Paper No. 11
Presentation Time: 10:45 AM
NORMAN L BOWEN AND EXPERIMENTAL PETROLOGY
Norman L Bowen (1887-1956) used a new discipline, experimental petrology, to revolutionize thinking about “The Evolution of the Igneous Rocks”, the title of a book he wrote in 1928. In it, he promoted crystallization differentiation as the principal cause of variation among both lavas and plutonic rocks. Educated in Canada and at MIT (PhD 1912), Bowen spent most of his career at the Geophysical Laboratory of the Carnegie Institution of Washington. There, with colleagues such as George Morey, J. Frank Schairer and O.F. Tuttle, he carried out crystallization experiments, usually at low pressure, in simple two- and three-component systems that were analogs to the more complex multi-component natural rocks. In his book and many papers, he described the pertinence of the resulting thermodynamically rigorous phase diagrams to natural systems. He wrote lucidly, often humorously, without equations, and at a level that could be understood by anyone acquainted with simple principles of geometry. Using experiments, he assayed the processes of equilibrium and fractional crystallization, accumulation of minerals in porphyritic lava, assimilation of country rock, fractional resorption, and the role of water in igneous systems. He critiqued hypotheses such as liquid immiscibility and magma mixing that nevertheless were at odds with experimental data. With Tuttle, he disproved Hess’s concept of “serpentinite magma” (1948), and established the primacy of igneous processes in the origin of granite over granitization (1958). Although much of his hypothesis of fractional crystallization utilizes gravity as the principal agent of separation of minerals from melts, one little-remembered paper, “Differentiation by Deformation” (PNAS 1920) points to the intertwined role of compaction and deformation of nearly crystalline magma as a means of producing monomineralic rocks “of extreme purity” such as dunite and anorthosite. Modern techniques have rendered many of his arguments obsolete, especially concerning the origin of common rocks in island arcs. Nevertheless, application of the fundamental physical principles of crystallization to igneous problems remains the backbone of petrological studies, and is his lasting contribution to our science.