Abstract

Xenoliths of rocks from the Earth's mantle are commonly included in alkalic basalt and kimberlite diatremes. The xenoliths, many of them highly deformed by plastic flow, consist primarily of olivine and pyroxene and may be derived from depths as great as the seismic low-velocity zone. Large numbers of very small bubbles of CO2-rich fluid are within the major phases. The combined techniques of optical and high-voltage electron petrography demonstrate that the smallest bubbles, many of them below optical resolution, are attached to crystal defects induced by deformation and exsolution and to grain boundaries. It is concluded that formation of the smallest bubbles precedes incorporation of the xenoliths into the host magma and occurs by solid state precipitation on the deformation and exsolution features. The bubbles then collect on migrating grain boundaries during syntectonic recrystallization.

The occurrence of bubble formation before, xenolith incorporation into the magma strengthens previous suggestions that the tectonite xenoliths may be the magma source rock or residuum and that the asthenosphere is not partially melted. Furthermore, since the presence of analogous “bubble structure” in ceramics and metals is known to weaken significantly the high-temperature creep resistance of such materials, it appears likely that the mode of flow in mantle rocks may be influenced similarly.

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