Marco A.G. Andreoli

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.

Geochemistry across an exposed section of Archaean crust at Vredefort, South Africa: with implications for mid-crustal discontinuities☆

Abstract The central region of the Vredefort structure consists of a semi-circular multi-layered sequence of crystalline rocks which are nearly vertical in attitude, and which increase in metamorphic grade towards the core of the structure. Together with the overlying Precambrian strata, this sequence provides a cross-section through almost the entire crustal section of the Kaapvaal craton (36 km). The upper part of the Vredefort crystalline crust consists of a 3.0-Ga sequence of differentiated felsic rocks in amphibolite facies. The lower crust consists of a complex and heterogeneous (both chemically and isotopically) high-grade metamorphic terrain of charnockites, granulites (mafic and felsic) and supracrustal rocks. The upper crust is separated from the lower crust by the Vredefort discontinuity, a brittle-ductile shear zone characterised by a high concentration of pseudotachylite and brecciated rock. Petrographic, chemical and isotopic evidence suggest that the upper and lower crust have undergone very different styles of evolution. This is indicative of different geological environments prior to their present juxtaposition. We speculate that the upper and the lower parts of the Vredefort crystalline crust were juxtaposed during intracratonic thrusting ∼2.8 Ga ago, and that the entire 36-km section of crust, including the Vredefort discontinuity, were rotated and uplifted into their present vertical position during the 2.0-Ga Vredefort catastrophe.

Late Jurassic age for the Morokweng impact structure, southern Africa

Abstract A roughly 70 km diameter circular feature buried beneath the Kalahari sands in South Africa is revealed on regional aeromagnetic maps. Boreholes drilled into the centre of the structure intercept a ∼ 250 m thick sheet of quartz norite, interpreted as an impact melt, which overlies brecciated and shock metamorphosed basement granite. Zircons recovered from the quartz norite, yield U-Pb ages of 145 ± 0.8 Ma, and biotites provide Ar-Ar ages of 144 ± 4 Ma. These data provide strong evidence for the occurrence of a Late Jurassic impact crater (the Morokweng impact structure) ∼ 100 m beneath the surface.

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.

Magnetic anomaly near the center of the Vredefort structure: Implications for impact-related magnetic signatures

A strong magnetic anomaly near the center of the ancient and deeply eroded Vredefort structure is attributed to remanent magnetization caused by a large meteorite impact at ∼2.0 Ga. The rocks underlying the anomaly are Archean gneisses thought to represent mid-crustal depths that were uplifted to the surface during the postulated impact event. Measurements of the remanent magnetization of the basement rocks yielded consistent vectors of declination = 25° , inclination = 56° , k = 18, α = 16 that correspond to the paleomagnetic pole position at time of impact. Petrologic studies indicate that during impact, large volumes of rock were heated enough to cause thermal remagnetization in the ambient field. Thermal effects of all large impacts on cratons may induce a remanent magnetization of sufficient intensity to cause anomalies in the geomagnetic field that are detectable even by satellites.

Archean age for the granulite facies metamorphism near the center of the Vredefort structure, South Africa

Granulite facies metamorphic assemblages in rocks exposed near the center of the 2.02 Ga Vredefort impact structure previously have been interpreted either as Early Proterozoic, genetically related to the 2060 Ma Bushveld Complex, or as Archean, and representative of lower crust that rebounded to upper crustal levels following an impact event. Zircon and monazite recovered from the granulite facies rocks record high-grade metamorphism at 3107 ± 9 Ma and a primary age of ≥ 3425 Ma for detrital zircon. A shock-deformed, but otherwise pristine, dolerite dike that intrudes the granulite terrane yields a U-Pb zircon age of ≥ 2560 Ma, providing a minimum age for the metamorphism. These isotopic age data are difficult to reconcile with a regional high-grade metamorphic event in the crust beneath Vredefort at 2060 Ma. Instead, the preimpact, high-temperature metamorphic history observed in the Vredefort lower crustal rocks indicates an enigmatic high-temperature event during the stabilization of diamondiferous Archean tectosphere.

