R3: High-pressure science and deep Earth’s material

4:45 PM - 5:00 PM JST(7:45 AM - 8:00 AM UTC)

[R3-04] Thermoelectric Properties of Ilmenite under High Pressure and Temperature: Implications for the Origin of the Lunar Paleomagnetic Field

*Takashi YOSHINO¹, Yoshihisa Mori², Koya Tsujimoto², Yuri Shimone², Hyu Takaki¹ (1. IPM, Okayama University, 2. Okayama University of Science)

Keywords:

ilmenite,magnetic field,Moon,electrical conductivity,Seebeck effect

The origin of the intense lunar paleomagnetic field (1~100 µT) during 3.9~3.0 Ga remains one of the most enduring paradoxes in lunar science, as conventional fluid core dynamo models struggle to sustain such high intensities in a small lunar core. Here, we propose an alternative, non-core-dynamo mechanism driven by the thermoelectric (Seebeck) effect within the ilmenite-rich low-velocity zone (LVZ) located directly above the lunar core-mantle boundary (CMB).We experimentally investigated the electrical conductivity (σ) and Seebeck coefficient (S) of ilmenite (FeTiO₃) at high temperatures and pressures under a highly reductive environment (ΔIW ~ -2). At lunar CMB temperatures (>1500 K), ilmenite exhibited a remarkably high conductivity of σ ~ 8,000 S/m and a robust negative Seebeck coefficient reaching -300 µV/K (n-type semiconductor behavior). While Fe²⁺ - Fe³⁺ electron hopping typically yields a positive (p-type) coefficient, our findings strongly imply that small-polaron hopping between Ti³⁺ - Ti⁴⁺, stabilized under lunar reductive conditions, dictates the dominant charge transport.Integrating these experimental properties into a spherical scaling model based on the Biot-Savart law, we evaluated the induced surface magnetic field (B). Assuming a geophysically constrained LVZ thickness (d = 100 ~ 200 km) and a hemispheric thermal asymmetry (ΔT ~ 400 K) driven by the Procellarum KREEP Terrane (PKT), the model successfully generates surface fields of 5~12 µT via a global thermoelectric current loop (52 MA). Furthermore, as the Moon cooled and the thermal gradient decayed to ΔT = 200 K, the predicted field naturally decreased to 1~3 µT, perfectly matching the observed temporal decline in Apollo paleomagnetic records. Our study demonstrates that the microscopic transport properties of titanium ions under planetary reductive conditions can efficiently modulate macro-scale planetary magnetic fields without invoking a core dynamo.