TY - JOUR
T1 - Initial 26Al/27Al in carbonaceous-chondrite chondrules
T2 - Too little 26Al to melt asteroids
AU - Kunihiro, Takuya
AU - Rubin, Alan E.
AU - McKeegan, Kevin D.
AU - Wasson, John T.
N1 - Funding Information:
We are greatly indebted to H. Kojima of the NIPR for provision of the Y-81020 section. We thank Frank Kyte for help with the electron microprobe and Haibo Zou with the ion microprobe. We also thank Andrew Davis, Ernst Zinner, and an anonymous reviewer for constructive reviews. Takuya Kunihiro appreciates helpful discussions with Motoo Ito. The UCLA ion probe was made possible by a gift from the W. M. Keck Foundation and is supported by National Science Foundation Grant EAR 01-13563 and National Aeronautics and Space Administration Grant NAG5-12898. This research is mainly supported by National Science Foundation Grant EAR-0074076.
PY - 2004/7/1
Y1 - 2004/7/1
N2 - We report 26Mg excesses correlated with Al/Mg ratios in five chondrules from the primitive CO3.0 chondrite Yamato 81020 that yield a mean initial 26Al/27Al ratio of only (3.8 ± 0.7) × 10-6, about half that of ordinary chondrite (OC) chondrules. Even if asteroids formed immediately after chondrule formation, this ratio and the mean Al content of CO chondrites is only capable of raising the temperature of a well-insulated CO asteroid to 940 K, which is more than 560 K too low to produce differentiation. The same ratio combined with the higher Al content of CV chondrites results in a CV asteroid temperature of 1100 K. We calculate that the mean initial 26Al/27Al ratio of about 7.4 × 10-6 found in LL chondrules is only able to produce small amounts of melting, too little to produce differentiation. These results cast serious doubt on the viability of 26Al as the heat source responsible for asteroid differentiation. Inclusion of 60Fe raises temperatures about 160 K, but this increment is not enough to cause differentiation, even of an LL-chondrite asteroid.
AB - We report 26Mg excesses correlated with Al/Mg ratios in five chondrules from the primitive CO3.0 chondrite Yamato 81020 that yield a mean initial 26Al/27Al ratio of only (3.8 ± 0.7) × 10-6, about half that of ordinary chondrite (OC) chondrules. Even if asteroids formed immediately after chondrule formation, this ratio and the mean Al content of CO chondrites is only capable of raising the temperature of a well-insulated CO asteroid to 940 K, which is more than 560 K too low to produce differentiation. The same ratio combined with the higher Al content of CV chondrites results in a CV asteroid temperature of 1100 K. We calculate that the mean initial 26Al/27Al ratio of about 7.4 × 10-6 found in LL chondrules is only able to produce small amounts of melting, too little to produce differentiation. These results cast serious doubt on the viability of 26Al as the heat source responsible for asteroid differentiation. Inclusion of 60Fe raises temperatures about 160 K, but this increment is not enough to cause differentiation, even of an LL-chondrite asteroid.
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U2 - 10.1016/j.gca.2004.02.006
DO - 10.1016/j.gca.2004.02.006
M3 - Article
AN - SCOPUS:3042688000
VL - 68
SP - 2947
EP - 2957
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
SN - 0016-7037
IS - 13
ER -