[S-CG48] Petrology, Mineralogy & Resource Geology

[SCG48-16] Electrical conductivity measurement of (Mg_0.81,Fe_0.19)O under high pressure and temperature

*Tatoshi Hiraoka¹, Yoshiyuki Okuda¹, Kei Hirose¹ (1.The University of Tokyo)

Keywords:electrical conductivity, ferropericlase, lower mantle, spin transition

(Mg,Fe)O ferropericlase (fp) is the second most abundant mineral in the lower mantle next to (Mg,Fe)SiO₃ bridgmanite (bdg). Fp is a weak phase, substantially weaker than bdg, (Girard et al., 2016), and so it may be interconnected in the Earth’s lower mantle. Where it is interconnected, the bulk electrical conductivity (EC) is controlled by fp whose EC has been reported to be ~2 orders of magnitude higher than bdg. It is known that the electronic state of iron in fp changes from high-spin to low-spin under lower-mantle pressure and temperature (P-T) conditions, resulting in a reduction in the room-temperature EC by one order of magnitude (Ohta et al., 2007; Lin et al., 2007). However, since a pressure range of such spin transition broadens at higher temperature (Tsuchiya et al., 2006), the effect of the iron spin transition on the EC of fp may be different between high and room temperatures. The previous multi-anvil experiments performed by Yoshino et al. (2011) measured the impedance of (Mg_1-x,Fe_x)O fp (x = 0.07, 0.10, 0.13, 0.17, and 0.24) up to only 600 K and 50 GPa. On the other hand, previous DAC experiments demonstrated the direct current (DC) resistance of (Mg_0.81,Fe_0.19)O fp to 131 GPa and 2730 K(Ohta et al., 2017), but the DC resistance may overestimate the sample resistance. Moreover, both studies performed only a single run for each sample composition. Here we measured the high P-T impedance of (Mg_0.81,Fe_0.19)O fp measured in a laser-heated diamond-anvil cell (DAC) up to 45 GPa and high temperatures. We employed a rhenium gasket insulated by cBN powder, a 5 μm-thick platinum foil sputtered with 500 nm-thick gold or a 3 μm-thick boron-doped diamond as electrodes, and ZrO₂ pressure medium which acts also as a thermal insulator. The electrodes were placed on both side of the sample (Okuda et al., 2022). The sample geometry and thickness were obtained on its cross section prepared by a focused ion beam (FIB) after releasing pressure. The results demonstrate that the room-temperature EC of fp increased linearly with increasing pressure to ~30 GPa and then the increasing rate dropped at higher pressures, which is consistent with previous studies (Ohta et al., 2007; Yoshino et al., 2011). Such drop in the increasing rate is likely attributed to the onset of spin transition of iron in fp. We also observed that the measured EC diminished by a few orders of magnitude with increasing temperature from 300 K to a few thousand kelvins. We will discuss more details in the presentation.