[meteorite-list] CAI's - Formation Age

From: Shawn Alan <photophlow_at_meteoritecentral.com>
Date: Sun, 28 Feb 2010 15:12:15 -0800 (PST)
Message-ID: <489988.73212.qm_at_web113606.mail.gq1.yahoo.com>

Mike G and Listers,



I would put CAI's at (4567.2 ? 0.6 Ma) which is 4.5672 billion years+- .6Ma. This date is sourced from.....



Meteoritics & Planetary Science 42, Nr 7/8, 1321?1335 (2007)
Abstract available online at http://meteoritics.org


Pb isotopic age of the Allende chondrules

Yuri AMELIN1?* and Alexander KROT2

1Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada, K1A 0E8
2Hawai?i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology,
University of Hawai?i at Manoa, Honolulu, Hawai?i 96822, USA
?Present address: Research School of Earth Sciences, The Australian National University, 61 Mills Road, Canberra, ACT 0200, Australia
*Corresponding author. E-mail: yuri.amelin at anu.edu.au


(Received 20 September 2006; revision accepted 23 February 2007)

The appendix table for this article is available online at http://meteoritics.org.




Abstract?We have studied Pb-isotope systematics of chondrules from the oxidized CV3
carbonaceous chondrite Allende. The chondrules contain variably radiogenic Pb with a 206Pb/204Pb ratio between 19.5?268. Pb-Pb isochron regression for eight most radiogenic analyses yielded the date of 4566.2 ? 2.5 Ma. Internal residue-leachate isochrons for eight chondrule fractions yielded consistent dates with a weighted average of 4566.6 ? 1.0 Ma, our best estimate for an average age of Allende chondrule formation. This Pb-Pb age is consistent with the range of model 26Al-26Mg ages of bulk Allende chondrules reported by Bizzarro et al. (2004) and is indistinguishable from Pb-Pb ages of Ca-Al-rich inclusions (CAIs) from CV chondrites (4567.2 ? 0.6 Ma) (Amelin et al. 2002) and the oldest basaltic meteorites. We infer that chondrule formation started contemporaneously with or shortly after formation of CV CAIs and overlapped in time with formation of the basaltic crust and
iron cores of differentiated asteroids. The entire period of chondrule formation lasted from 4566.6 ? 1.0 Ma (Allende) to 4564.7 ? 0.6 Ma (CR chondrite Acfer 059) to 4562.7 ? 0.5 Ma (CB chondrite Gujba) and was either continuous or consisted of at least three discrete episodes. Since chondrules in CB chondrites appear to have formed from a vapor-melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated (Krot et al. 2005), there were possibly a variety of processes in the early solar system occurring over at least 4?5 Myr that we
now combine under the umbrella name of ?chondrule formation.?



Shawn Alan





Galactic Stone & Ironworks meteoritemike at gmail.com
Sun Feb 28 09:10:50 EST 2010


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Hi List,

I can't recall exactly how old most CAI's are. I know they were
formed a few million years before chondrules, but does anyone have a
more precise age for these? I think it was around 4.6+ billion years,
give or take a few minutes....?

Best regards,

MikeG

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Received on Sun 28 Feb 2010 06:12:15 PM PST