Abstract

TY - JOUR A1 - Abeykoon, Sumith A1 - Howard, Christopher A1 - Dominijanni, Serena A1 - Eberhard, Lisa A1 - Kurnosov, Alexander A1 - Frost, Daniel J. A1 - Ballaran, Tiziana Boffa A1 - Terasaki, Hidenori A1 - Sakamaki, Tatsuya A1 - Suzuki, Akio A1 - Ohtani, Eiji A1 - Sano‐Furukawa, Asami A1 - Abe, Jun T1 - Deuterium Content and Site Occupancy in Iron Sulfide at High Pressure and Temperature Determined Using In Situ Neutron Diffraction Measurements Y1 - 2023-09-21 VL - 128 IS - 9 SP - EP - JF - Journal of Geophysical Research: Solid Earth DO - 10.1029/2023JB026710 PB - N2 - Abstract

We have performed in situ time‐of‐flight neutron diffraction experiments to examine the uptake of deuterium in iron monosulfide at pressures up to 11.4 GPa and temperatures to 1300 K. A D₂ fluid was formed in the experiments through the decomposition of ND₃BD₃, resulting in an oxygen fugacity of approximately 1.2 log units below the iron‐wüstite buffer. Deuterium positions and site occupancies were determined in FeS V, using Rietveld refinements of the powder neutron diffraction patterns. Our structural model indicates that two normally unoccupied sites in the P6₃/mmc FeS V structure, at Wyckoff positions 6h and 4f, are partially occupied by D atoms, with the latter being more dominant. The deuterium content D_x in FeSD_X increases with both pressure and temperature over the experimental conditions explored, from 0.126 (14) at 2.3 GPa and 787 K to 1.20 (16) at 9.7 GPa and 1300 K. The unit‐cell volume expansion per deuterium atom is 1.53 ± 0.16 Å³ at 6.9 GPa and 960 K, which is smaller than that determined for metallic iron phases at similar conditions. The variation in unit‐cell volume indicates that most deuterium is lost from FeS V upon temperature quenching at high‐pressures. By fitting the obtained FeS V deuterium site occupancies to a thermodynamic model, estimates for the hydrogen contents of iron monosulfide at conditions and oxygen fugacities consistent with the base of the cratonic lithosphere can be made. This results in values in the range of 1,700–2,700 ppm, which contribute to approximately 2–3 ppm hydrogen in the bulk mantle.

N2 - Plain Language Summary: Small amounts of iron sulfide minerals are found in most rocks from the Earth's mantle and as inclusions trapped in natural diamonds. Hydrogen may dissolve into iron sulfide minerals under high pressures and temperature, but is most likely lost once pressure and temperature are removed. In this study, we determined deuterium contents (deuterium was used as a proxy for hydrogen as it has better neutron scattering properties) in iron sulfide, held under high pressure and temperature conditions, using neutron diffraction measurements. Our results show that the amount of deuterium in iron sulfide increases with both pressure and temperature, but the deuterium is lost on recovery to room conditions. The results are used to estimate hydrogen contents of iron sulfide minerals in the deep continental lithospheric mantle, which are found to be in the range 1,700–2,700 ppm. This corresponds to approximately 2–3 ppm of hydrogen in the bulk mantle.

N2 - Key Points:

Deuterium contents in iron sulfide were measured at high‐P, up to 11.4 GPa and high‐T to 1300 K in in situ neutron diffraction experiments

The total deuterium content, D_x in FeSD_X, increases with both P and T, from 0.126 (14) at 2.3 GPa and 787 K to 1.20 (16) at 9.7 GPa and 1300 K

A thermodynamic model shows that the hydrogen contents of iron monosulfide at the base of the cratonic lithosphere could be 1,700–2,700 ppm

UR - http://resolver.sub.uni-goettingen.de/purl?gldocs-11858/11390 ER -