Iain McDonald

Thioredoxin reductase 2 is essential for keeping low levels of H2O2 emission from isolated heart mitochondria

Respiring mitochondria produce H2O2 continuously. When production exceeds scavenging, H2O2 emission occurs, endangering cell functions. The mitochondrial peroxidase peroxiredoxin-3 reduces H2O2 to water using reducing equivalents from NADPH supplied by thioredoxin-2 (Trx2) and, ultimately, thioredoxin reductase-2 (TrxR2). Here, the contribution of this mitochondrial thioredoxin system to the control of H2O2 emission was studied in isolated mitochondria and cardiomyocytes from mouse or guinea pig heart. Energization of mitochondria by the addition of glutamate/malate resulted in a 10-fold decrease in the ratio of oxidized to reduced Trx2. This shift in redox state was accompanied by an increase in NAD(P)H and was dependent on TrxR2 activity. Inhibition of TrxR2 in isolated mitochondria by auranofin resulted in increased H2O2 emission, an effect that was seen under both forward and reverse electron transport. This effect was independent of changes in NAD(P)H or membrane potential. The effects of auranofin were reproduced in cardiomyocytes; superoxide and H2O2 levels increased, but similarly, there was no effect on NAD(P)H or membrane potential. These data show that energization of mitochondria increases the antioxidant potential of the TrxR2/Trx2 system and that inhibition of TrxR2 results in increased H2O2 emission through a mechanism that is independent of changes in other redox couples.

Platinum-group elements in the Morokweng impact structure, South Africa : evidence for the impact of a large ordinary chondrite projectile at the Jurassic-Cretaceous boundary

Radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to overlap in time with major boundaries in Earth history [e.g., the Cretaceous–Tertiary (K/T) and Jurassic–Cretaceous (J/K) boundaries]. The Morokweng impact crater in South Africa is coincident in age with the J/K boundary. However, the types of objects that generate large craters are poorly known because it is difficult to unambiguously identify the projectile from the signature it imparts into the impact rocks. Meteorites are highly enriched in the platinum-group elements (PGE), which have been widely used as a tool for identifying the presence of a meteorite signature. Here we present new PGE analyses from the Morokweng impact melt sheet. Our data reveal high PGE concentrations and high degree of PGE correlation through the melt sheet. Regression analysis was used to determine the projectile PGE signature and constrain input from the terrestrial target rocks. The closest match to Morokweng is the PGE signature of ordinary (L or LL) chondrite meteorites, which is broadly in agreement with the results of an earlier Cr isotope study. The results of these independent studies provide strong evidence that a large, ordinary chondrite projectile struck the area of Morokweng in the late Jurassic.

A review of the behaviour of Platinum Group Elements within natural magmatic sulfide ore systems

The largest and most significant type of geological deposit of platinum group elements (PGEs) is that associated with magmatic base metal sulfide minerals in layered mafic or ultramafic igneous intrusions. The common association of PGEs with sulfide minerals is a result of processes of magmatic and sulfide liquid segregation and fractionation. The mineralogical nature of the ores is dependent on a number of factors during sulfide liquid fractionation. The most significant of these with regard to the mineralogy of the two most important metals, platinum and palladium, is the presence and concentration of semimetals such as bismuth and tellurium within the mineralising sulfide liquid. Whereas rhodium, iridium, osmium and ruthenium are almost always present in solid solution within the resultant base metal sulfide minerals; should sufficient semimetals be present, Pd and especially Pt will form discrete minerals (such as platinum bismuthides) around the margins of, and possibly away from, the sulfides.

High magmatic flux during Alpine-Himalayan collision: Constraints from the Kal-e-Kafi complex, central Iran

Voluminous Eocene gabbros to granites of the Kal-e-Kafi backarc composite intrusion were emplaced prior to the Alpine-Himalayan collision in the central Iranian backarc, but the reasons for precollisional high arc and backarc magmatic productivity (60–53 Ma) are unclear. Diagnostic geochemical signatures are high K2O-Sr contents and successively depleted middle rare earth element (REE) patterns, reflecting a highly metasomatized source and an increasing role for amphibole and garnet (0%–10%) in the relatively younger granites. Release of concealed K-Sr-rich fluids from oceanic fractures and faults during buckling and bulging of a precollisional choking oceanic slab, and melting of phlogopite-bearing lithosphere with subsequent interaction of the melt with lower crustal garnet-amphibolite of a ~40-km-thick crust, can explain the Kal-e-Kafi geochemical and isotopic signatures. Gravimetric data indicating a ~39 km present-day backarc crustal thickness are consistent with geochemical results but also imply little if any collisional crustal thickening of the central Iranian Plateau. High Eocene arc-backarc melt flux prior to collision in this region reflects vigorous thermal convection, which may in fact be diagnostic of collisional magmatism, explaining the presence of postcollisional shoshonitic melts in this and other collisional orogenic settings.

