Experimental Petrology: Earth Materials Science

Abstract

Petrology is the science of rocks. Geologists map rocks in the field, and bring selected .samples back to the laboratory for detailed petrographic analysis, mineralogical study and chemical analysis. On the basis of these studies, existing hypotheses for the origin of the rocks are tested, or new hypotheses are erected. From examination of the rocks in field and laboratory, geologists then attempt to deduce their histories. Experimental petrology involves further laboratory experiments which reproduce the conditions within the Earth during the generation and evolution of a rock or rock suite. This involves subjecting minerals and rocks, or their synthetic equivalents, to high pressures and temperatures under varied but precisely controlled conditions. Determination of the reactions which occur under the known conditions in the laboratory provides calibrations for the processes involved in formation of the rocks in nature, defining the actual conditions, and facilitating selection among competing hypotheses of origin. Furthermore, exploration of reactions under various conditions within the laboratory may reveal processes operating within the Earth which were previously unsuspected Experimental petrology had its beginnings in adventurous experiments on minerals and rocks using furnaces or cannon barrels. It became a force in Earth sciences starting in the early 1900s with the systematic determination of high-temperature phase equilibria involving the crystallisation of synthetic silicate liquids, which included representatives of the common rock-forming minerals. These investigations brought the rigour of thermodynamics to the processes of partial melting of rocks and the crystallisation of magmas, and elucidated many problems in igneous petrology. Only in the 1950s did phase equilibrium experiments with simultaneously maintained high temperatures and pressures become routine. At first the experiments reproduced conditions only within the continental crust, then during the 1960s the experimental range was extended to high-pressure conditions within the mantle, equivalent to about 100 km depth. During the 1980s one type of large-volume apparatus reproduced conditions down to 650 km in the Earth, and a miniature device has reproduced conditions corresponding to 2000 km depth, not far short of the mantle-core boundary of the Earth. The equipment mentioned above is static, with samples being held at constant pressure and temperature as reactions occur in the samples. A dynamic experimental approach reproduces conditions to the centre of the Earth; a sample is shattered by the passage of a shock wave, and its properties under extremely high pressures are measured during the last nanoseconds before its destruction.