Acceptance speech for the 1993 Clarke Award

Abstract

Thank you, Ed (and Dave, as it was then), for the kind words. I am deeply honored to receive this unexpected award from the Geochemical Society and overwhelmed to have been placed in the company of Clarke medalists, many of whom 1 know and admire. Two of them, Dave Walker and Ed Stolper, have had a profound influence on my career. It humbles me to think about so many worthy peers and to know that even some great minds of geochemistry have not received this award. It is indeed a privilege and honor just to do sciences, not only because you are paid for doing what interests you, but also because you are not restricted in the choices of research projects. I have certainly enjoyed this freedom and have worked on a diverse variety of problems related to diffusion in silicate melts, glasses and minerals, the degassing history of the Earth, island arc petrogenesis, and volcanic eruptions. The progress of science is always built upon previous work. This is especially true in my case. Let me give you an example. Water diffusion in silicate melts and glasses is an interesting problem that has attracted many workers. I was especially interested in the project because it was the first diffusion problem in which the concentration of the several species involved in the diffusion of a component can be measured directly, and hence it provides an ideal case to study the role of speciation during diffusion. When I started working on this, much had already been learned about water in silicate melts and glasses, and therefore the time was ripe to gain a deeper understanding of this problem. For example, the diffusivity of water in rhyolitic melts had been found by Shaw, Jambon, Karsten, and Delaney, and other workers to increase with total water concentration. The work of Stolper and his group, including Newman, Silver, and others using infrared spectroscopy had established the presence of two species of dissolved water, OH groups and H20 molecules. They had also calibrated the infrared technique so that measuring the concentration of the two species in a small area could be done in a routine manner. We knew by then that OH groups and H20 molecules interconvert in the structure and that the ratio of molecular H20 to OH groups increases with total water concentration. The early work of Doremus and the more recent work of Wasserburg had proposed that molecular Hz0 is the diffusing species for water diffusion in silicate melts and glasses, even though some other workers disagreed. It was then that I arrived at Caltech to study water diffusion and to attack the more general problem of the role of speciation during diffusion. From our experimental data and theoretical analysis, Ed Stolper, Jerry Wasserburg, and 1 were able to show that molecular H20 is the diffusing species for water diffusion in our rhyolitic glasses. We were also able to