In Situ Measurements of Sample Thickness in Diamond Anvil Cells Suggest Large Systematic Errors in High-Pressure Conductivities

Abstract

The thermal and electrical conductivities of materials in planetary mantles and cores are crucial for understanding planetary evolution and dynamics. The experimental determination of conductivities at mantle and core pressure-temperature conditions in the diamond anvil cell requires that the sample thickness is precisely known. The standard approach has been to estimate sample thickness from the equations of state of the sample, assuming isotropic contraction (upon compression) or expansion (upon decompression). Recently, however, it has been shown [1] that common pressure media used in diamond anvil cell experiments thin in a strongly non-isotropic manner. If samples embedded in pressure media also deform non-isotropically, then the extant experimental estimates of mantle and core conductivity may contain systematic errors of approximately 30-50% [1]. In situ measurements of sample thickness are needed to verify this inference.   We will report on the first in situ interferometric measurements of Fe foil thickness in a diamond anvil cell with sample configurations resembling that of previous experiments to measure the conductivity of Fe. Our preliminary data show that the contraction and expansion of Fe is strongly non-isotropic, potentially explaining the discrepancies in the reported iron conductivity at core-mantle boundary conditions [2, 3]. We will also discuss practical aspects of future measurements of thermal and electrical conductivity of mantle and core materials in a diamond anvil cell.   [1] Lobanov, S. S., & Geballe, Z. M. (2022). Non-isotropic contraction and expansion of samples in diamond anvil cells: Implications for thermal conductivity at the core-mantle boundary. Geophysical Research Letters, 49, e2022GL100379. [2] Zhang Y., Hou M., Liu G., Zhang C., Prakapenka V.B., Greenberg E., Fei Y., Cohen R. E., and Lin J.-F (2020) Reconciliation of Experiments and Theory on Transport Properties of Iron and the Geodynamo. Phys. Rev. Lett. 125, 078501. [3] Ohta, K., Kuwayama, Y., Hirose, K. et al. (2016). Experimental determination of the electrical resistivity of iron at Earth’s core conditions. Nature 534, 95–98.