates. Where flows rapidly lose momentum and deposit across the grain-size spectrum, i.e., in proximal areas, levees tend to be steep; farther down the channel, the steep levee slope gives way to a more gradually tapering deposit. In more distal parts of the channel, deposition is directly related to sediment settling velocity (rather than the suspended load exceeding flow transport capacity as is the case in proximal areas), the deposit reflects this with relatively simple exponential thickness decays. Additionally, small-scale sediment waves developed under lee wave conditions on the inner-bend overbank. The waves initially migrated slightly towards the channel, but as the style of overspill evolved due to intra-channel deposition, flows moved out of the lee wave window and sedimentation became out of phase with the wavelength of the features and the topography was healed.