Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.531k&link_type=abstract
Meteoritics, vol. 30, no. 5, page 531
Other
1
Ar-Ar Ages, Chondrites, Bluff, Ordinary, Rose City, Dating, Impacts, Shock
Scientific paper
Since the beginning of crater dating it has been a matter of debate which chronometer should be applied on which kind of impact-metamorphic rock to obtain the true age of a crater. The transition of even a high pressure shock wave in itself appears to be insufficient to fully reset radiometric isotope systems although the K-Ar system suffers partial 40Ar loss and redistribution [1,2]. But rocks that were demonstrably molten in the course of the impact are first choice samples for crater dating. This selection criterion has proven successful to date terrestrial and lunar impact structures by stepwise heating 40Ar-39Ar analysis [e.g. 3-5]. On the other hand, some observations seem to indicate that even totally molten material can retain a significant amount of previously accumulated radiogenic 40Ar [6,7]. In our ongoing effort to contribute to the solution of crater dating problems, we have performed high-resolution 40Ar-39Ar analyses of a suite of highly shocked ordinary chondrites and here report as examples the results of Bluff (L5/S6) and Rose City (H5/S6) The melt samples from Bluff and Rose City yield plateau ages of 0.85 + 0.01 Ga (9 fractions) and 3.62 +/- 0.01 Ga (16 fractions), respectively, for the high temperature release above 1300 degrees C comprising ~65% of the total 39Ar content (Fig. 1). These are the times of the last total degassing of the rocks. Low ages below 1000 degrees C extraction temperature clearly indicate some later disturbances that are more pronounced in the spectra of the host rocks (ages <= 0.5 Ga). The high spectral resolution of the analyses, i.e. the large number of degassing steps, rules out that the plateau ages result from partial degassing of ~4.5 Ga old rock by a thermal or shock-induced overprint as late as ~0.5 Ga. For this reason at least two events affecting the K-Ar systems in these meteorites are needed to explain the 40Ar-39Ar spectra. We associate the plateau ages with the formation of the melts and thus with the time of severe shock metamorphism. Also impact related, but weaker disturbances thereafter caused partial resetting of the radiometric clock affecting the host rocks more severely than the melts. This interpretation is supported by the fact that the main 39Ar release occurs above 1300 degrees C from the melts while below 1000 degrees C from the host rocks. Thus, the melt is more retentive than the host rock and consequently less affected by the late event. This degassing behavior contrasts to the observation that melts are crystallized on a, micrometer-scale and expectedly should release argon earlier than the host rocks. TEM information on the K-bearing sites in the melts is needed for model calculations of the age spectra in order to test our conclusion and to obtain evidence on the history of the meteorites and their parent bodies. References: [1] Bogard D. et al. (1987) GCA, 51, 2035-2044. [2] Trieloff M. et al. (1994) Meteoritics, 29, 541. [3] Jessberger E. K. et al. (1977) Proc. LSC 8th, 2567-2580. [4] Staudacher T. et al. (1982) JGR, 51, 1-11. [5] Stephan T. and Jessberger E. K. (1992) GCA, 56, 1591-1605. [6] Begemann F. et al. (1992) Meteoritics, 27, 174-178. [7] Bogard D. D. et al. (1995) GCA, 59, 1383-1399.
Falter Martina
Jessberger Elmar K.
Kunz Jacob
Stäffler Dieter
Trieloff Mario
No associations
LandOfFree
Dating Impacts: New Constraints from 40Ar-39Ar Analyses of Shocked Chondrites 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 Dating Impacts: New Constraints from 40Ar-39Ar Analyses of Shocked Chondrites, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dating Impacts: New Constraints from 40Ar-39Ar Analyses of Shocked Chondrites will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-830087