New age estimation of the Monturaqui impact crater

New age estimation of the Monturaqui impact crater

Valenzuela, M.; Bourlès, D. L.; Braucher, R.; Faestermann, T.; Finkel, R. C.; Gattacceca, J.; Korschinek, G.; Merchel, S.; Morata, D.; Poutivtsev, M.; Rochette, P.; Rugel, G.; Suavet, C.

The Monturaqui impact crater is the only meteorite impact related structure yet found in Chile. It is localized at 3,015 m altitude in the precordillera near the southern end of Salar de Atacama. It is a simple crater of ~400 m diameter and ~34 m of depth [1], first referred as an impact crater by [2]. The age of the crater was estimated as older than 0.1 Ma by [3] by thermoluminiscence analysis.
We are reporting the first absolute ages of the Monturaqui impact crater following two approaches: a) the terrestrial age of the impactor by measuring the residual activities of 10Be, 26Al, 36Cl, 41Ca, 59Ni, 60Fe, and 53Mn in selected iron shale samples, which corresponds to the remaining altered fragments of the impactor, inferred to be an iron meteorite, and b) in-situ ages obtained through the use of long-lived terrestrial cosmogenic radionuclides 10Be and 26Al in the granite outcrops exposed to cosmic radiation starting after the impact.
Chemical preparation of targets suitable for accelerator mass spectrometry (AMS) have been performed after [4] (for the iron meteorite sample) and a combination of slight modifications of [4] and [5] (for the granite samples).
AMS measurements of 10Be and 26Al have been performed at the French 5 MV-AMS facility ASTER, 36Cl at CAMS, LLNL, USA, and 53Mn at the Maier-Leibnitz-Laboratory (MLL).
We can compare our measured radioactivities with depth-depending production rates from sophisticated theoretical Monte-Carlo calculations [ I. Leya]. As these production rates are a function of the chemical composition (of the impactor in space), remaining fragments are highly altered and precise chemical analyses could not yet be achieved, certain further assumptions are influencing the following discussion of our, thus preliminary, data.
The longest-lived radionuclide 53Mn (t1/2=3.7 Ma), normalized to a fully corroded Fe2O3-sample, is the least sensitive nuclide to a varying terrestrial age, thus, providing us with a shielding depth of 62-71 cm. The best fit of the measured shortest-lived radionuclide 36Cl (t1/2=0.3 Ma) with theoretical production rate at that depths is for a terrestrial age of 500-600 ka. The 26Al-activity goes along with that age. Though, the measured 10Be-concentration is far too high in comparison to the theoretical production rate, which are based on an average carbon-content of 0.1% (as Canyon Diablo). As earlier studies [6,7] demonstrated the great influence of inhomogeneous distributed trace elements like C, S, and P on the production rates of lighter cosmogenic radionuclides in iron meteorite samples. Finally, under the contrary assumption of no corrosion of the impactor, the whole discussion changes only slightly: Deeper shielding position (66-80 cm), but as production rates of 53Mn and 36Cl are influenced the same way, the terrestrial age will not change.
Our second approach using terrestrial cosmogenic radionuclides leads to concordant results for 10Be only: The minimum in-situ exposure age of two samples from the crater wall could be calculated to 200-250 ka. A larger age of excavation is very likely due to the subsequent erosion of the crater walls.
We are looking forward to measurements of the most sensible 41Ca (t1/2=0.1 ka) that might improve the accuracy of this age.

References: [1] H. Ugalde et al., MAPS 42 (2007) 2153. [2] J. Sanchez, W. Cassidy, J. Geophys. Res. 71 (1966) 4891. [3] V.F. Buchwald, Handbook of iron meteorites, Univ. of California Press, Berkeley. Vol. 1 (1975) 262. [4] S. Merchel, U. Herpers, RCA 84 (1999) 215. [5] E. T. Brown et al., GCA 55 (1991) 2269. [6] I. Leya, Michel R., Lunar Planet. Sci. 29 (1998) #1172. [7] I. Leya et al., MAPS 32 (1997) A78.

Keywords: impact; terrestrial cosmogenic nuclides; accelerator mass spectrometry; meteorite; exposure age

  • Contribution to external collection
    in: Annual report of the Maier-Leibnitz-Laboratorium für Kern- und Teilchenphysik der Ludwig- Maximilians-Universität München und der Technischen Universität München (MLL), München: Maier-Leibnitz-Laboratorium, 2008, 27

Publ.-Id: 12708