Abstract

Iron-bearing pyrope, an abundant silicate mineral in the Earth’s upper mantle, exhibits the largest quadrupole splitting (QS) in Mössbauer spectra among all common Fe-bearing rock-forming silicate minerals at ambient conditions, with a value of ~3.5 mm/s. Knowledge regarding the hyperfine QS of mantle minerals at relevant pressures and temperatures (P-T) is needed to aid our understanding of the electronic spin and valence states of iron and local site distortion in major mantle silicate minerals. The latter, in turn, is relevant for coherent model of electronic and mechanical properties of the Earth’s mantle. Here we have measured synchrotron Mössbauer spectra (SMS) of the high-spin Fe2+ in Fe-bearing pyrope garnet with two distinct compositions, (Mg0.8Fe0.2)3Al2Si3O12 (py80alm20) and Fe3Al2Si3O12 (alm100), up to 30 GPa and 750 K. Analyses of the SMS spectra revealed that the high-spin Fe2+ ions in the distorted dodecahedral site exhibit extremely high QS of ~3.4–3.6 mm/s and relatively high chemical shifts (CS) of ~1.2–1.3 mm/s at high P-T, indicating that the Fe2+ ions remain in the high-spin state. An increase in the Fe content in the pyrope-almandine series only slightly decreases the QS and CS of the Fe2+ ions. To decipher the energy separation (Δ) between the two lowest energy levels of the 3d electrons of the Fe2+ ions in the sample, the dxy and dz2 orbitals, the QS values of py80alm20 at high P-T were further evaluated using Huggins model. Our modeled results show that the Δ of the Fe2+ ions in py80alm20 is ~156 meV at high P-T, and may be correlated to the change of the crystal-field energy splitting (ΔC). Comparison of the QS, Δ, and ΔC values of Fe2+ ions in the distorted dodecahedral sites of pyrope and silicate perovskite indicates that the high-spin Fe2+ with the extremely high QS can remain stable at high P-T conditions, consistent with recent theoretical predictions. Our results thus contribute to our current understanding of the hyperfine parameters and spin and valence states of iron in the mantle silicate minerals at high P-T.

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