Microstructures and OH-bearing nano-inclusions in "wet" olivine xenocrysts from the Udachnaya kimberlite

Abstract

Mantle olivine xenocrysts from the Udachnaya kimberlite pipe (Yakutiya, Siberia) were investigated by optical microscopic and TEM methods. From optical observations two types of olivine xenocrysts are discriminated. Crystals exhibiting an optically visible cross-hatched microstructure, caused by linear features oriented parallel to [100], [001], [101] and [−101] directions in the olivine host are assigned to type 1, and the optically clear ones are assigned to type 2. TEM investigation showed that both type 1 and type 2 samples contain OH-bearing nano-inclusions with a grain size between several tens and several hundreds of nanometres, composed of hydrous magnesium silicates such as “hydrous olivine” (occasionally), the 10-A phase, serpentine and talc. In the investigated samples, the 10-A phase is to a large extent replaced by the low-pressure assemblage serpentine + talc. On TEM scale, the difference between type 1 and type 2 olivine samples is that the inclusions in type 1 are grouped in arrays (high density of precipitation), while in type 2 they are randomly arranged (low density of precipitation). The cross-hatched linear features in type 1 correspond to arrays of the OH-bearing nano-inclusions precipitated in (001), (100), (101) and (−101) planes of olivine. The presented data confirm that nano-inclusions filled by hydrous magnesium silicates, both high-pressure (“hydrous olivine” and 10-A phase) and low-pressure ones (serpentine and talc), are much more frequent and typical for olivine nodules in kimberlites than it would have been expected earlier. The amount of water in the investigated olivine samples ranges from 37 to 300 ppm (Koch-Muller et al. , 2006) and does not vary significantly depending on whether the samples exhibit an optically visible microstructure or not. We conclude that the xenocrysts crystallized as “wet” olivine in a mantle environment, whereas the OH-bearing nano-inclusions formed later during eruption and transport of olivine fragments from depths to the surface. The origin of nano-inclusions is discussed in terms of subsolidus aggregation of intrinsic OH-bearing point defects in olivine; this event has been initiated by deformation processes in the kimberlite pipe during eruption.