Geoscience Reference
In-Depth Information
ate perovskite. In a few cases, the inclusions are rich in aluminium which, under upper
mantle conditions, is locked up in garnet. And some inclusions are iron-rich, suggesting
that they might have originated very deep in the mantle, close to the core mantle boundary.
These deep diamonds also have a different carbon isotope signature, believed to be charac-
teristic of deep mantle rock rather than subducted ocean lithosphere. Estimates of the age
of diamonds and the rock that surrounds them suggests that some have had a very long and
tortuous passage through the mantle that may have taken them more than a billion years.
But it is convincing evidence of at least some transfer between the lower and upper mantle.
Almost as fascinating as the diamonds themselves is the rock in which they are found. It's
called kimberlite after the South African diamond-mining town of Kimberley. The rock it-
self is a mess! Apart from the diamonds, it contains a whole range of angular lumps and
pulverized fragments of different rocks; a so-called breccia. It is volcanic and tends to form
the carrot-shaped plugs of ancient volcanic vents. It is hard to determine its exact composi-
tion because it contains so much pulverized debris from its passage through the lithosphere,
but the original magma must have been mostly olivine from the mantle together with an
unusual amount of volatile material now in the form of mica. If it had found its way up
slowly from the mantle, we would not have diamonds today. Diamond is unstable at pres-
sures found less than 100 kilometres underground and, given time, would dissolve in the
magma. But kimberlite volcanoes did not keep it waiting. It is estimated that the average
speed of material through the lithosphere was about 70 kilometres per hour. The widening
neck of the vent as it approaches the surface suggests that volatile material was expanding
explosively and the surface eruption speed could have been supersonic. As a result, all the
rock fragments collected on the way up have been quenched, frozen in time, so that they
represent samples from deep in the lithosphere and even the mantle.
The base of the mantle
Recent analysis of seismic data from around the world has revealed a thin layer at the base
of the mantle, the D″ layer, up to 200 kilometres thick. It is not a continuous layer but seems
more like a series of slabs, a bit like continents on the underside of the mantle. This could
be regions where silicate rocks in the mantle are partly mixed with iron-rich material from
the core. But another explanation is that this is where ancient ocean lithosphere comes to
rest. After its descent through the mantle, the slab is still cold and dense so it spreads out at
the base of the mantle and is slowly heated by the core until, perhaps a billion years later,
it rises again in a mantle plume to form new ocean crust.
Clues to the deep interior of the Earth also come from measuring tiny variations in day
length. Our spinning planet is gradually slowing down due to the pull of the moon on the
tides and to the rising of land compressed by ice in the last Ice Age. But there are other
