Does cation ordering in omphacite influence development of lattice-preferred orientation?

Three mantle eclogite xenoliths from the Robert Victor mine in South Africa were studied. The calculated pT conditions are p >= 4.5 GPa and T between 950 and 1145 T. The clinopyroxene has a jadeite component between 0.19 and 0.52. In-situ, at these compositions and temperatures clinopyroxene (omphacite) has the high temperature C2/c structure, which has perfect cation disordering. The lattice preferred orientation (LPO) of clinopyroxene in the samples of different microstructure and composition has been measured by EBSD. The LPO for all three samples is characterized by a strong alignment of the [001] axes with the lineation, typical of the 'L-type' or 'LS-type' patterns for omphacite. These data disagree with a cation-ordering model for dislocation slip, which predicts a 'S-type' LPO pattern with the [00 1] axes forming a girdle in the foliation plane. Numerical simulation using the VPSC model has been undertaken to show the influence of the < 110 >{110} slip system on the LPO development in axially symmetric compression and extension, and pure and simple shear. The activity of the < 110 >{110} system should be very sensitive to changes in the cation ordering as the clinopyroxene goes towards the low temperature fully ordered P2/n structure. Increasing the critical resolved shear stress on this system from I to 10, resulted in a decrease of the slip activity of 80%, but produced no significant changes to the LPO. The most important slip systems for LPO development are [001](100), [ 100](010) and [001](010) in all simulations. The LS-index is introduced to quantify the symmetry of omphacite LPO. The index is based on the eigenvalues of the (010) and [001] pole figures, and has a value of one for the end-member L-type, zero for the end-member S-type and intermediate values for LS-types. The naturally deformed omphacite LPOs have LS-indexes of between 0.61 and 0.85 confirming they are LS-types, but closer to the L-type end-member. In the VPSC models the S-type end-member develops in axially symmetric compression (LS-index approximate to 0), whereas the L-type end-member develops in axially symmetric extension (LS-index approximate to 1). In simple and pure shear the LS-type fabric develops with LS-index of between 0.55 and 0.60. The VPSC simulations show that the LS-index is nearly constant with increasing strain for a constant macroscopic velocity gradient, which prescribes the constant strain path. We conclude that development of S- and L-type LPOs in omphacite is controlled by the strain path and that the LS-index is a useful quantitative indicator of fabric symmetry. (c) 2005 Elsevier Ltd. All rights reserved.