Mathematics – Logic
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.517p&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 517-518
Mathematics
Logic
2
Astronomical Models, Chondrites, Melts (Crystal Growth), Petrology, Temperature Effects, Asteroids, Cooling, Radioactive Age Determination, Regolith
Scientific paper
Recent Pb-Pb, Ar-Ar, and Pu fission track results, together with petrological constraints allow us to describe the thermal evolution of the A-L asteroid at the Acapulco location. By extrapolating back in time the Pu content of bulk phosphates, an initial value of approximately 70 ppb is found, corresponding (4565 Ma) to a Pu/U ration of approximately 1.3 x 10-2 (valid only for our sample). In the various temperature regimes (1300-720, 720-550, 550-360 K) the Acapulco cooling rates were approximately 100, approximately 7 and approximately 1.6 K/m.y., respectively. While slight thermal differences have been observed between Monument Draw and Gibson, all the 17 A-L objects appear to have cooled fast at the beginning. Since Acapulco is buried at some depth in the parent body, the fast early cooling strongly suggests an originally small body, distinctly smaller than the estimated dimensions of OC asteroids. The range of peak temperatures (1250-1350 K) required to explain the various degrees of partial melting observed in A-L chondrites would most likely indicate some original heterogeneities in the distributions of Al-26 and Fe-60, which appear to have been the effective heat sources. A parent body model was developed to fit the chronological and petrological constraints derived from Acapulco, taking into account plausible values for the various thermal parameters. The model implies that no regolith was initially present. In our model the pre-Acapulco material is located at a depth of 20 km(0.5 R). The regolith formation had to start 10-20 m.y. after the accretion, at a rate of 100 m/m.y., unitl a thickness of 900 m was reached. Sintering of the regolith does begin 15-20 m.y. after the formation. With these conditions, a satisfying accord between the experimental and modeled cooling curves was obtained.
Pellas Paul
Rasmussen Lars K.
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