Marian Tredoux

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.

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.

Chemostratigraphy across the Cretaceous-Tertiary Boundary and a Critical Assessment of the Iridium Anomaly

The elevated concentration of iridium-one of the platinum-group elements (PGE)-at the Cretaceous-Tertiary boundary is still the most generally accepted evidence that a large bolide struck the earth at the time of the end-Cretaceous mass extinctions. New chemostratigraphic data for cross-boundary sections from both hemispheres are not easily explained in terms of such an impact event, for example the observation that the PGE patterns show marked differences between the hemispheres. The new constraints indicate that models of mantle-derived PGE should be seriously considered, and that PGE anomalies might not be as useful as previously thought as unambiguous identifiers of large impact events in the earths history.

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.

Determination of the platinum-group elements in South African kimberlites by nickel sulphide fire-assay and neutron activation analysis

Abstract Ten kimberlites from various localities in South Africa have been analysed for all of the platinum-group elements (PGEs) and gold using a nickel sulphide fire-assay preconcentration followed by neutron activation analysis (NAA). Problems encountered during the analysis of these samples prompted a radio tracer study to test the recovery of the precious metals during firing, and then the subsequent dissolution of the assay button. The results of this study suggest solutions to the potential problems of incomplete melting of MgO-rich and CO2-rich during fire-assay, and minimising losses of Pt, Pd and Au during dissolution. Furthermore, this improved procedure offers lower limits of detection than previous methods which combined fire-assay and NAA. The concentrations of PGEs determined in this study of South African kimberlites are compared with previous partial analyses from the literature, indicating that earlier analyses may have seriously overestimated the concentrations of the some PGEs in kimberlites.

Ultramafic rocks at the center of the Vredefort structure: Further evidence for the crust on edge model

Ultramafic rocks from the center of the Vredefort structure previously have been interpreted either as Early Proterozoic igneous rocks genetically related to the Bushveld complex, or as Archean upper mantle rocks. Re-Os isotopic data from these rocks display very low 187 Os/ 188 Os and 187 Re/ 188 Os, suggesting that they are not magma chamber cumulates, but rather the residues of melt extraction, similar in Re-Os systematics to subcontinental lithospheric mantle. Whole-rock Re depletion model ages for two samples are 3.3 and 3.5 Ga, indicating that the postulated melt extraction event must have occurred in the Archean. On the basis of these data we conclude that the ultramafic rocks are not related to the Bushveld event, but rather represent upper mantle that rebounded to crustal levels following an impact event. This conclusion strengthens evidence for a model whereby the rocks exposed in the core of the Vredefort structure represent the lower parts of a crust on edge profile that possibly contains a paleo-Moho very near the Earth9s surface.

New PGE and Re/Os-isotope data from lower crustal sections of the Vredefort Dome and a reinterpretation of its “crust on edge” profile

Granulites exposed in the core of the Vredefort Dome are generally thought to represent components of the Archean middle crust of the central Kaapvaal Craton. These rocks were exhumed from depths of at least 20 km following an ~2.02 Ga meteorite impact event, exposing a “crust on edge” section believed to represent a continuous magmatically-differentiated sequence from upper crust possibly down to Moho. Platinum group element concentrations suggest that mafic and ultramafic enclaves in the lower most sections are fragments of greenstone belt material metamorphosed at granulite grade. Re/Os isotopic data are consistent with previously published U-Pb zircon data, which suggest that the mafic and ultramafic rocks have a primary age of ~3.5 Ga and were metamorphosed to granulite grade at ~3.1 Ga. However, near the core of the Vredefort Dome across a major shear zone, other ~3.5 Ga greenstone belt remnants are at distinctly lower metamorphic grade. The juxtaposition of mafic-ultramafic rocks with a different metamorphic history in the core of the Vredefort structure requires a modified “crust on edge” model that incorporates tectonic stacking of different crustal segments at ~3.1 Ga.

Refractory trace elements in diamond inclusions: Further clues to the origins of the ancient cratons

Diamond inclusions are thought to preserve the characteristics of rocks from beneath the Archean cratons. We have found that refractory trace element ratios in these inclusions are similar to those measured in both rocks of the present-day oceanic lithosphere and some Archean greenstone belts. These observations are consistent with a model in which Archean continental lithosphere evolved by tectonic stacking of oceanic crust and its underlying mantle during intraoceanic collisions. We conceive the continental lithosphere to consist of alternating layers of basalt and peridotite derived from subducted and obducted Archean oceanic lithosphere. This process provides a mechanism for the genesis of diamonds in the metamorphic equivalent of these Archean rocks.