Electronic Structure of Iron in (Mg,Fe)(Si,Al)O3 Glass up to 93 GPa

Recent studies have found changes in the electronic spin state of iron in crystalline mantle silicate and oxide phases. However, little is known for the spin state of iron in melts in the mantle. We have measured the Mössbauer spectra of (Mg_0.8Fe_0.2)(Si_0.9Al_0.1)O₃ glass up to 93 GPa at 300 K in the diamond-anvil cell at Sector 3 of the APS. At ambient conditions, 64% of iron in the glass has a high quadrupole splitting value (QS = 2.2 mm/s) consistent with high-spin Fe²⁺ and 36% has a low QS value (= 0.7 mm/s) consistent with high-spin Fe³⁺. The ratio of high QS to low QS populations is unchanged by 6 GPa. The population of the low QS site increases rapidly from 34% to 57% between 6 and 32 GPa, followed by a more gradual increase to 68% between 32 and 93 GPa, indicating that 50% of Fe²⁺ changes from high QS to low QS in the pressure range. According to recent first-principles calculations, low- or intermediate-spin Fe²⁺ has QS = 0.5 ∼1.5~mm/s which overlaps with the QS of high-spin Fe³⁺. Therefore, the increase in the low QS site population can be related to a decrease in the spin moment of 50% of Fe²⁺ between 6 and 93 GPa. If our findings are applicable for silicate melts, the spin transition in silicate melts may occur over a much wider depth range (200--2500 km) starting at a much shallow depth (200 km) than those in perovskite and ferropericlase in the mantle.