Showing papers by "Fred Jourdan published in 2008"
TL;DR: In this paper, the authors present 16 new 40Ar/39Ar analyses carried out on fresh plagioclase and biotite separates from 15 sill samples collected along a N-S trend in the eastern part of the Main Karoo sedimentary basin.
Abstract: [1] Reliable geochronological results gathered so far (n = 76) have considerably constrained the timing of the emplacement of the Karoo large igneous province (LIP). Yet strikingly missing from this dating effort is the huge southern sill complex cropping out in the >0.6 × 106 km2 Main Karoo sedimentary basin. We present 16 new 40Ar/39Ar analyses carried out on fresh plagioclase and biotite separates from 15 sill samples collected along a N–S trend in the eastern part of the basin. The results show a large range of plateau and miniplateau ages (176.2 ± 1.3 to 183.8 ± 2.4 Ma), with most dates suggesting a ∼3 Ma (181–184 Ma) duration for the main sill events. The available age database allows correlation of the Karoo LIP emplacement with the Pliensbachian-Toarcian second-order biotic extinction, the global warming, and the Toarcian anoxic event (provided that adequate calibration between the 40K and 238U decay constant is made). The mass extinction and the isotopic excursions recorded at the base of the Toarcian appear to be synchronous with both the increase of magma emission of the Karoo LIP and the emplacement of the sills. The CO2 and SO2 derived from both volcanic emissions as well as carbon-rich sedimentary layers intruded by sills might be the main culprits of the Pliensbachian-Toarcian climate perturbations. We propose that the relatively low eruption rate of the Karoo LIP is one of the main reasons explaining why its impact on the biosphere is relatively low contrary to, e.g., the CAMP (Triassic-Jurassic) and Siberia (Permo-Triassic) LIPs.
118 citations
TL;DR: In this article, the authors presented new 40 Ar/39 Ar data for the Janisjarvi impact melt rock and obtained five concordant isochron ages that yield a combined estimated age of 682 −± −4 −Ma (2 σ ) with a MSWD of 1.2, P ǫ = 0.14, and 40 Ar / 36 Ar intercept of 475 −±−3.
28 citations
01 Jan 2008
TL;DR: In this article, the authors presented new 40 Ar/39 Ar data for the Janisjarvi impact melt rock and obtained five concordant isochron ages that yield a combined estimated age of 682 −± −4 −Ma (2 σ ) with a MSWD of 1.2, P ǫ = 0.14, and 40 Ar 36 Ar intercept of 475 −±−3.
Abstract: Abstract The ∼ 14 km diameter Janisjarvi impact structure is located in Svecofennian Proterozoic terrain in the southeastern part of the Baltic shield, Karelia, Russia. Previous radioisotopic dating attempts gave K/Ar and 40 Ar/ 39 Ar ages of 700 ± 5 Ma and 698 ± 22 Ma, respectively, with both results being difficult to interpret. Recent paleomagnetic results have challenged these ages and proposed instead ages of either 500 Ma or 850–900 Ma. In order to better constrain the age of the Janisjarvi impact structure, we present new 40 Ar/ 39 Ar data for the Janisjarvi impact melt rock. We obtained five concordant isochron ages that yield a combined isochron age of 682 ± 4 Ma (2 σ ) with a MSWD of 1.2, P = 0.14, and 40 Ar/ 36 Ar intercept of 475 ± 3. We suggest that this date indicates the age of the impact and therefore can be used in conjunction with existing paleomagnetic results to define the position of the Baltica paleocontinent at that time. Argon isotopic results imply that melt homogenization was achieved at the hundred-micrometer scale certainly, because of the low-silica content of the molten target rock that allows fast 40 Ar ⁎ diffusion in the melt. However, the large range of F ( 40 Ar ⁎ inherited ) (4.1% to 11.0%) observed for seven grains shows that complete isotopic homogenization was not reached at the centimeter and perhaps the millimeter scale. The F ( 40 Ar ⁎ inherited ) results are also in good agreement with previous Rb and Sr isotopic data.
25 citations