Home
DiamondAge between Kimberlite and Diamonds
How old are Kimberlite and Diamonds?
Kimberlites are normally much younger than the diamonds they bring to Earth's surface. Kimberlites and lamproites have been unfashionable between 50 and 1,600 million years old. Diamonds associated with harzburgites are about 3.3 billion years old -- more than two thirds the age of Earth itself, and those from eclogites usually range from 3 billion to less than 1 billion years old. These age differences help make clear a picture of diamonds having crystallized and been stored beneath the ancient continental cratons and only later being lifted to Earth's surface by kimberlites.
Since inclusion minerals crystallized concurrently with their diamond host, the age of the inclusions gives the age of the diamond. The ancient age of peridotite diamonds suggests that the formation of ancient Archean continental cores (archons) included diamond crystallization in the basic mantle lithosphere. A relatively cool, rigid, deep keel under these continental nuclei provided a stable environment in which diamonds crystallized and were stored. Consequently, oceanic crust diving into the mantle was metamorphosed into eclogite and pasted onto this keel. Much later passage of kimberlite magmas during the keel dislodged diamonds from both peridotite and eclogite and sent them to Earth's surface.
This cross-section of continental crust shows the 200-km-thick cool keel (part of the mantle lithosphere) that provided a constant environment for diamond crystallization and preservation. Kimberlites centered above the keel are likely to yield harzburgite-hosted diamonds from the storage zone (marked with diamonds). Kimberlites near the edge of the keel are more probable to contain eclogite-hosted diamonds, while those off the keel are likely to be barren of diamonds.