A unique combination of diamond with coesite -- a dense variety of quartz -- is providing scientists with a new means for determining the pressure at which a rock or mineral forms deep within our planet.
This intriguing combination of minerals retains the high pressures surrounding the rock once present inside the Earth, yielding a "fossilized pressure."
Investigators from the Geophysical Laboratory of the Carnegie Institution of Washington and the Russian Academy of Sciences report their research in the October 17, 2000, issue of the Proceedings of the National Academy of Sciences.
Although these dense minerals, diamond and coesite, must be formed under high pressure, thus implying a deep origin, the actual pressure would normally have been released long since the specimens reached the Earth's surface.
A diamond-coesite sample from Venezuela, however, is a rare find that actually contains the pressure present at early epochs in the history of our planet.
The combination of the two materials is excellent for the preservation and determination of fossilized pressure because the extremely strong, non-yielding diamond container prevents the highly compressed, chemically simple coesite from expanding and releasing the pressure.
The maximum pressure is therefore preserved, and the scientists can analyze the sample without the chemical variability that is associated with other inclusions such as garnet or olivine.
In their analysis, the scientists used two techniques -- micro-Raman and micro-x-ray diffraction. They focused lasers and x-rays to micrometer-sized beams (less than a tenth of the diameter of a human hair) to probe the 60-micrometer microscopic inclusion of the mineral lodged within the two-millimeter diamond crystal.
The results from both techniques agreed well, yielding a pressure at the site of the inclusion of 3.62 gigapascals -- enough pressure to squeeze charcoal into diamond.
According to Dave Mao, "the preserved pressure depends upon the difference between the compressibility and expansivity of the host diamond to the inclusion. "From the fossilized pressure, we can retrace the exact pressure and temperature at which the diamond-containing rock was formed and the journey that it went through to reach the Earth's surface. This enlightens our understanding of Earth's interior at depths exceeding 120-150 kilometers."
The Carnegie Institution of Washington
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