Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.354g&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 354
Mathematics
Logic
H Chondrites, Isotopes, Metamorphism, Parent Body, Phosphates, Radiogenic Ages
Scientific paper
Pb/Pb ages of phosphates separated from equilibrated H chondrites vary from 4.563 to 4.502 aeons and show a negative correlation with the metamorphic grade of the host meteorite. This is the first clear relationship in ordinary chondrites ever observed between a long-lived chronometer and the intensity of metamorphism [1]. The range of the Pb/Pb ages, 6 x 10^7 y indicates the period for the thermal processing of equilibrated chondrites. The U/Pb systematics do however not allow to distinguish if this thermal activity has an episodic character (metamorphic event) or if it was a continuous process (internal heating followed by slow cooling). In the case of the second hypothesis the precision of the Pb/Pb chronometer with an age resolution of about 1 myr and the estimate of the temperature for the thermal U/Pb closure deduced from experimental studies of Pb diffusion in apatites [2] allow us to derive sharp and absolute chronological constraints for the thermal evolution of the H-chondrite parent body. The calculated temperature for the U/Pb closure in phosphates in chondrites of grade 5-6 is 710 +- 20 K for a cooling rate of 5 +- 2 K/10^6 y. We consider a classical modelization for the thermal evolution of the H parent body assuming that the accretion of the primitive material occured at low temperatures and in a short time interval relative to the mean life of ^26Al. The inferred value of 1150 K for the peak temperature of type 6 chondrites defines a value for the ^27Al/^26A = 5 +- 1 x 10^-6 at the time of accretion. The Pb/Pb systematics indicate that the accretion of the H chondrite parent body occurred at 4.564 aeons. The time interval between the Pb/Pb age of the Allende refractory inclusions and the accretion of the H chondrite parent body (Delta T = 3.0 +- 2.6 x 10^6 y) is compatible with this ^26Al time constraint. The radius of the H chondrite parent body is about 80 km, similar to that previously defined by [3]. The Pb/Pb chronology of phosphates from equilibrated chondrites supports the model of a ~10^2 km large parent body, which preserved its layered structure and, which was heated by heat furnished from ^26Al in situ decay if the Pb/Pb ages are interpreted as indicating the accurate time of the thermal U/Pb closure. This interpretation is, however, not coherent with the few internal Pb/Pb ages determined in unequilibrated chondrites, which show ages lying around 4.48 aeons [4]. Moreover, this model postulates that the most equilibrated material spent about 3 x 10^6 y at temperatures higher than 1100 K. Such a long period and slow cooling at high temperatures however is incompatible with petrographic observations made in this material, they require a very short time interval and a fast cooling period at temperatures above 1000 K. References: [1] Gopel C. et al. (1990) Meteoritics, 25, 367-368. [2] Cherniak D. et al. (1991) GCA, 55, 1663-1673. [3] Miyamato M. et al. (1981) LPS XII, 1145-1152. [4] Hanan B. B. and Tilton G. R. (1985) EPSL, 74, 209219.
Allègre Claude J.
Göpel Christa
Manhes Gerard
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