Geological Aspects of High-Pressure Research

The low-density minerals that make up the bulk of rocks in the earth's crust, such as quartz and the feldspars, are transformed by high pressure into much denser phases. In some cases the products of these transitions are new phases that were first discovered in the laboratory; in other cases they are minerals such as kyanite, jadeite, and pyrope, which have long been known as constituents of metamorphic rocks. Determinations of the stability fields of these high-pressure minerals show that either metamorphism of sedimentary rocks takes place at much greater depth than has hitherto been supposed or pressures generated by orogenic forces may have significantly augmented the hydrostatic pressure. The second alternative seems unlikely, but lack of information on the strength of rocks during metamorphism makes the matter uncertain. Geophysical and petrological observations indicate that the dominant rock type in the upper mantle is garnet peridotite. However, there is reason to believe that the mantle is inhomogeneous and that a variety of rocks ranging in bulk composition from eclogite to peridotite are present. Hydrous phases, such as amphiboles, are possible constituents in the upper 100 kilometers. The hypothesis that the Mohorovičić discontinuity is a dynamic equilibrium between basalt and eclogite seems improbable. The transition zone between the upper and lower mantle can be explained as a series of reactions in which silicates with the silicon ion in fourfold coordination are transformed into phases in which silicon is in six-fold coordination. This interpretation is supported by synthesis of stishovite, a polymorph of SiO2 with rutile structure, and by syntheses of germanate pyroxenes with ilmenite structure. Data on the melting of silicates at pressures up to 50 kilobars show that the initial dT/dP slopes of silicate melting curves are much steeper than those of metals but that they show considerable curvature. The increase of melting temperature with pressure should be much more pronounced near the top of the mantle than at greater depth.