The Ordinary Chondrite Parent Body: A Model Comparing Hot and Cold Accretion

Details The Ordinary Chondrite Parent Body: A Model Comparing Hot and Cold Accretion The Ordinary Chondrite Parent Body: A Model Comparing Hot and Cold Accretion

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.263m&link_type=abstract

Meteoritics, vol. 27, no. 3, volume 27, page 263-266

Computer Science

Scientific paper

Using the heat diffusion equations of Carslaw and Jaeger (1959), a model for the thermal history of small bodies has been constructed, involving variations in initial temperature, initial ^26Al/^27Al ratio, and planetesimal radius. Two models, one of cold accretion (initial temperature T(sub)0 = 200 K), and one of hot accretion (T(sub)0 = 1100 K) were compared. T(sub)O for the hot accretion model approximates to the type 6 equilibration temperature as measured by various mineral thermometers. Constraints are introduced by considering maximum central temperatures (related to the ^26Al/^27Al ratio and the planetesimal radius) and radioactive closure ages, which exhibit synchroneity; the Rb-Sr system (closes at 400 K within 100 m.y.), the Ar-Ar system (640 K/60 m.y.), and the I-Xe system (1000 K/10 m.y). The I-Xe system constrains the hot accretion model. Since in the cold accretion model, some material does not reach even 640 K, it is constrained by the Rb-Sr system. The models are also constrained by the ^26Al/^27Al ratio. Since this provides heating energy, high ^26Al/^27Al ratios will cause overheating and melting, while low ^26Al/^27Al ratios will not heat material sufficiently to form the textures seen. The cold accretion model is possible between 5x10^-6 < ^26Al/^27Al < 7x10^-6, the hot accretion model ^26Al/^27Al < 7x10^-7. The maximum radius of the hot accretion model is ~50 km; cold accretion allows a radius of ~100 km. The hot accretion model has many more favorable solutions than the cold accretion model, and is preferred. If the "onion-skin" model of ordinary chondrite formation is correct (type 6 at the centre, type 3 at the perimeter), and the ^26Al/^27Al ratio is homogeneous within the early solar system, then the two models are mutually exclusive. Carslaw, M.S. and Jaeger, J.C. (1959) Conduction of heat in solids, 2nd ed. Oxford Uni. Pr., N.Y., 510 pp.

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