Thermal history modelling of the H chondrite parent body

¹ Institut für Theoretische Astrophysik, Zentrum für Astronomie, Universität Heidelberg, Albert-Überle-Str. 2, 69120 Heidelberg, Germany
e-mail: gail@uni-heidelberg.de
² Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany
³ Institut für Planetologie, Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany

Received: 23 February 2012
Accepted: 25 August 2012

Abstract

Context. The cooling histories of individual meteorites can be empirically reconstructed by using ages obtained from different radioisotopic chronometers having distinct closure temperatures. For a given group of meteorites derived from a single parent body such data permit the detailed reconstruction of the cooling history of that body. Particularly suited for this purpose are H chondrites because (i) all of them are thought to derive from a single parent body (possibly asteroid (6) Hebe) and (ii) for several specimens precise radiometric ages over a wide range of closure temperatures are available.

Aims. A thermal evolution model for the H chondrite parent body is constructed by using the cooling histories of all H chondrites for which at least three different precise radiometric ages are available. The thermal model thus obtained is then used to constrain some important basic properties of the H chondrite parent body.

Methods. Thermal evolution models are calculated using our previously developed code, which incorporates the effects of sintering and uses new thermal conductivity data for porous materials. Several key parameters determining the thermal evolution of the H chondrite parent body are varied together with the unknown original location of the H chondrites within their parent body until an optimal fit between the radiometric age data and the properties of the model is obtained. The fit is performed in an automated way based on an “evolution algorithm” to allow for a simultaneous fit of a large number of data, which depend in a complex way on several parameters. Empirical data for the cooling history of H chondrites are taken from the literature and the thermal model is optimised for eight samples for which radiometric ages are available for at least three different closure temperatures.

Results. A set of parameters for the H chondrite parent body is found that yields excellent agreement (within error bounds) between the thermal evolution model and empirical data for the cooling histories of six of the examined eight H chondrites. For two of the samples significant discrepancies exist between model and empirical data, most likely reflecting inconsistencies in the empirical cooling data. The new thermal model constrains the radius and formation time of the H chondrite parent body, and the initial burial depths of the individual H chondrites. In addition, the model provides an estimate for the average surface temperature of the body, the average initial porosity of the material the body accreted from, and the initial ⁶⁰Fe content of the H chondrite parent body.