Oldhamite: a new link in upper mantle for C–O–S–Ca cycles and an indicator for planetary habitability

Abstract

In the solar system, oldhamite (CaS) is generally considered to be formed by the condensation of solar nebula gas. Enstatite chondrites, one of the most important repositories of oldhamite, are believed to be the representative of the material which formed Earth. Thus, the formation mechanism and the evolution process of oldhamite are of great significance to the deep understanding of the solar nebula, meteorites, the origin of Earth, and the C-O-S-Ca cycles of Earth. Until now, oldhamite has not been reported to occur in mantle rock. However, here we show the formation of oldhamite through the reaction between sulfide-bearing orthopyroxenite and molten CaCO3 at 1.5 GPa/1510 K, 0.5 GPa/1320 K, and 0.3 GPa/1273 K. Importantly, this reaction occurs at oxygen fugacities within the range of upper mantle conditions, 6 orders of magnitude higher than that of the solar nebula mechanism. Oldhamite is easily oxidized to CaSO4 or hydrolyzed to produce calcium hydroxide. Low oxygen fugacity of magma, extremely low oxygen content of the atmosphere, and the lack of a large amount of liquid water on the planet's surface are necessary for the widespread existence of oldhamite on the surface of a planet; otherwise, anhydrite or gypsum will exist in large quantities. Oldhamites may exist in the upper mantle beneath mid-ocean ridges. Additionally, oldhamites may have been a contributing factor to the early Earth's atmospheric hypoxia environment, and the transient existence of oldhamites during the interaction between reducing sulfur-bearing magma and carbonate could have had an impact on the changes in atmospheric composition during the Permian-Triassic Boundary.

Commentary on ‘Oldhamite: a new link in upper mantle for C-O-S-Ca cycles and an indicator for planetary habitability’

“…In addition, it was demonstrated that this rare mineral is stable at temperatures greater than 700°C [ 2 ] (thermal expansion coefficient of 4.03 × 10 −5 K −1 ). I agree with the authors that oldhamite could have been involved as a transient phase in important geological processes [ 1 ] as the examples reported by them: i) the mantle metasomatism by carbonate melts beneath the MORB region and ii) the crustal calcite contamination of mantle-derived magma during the formation of some magmatic Cu-Ni-PGE sulfide deposits. The possibility to consider CaS as a transient phase as a function of oxygen fugacity opens new scenarios on deep geological processes where S is involved and we cannot exclude the presence of further rare sulfides both within the Earth and on other Planetary bodies.…”

“…The paper by Liu et al. [ 1 ] is aimed to discuss the role of oldhamite (CaS) in the understanding of the C-O-S-Ca cycles of the Earth. The results of this manuscript also demonstrate the meaning of the presence of oldhamite on the surface and interior of a planet by taking a view of its stability on Earth as well as on other bodies in the Solar System.…”

“…Oldhamite ( Fm -3 m, a = 5.69 Å with V/10 6 pm 3 = 184.88) is stable in highly reducing conditions with an oxygen fugacity below IW + 3.3 (IW = iron-wüstite redox buffer, in lgf o 2 ) [ 1 ]. In addition, it was demonstrated that this rare mineral is stable at temperatures greater than 700°C [ 2 ] (thermal expansion coefficient of 4.03 × 10 −5 K −1 ).…”

“…This sulfide is well-known in meteorites (e.g. enstatite chondrites, aubrites, lunar meteorites [ 1 ], and ungrouped achondrites [ 6 ]) and as one of the major components of hollows and pits on the surface of Mercury [ 2 , 7 ]. However, even if I agree that investigating the role of CaS is crucial for the understanding of the C-O-S-Ca cycles, at the same time, it would be of interest to conduct similar experiments on niningerite, MgS.…”

Laboratory Emissivity Spectra of Sulphide-Bearing Samples, New Constraints for the Surface of Mercury: Oldhamite in Mafic Aggregates

“…Oldhamite has been reported in meteorites [52,53], and its presence is also suggested for the Hermean surface (e.g., [1]). Moreover, oldhamite stability is demonstrated in highly reducing conditions [52] and at high temperatures [22]. On Mercury's surface, CaS is possibly mixed with volcanic bedrock with mafic mineralogy enriched in Mg.…”