Abstract Heterogeneity in Earth’s mantle is a record of chemical and dynamic processes over Earth’s history. The geophysical signatures of heterogeneity can only be interpreted with quantitative constraints on effects of major elements such as iron on physical properties including density, compressibility, and electrical conductivity. However, deconvolution of the effects of multiple valence and spin states of iron in bridgmanite (Bdg), the most abundant mineral in the lower mantle, has been challenging. Here we show through a study of a ferric-iron-only (Mg 0.46 Fe 3+ 0.53 )(Si 0.49 Fe 3+ 0.51 )O 3 Bdg that Fe 3+ in the octahedral site undergoes a spin transition between 43 and 53 GPa at 300 K. The resolved effects of the spin transition on density, bulk sound velocity, and electrical conductivity are smaller than previous estimations, consistent with the smooth depth profiles from geophysical observations. For likely mantle compositions, the valence state of iron has minor effects on density and sound velocities relative to major cation composition.
Abstract The high-pressure behavior of iron nitrides has garnered significant attention due to the possibility of deep nitrogen reservoirs within the Earth’s interior. Here, we investigate the magnetic, structural, electrical, and thermal properties of Fe3N up to 62 GPa and 2100 K, using multiple probes coupled with the diamond-anvil cell technique (including synchrotron X-ray diffraction, synchrotron Mössbauer spectroscopy, and electrical measurements). Fe3N undergoes a magnetic phase transformation from the ferromagnetic to paramagnetic state at ~17-20 GPa, 300 K. The equation of state was determined as, V0/Z = 42.8(1) Å3, and K0 = 151.8(1) GPa, with K′ fixed at 4. Additionally, Fe3N exhibits unexpectedly low electrical and thermal conductivity under high-pressure and high-temperature conditions. This result suggests that deep nitrogen cycling may contribute to the thermal evolution of the deep interiors of Earth and other terrestrial bodies.
Abstract Fe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg 0.5 Fe 3+ 0.5 )(Al 0.5 Si 0.5 )O 3 bridgmanite (FA50) with the highest Fe 3+ ‐Al 3+ coupled substitution known to date. X‐ray diffraction measurements showed that at ambient conditions, the FA50 adopted the LiNbO 3 structure. Upon compression at room temperature to 18 GPa, it transformed back into the bridgmanite structure, which remained stable up to 102 GPa and 2,600 K. Fitting Birch‐Murnaghan equation of state of FA50 bridgmanite yields V 0 = 172.1(4) Å 3 , K 0 = 229(4) GPa with K 0 ′ = 4(fixed). The calculated bulk sound velocity of the FA50 bridgmanite is ~7.7% lower than MgSiO 3 bridgmanite, mainly because the presence of ferric iron increases the unit‐cell mass by 15.5%. This difference likely represents the upper limit of sound velocity anomaly introduced by Fe 3+ ‐Al 3+ substitution. X‐ray emission and synchrotron Mössbauer spectroscopy measurements showed that after laser annealing, ~6% of Fe 3+ cations exchanged with Al 3+ and underwent the high‐ to low‐spin transition at 59 GPa. The low‐spin proportion of Fe 3+ increased gradually with pressure and reached 17–31% at 80 GPa. Since the cation exchange and spin transition in this Fe 3+ ‐Al 3+ ‐enriched bridgmanite do not cause resolvable unit‐cell volume reduction, and the increase of low‐spin Fe 3+ fraction with pressure occurs gradually, the spin transition would not produce a distinct seismic signature in the lower mantle. However, it may influence iron partitioning and isotopic fractionation, thus introducing chemical heterogeneity in the lower mantle.
Significance Seismic studies revealed that shear wave ( S wave) travels through the inner core at an anomalously low speed, thus challenging the notion of its solidity. Here we show that for the candidate inner core component Fe 7 C 3 , shear softening associated with a pressure-induced spin-pairing transition leads to exceptionally low S -wave velocity ( v S ) in its low-spin and nonmagnetic phase. An Fe 7 C 3 -dominant inner core would match seismic observations and imply a major carbon reservoir in Earth’s deepest interior.
Abstract Electronic states of iron in the lower mantle's dominant mineral, (Mg,Fe,Al)(Fe,Al,Si)O3 bridgmanite, control physical properties of the mantle including density, elasticity, and electrical and thermal conductivity. However, the determination of electronic states of iron has been controversial, in part due to different interpretations of Mössbauer spectroscopy results used to identify spin state, valence state, and site occupancy of iron. We applied energy-domain Mössbauer spectroscopy to a set of four bridgmanite samples spanning a wide range of compositions: 10–50% Fe/total cations, 0–25% Al/total cations, 12–100% Fe3+/total Fe. Measurements performed in the diamond-anvil cell at pressures up to 76 GPa below and above the high to low spin transition in Fe3+ provide a Mössbauer reference library for bridgmanite and demonstrate the effects of pressure and composition on electronic states of iron. Results indicate that although the spin transition in Fe3+ in the bridgmanite B-site occurs as predicted, it does not strongly affect the observed quadrupole splitting of 1.4 mm/s, and only decreases center shift for this site to 0 mm/s at ~70 GPa. Thus center shift can easily distinguish Fe3+ from Fe2+ at high pressure, which exhibits two distinct Mössbauer sites with center shift ~1 mm/s and quadrupole splitting 2.4–3.1 and 3.9 mm/s at ~70 GPa. Correct quantification of Fe3+/total Fe in bridgmanite is required to constrain the effects of composition and redox states in experimental measurements of seismic properties of bridgmanite. In Fe-rich, mixed-valence bridgmanite at deep-mantle-relevant pressures, up to ~20% of the Fe may be a Fe2.5+ charge transfer component, which should enhance electrical and thermal conductivity in Fe-rich heterogeneities at the base of Earth's mantle.
Among redox sensitive elements, carbon is particularly important because it may have been a driver rather than a passive recorder of Earth's redox evolution.The extent to which the isotopic composition of carbon records the redox processes that shaped the Earth is still debated.In particular, the highly reduced deep mantle may be metal-saturated, however, it is still unclear how the presence of metallic phases influences the carbon isotopic compositions of super-deep diamonds.Here we report ab initio results for the vibrational properties of carbon in carbonates, diamond, and Fe 3 C under pressure and temperature conditions relevant to super-deep diamond formation.Previous work on this question neglected the effect of pressure on the equilibrium carbon isotopic fractionation between diamond and Fe 3 C but our calculations show that this assumption overestimates the fractionation by a factor of ~1.3.Our calculated probability density functions for the carbon isotopic compositions of super-deep diamonds derived from metallic melt can readily explain the very light carbon isotopic compositions observed in some super-deep diamonds.Our results therefore support the view that metallic phases are present during the formation of super-deep diamonds in the mantle below ~250 km.
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