Geoscience Reference
In-Depth Information
6. The basic circulation in the Earth's mantle and how it is reflected in lithospheric
plate motions and plate boundaries. For clarity, motions are simplified and the vertic-
al scale of the lithosphere is greatly exaggerated.
Message in a diamond
Diamond is the high-pressure form of carbon. It can only form in the Earth at depths of
over 100 kilometres, sometimes well over this. Isotope ratios in diamonds suggest that they
often form from carbon in subducted ocean crust, maybe carbonate from ocean sediments.
Sometimes, there are tiny inclusions of other minerals within a diamond. It is not a feature
that is popular among gem stone dealers, but it is just what geochemists are searching for.
Minute analysis of those inclusions can tell the long and sometimes tortuous history of the
diamond's formation and passage through the mantle.
Some of the inclusions are of a mineral called enstatite, which is a form of magnesium
silicate. Some researchers believe that it was originally magnesium silicate perovskite and
comes from the lower mantle. Their evidence comes from the observation that it contains
only one-tenth as much nickel as would be expected in the upper mantle. At lower mantle
temperatures and pressures, nickel gets taken up into a mineral called ferropericlase, which
is also a common inclusion of diamonds, leaving very little nickel left in magnesium silic-
