DISLOCATION STRUCTURES IN OLIVINE AT THE NANOSCALE

Many tectonic processes are strongly influenced by the solid-state flow of minerals in the lower crust and upper mantle. One mechanism for this flow is plastic deformation, which typically involves the migration of dislocations through crystals. Dislocation structures can have profound effects on the mechanical and electrical properties of their host minerals. A full understanding of dislocation motion and the effect of dislocations on material properties requires detailed knowledge of their atomic structures. Also, experimental studies indicate significant quantities of “water” could be incorporated into the upper mantle as hydrogen point defects in olivine, and hydrogen has been detected in mantle olivine by IR spectroscopy. Some fraction of this hydrogen could be stored in extended defects such as dislocations and stacking faults that are prevalent in the deformed olivine of the upper mantle.

We are using high-resolution transmission electron microscopy to acquire images at the subnanometer scale of dislocation structures in olivine from xenoliths from San Carlos, Arizona, USA. Subgrain boundaries occur in both [010]- and [100]-oriented samples and consist of arrays of edge dislocations arranged parallel to (101) and (02-1), respectively. Observed burgers vectors include [100], [001], [101], and [01-1]. With the exception of the [100] dislocations, all the observed dislocations are dissociated into two or more partial dislocations. A combination of image analysis and crystal structure constraints reveals probable structures for the dislocation cores and stacking faults that separate the partials. We will present structures of the observed dislocations and discuss the effects of these structures on dislocation mobility and possible water (hydrogen) storage in the upper mantle.