Bevan, AWR

Bland, PA

Johnson, JA

Jull, AJT

Kim, KJ

McHargue, LR

Meteoritics and Planetary Science

Spiral - Imperial College Digital Repository

STUDIES OF 14C AND 10BE PRODUCTION RATES AND TERRESTRIAL AGES OF DESERT METEORITES.  A. J. T. Jull1, L. R. McHargue1, K. J. Kim1, J. A. Johnson1, P. A. Bland2 & A. W. R. Bevan3. 1 NSF Arizona AMS Laboratory, University of Arizona, Tucson, AZ 85721, USA. E-mail: jull@email.arizona.edu 2Department of Geology, Imperial Col-lege, London SW7, UK.  3Western Australian Museum, Francis St., Perth, WA 6000, Australia.    Introduction: We have investigated the terrestrial ages, or residence time on the Earth’s surface, of 76 meteorites from Western Australia, and one from South Australia, using both 14C measurements and also 14C/10Be.   Some of the 14C measurements have been reported previously [1].  We have now included 10Be measurements from 30 meteorites.   We find that the 14C/10Be terrestrial ages are more precise than 14C alone, since we can cor-rect for shielding effects.  We compared the two different age determinations, where measured, which generally agreed within 1-2ka.    Production rates:  Kring et al. [2] and Welten et al. [3,4] assumed a constant production ratio of 14C to 10Be, in order to use 10Be as a shielding correction for 14C terrestrial ages of meteorites of 2.5 to 2.6.  In general, ordinary chondrites all have saturated 10Be (exposure ages >5Myr) and any low value of 10Be is due to shielding [5].   For meteorites of lower exposure age, it is impor-tant to correct for the exposure age. We have modeled the behav-ior of 14C/10Be vs. 10Be for different-sized meteoroids.  Our re-sults indicate that for meteoroids of radius (R) > 30cm, 14C/10Be increases with depth into the object.  For larger objects, R>50cm, 14C/10Be is highest close to the center, 2.5 to 3.0 and declines to ~2.0 at the surface of these objects.  For smaller objects, R<25cm, 14C/10Be ratios are 2.0 or less.    Experimental:  We have compared the modeling to 14C and 10Be results from known falls and to 12 Western Australian (WA) meteorites of low terrestrial age (Tterr<2kyr).   An intriguing re-sult is that we found that these WA meteorites studied must have come from object with R>45cm.    We also found one meteorite (Mundrabilla 005) which has an unusually high value of 14C/10Be. Summary:  In principle, improved precision on 14C terres-trial ages can be achieved by using 14C/10Be to correct for shield-ing [2,3,4], or using 22Ne/21Ne as noted by Schultz et al. [6].  However, it is clear a better understanding of the systematics of changes of these various nuclides as a function of depth in the meteoroid must also be taken into account. References: [1] Bland P. A. et al.  2000. Quaternary Re-search 53:131. [2] Kring D. A. et al. 2001. Meteoritics and Planetary Science 36:1057. [3] Welten K. C. et al. 2001. Meteor-itics and Planetary Science 36: 201.   [4]  Welten K. C. et al. 2004. Meteoritics and Planetary Science 39: 481. [5] Graf Th. et al. 1990.  Geochimica et Cosmochimica Acta 54: 2521. [6] Schultz L. et al. 2005. Meteoritics and Planetary Science 40, in press.  68th Annual Meteoritical Society Meeting (2005) 5181.pdf

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Studies of C-14 and Be-10 production rates and terrestrial ages of desert meteorites

Studies of C-14 and Be-10 production rates and terrestrial ages of desert meteorites

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