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High-Pressure Geophysics: A Window on the Lower Mantle

Information about the earth's deep interior is scanty, and even inferences drawn from seismic and electrical conductivity measurements include assumptions about the relevant mineral species and their properties. Creating in the laboratory pressures and temperatures characteristic of the lower mantle (300 to 1500 kilobars) has proved difficult, except under transient conditions in shock wave apparatus. But these barriers appear to be falling as geophysicists apply an improved technique for studying materials under extremely high pressures and temperatures. Already investigators have found evidence that suggests major revisions in the accepted picture of the lower mantle. The most recent discovery bears on the chemical evolution of the earth, specifically the separation of a metallic iron core from the iron oxide of the mantle; the new evidence, which has attracted considerable attention among geophysicists, would appear to strengthen the hand of those who believe the earth evolved from an initially homogeneous mass. The key to these new results is a device known as a diamond-anvil or diamondwindow pressure cell. It is not new, having been developed in the late 1950's by a group at the National Bureau of Standards, but it is only recent improvementsin the cell itself and in means of measuring high pressures and of heating samples within the cell that have made possible significant applications to geophysical and other research problems. If its development has been evolutionary rather than revolutionary, however, the capability of the technique is nonetheless remarkable. Two laboratories have now reported measurements at pressures of 500 kilobars equivalent to the pressures found at a depth of about 1000 kilometers within the earth. Perhaps the most striking feature of a diamond-anvil cell to one who is unfamiliar with it is the extremely small size of an apparatus capable of such high pressures (Fig. 1). The small size contrasts with more established high-pressure techniques based on tungsten carbide anvils, which are massive and expensive, although they permit the use of larger samples. In the diamond cell, a spring-lever arm or similar mechanism presses a hardened steel piston containing an anvil-shaped diamond against an opposing diamond anvil. The experimental sample is contained in a metal gasket between the two diamonds, which are cut from gem quality stones of about