Platinum-group Elements and Microstructures of Normal Merensky Reef from Impala Platinum Mines, Bushveld Complex

The Merensky Reef of the Bushveld Complex contains one of theworld’s largest concentrations of platinum-group elements(PGE). We have investigated ‘normal’ reef, its footwalland its hanging wall at Impala Platinum Mines. The Reef is 46cm thick and consists from bottom to top of leuconorite, anorthosite,chromitite and a very coarse-grained melanorite. The footwallis leuconorite and the hanging wall is melanorite. The onlyhydrous mineral present is biotite, which amounts to 1%, orless, of the rock. All of the rocks contain 0·1–5%interstitial sulphides (pyrrhotite, pentlandite and chalcopyrite),with the Reef rocks containing the most sulphides (1–5%).Lithophile inter-element ratios suggest that the magma fromwhich the rocks formed was a mixture of the two parental magmasof the Bushveld Complex (a high-Mg basaltic andesite and a tholeiiticbasalt). The Reef rocks have low incompatible element contentsindicating that they contain 10% or less melt fraction. Nickel,Cu, Se, Ag, Au and the PGE show good correlations with S inthe silicate rocks, suggesting control of the abundance of thesemetals by sulphides. The concentration of the chalcophile elementsand PGE in the silicate rocks may be modelled by assuming thatthe rocks contain sulphide liquid formed in equilibrium withthe evolving silicate magma. It is, however, difficult to modelthe Os, Ir, Ru, Rh and Pt concentrations in the chromititesby sulphide liquid collection alone, as the rocks contain 3–4times more Os, Ir, Ru, Rh and Pt than the sulphide-collectionmodel would predict. Two possible solutions to this are: (1)platinum-group minerals (PGM) crystallize from the sulphideliquid in the chromitites; (2) PGM crystallize directly fromthe silicate magma. To model the concentrations of Os, Ir, Ru,Rh and Pt in the chromitites it is necessary to postulate thatin addition to the 1% sulphides in the chromitites there isa small quantity (0·005%) of cumulus PGM (laurite, cooperiteand malanite) present. Sulphide liquids do crystallize PGM atlow f_S2. Possibly the sulphide liquid that was trapped betweenthe chromite grains lost some Fe and S by reaction with thechromite and this provoked the crystallization of PGM from thesulphide liquid. Alternatively, the PGM could have crystallizeddirectly from the silicate magma when it became saturated inchromite. A weakness of this model is that at present the exactmechanism of how and why the magma becomes saturated in PGMand chromite synchronously is not understood. A third modelfor the concentration of PGE in the Reef is that the PGE arecollected from the underlying cumulus pile by Cl-rich hydrousfluids and concentrated in the Reef at a reaction front. Althoughthere is ample evidence of compaction and intercumulus meltmigration in the Impala rocks, we do not think that the PGEwere introduced into the Reef from below, because the rocksunderlying the Reef are not depleted in PGE, whereas those overlyingthe Reef are depleted. This distribution pattern is inconsistentwith a model that requires introduction of PGE by intercumulusfluid percolation from below. KEY WORDS: Merensky Reef; platinum-group elements; chalcophile elements; microstructures     

The early Holocene Saksunarvatn tephra found in lake sediments in NW Germany

A basaltic tephra layer consisting of brownish-olive glass shards. and about 0.2 mm thick. was found in cores from four lakes in northwest Germany. According to pollen analysis it was deposited during the early Boreal period (corresponding to about 8700 BP). The petrographic properties. the geochemical composition and the age agree with those of the Saksunarvatn tephra. which was first found on the Faroe Islands. The position of the tephra layer in the pollen stratigraphy and in the absolute time-scale is discussed. Procedures for locating the tephra in other cores are suggested.     

Modern carbonate and terrigenous clastic sediments on a cool water, high energy, mid-latitude shelf: Lacepede, southern Australia

