N.E.R.D. - Normative Estimator of Rock Diffusivity: a tool for calculating the temperature-dependence of crystalline rock thermal transport properties

Natural variations in the thermal transport properties thermal diffusivity (D), thermal conductivity, and heat capacity on the order of a factor of 10 have been observed for common rocks and minerals in the crust. For example, an increase in temperature from ambient to near Moho (~500-1000°C) can result in thermal conductivity for a rock that is on the order of 50% lower. Likewise, at ambient temperature, variability in D between common rocks types can be as much as a factor of 10, and a factor of 5 between rocks that fall in the same field in rock classification diagrams (e.g. granite, granodiorite).

To provide new insight into the effects of natural variations in conductive properties, we have created a tool which, when common rock properties are integrated, will calculate rock thermal transport properties at temperatures up to 1300°C. This Microsoft Excel© spreadsheet, called N.E.R.D. - Normative Estimator of Rock Diffusivity, was tested against 122 disks of 55 rock samples measured at ambient temperatures, and up to 1250°C using the Laser Flash Analysis method (LFA). Sample characteristics were evaluated directly on LFA disks using XRD, SEM, and optical methods, and additional rock characteristics were observed using Electron Microprobe, density and porosity measurements, and ICP-OA.

When primary thermal conduction sample characteristics such as mineralogy and porosity are accounted for, rock Dcan be calculated using NERD to within the analytical error of the LFA method at ambient temperatures. Furthermore, sample Dmeasured to high temperature is often well-reproduced by NERD, but additional experimental errors are apparent for many samples.

Novel applications for the NERD applicator are discussed, ranging from calculating the effects of metamorphism on the ability of the crust to conduct heat, to generating profiles of thermal diffusivity across a transect of exposed rock in Yosemite national park, to calculating the ability of Martian surface rock to conduct heat using data from the Mars rover.