Bull. Minéral. (1979), 102, 178-183.

Dislocations in naturally deformed terrestrial olivine : classification, interpretation, applications

Les dislocations dans I' olivine déformée naturellement : classification, interprétation, applications.

Introduction

The geophysical importance of olivine explains the great number of physical studies on this mineral. In recent years, plastic deformation of olivine have been extensively investigated. The first analysis used optical means (Raleigh, 1968) later electron micro¬ scopy methods were employed (Boland et al., 1971 ; Green and Radcliffe, 1972 ; Phakey et al., 1972). Glide systems were identified and flow laws determi¬ ned (Carter and Ave Lallemant, 1970 ; Post, 1973 ; Kohlstedt and Goetze, 1974 ; Durham et al., 1977). The introduction of a simple and powerful decoration technique (Kohlstedt et al., 1976) has made possible systematic studies of dislocation microstructures in mantle peridotite xenoliths (Guegen, 19 77). Compa¬ rable studies have been performed using T. E. M. (Buis-kool Toxopeus, 1977), but this technique limits the investigation because it is time consuming and only very small areas are observed.

The decoration technique enables study of two important problems : (1) It gives a representative picture of the microstructures, and thus leads to important conclusions on internal stresses, recovery, and so on. (2) When dislocation densities are very low, a detailed analysis provides a means of deciphering the elements of flow mechanisms in olivine (glide systems, creep controlling factors, etc.).

This study presents new data on xenoliths and peridotite massifs. 115 xenoliths have been examined, most of which have already been used in a previous

work (Gueguen, 19 77), 63 are xenoliths from basalts and 52 from kimberlites. They cover many different areas : Arabia Plateau, Hawaï, America, Europe, South Africa. 39 samples from massifs have been studied, mainly from Lanzo (Italian Alps) and Alm¬ klovdalen (Norway). Methods and techniques of observations have already been described in Gueguen (19 77). This last paper has shown that the classifica¬ tions of xenoliths of Mercier and Nicolas (1975) or Boullier and Nicolas (1975) were not satisfactory for dislocations. This result is not surprising. These authors have classified deformation induced micro-structures based on preferred orientations and re-crystallization, which occured at a high strain (more than 20-30 %). Dislocation microstructures correspond to the last 1-5 % strain and thus record some phases which are not observable at the scale of a thin section. A classification of dislocation microstructures into 6 groups is presented in this paper. Interpretations are discussed by comparison to experimental and theoretical models. The field of application of such analysis is delineated and the main applications are developped.

I. Classification and interpretation of the microstructures

1. [100] organization

This first microstructure is the simplest one : it consists of [100] screw dislocations crossing from