The Big Hole: Information capsules from the deep.

Rocks brought up by kimberlites from the bowels of the earth are called Xenoliths, the most common of these deep xenoliths in Kimberley are Peridotites and then to a lesser degree Eclogites.

Peridotite is the dominant rock of the upper part of the Earth's mantle. When weathered its rather drab.

When polished the Olivene and Orthopyroxene(green) and garnet(purple and red) crystals are readily apparent.

Orthopyroxene and pyrope garnet only form a specific temperatures and pressures. They also may contain chromite, plagioclase, and amphibole.

This is able to fill in the gaps... we know a lot about densities in the various layers of the earth from seismology and earthquakes.

Having samples that we can position based on formation temperatures and pressures helps complete the picture.

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Eclogites are less common in kimbelites and are quite different internally.

Even though they are similarly drab on the weathered exterior.

They consist of only two minerals Orange Garnet and dark green Diopside.

These are likely to be slabs of ocean crust that have been pushed deep under continents.

Other posts in this series:

The Big Hole. What is the big hole and where is it?
The Big Hole: Part of a greater whole
The Big Hole: Kimberlites, how do they form.
The Big Hole: Diamonds are a girls best friend...
The Big Hole: The other beauty in kimberlites and a wealth of information.