Movement of heat within Earths outer layers is largely governed by the vibrational component of thermal diffusivity (D) and how this depends on structure, composition and temperature. Recent transfer of the accurate (±2%) laser-flash analysis (LFA) technique from engineering to Earth science has shown that methods previously applied to Earth materials systematically underdetermined D by ~15 to 100%, underestimated anisotropy in D, and gave incorrect dD/dT. New LFA data on garnet, olivine, quartz, and two glasses show that hydration lowers D and the closely related property, lattice thermal conductivity klat for these phases. If this effect is universal, then significant depression of klat should occur in wet regions of the upper mantle. If this is the case, hydration will promote heat retention and magma generation in the mantle wedge and under mid-ocean ridges, and provides positive feedback for hydrolytic weakening and mantle heterogeneities. To test if this relationship is general, accurate measurements of the dependence of D on T and volatile content are proposed for relevant minerals, rocks, and melts. These new data will be made available for use in thermal models. The PI's efforts will immediately improve understanding of the flow of heat on a microscopic basis (i.e., the results pertain directly to physical behavior of solids) and in the long run will improve our understanding of conductive heat flow in the crust and upper mantle (i.e. the data are needed for various applications in Earth science). Broader impacts include relevance to materials science, and training of undergrad and graduate students in a state-of-the-art technology. The investigators will present results at conferences and public lectures, which combines student training with outreach. Public speaking helps the female PI to serve as a role model. The PI may provide inspiration to young women as her career was impeded for a decade due to child-raising, but her productivity has rebounded.
Experimental details are as follows: The effect of volatiles and T on D will be quantified through LFA measurements from ~25ºC up to melting or ~2000ºC of an appropriately diverse suite of samples: major upper mantle minerals and rocks, and re-melted lavas and synthetic analog melts. Oriented single-crystals will be studied. Liquids will be characterized by gathering data above the glass transition temperature. Volatile concentration, speciation, and dehydration rates will be characterized using IR spectroscopy (for OH, H2O, CO2, also Fe2+ content) and microprobe analyses (for F and Fe total). Using synthetics allows isolation of chemical effects.
Suites of appropriate rocks (mantle xenoliths, basalts, andesites) and mineral samples are available. These three suites, minerals, and the glasses provide sub-projects suitable for research by students ranging from undergraduate to PhD level. The team will focus on isolating the effects of volatiles, and on quantifying the temperature dependence and structural effects because in the upper mantle, pressure effects on D are low and predictable. The effect of grain-size will be discerned by comparing mineral to rock data.
