Dating the shock wave and thermal imprint of the giant Vredefort impact, South Africa
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Research Article| January 01, 1997 Dating the shock wave and thermal imprint of the giant Vredefort impact, South Africa D. E. Moser D. E. Moser 1Jack Satterly Geochronology Laboratory, Royal Ontario Museum, Toronto M5S 2C6, Canada Search for other works by this author on: GSW Google Scholar Geology (1997) 25 (1): 7–10. https://doi.org/10.1130/0091-7613(1997)025<0007:DTSWAT>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation D. E. Moser; Dating the shock wave and thermal imprint of the giant Vredefort impact, South Africa. Geology 1997;; 25 (1): 7–10. doi: https://doi.org/10.1130/0091-7613(1997)025<0007:DTSWAT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract U-Pb geochronology of single grains of zircon and monazite has been used to date an episode of intense postimpact metamorphism in the core of the deeply eroded Vredefort impact structure of South Africa. Results from two basement units exposed in the uplifted central region indicate that the impact and a later pyroxene hornfels metamorphic event were penecontemporaneous at 2020 ± 3 Ma. Discovery of a synimpact to postimpact dike of norite that intruded at 2019 ± 2 Ma is the first recognition of mafic igneous activity related to impact. The dike is either derived from a Sudbury-type impact melt layer (since eroded) or is the product of decompression melting of Kaapvaal mantle in response to the ablation of >15 km of crust at the center of the crater. The combined heating effects of the shock wave and impact-triggered magmas are thought to have created the 300 km2 thermal imprint of the asteroid collision with Kaapvaal craton, and account for the nearly coeval timing relationship between core metamorphism and shock revealed by this study. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.Keywords:
Geochronology
Large igneous province
Pyroxene
Radiometric dating
Optical, x-ray, and differential thermal methods are used to compare monazite recently discovered near Chester, New Jersey, with monazite from nine other localities. lndices of refraction, 2V and birefringence have been determined. X-ray-diffraction patterns have been measured, indexed, and their relationships determined. Micro-camera diffraction patterns have provided information on the alteration of monazite. The application of x-ray fluorescence to the quantitative analysis of rare earth elements in monazite is shown to be feasible through sensitivity to minor variations caused by crystal fractionation. Theoretical factors which interfere with precise x-ray fluorescence, quantitative analysis of rare earth elements in monazite are examined. The effect of two types of alteration, intercrystalline and intracrystalline, upon the differential thermal pattern of monazite is observed. (auth)
Differential thermal analysis
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Abstract The content of monazite in the Egyptian black beach sand and coastal sand dunes is normally equal to or below 0.01 wt.%. The obtained high grade monazite concentrate includes three minor monazite groups in addition to major canary and lemon yellow coloured monazite: (i) the colourless to pinkish white coloured monazite; (ii) the opaque light to dark resinous, reddish brown and dark brown coloured monazite; and (iii) opaque yellowish red to brownish red coloured monazite grains group. These groups represent 3%, 4% and 2%, respectively, in the high grade monazite concentrate. A negligible amount of euhedral to subhedral black to brownish black chevkinite/perrierite mineral crystals was detected in the obtained monazite concentrate. The presence of these minor mineral groups affects the chemical composition of the obtained high grade monazite concentrate. The Ce 2 O 3 is the main REE in the studied monazite. In the colourless‐pinkish monazite grains, the analyzed REE are the following, in order of abundance; Ce > La > Nd > Pr > Sm > Gd > Dy. UO 2 ranges between 0.11 and 1.74 wt.%. The contents of Eu 2 O 3 is under the limit of detection while ThO 2 ranges between 3.99 and 8.58 wt.% with an average value of 5.57 wt.%. These grains are most probably igneous monazite from a highly differentiated granite. The resinous, brown monazite grains have lower Ce 2 O 3 content (24.63 wt.%) and much lower La 2 O 3 content (6.00 wt.%) but greater content of Eu 2 O 3 (0.41 wt.%) than those of the colourless‐pinkish monazite. These monazites have the lowest contents of Th, U and Ca among the three groups. The resinous, brown monazites are most probably formed by metamorphism or alteration leading to leaching or replacement of pre‐existing minerals. The red monazite group has a lower average Ce 2 O 3 content (25.28 wt.%) than the colourless‐pinkish variety (28.02 wt.%) but slightly greater than that of the resinous, brown ones. The red monazite group has the highest ThO 2 and UO 2 contents; 5.84 wt.% and 1.24 wt.%, respectively. It has the lowest monazite component mole fraction (0.75). The red monazite seems to have been formed by hydrothermal alteration of pre‐existing monazite and other mineral species bearing for Y, REE, Ca, Th and U. The two coupled substitution mechanisms: (Th, U) 4+ + Ca 2+ 2REE 3+ , and (Th, U) 4+ + Si 4+ REE 3+ + P 5+ , are obvious in the studied colourless‐pink monazite.
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In determining lead in monazite [(Ce,La,Th)PO4]--to be used as the basis for geologic age measurements--it was necessary to eliminate interferences due to the presences of phosphates of thorium and the rare-earth metals. The method, in which monazite samples are attacked with hot concentrated sulfuric acid, taken up with dilute nitric acid, lead extracted as the dithizonate and then determined spectrophotometrically at 520 mμ, was successfully applied to a series of monazite samples. Rapid determinations were made with good reproducibility.
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