Discovery of a 25-cm asteroid clast in the giant Morokweng impact crater, South Africa.

Meteorites provide a sample of Solar System bodies and so constrain the types of objects that have collided with Earth over time. Meteorites analysed to date, however, are unlikely to be representative of the entire population and it is also possible that changes in their nature have occurred with time. Large objects are widely believed to be completely melted or vaporized during high-angle impact with the Earth. Consequently, identification of large impactors relies on indirect chemical tracers, notably the platinum-group elements. Here we report the discovery of a large (25-cm), unaltered, fossil meteorite, and several smaller fragments within the impact melt of the giant (> 70 km diameter), 145-Myr-old Morokweng crater, South Africa. The large fragment (clast) resembles an LL6 chondrite breccia, but contains anomalously iron-rich silicates, Fe-Ni sulphides, and no troilite or metal. It has chondritic chromium isotope ratios and identical platinum-group element ratios to the bulk impact melt. These features allow the unambiguous characterization of an impactor at a large crater. Furthermore, the unusual composition of the meteorite suggests that the Morokweng asteroid incorporated part of the LL chondrite parent body not represented by objects at present reaching the Earth.

Platinum-group mineral assemblages in the Platreef at the Sandsloot Mine, northern Bushveld Complex, South Africa

Abstract Platinum group mineral (PGM) assemblages in the Platreef at Sandsloot, northern Bushveld Complex, in a variety of lithologies reveal a complex multi-stage mineralization history. During crystallization of the Platreef pyroxenites, platinum group elements (PGE) and base-metal sulphides (BMS) were distributed thoughout the interstitial liquid forming a telluride-dominant assemblage devoid of PGE sulphides. Redistribution of PGE into the metamorphic footwall by hydrothermal fluids has formed arsenide-, alloy- and antimonide-dominant assemblages, indicating a significant volatile influence during crystallization. Serpentinization of the footwall has produced an antimonide-dominant PGM assemblage. Parts of the igneous reef were subjected to alteration by a late-stage, Fe-rich fluid, producing ultramafic zones where the telluride-dominant assemblage has been recrystallized to an alloy-dominant one, particularly rich in Pt-Fe and Pd-Pb alloys. A thin, small-volume zone of PGE- BMS mineralization along the base of the hangingwall contains a primary PGM assemblage that is locally altered to one dominated by Pt/Pd germanides. This is thought to have formed when the new pulse of Main Zone magma entered the chamber, and scavenged PGE from the underlying Platreef pyroxenites. That each major rock type at Sandsloot contains a distinctive PGM assemblage reflects the importance of syn- and post-emplacement fluid and magmatic processes on the development of Platreef mineralization.

A baseline survey of the distribution and origin of platinum group elements in contemporary fluvial sediments of the Kentish Stour, England

The extensive anthropogenic use of the platinum group elements (PGE: Ru, Rh, Pd, Os, Ir and Pt) is the main factor responsible for the widespread dispersion of these elements throughout the environment. Significant quantities of the PGE enter fluvial systems via road runoff, storm drains and wastewater and sewage treatment systems, and may accumulate in fluvial sediments by physical and chemical processes. A baseline survey of contemporary fluvial sediments in the Kentish Stour river, east Kent, England, has been undertaken to document the sources and distribution of anthropogenic PGE in an attempt to constrain some of the physical and chemical parameters that may influence the distribution of these elements. The geology of the catchment of the Kentish Stour is dominated by carbonate and silicate sedimentary rocks, and the river passes through urban and rural land and receives inputs of waste from sewage works. Nine sedimentary rocks, 4 motorway-runoff sediments and 22 river sediments were analysed for PGE by NiS fire assay preconcentration and ICP–MS. The highest element abundances occur in the motorway-runoff sediments (maximum total PGE content of 55 ng/g), whilst the lowest values were recorded in the sedimentary rocks, where some samples contain PGE at concentrations below the limit of detection. The total PGE content of the river sediments ranged from 0.4 to 10.8 ng/g. The distribution and variation in concentrations and ratios of the PGE in the contemporary fluvial sediments of the Kentish Stour correspond strongly with land-use changes (urban versus rural) and with points of discharge from sewage works. The absence of a pure catalytic converter signature in the river sediments, however, indicates that source signatures may be mixed in sewage works or that PGE may chemically fractionate in the fluvial environment.

