Thermal evolution and sintering of chondritic planetesimals

II. Improved treatment of the compaction process^⋆

¹ Institut für Theoretische Astrophysik, Zentrum für Astronomie, Universität Heidelberg, Albert-Ueberle-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 Klaus-Tschira-Labor für Kosmochemie, Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany

Received: 26 May 2014
Accepted: 15 January 2015

Abstract

Context. Reconstruction of the thermal history of individual meteorites which can be assigned to the same parent body allows us to derive general characteristics of the parent body, such as its size and formation time, which hold important clues on the planetary formation process. This requires us to construct a detailed model of the heating of such a body by short lived radioactives, in particular by ²⁶Al, and its cooling by heat conduction, which may then be compared to the reconstructed cooling histories of the meteorites.

Aims. The heat conductivity of the material from which planetesimals are composed depends critically on the porosity of the chondritic material. This changes during the process of compaction (also called sintering) of the material at elevated temperatures and pressures. Therefore, compaction of an initially granular material is a key process determining the thermal history of the parent bodies of meteorites. The most realistic modelling of sintering of chondritic material is required.

Methods. The modelling of the compaction process is improved by applying concepts originally developed for the modelling of hot isostatic pressing in metallurgical processes, and by collecting data available from geosciences for the materials of interest. It is extended to a binary mixture of granular components of very different diameters – matrix and chondrules – as observed in chondrites.

Results. By comparison with some published data on sintering experiments it is shown that the algorithm to follow the decrease of porosity of granular material during progressive sintering allows a sufficiently accurate modelling of the compaction of silicate material. The dependence of the compaction process on the nature of the precursor material, either matrix-dominated or chondrule-dominated, is discussed. It is shown that the characteristic temperature at which sintering occurs is different for matrix or chondrule-dominated precursor material. We apply the new method for calculating compaction to the evolution of the parent body of the H chondrites and determine an improved optimised set of model parameters for this body.

Conclusions.