Tubular symplectic inclusions in olivine from the fukang pallasite

Olivine from the Fukang meteorite, like that from many other pallasites, contains distinctive arrays of parallel, straight, tubular inclusions. They differ in their extension and linearity from those in terrestrial olivines. They comprise approximately 1% of the total volume. Most have lens-shaped cross-sections, but some are rounded. The major axis of the lens-shaped inclusions is rigorously oriented along olivine [001], and the rounded ones lie along olivine [010] and a few along [100]. The linear nature and orientations of the inclusions suggest that they nucleated on screw dislocations, perhaps formed through shock triggering. High-resolution transmission electron microscopy (TEM) and energy-dispersive x-ray spectroscopy show that the inclusions consist of symplectic intergrowths of chromite, diopside, and silica that appear to have formed by exsolution from the host olivine. The symplectites consist of chromite lamellae with approximately 35-nm spacings that grew outward from a central plane, with interstitial diopside and silica. Contrast modulations having an average spacing of 4.4 nm occur within the chromite lamellae. Using a reaction-front model, we estimate that exsolution occurred over a period of 30 to 100 min, suggesting rapid cooling at high temperature. The crystallographic observations and inferences on growth rate are consistent with the hypothesis that the inclusions nucleated during heating following dislocation formation in a shock event, perhaps concurrent with that proposed to have disrupted the pallasite parent body.