Platinum-group element mineralization in the Platreef and calc-silicate footwall at Sandsloot, Potgietersrus District, South Africa

Abstract The Platreef is a platinum-group element (PGE) deposit in the form of a mafic-ultramafic, tabular body at the base of the northern (Potgietersrus) limb of the 2050 m.y. Bushveld Igneous Complex. The reef transgresses sedimentary floor rocks (footwall) of the 2600-2200 m.y. Transvaal Supergroup and the Archaean granite basement. The roof rocks (hanging-wall) of the reef are PGE-free Main Zone gabbronorites of the Rustenburg Layered Suite. At the Sandsloot open-pit mine the Platreef consists of coarse to pegmatoidal pyroxenites and gabbros with accessory phlogopite, base-metal sulphides and oxides. Thermal metamorphism of siliceous dolomites that form the footwall has produced clinopyroxenites and calcsilicate hornfelses with a variety of skarn assemblages. These were subjected to later hydrothermal alteration and serpentinization that also affected parts of the Platreef. The link between sulphides and PGE in the Platreef has led previous authors to consider the mineralization as an orthomagmatic sulphide deposit, where sulphide separation collected PGE from a large volume of melt. In the reef and footwall, however, the development of extensive alteration zones with high concentrations of PGE- and semi-metal (Te, Sb, Se, Bi and Ge)-bearing platinum-group minerals that are typical of many low-temperature PGE deposits suggests syn- to post-magmatic crystallization or redistribution of PGE by hydrothermal fluids. The results obtained to date in a new study suggest that the Platreef at Sandsloot is a complex PGE deposit that has been subject to a number of different processes during its development.

Geochemistry and petrology of Witwatersrand and Dwyka diamictites from South Africa: search for an extraterrestrial component

Abstract Diamictites are poorly sorted sediments characteristically carrying coarse-grained clasts in a fine-grained matrix. They have generally been considered of glaciogenic or glaciomarine origin. Recently, however, it has been suggested that some massive tillite/diamictite layers could represent impact breccias. An earlier petrographic study of rock and mineral clasts from Dwyka Group diamictites revealed no evidence for shock metamorphism, such as planar deformation features. Detailed geochemical studies of diamictite samples from the Archean Witwatersrand Supergroup and the Dwyka Group of the Mesozoic Karoo Supergroup from South Africa are reported. We studied the contents of the siderophile elements in these breccias, as elevated abundances of such elements, especially iridium, could be indicative for an impact origin. By use of γ-γ coincidence spectrometry and other trace element analysis, geochemical tracers of extraterrestrial components were sought. However, no enrichments of indicator elements for extraterrestrial components, compared with ordinary continental crust, were found. Thus, neither geochemical nor petrographic evidence supports an impact origin of the diamictites from the Dwyka Group and the Witwatersrand Supergroup in South Africa.

Mineral Deposits and Earth Evolution

Mineral deposits are not only primary sources of wealth generation, but also act as windows through which to view the evolution and interrelationships of the Earth system. Deposits formed throughout the last 3.8 billion years of the Earth’s history preserve key evidence with which to test fundamental questions about the evolution of the Earth. These include: the nature of early magmatic and tectonic processes, supercontinent reconstructions, the state of the atmosphere and hydrosphere with time, and the emergence and development of life. The interlinking processes that form mineral deposits have always sat at the heart of the Earth system and the potential for using deposits as tools to understand that evolving system over geological time is increasingly recognized. This volume contains research aimed both at understanding the origins of mineral deposits and at using mineral deposits as tools to explore different long-term Earth processes.