T. E. M. investigation of chromites from New-Caledonia

Abstract

A T.E.M. investigation on chromite from New-Caledonian ophiolites has been undertaken in an effort to elucidate the origin of structures in chromite ore bodies. These bodies have a podiform shape and internally the chromite ore and its olivine matrix are strongly deformed. It is a question with important economical bearings to decide whether these structures were directly obtained in a magma chamber during the course of a turbulent magmatic sedimentation of chromite and olivine or if they result from plastic deformation postdating the magmatic event. This plastic deformation is registered in the surrounding peridotites and in the olivine matrix of the chromite ore. It occurred around 1200° C for stresses lower than a few hundred bars. Chromites consist in large grains with a very low density of free dislocations and well organized sub-grain boundaries. Stereographic analysis of some of these subgrain boundaries indicate that a large amount of climb occurred because in most cases, the constitutive dislocations no more lie in the usual glide planes of the spinel structure. These features are characteristic of a temperature well above 1/2 TMj that is probably higher than the 1200° C estimated for plastic flow conditions. Optical observations of thin sections show that during this plastic flow chromite grains have accommodated strain by brittle fracture andT.E.M. observations at the tip of the cracks show that the zone which has been plastically deformed at the front crack does not exceed some microns. These remarks are in agreement with the mechanical properties of other spinels which begin to be correctly known. All seem to have a very high elastic limit, even at 1/2 TM. These observations suggest that the dislocation structure in this chromite has not been produced by the plastic deformation observed in surrounding olivine bearing rocks but is possibly due to its magmatic history.