Aspects of the dynamic and thermal metamorphic history of the Vredefort cryptoexplosion structure: implications for its origin

Abstract The Vredefort structure is the oldest and the largest known cryptoexplosion structure on earth. An approximately 36 km deep section through the Archean sialic crust and the overlying Precambrian strata of the Kaapvaal craton is exposed in the core of the structure. The geology presented in the exposed section includes all the principal metamorphic facies in the crust and records a long and complex thermo-tectonic history which dates back to at least 3.5 Ga. The petrographie and geological observations in the basement rocks indicate that there is a complex interrelationship between the Archean geology and the 2.0 Ga dynamic and thermal metamorphic overprint (some of which are postulated to be indicative of impact processes). The dynamic and thermal metamorphic effects do not increase progressively towards the centre of the structure as found at known impact structures. In particular, dynamic deformation phenomena such as pseudotachylite and planar features in quartz reach maximum intensity in the rocks close to the Vredefort discontinuity, a brittle-ductile shear zone which separates upper crustal amphibolite facies rocks from lower crustal granulites. In certain other lithological zones, deformation phenomena are noticeably absent or diminished. We suggest that changes in the physical and chemical properties of the rocks from margin to centre of the basement may account for the variation in the intensity of the 2.0 Ga thermal and dynamic metamorphic effects observed at Vredefort. In conclusion, our overall impression of the Vredefort structure is that it is a relic of an ancient meteorite impact crater, but that there were thermo-tectonic events which occurred both prior to and after the postulated impact event, which complicates the interpretation of its origin.

Denudation along the Atlantic passive margin: new insights from apatite fission-track analysis on the western coast of South Africa

Abstract Apatite fission-track (AFT) data from two traverses across the Great Escarpment of the western coast of South Africa are used to reconstruct the tectonic evolution and denudation history of this sector of the Atlantic passive margin. Fission-track ages range between 180 and 86 Ma. Modelling of this data identifies two distinct cooling events. The first event, between 160 and 138 Ma, is recorded only by the rocks above the escarpment in the Karoo area, and is tentatively linked to post-Karoo magmatism (c. 180 Ma) thermal relaxation. The second, between 115 and 90 Ma, results instead from a tectonically induced denudation episode responsible for the removal of up to 2.5 km of crust across the coastal zone in front of the escarpment and less than 1 km on the elevated interior plateau. Based on these results, it is suggested that the Cretaceous is the time when most of the elevated topography of Southern Africa was generated, with only a minor Cenozoic contribution.

The degradation of monazite Implications for the mobility of rare-earth and actinide elements during low-temperature alteration

Monazite is an important economic source of thorium, the rare-earth elements and uranium. This and its chemically inert nature has led to speculation that artificial phosphate-based matrices similar in composition to monazite may prove useful as wasteforms for high-level radioactive waste. In order to assess the long-term degradation behaviour of monazite, an integrated geological, hydrogeological and mineralogical study was undertaken at the Steenkampskraal monazite mine, South Africa. Steenkampskraal is among the richest monazite ore bodies in the world comprising up to 45 weight % rare-earth oxides, 8.8% thorium oxides and 600 ppm uranium. Optical and electron-microprobe analyses of the ore reveal distinctive alteration patterns with uranium appearing to be lost preferentially. The heavy rare earths are also preferentially removed from the ore, giving rise to a marked fractionation in altered grains. Thorium, although leached, is re-concentrated together with the heavier rare-earth elements in microcrystalline silicate and oxide alteration products within the host rock. The implications for waste encapsulation are discussed from the perspective of potential groundwater transport away from the source.

Siderophile-rich inclusions from the Morokweng impact melt sheet, South Africa: possible fragments of a chondritic meteorite

An 870 m thick (diameter ∼30 km) melt sheet associated with the Morokweng impact structure, South Africa, contains Cr-rich silicate inclusions (up to 30 mm in diameter) as well as disseminated Ni-rich sulphides and oxides that commonly occur in mutual intergrowths. Although the silicate inclusions are highly altered, the mineral chemistry and petrographic evidence, along with broadly chondritic Re–Os systematics and platinum-group element (PGE) signatures, all support the view that the silicate inclusions represent relict fragments of an impactor with compositions similar to ordinary chondrites. The Ni-rich sulphides and oxides are significantly enriched (up to 35× chondrites) in PGE, Ni and Cr. Although the bulk PGE signature of the melt sheet is chondritic, the sulphides and oxides display mm and sub-mm scale fractionation of PGE into different inclusion components. A model is presented whereby pieces of the projectile react with the relatively more oxidising impact melt into which they fell, resulting in oxidation and fractionation of the metal–silicate fragments to form the inclusion assemblages. This may be the first reported occurrence of a large melt sheet (300 km3) contaminated by swarms of discrete pebble-sized fragments of asteroid material. The high proportion of meteorite fragments in the Morokweng melt sheet suggests either that the structure was formed by a relatively low velocity (<18 km/s) impact event, or that the angle of impact was optimum for retaining maximum amounts of meteoritic material within the crater.