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xenoliths

Xenoliths - hitchikers from the center of the earth

Kimberlite magmas carry foreign rocks -- xenoliths -- from Earth's mantle to the surface. Xenoliths are geologists' only samples from the deep Earth, and carry information about diamond growth conditions. The 2 most common types of xenoliths are peridotites and eclogites. Peridotite is the main constituent of the mantle beneath the crust and consists primarily of olivine -- the gem variety is peridot. Eclogite, consisting primarily of garnet and a green pyroxene, is formed by plate tectonics when basalt of the ocean crust founders into the mantle. Certain kinds of xenoliths contain diamonds.

 

These diagrams show the compositions of mantle xenoliths.

Lherzolite is a variety of peridotite thought to form most of the upper mantle.

 

 
 

Harzburgite is another kind of peridotite with less clinopyroxene. Garnet harzburgites contain red garnet and, occasionally, diamonds.

 

 

 

 
 

Eclogite, a very different rock, consists of garnet and sodium-rich pyroxene; some also contains diamonds.

 

 

 

 

 
 

Diamond inclusions

Diamonds with inclusions are like little space capsules from the mantle: pristine mineral samples are protected by the diamond's indomitable embrace and transported to the surface by a volcanic rocket.

Inclusions capture a picture of the rock and environment in which diamonds grow and indicate that garnet harzburgite (a type of peridotite) and eclogite are the most common rocks in which diamonds have grown.

A single mineral inclusion rarely defines a specific rock, but two or more minerals may enable interpretation of rock associations and origin. Some inclusion minerals are virtually unique to diamond sources and are thus sought in the exploration for diamonds.

 

A purple pyrope garnet, an indicator of garnet harzburgite, in a brownish diamond octahedron from the Udachnaya pipe, Sakha Republic, Russia (about 0.8 mm across).