TSK 11 Göttingen 2006 Sachau & Koehn
 Influence of viscosity on
 the growth of high pressure
 phases in computer experi-
 ments Poster
 Till Sachau1 Daniel Koehn1
 Introduction
 The general aim of the project is the
 examination of microstructures that de-
 velop under HP conditions in computer
 experiments. Starting point is an inter-
 est in the dynamics of HP phase transi-
 tions, as for instance the probably catas-
 trophic phase-change event of olivine
 to spinel in the upper mantle. This
 is either explained by large overpres-
 sure or failure during the development
 of micro-structures during the growth of
 the spinel phase. Experimental results
 on this subject are rare, and do not lead
 by themselves to a deeper insight into
 the complicated stress/strain/volume-
 change/micro-crack relationships of the
 transition. We developed a central force
 spring model, where particles can un-
 dergo a phase change using parameters
 of olivine and spinel. The algorithm is
 capable of simulating the local growth
 of the mentioned phases on the basis of
 direction-dependant rate laws.
 In the current context newtonian vis-
 cosity is added to the previously solely
 elastic system, since under HP/HT con-
 ditions the viscous flow within the ma-
 terial will have a large influence on the
 distribution of elastic energies, which in
 turn have an important influence on the
 driving force of the transition. Thus
 we are dealing with a visco-elastic sys-
 tem, which will be subjected to time-
 dependant strain.
 1 Institut für Geowissenschaften, Universität
 Mainz
 Figure 1: Spinel (black) growing within
 olivine (grey) in a computer simulation. In
 run a) the reaction is sluggish due to a
 low high activation energy. Run b) uses
 a high activation energy. The resolution
 is 100 × 100 particles, the system size is
 0.5mm
 Mechanism
 A solely brittle rock will fracture if a cer-
 tain yield stress is reached, while a rock
 that behaves only viscously, deforms like
 a fluid and will compensate stress by
 flow. Combining these properties leads
 to a viscoelastic model, which is elas-
 tic for small strain rates but viscous for
 large ones. Whether or not a rock will
 fracture or flow depends therefore on the
 interplay of these properties. In the con-
 text of stress-driven rapid phase tran-
 sitions, this means a large viscous de-
 formation during deformation previous
 to the reaction, and brittle fracturing
 during the rapid volume reduction after
 the reaction started. The system itself
 is sensitive to the starting conditions,
 which will be influenced by the reduced
 differential stresses due to viscous flow.
 An important role in the development
 of microstructures plays also the veloc-
 ity of the transition, since this controls
 1
Sachau & Koehn TSK 11 Göttingen 2006
 the interplay with the latent heat release
 and the subsequent heat conduction (see
 Fig. 1).
 2