Physics
Scientific paper
Dec 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990gecoa..54.3435s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 54, Issue 12, pp.3435-3447
Physics
10
Scientific paper
Minerals and whole rocks from crystalline fragments collected in the allochthonous polymict breccia of the 23-Ma-old Haughton impact crater on Devon Island (Arctic Canada) were analysed for U-Pb, K and Rb-Sr in order to constrain degrees of shock-wave induced perturbations in these systems. The results show that shock-wave metamorphism at 50 GPa with onset of total melting does not significantly affect the U-Pb chronometers in monazite and zircon. Monazite preserved its age of crystallization during high-grade metamorphism at 1928.2 (+2.1/-2.0) Ma (2 -errors) as dated by the upper intercept of 2-9% discordant monazites. The same analyses yield a lower intercept age of 277.5 (+35.1/ -35.0) Ma in the concordia diagram, demonstrating that discordancy is not a direct effect of shock-wave metamorphism but a product of time integrated Pb-diffusion prior to the impact event, which occurred 23 Ma ago. This constrains maximal shock-induced Pb-loss to less than 3% including possible alteration related Pb diffusion from the shock damaged grains. In 1903.1 (+1.4/-1.1) Ma old monazite from an unshocked reference gneiss, recent Pb-loss also reaches 3% corroborating the occurrence of weak alteration-related U / Pb fractionation. The data substantiate that U-Pb discordancy patterns are not a reliable chronometer to date impact events. Small rock fragments and a coherent block of shock-stage I to III/VI consist of heterogeneous, unequilibrated Rb-Sr sub-systems on the 10 m-scale, with drastically varying I Sr . Shock-related Rb / Sr fractionation is very limited, and, in the large block (stage III), three biotite fractions yield an Rb-Sr isochron age of 1791 ± 34 Ma (2 ), which is undistinguishable from the 1814 ± 8 Ma age of biotite in the unshocked basement gneiss. In contrast to this m 3 -sized fragment, Rb-Sr systematics in a fist-size sample of shock stage III do not yield any reliable age information due to small-scale (< 1 cm 3 ) isotopic disequilibrium. It appears that such small-scale disequilibrium is essentially due to a take-up of radiogenic 87 Sr in feldspar-glass from biotite during shock-wave passage. This prevents dating of the impact events by the Rb-Sr "internal-isochron" method, and it can be concluded that total rock melting is the first condition necessary to reset the Rb-Sr chronometer in shocked crystalline rocks.
Deutsch Alex
Scharer Urs
No associations
LandOfFree
Isotope systematics and shock-wave metamorphism: II. U-Pb and Rb-Sr in naturally shocked rocks; the Haughton Impact Structure, Canada does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Isotope systematics and shock-wave metamorphism: II. U-Pb and Rb-Sr in naturally shocked rocks; the Haughton Impact Structure, Canada, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Isotope systematics and shock-wave metamorphism: II. U-Pb and Rb-Sr in naturally shocked rocks; the Haughton Impact Structure, Canada will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1630564