T273 Advances in Mineral Sciences from Atomistic to Macro Perspectives II: Recognition of Dr. Nancy L. Ross, 2024 Roebling Medalist of the Mineralogical Society of America

Wednesday, 25 September 2024
1:30 PM - 5:30 PM
206B

SPIN TRANSITION LINKS WATER AND FERRIC IRON IN EARTH’S LOWER MANTLE

Paper number: T273-10
Program: Technical Sessions
Time: 3:35 PM – 3:50 PM

Authors

Abstract

We are pleased to contribute to the GSA special session honoring Dr. Nancy Ross by bridging a theme of the annual meeting (water) with one of Nancy’s (lattice dynamics). The transport of volatile species, particularly hydrogen and sulfur, influences thermochemical heterogeneity within Earth and other planetary bodies1, playing a major role in dynamic processes, such as magma mixing and recharge2. Nevertheless, significant gaps in our understanding remain, including the depth extent of this transport, storage capacity, and detectability of such volatile-bearing hosts in the mantle. The discovery of hydrous inclusions in sublithospheric diamonds suggest metasomatism and locally hydrous conditions at the mantle transition zone base. Here, oxyhydroxide phases in the (Al,Fe3+)OOH–MgSiO2(OH)2 system can form within oceanic lithosphere3 and are capable of retaining hydrogen in their crystal structures at lower-mantle PT conditions.

By utilizing a suite of scattering techniques, including X-ray diffraction (XRD), infrared spectroscopy, 57Fe nuclear resonant forward (Mössbauer spectroscopy) and inelastic X-ray scattering (NRIXS), at a range of PT conditions, we evaluate the link between a spin crossover in ferric iron4-5 and increased hydrogen storage capacity. In particular, nuclear resonant scattering methods are powerful techniques capable of assessing the electronic, vibrational and elastic properties of Fe-bearing phases. When combined with XRD, an informative link is created between lattice dynamics (microscopic) and structural (macroscopic) analyses6, which informs theoretical calculations and, in the example provided here, our understanding of Earth’s water cycle. Based on our and previous7 experimental results, we constructed a thermodynamic model to evaluate the impact of changes in energy associated with the spin transition on phase equilibria that control deep ”water” storage. By stabilizing Fe3+-bearing oxyhydroxides at lower water activities and to higher temperatures, the spin transition of ferric iron may facilitate water storage in the lower mantle.

1 Pardo et al. (2023) Am Mineral 108: 476-484

2 Ratschbacher et al. (2023) Am Mineral 108: 70-86

3 Walter et al. (2015) Chem Geo 418: 16–29

4 Strozewski et al. (2023) JGR 128

5 Ohira et al. (2019) Am Mineral 104: 1273–1284

6 Buchen et al. (2021) EJM 33: 485–502

7 Ohira et al. (2021) Sci Rep 11: 12036

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-404371

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