[S-IT16] Deep Earth Sciences

[SIT16-P12] Electrical conductivity of iron-bearing enstatite glasses up to the lowermost mantle pressure

*Izumi Mashino¹, Takashi Yoshino¹, Shinji Kitao², Takaya Mitsui³, Ryo Masuda⁴, Makoto Seto² (1.Institute for Planetary Materials, Okayama University, 2.Institute for Integrated Radiation and Nuclear Science, Kyoto University, 3.National Institutes for Quantum Science and Technology, 4.Hirosaki University)

Keywords:Silicate glass, The lower mantle, Electrical conductivity

The possible presence of silicate melts can explain seismic and conductivity anomalies in the mantle. The silicate melts is expected to exist at the bottom of the mantle, but its stability is still under debate. The gravitational stability of the silicate melt depends on the relationship between the melt and co-existing minerals. Although a density crossover between melt and minerals has been expected to occur within the mantle, the occurrence of the density crossover and its depth depends on the chemical composition of both melt and minerals, especially the partitioning of iron which is influenced by the spin transition of iron.
Here we show the results of in-situ high-pressure electrical conductivity measurements of both Fe²⁺- and Fe³⁺-bearing enstatite glasses, which have been considered as good analogues of silicate melts, with three different amounts of iron. The electrical conductivity of the iron-bearing enstatite glasses monotonically increases with increasing pressure, whereas decreases once above 70 GPa and then increases or becomes constant at higher pressure. The sudden decrease of conductivity above 70 GPa would correspond to a change of high to low/intermediate spin state of iron because similar trend change of electrical conductivity induced by the spin transition was observed in previous studies on mantle minerals. The spin state change of iron is thought to affect the iron partitioning, leading to strong iron enrichment in melts. This implies that the silicate melts might become denser and more stable at the bottom of the mantle, which can be the cause of the ultralow-velocity zones and/or high electrical conductivity regions observed at the core-mantle boundary.