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- Introduction
- VREDEFORT DOME - World Heritage Datasheet
- Earth's oldest asteroid impact 'may have ended ice age'
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The crust rose up like a dome — picture the slow-motion videos of a raindrop hitting a bowl of water. Moser published his discovery in Geology in , and immediately sparked a battle over whether the dikes, filled with a rock called gabbronorite, were truly remains of Vredefort's impact melt. That battle continues today. Some researchers objected because the magma had an unusual layered appearance called foliation, common in altered rocks. Perhaps it was simply another pseudotachylite, or part of the Earth's original crust, the critics said.
Others suggested the young zircons could have crystallized in pre-existing rocks from the impact's heat. So Moser and Cupelli recently returned to South Africa and searched for definitive evidence that the magma dikes were as young as the crater itself. Now Cupelli, who led the new study, thinks the team can prove the magmas were born in Vredefort's impact melt. The zircons are randomly distributed and interlaced with their surrounding minerals — they couldn't have grown from the heat of impact later than their neighbors, she said.
The zircons also crystallized between 1, to 1, degrees Fahrenheit to degrees Celsius , hotter than normal on Earth, but the same temperature as in Sudbury's impact melt. Finally, levels of the element hafnium suggest that the magma melted from the 3-billion-year-old rocks originally overlying the crater the same sedimentary and volcanic rocks in nearby Witwatersrand Basin , not from the very deep crust now exposed by 2 billion years of erosion.
The new study has already kicked off a search for Moser's rocks by other Vredefort researchers, who hope to confirm or deny the results. Reimold is firmly in the anti-impact-melt camp, though he praised the study's zircon chemical techniques. But Moser thinks the unusual appearance of Vredefort's impact melt could also help researchers search for older impact craters, which have been confirmed only through discovery of impact ejecta.
Ramdohr P New observations on the ores of the Witwatersrand in South Africa and their genetic significance. Thesis, University of Cape Town, 59 pp unpublished. Robb LJ, Meyer FM A contribution to recent debate concerning epigenetic versus syngenetic mineralization processes in the Witwatersrand basin.
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Robinson P, Ross M, Jaffe HW Composition of the anthophyllite-gedrite series, comparisons of gedrite and hornblende, and the anthophyllite-gedrite solvus. Am Mineral — Rev Mineral 9B: 1— Mineral Soc Amer, Spec Paper 2: — Mineral Deposita 3: — J Afr Earth Sci 65— Springer, New York, pp. Velde B, Medhioub M Approach to chemical equilibrium in diagenetic chlorites. Download references. Reprints and Permissions. Frimmel, H. Witwatersrand iron-formations and their significance for gold genesis and the composition limits of orthoamphibole.
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Mineralogy and Petrology 56, — Download citation. Received : 16 August Accepted : 28 June Issue Date : September Search SpringerLink Search. Summary In the West Rand Group of the 3. Zusammenfassung Die West Rand Group innerhalb der 3. Immediate online access to all issues from Texture component TC maps display variations in crystal orientation relative to a user-selected reference point indicated by the red cross in Figs.
Inverse pole figure IPF maps are color-coded according to Miller index to identify variations in crystallographic orientation. The grain boundary GB function inserts a colored line red along the boundary of adjacent pixels that form a specific misorientation about a specific axis. Red cross indicates reference location.
Introduction
Images of grain 14VD Interior images of grain 09VD U-Th-Pb geochronology data were collected for three shocked zircon grains. Four analyses of grain 09VD were performed using a 25 mm primary beam. A total of eight analyses were conducted on the two zircon grains, including five analyses of zircon 14VD and three analyses of zircon 14VD A total of 11 detrital zircon grains from the three disparate geographic areas of South Africa were surveyed to search for shock features. Shocked grains were only identified in three samples near the Orange River mouth on the Atlantic coast Fig.
A summary of the imaging and geochronological results is presented below. From the middle Orange River a total of detrital zircon grains from five modern alluvium samples were surveyed. Sample 14VD69 was collected from a sandbar 2 km downstream from the Vaal-Orange river confluence. No shocked zircon grains were found in the populations investigated, which ranged from 41 09VD45 to 14VD69 grains per sample Table 1.