The wide Lacepede Shelf and narrow Bonney Shelf are contiguous parts of the south-eastern passive continental margin of Australia. The shelves are open, generally deeper than 40 m, covered by waters cooler than 18°C and swept by oceanic swells that move sediments to depths of 140 m. The Lacepede Shelf is proximal to the ‘delta’of the River Murray and the Coorong Lagoon. Shelf and upper slope sediments are a variable mixture of Holocene and late Pleistocene quartzose terrigenous clastic and bryozoa-dominated carbonate particles. Bryozoa grow in abundance to depths of 250 m and are conspicuous to depths of 350 m. They can be grouped into four depth-related assemblages. Coralline algae, the only calcareous phototrophs, are important sediment producers to depths of 70 m. Active benthic carbonate sediment production occurs to depths of 350 m, but carbonate sediment accumulation is reduced on the open shelf by continuous high energy conditions. The shelf is separated into five zones. The strandline is typified by accretionary sequences of steep shoreface, beach and dune carbonate/siliciclastic sediments. Similar shoreline facies of relict bivalve/limestone cobble ridges are stranded on the open shelf. The shallow shelf, c.40–70 m deep, is a wide, extremely flat plain with only subtle local relief. It is a mosaic of grainy, quartzose, palimpsest facies which reflect the complex interaction of modern bioclastic sediment production (dominated by bryozoa and molluscs), numerous highstands of sea level over the last 80 000 years, modern mixing of sediments from relatively recent highstands and local introduction of quartz-rich sediments during lowstands. The middle shelf, c.70–140 m deep, is a gentle incline with subtle relief where Holocene carbonates veneer seaward-dipping bedrock clinoforms and local lowstand beach complexes. Carbonates are mostly modern, uniform, clean, coarse grained sands dominated by a diverse suite of robust to delicate bryozoa particles produced primarily in situ but swept into subaqueous dunes. The deep shelf edge, c. 140–250 m deep, is a site of diverse and active bryozoa growth. Resulting accumulations are characteristically muddy and distinguished by large numbers of delicate, branching bryozoa. The upper slope, between 250 and 350 m depth, contains the deepest platform-related sediments, which are very muddy and contain a low diversity suite of delicate, branching cyclostome bryozoa. This study provides fundamental environmental information critical for the interpretation of Cenozoic cool water carbonates and the region is a good model for older mixed carbonate-terrigenous clastic successions which were deposited on unrimmed shelves.     

EARLY VOID-FILLING CEMENTATION IN DEVONIAN FORE-REEF LIMESTONES (GERMANY)

SUMMARY
Fibrous calcite is the main cement in the voids and cavities of Devonian fore-reef limestones in Germany. Cementation occurs in some cases simultaneously with encrustation by blue-green algae as well as internal deposition of calcarenites and calcilutites. These observations lead to the conclusion that the fore-reef voids were exposed to light and were still in free communication with the sea, when the cementation began. Furthermore, redeposited fragments of former cavity fillings indicate that the formation of cavity-filling fibrous calcium carbonate cement was already completed before the sea-ward erosion took place. It is believed that the precipitation of fibrous calcium carbonate in most of the Devonian fore-reef limestones, which show no signs of dissolution, occurred in a shallow subtidal environment. Only the voids showing larger solution phenomena were probably exposed to fresh water in the inter- to supratidal zone.     

Environmental setting and microstructure of subfossil lithified stromatolites associated with evaporites, Marion Lake, South Australia

A variety of finely laminated, subfossil, aragonitic stromatolites and oncolites occur on a regressive marginal flat surrounding Marion Lake, South Australia. These algal forms overlie a substrate of coarse, highly porous, moldic aragonitic limestone which passes progressively towards the take centre through a zone of interstatified aragonite and gypsum and ultimately to pure crystalline gypsum. All of these facies overlie Holocene marine carbonate bank sediments which unconformably overlie at least one upper Pleistocene marine unit. Detailed petrographic and stratigraphic studies, combined with comparative studies of related nearby lakes containing a variety of living aragonitic cryptalgalaminates, provide a model for development of the Holocene sedimentary sequence. Marion Lake last became inundated by the sea around 6500 years ago during the Holocene transgression, when a protected marine environment was initiated. Lateral sediment accretion sealed marine passes into the resulting lagoon system soon after sealvel stabilized, and a variety of gypsum and gypsum-carbonate-algal facies evolved. Pure gypsum was deposited in waters 2–3 m deep in the central basin area concurrently with formation of seasonally alternating gypsum and aragonite layers towards basin margins. Blue-green filamentous algae thrived in the shallower marginal areas and at least partly controlled carbonate deposition, which must have occurred during seasonal outflow of carbonate-rich ground water from the calcareous dune aquifer over denser gypsum-saturated waters. These systems eventually migrated towards the centre of the lake to produce the relationships preserved today. The fresher waters also leached the gypsum from the marginal gypsum-carbonate facies. Collapse due to gypsum dissolution, along with aragonite crystallization, combined to form a lake-marginal mega-polygonal facies. Teepee structures formed around polygon margins, with optimum conditions for stromatolite development occurring on the teepee crests. The actual stromatolites which occur around Marion Lake are strongly indurated and involve a variety of morphologies, the most common of which are laterally linked hemispheroids. Stacked hemispheroids and oncolites are also relatively common, along with irregular forms, many of which encrust a variety of substrate irregularities. Vertical relief of the stromatolites varies from centimetres to tens of centimetres and all forms are characterized by extremely fine internal interlaminations of alternate light and dark grey laminae which typically occur several per millimetre. The microstructure comprises micritic aragonite crystals with fibrous habit associated with organic matter, and occasional zones of abundant algal filament molds which are generally oriented normal to the laminae.     

