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Magmas readily react with their wall-rocks forming metamorphic contact aureoles. Sulphur and possibly metal mobilization within these contact aureoles is essential in the formation of economic magmatic sulphide deposits. We performed heating and partial melting experiments on a black shale sample from the Paleoproterozoic Virginia Formation, which is the main source of sulphur for the world-class Cu-Ni sulphide deposits of the 1.1 Ga Duluth Complex, Minnesota. These experiments show that an autochthonous devolatilization fluid effectively mobilizes carbon, sulphur, and copper in the black shale within subsolidus conditions (≤ 700 °C). Further mobilization occurs when the black shale melts and droplets of Cu-rich sulphide melt and pyrrhotite form at ∼1000 °C. The sulphide droplets attach to bubbles of devolatilization fluid, which promotes buoyancy-driven transportation in silicate melt. Our study shows that devolatilization fluids can supply large proportions of sulphur and copper in mafic-ultramafic layered intrusion-hosted Cu-Ni sulphide deposits., (© 2021. The Author(s).)

Very low seismic velocity anomalies in the Earth's mantle may reflect small amounts of melt present in the peridotite matrix, and the onset of melting in the Earth's upper mantle is likely to be triggered by the presence of small amounts of carbonate. Such carbonates stem from subducted oceanic lithosphere in part buried to depths below the 660-kilometre discontinuity and remixed into the mantle. Here we demonstrate that carbonate-induced melting may occur in deeply subducted lithosphere at near-adiabatic temperatures in the Earth's transition zone and lower mantle. We show experimentally that these carbonatite melts are unstable when infiltrating ambient mantle and are reduced to immobile diamond when recycled at depths greater than ∼250 kilometres, where mantle redox conditions are determined by the presence of an (Fe,Ni) metal phase. This 'redox freezing' process leads to diamond-enriched mantle domains in which the Fe(0), resulting from Fe(2+) disproportionation in perovskites and garnet, is consumed but the Fe(3+) preserved. When such carbon-enriched mantle heterogeneities become part of the upwelling mantle, diamond will inevitably react with the Fe(3+) leading to true carbonatite redox melting at ∼660 and ∼250 kilometres depth to form deep-seated melts in the Earth's mantle.

Magmatic production on Earth is dominated by asthenospheric melts of basaltic composition that have mostly erupted at mid-ocean ridges. The timescale for segregation and transport of these melts, which are ultimately responsible for formation of the Earth's crust, is critically dependent on the permeability of the partly molten asthenospheric mantle, yet this permeability is known mainly from semi-empirical and analogue models. Here we use a high-pressure, high-temperature centrifuge, at accelerations of 400g-700g, to measure the rate of basalt melt flow in olivine aggregates with porosities of 5-12 per cent. The resulting permeabilities are consistent with a microscopic model in which melt is completely connected, and are one to two orders of magnitude larger than predicted by current parameterizations. Extrapolation of the measurements to conditions characteristic of asthenosphere below mid-ocean ridges yields proportionally higher transport speeds. Application of these results in a model of porous-media channelling instabilities yields melt transport times of approximately 1-2.5 kyr across the entire asthenosphere, which is sufficient to preserve the observed (230)Th excess of mid-ocean-ridge basalts and the mantle signatures of even shorter-lived isotopes such as (226)Ra (refs 5,11-14).

The evolution of the martian core is widely assumed to mirror the characteristics observed for Earth's core. Data from experiments performed on iron-sulfur and iron-nickel-sulfur systems at pressures corresponding to the center of Mars indicate that its core is presently completely liquid and that it will not form an outwardly crystallizing iron-rich inner core, as does Earth. Instead, planetary cooling will lead to core crystallization following either a "snowing-core" model, whereby iron-rich solids nucleate in the outer portions of the core and sink toward the center, or a "sulfide inner-core" model, where an iron-sulfide phase crystallizes to form a solid inner core.