Ancient and recent exposure history of chondrules from two highly primitive meteorites,

Ancient and recent exposure history of chondrules from two highly primitive meteorites,

Ott, U.; Merchel, S.; Beyersdorf-Kuis, U.; Akhmadaliev, S.; Pavetich, S.; Rugel, G.; Ziegenrücker, R.

Chondrules may have spent several million years as free-floating particles in the solar nebula [1], and if so, been exposed to an early cosmic ray irradiation. The search for “pre-irradiation” in noble gas isotopic signatures has, thus, been actively pursued recently [2-4]. Results for two highly primitive CR3 chondrites (MET00426 & QUE99177) [5] are intriguing: 1) They are among the most unmetamorphosed meteorites, most likely to have retained any pre-irradiation record. 2) Target elements were determined by Instrumental Neutron Activation Analysis on the same material used for noble gas analysis. 3) QUE99177 shows no hint for having been part of an asteroidal regolith. 4) Chondrules show both higher and lower cosmic ray exposure than identically-shielded matrix samples.
The shortest cosmic ray exposure determined via stable noble gases is an upper limit to the recent cosmic ray exposure age. Further constraints can be obtained via radionuclides such as 10Be, 26Al, 36Cl, which have been analyzed at DREAMS [6,7]. Despite sample masses of only 1.6-1.8 mg for single chondrules, ratios are as high as 1-3x10-12 for 10Be/9Be and 26Al/27Al, and 1x10-13 for 36Cl/35Cl, clearly distinguishable from blanks. Preliminary evaluation shows that the radionuclides are not in saturation. However, since the meteorites are finds from Antarctica, one also has to consider decay during terrestrial residence. To better constrain this, AMS of 41Ca and 53Mn is scheduled at DREAMS and ANU, respectively.
Ref.: [1] Cuzzi, Nat. Geosci. 4 (2011) 219. [2] Eugster et al., MAPS 42 (2007) 1351. [3] Das & Murty, MAPS 44 (2009) 1797. [4] Roth et al., MAPS 46 (2011) 989. [5] Beyersdorf-Kuis et al., 44th LPSC (2013) 1999. [6] Akhmadaliev et al., NIMB 294 (2013) 5. [7] Rugel et al., AMS-13.

Keywords: accelerator mass spectrometry; meteorites; solar system

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Publ.-Id: 20123