The Fe-FeSi phase diagram at Mercury's core conditions

A diamond-bearing core-mantle boundary on Mercury

Abundant carbon was identified on Mercury by MESSENGER, which is interpreted as the remnant of a primordial graphite flotation crust, suggesting that the magma ocean and core were saturated in carbon. We re-evaluate carbon speciation in Mercury's interior in light of the high pressure-temperature experiments, thermodynamic models and the most recent geophysical models of the internal structure of the planet. Although a sulfur-free melt would have been in the stability field of graphite, sulfur dissolution in the melt under the unique reduced conditions depressed the sulfur-rich liquidus to temperatures spanning the graphite-diamond transition. Here we show it is possible, though statistically unlikely, that diamond was stable in the magma ocean. However, the formation of a solid inner core caused diamond to crystallize from the cooling molten core and formation of a diamond layer becoming thicker with time.

Core origin of seismic velocity anomalies at Earth's core-mantle boundary

Seismic studies have found fine-scale anomalies at the core-mantle boundary (CMB), such as ultralow velocity zones (ULVZs)1,2 and the core rigidity zone3,4. ULVZs have been attributed to mantle-related processes5-10, but little is known about a possible core origin. The precipitation of light elements in the outer core has been proposed to explain the core rigidity zone3, but it remains unclear what processes can lead to such precipitation. Despite its importance for the outer core11, the melting behaviour of Fe-Si-H at relevant pressure-temperature conditions is not well understood. Here we report observations of the crystallization of B2 FeSi from Fe-9wt%Si melted in the presence of hydrogen up to 125 GPa and 3,700 K by using laser-heated diamond anvil cells. Hydrogen dramatically increases the Si concentration in the B2 crystals to a molar ratio of Si:Fe ≈ 1, whereas it mostly remains in the coexisting Fe liquid. The high Si content in the B2 phase makes it stable in a solid form at the outermost core temperatures and less dense than the surrounding liquids. Consequently, the Si-rich crystallites could form, float and be sedimented to the underside of the CMB interface, and that well explains the core side rigidity anomalies3,4. If a small amount of the FeSi crystals can be incorporated into the mantle, they would form dense low-velocity structures above the CMB, which may account for some ULVZs10. The B2 FeSi precipitation promoted by H in the outermost core provides a single core-driven origin for two types of anomalies at the CMB. Such a scenario could also explain the core-like tungsten isotope signatures in ocean island basalts12, after the materials equilibrated with the precipitates are entrained to the uppermost mantle by the mantle plumes connected to ULVZs.

Electrical resistivity of the Fe–Si–S ternary system: implications for timing of thermal convection shutdown in the lunar core

The composition of the lunar core has been suggested to be Fe-rich with varying amounts of lighter elements, such as Si and S. Presence of Si and S affects electrical and thermal transport properties and thus influences core thermal processes and evolution. Paleomagnetic observations constrain a high intensity magnetic field that ceases shortly after formation of the moon (~ 3.5–4.2 Ga year ago), and thermal convection in the core may contribute to generation of this field. In this study, the electrical resistivity of Fe-14 wt% Si-3 wt% S was measured in both solid and molten states at pressures up to 5 GPa and thermal conductivity was calculated via the Wiedemann–Franz Law from the electrical measurements. The results were used to estimate the adiabatic conductive heat flux of a molten Fe-14 wt% Si-3 wt% S lunar core and compared to a Fe-2-17 wt% Si lunar core, which showed that thermal convection of either core composition shuts down within the duration of the high intensity magnetic field: (1) 3.17–3.72 Ga year ago for a Fe-14 wt% Si-3 wt% S core; and (ii) 3.38–3.86 Ga years ago for a Fe-2-17 wt% Si core. Results favouring compatibility of these core compositions with paleomagnetic observations are strongly dependent on the temperature of the core-mantle boundary and time-dependent mantle-side heat flux.