Giant Holocene Freshwater Microbialites,Laguna Bacalar,Quintana Roo,Mexico

With more than 10 km of total length, Holocene microbialites in Laguna Bacalar, Mexico, belong to the largest freshwater microbialite occurrences. Microbialites include domes, ledges and oncolites. Domal forms can grow to diameters and heights of 3 m. Microbialites are composed of low magnesium calcite which is, to a large extent, precipitated due to the metabolic activity of the cyanobacteria Homeothrix and Leptolyngbya, and associated diatoms. Photosynthesis removes carbon dioxide and triggers carbonate precipitation. Also, an elevated carbonate concentration in lagoon waters, derived from dissolution of Cenozoic limestone in a karst system, supports carbonate precipitation. Trapping and binding of detrital grains is also observed, but is not as common as precipitation. Bacalar microbialites are largely thrombolitic, however, stromatolitic sections occur as well. The bulk of Bacalar microbialites probably formed in the Late Holocene (ca 1 kyr BP until present). According to ¹⁴C dating, microbialites accreted 9 to 8 cal kyr BP; however, these ages may be too old as a result of a strong hard water effect. This effect is seen in ¹⁴C ages of living bivalve and gastropod mollusc shells from Bacalar Lagoon, which are 8 to 7 cal kyr BP. The modern associated fauna of microbialites is characterized by low diversity and high abundance of the bivalve mollusc Dreissena sp. and the gastropod Pomacea sp. The abundant grazing gastropods presumably hamper modern microbialite formation. A comparison of Bacalar microbialites with other modern microbialite occurrences worldwide shows only a few patterns: sizes, shapes, microbial taxa, mineralogy, type of accretion and settings including water properties of microbialite occurrences exhibit high variability. A trend may be seen in the grazing metazoa, which are rare to absent in the marine and brackish examples, but apparently present in all the freshwater occurrences of microbialites. Also, freshwater examples are usually characterized by elevated concentrations of carbonate and/or calcium ions in the surrounding waters.     

DIE STRATIGRAPHISCHE UND PALAEONTOLOGISCHE BEDEUTUNG DER FOSSILEN NAGETIERE CHINAS

Ⅰ. VORBEMERKUNG. Die Ordnung Nagetiere umfasst sehr viele Arten und ist die verbreitetste Tiergruppe innerhalb der Saeugetiere. Sie macht mit ihren etwa 2800 Species mehr als ein Drittel der heutigen Landsaeugetiere aus. Die Nagetiere sind auf der ganzen Welt bis zu den Polen und dem vereisten Hochgebirge an-     

Reduction of biosphere life span as a consequence of geodynamics

The long‐term co‐evolution of the geosphere–biospere complex from the Proterozoic up to 1.5 billion years into the planet's future is investigated using a conceptual earth system model including the basic geodynamic processes. The model focusses on the global carbon cycle as mediated by life and driven by increasing solar luminosity and plate tectonics. The main CO₂ sink, the weathering of silicates, is calculated as a function of biologic activity, global run‐off and continental growth. The main CO₂ source, tectonic processes dominated by sea‐floor spreading, is determined using a novel semi‐empirical scheme. Thus, a geodynamic extension of previous geostatic approaches can be achieved. As a major result of extensive numerical investigations, the "terrestrial life corridor", i.e., the biogeophysical domain supporting a photosynthesis‐based ecosphere in the planetary past and in the future, can be identified. Our findings imply, in particular, that the remaining life‐span of the biosphere is considerably shorter (by a few hundred million years) than the value computed with geostatic models by other groups. The "habitable‐zone concept" is also revisited, revealing the band of orbital distances from the sun warranting earth‐like conditions. It turns out that this habitable zone collapses completely in some 1.4 billion years from now as a consequence of geodynamics.     

Expert system for the selection of methods to calculate design flood flows

Abstract

This paper presents an expert system to help select the best method to estimate design flood flows for civil engineering works based upon the procedures available, the nature and characteristics of the basin and existing hydrological records. The system presents the user with a list of possible methods ranked in descending grade order and optionally presents explanations which support the selected choices. Ordering is achieved using the knowledge base provided by the expert. The system recommends procedures for both preliminary estimates and final designs. The system also constitutes a valuable aid for junior engineers and experienced hydrologists in the selection of methods. Its conceptual structure can be easily generalized to treat other problems of a similar nature in the field of hydrology and water resources.