age of diamonds
Age of Diamonds and Kimberlites
Kimberlites are generally much younger than the diamonds they bring to Earth's surface. Kimberlites and lamproites have been dated 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 generally range from 3 billion to less than 1 billion years old. These age differences help clarify 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 simultaneously
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 underlying mantle lithosphere. A relatively cool, rigid, deep
keel
beneath these continental nuclei provided a stable environment in which
diamonds crystallized and were stored. Subsequently, oceanic crust
diving
into the mantle was metamorphosed into eclogite and pasted onto this
keel.
Much later passage of kimberlite magmas through 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
stable environment for diamond crystallization and preservation.
Kimberlites
centered over 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 likely
to contain eclogite-hosted diamonds, while those off the keel are
likely
to be barren of diamonds.