A total of zircon grains were surveyed from two modern alluvium samples collected from sandbars 14VD76, 14VD77 , and two modern beach sand samples 09VD48, 14VD80 at and near the mouth of the Orange River, approximately km downriver from the Vredefort Dome. One of the grains was found in a sandbar 15 km upriver from the Atlantic coast 14VD Figs.
Each of the three sediment samples yielded a single detrital shocked zircon, which equates to abundances of 0.
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Shock features are not obvious in the CL image of this grain, which shows both oscillatory and sector zoning Fig. PFs are faintly visible in CL Fig. A total of detrital zircon grains were surveyed from 3 modern beach sand samples collected south of the Orange River mouth on the Atlantic coast. Sample 14VD89, located km south of the Orange River mouth, was sampled at the mouth of the Oliphants River, which represents the southern-most location of the paleo-Orange River mouth since the Cretaceous Dingle and Hedey However, no shocked zircon grains were identified in the three populations surveyed, which ranged from 14VD79 to 14VD89 grains per sample.
A total of detrital zircon grains from two samples of modern alluvium near the Morokweng impact structure were surveyed.
Sample 09VD51 was collected from the Phepane River, and sample 09VD52 was collected from the Mashowing River; these two rivers, both dry, drain the northern and southern regions of the Morokweng structure. Both fluvial systems are tributaries of the Molopo River, which is a right-bank tributary of the Orange River; the two samples are located and km upriver from the Orange River mouth, on the southern margin of the Kalahari desert Fig. Both samples yielded few zircon grains, resulting in a relatively small number of grains surveyed. No shocked zircon grains were found in either population, which ranged from 86 09VD51 to 67 09VD52 grains per sample Table 1.
Regressions in the top two panels are not anchored to the concordia curve. The three detrital zircon grains 09VD, 14VD, and 14VD discovered near the mouth of the Orange River at the Atlantic coast preserve unambiguous shock microstructures.
VREDEFORT DOME - World Heritage Datasheet
The grains contain multiple orientations of PFs visible on external surfaces, which correlate to features visible in polished section. The microstructures described above provide clear evidence of shock deformation, but cannot alone be used to determine the provenance of the grains.
The above ages correlate well with ages of bedrock exposed in the core of the Vredefort impact structure, which range from to Ma Kamo et al. Pb-loss within the analyzed zircon grains does not record the Ma Vredefort impact, and was likely caused by either the Neoproterozoic Pan-African orogeny or the 1. Similar Pb-loss ages have been reported in studies of shocked zircon from Vredefort bedrock Flowers et al. The possibility that the three detrital shocked zircon grains described here did not originate from the Vredefort structure was also considered.
Only one other impact crater has been confirmed in the Orange River basin. The Morokweng structure is within the Molopo River basin, a northern right-bank tributary whose confluence with the Orange River is km upriver from the Atlantic coast. The location of Morokweng inside the Orange basin allows for the possibility that detrital shocked zircon at the Orange River mouth may have originated from this structure. However an origin from Morokweng is considered unlikely for the following reasons.
Earth's oldest asteroid impact 'may have ended ice age'
The Morokweng structure is buried, and shocked bedrock is not exposed. Archean granitoids ranging from 3. Ephemeral streams crosscut the Morokweng structure, two of which, the Phepane and Mashowing dry rivers, were sampled for this study. Corner et al. However, our survey of 86 detrital zircon grains from the Mashowing River and 67 detrital zircon grains from the Phepane River did not identify any shocked zircon grains Table 1.
For the past years, a barchan dune has blocked the Molopo River 15 km upstream from its confluence with the Orange River; today the Molopo is a dry ephemeral river that only flows during infrequent floods Bremner et al. The Morokweng impact structure likely contributed detrital shocked minerals to local fluvial systems in the Kalahari region after its formation at Ma, and before subsequent burial.