Volcanic and igneous activity, which involve through production of melts known as magmas and their subsequent transport and cooling, play a crucial role in how Earth's layered structure has evolved and cooled over time. Melts differing in composition from their source produce various rock types found on the Earth. Earth's cooling rate is affected because melts can move, carrying heat with them, but impede heat conduction if stationary, because they have low thermal diffusivity (D). Up to the present, methods to accurately measure D were not available. The proposed work is timely because accurate measurements of D at high temperatures (T) are now possible. The proposed study is made possible by technology transfer from materials science to geoscience. Although the immediate use for these new data is to better understand the processes of rock melting and volcanism, and the balance of heat transport by flow and heat impedance by slow conduction, the findings will help us to understand microscopic processes and therefore reveal the basic physics of heat transport. Thus, this study thus will potentially contribute to our understanding of the Earth's history of internal differentiation.
Specifically, heat transport in melts well above melting temperature will be quantified using laser flash analysis (LFA) which lacks commonly occutring systematic errors of contact losses and spurious radiative transfer gains. Although LFA is the industry standard in materials science, the investigator's laboratory is the only one in Earth science pursuing this technique. Thus far they have measured D of a small number of glass compositions and to at most a few hundred degrees above the glass transition temperature (because flow terminates data collection from suspended samples), yet these few results have provided important insights into melt behavior. Results just above melting of a small number of compositions have provided new insight into microscopic behavior of complex magmas, but the findings of an heretofore unrecognized radiative mechanism involving IR photons, needs to be verified before accurate models can be constructed that permit extrapolation of these data to conditions in Earth's interior. It is proposed to: (1) Develop a new graphite cell enabling measurement of D for 'fragile' silicate liquids above liquidus over a large T range; (2) Independently measure dD/dT of several geologically important melts by applying their current technique to glasses with varying H2O contents, for which the glass transition temperature varies drastically; (3) Determine D as a function of T for diverse lavas from through the glass transition to superliquidus conditions; (4) Quantify effects of Si, Fe, and Ca which their present data indicate are important; (5) Construct a new microscopic model based on the results and formulae for Dmelt(T,X,P), for subsequent use for a variety of Earth Sciences applications that require thermal modeling.
