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 representative of the material that 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, six orders of magnitude higher than that of the solar nebula mechanism. Oldhamite is easily oxidized to CaSO4 or hydrolysed 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 celestial body's surface are necessary for the widespread existence of oldhamite on the surface of a celestial body 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.


INTRODUCTION
The alkaline-and alkaline-earth sulfides such as oldhamite (CaS) are extremely rare in terrestrial rocks.To date, only two studies have reported the potential occurrence of oldhamite in natural terrestrial rocks.One is in volcanic glass from the Arteni massif [ 1 ], and the other is from an impactite [ 2 ].The is no report about the existence of oldhamite in the mantle and during the mantle-crust interaction process.However, oldhamite is potentially abundant in hollows and pits on Mercury's surface [ 3 , 4 ], which is closest to the Sun among the eight planets in the solar system, and it is a common mineral in enstatite chondrites [ 5 ] and aubrites (enstatite achondrites) [ 6 ] (Table 1 ), which are considered to be formed near the center of the solar nebula within the orbit of Mercury [ 6 , 7 ].In both cases, the highly reducing conditions with an oxygen fugacity well below IW-2.7 (IW = iron-wüstite redox buffer, in lg f o 2 ) stabilize oldhamite [ 8 -10 ].It seems that the initial formation of most oldhamites was in the region close to the Sun, at Mercury's surface or within the orbit of Mercury.At present, there is no discussion about the origin of oldhamite on Earth, and previous studies have focused on the origin of this mineral in enstatite chondrites and aubrite.There are currently two views regarding this issue.
Most scholars hold the view that oldhamite is a product of condensation of the solar nebula gas.Laboratory smoke experiments demonstrate that pure CaS condenses from vapor phases of calcium and sulfur [ 17 ].This supports a solar nebula gas origin and, according to first-principles calculations at equi librium, old hamite is more easily enriched in light Ca isotopes than other solid minerals.In contrast, condensed Ca-bearing minerals from nebula gas are enriched in heavy Ca isotopes relative to the residual gaseous Ca [ 18 ].Oldhamite in enstatite chondrites is isotopically heavier than coexisting silicate materials, supporting the solar nebula gas origin.However, sulfur isotope data do not support the solar nebula gas origin.The correlation between 33 S and 36 S of some enstatite chondrites does not follow the trends of photochemistry in the solar nebula with 36 S = −2.98 33 S [ 19 ] and of cosmic-ray spallation with 36 S = 8 33 S [ 20 ].Some scholars argue that the oldhamite in enstatite chondrites and aubrite is of igneous origin rather than the solar nebula gas origin [ 16 , 21 ].Textural evidence includes apparent primary igneous grain boundaries between oldhamite and forsterite, and the presence of round, droplet-like Mn-Fe-Mg-Cr-Na sulfide inclusions within oldhamite, which appear to represent an immiscible sulfide liquid [ 16 , 21 ].
At the pressure ( P ) -temperature ( T ) conditions compatible with Earth's upper mantle and lower crust, the reaction experiments between sulfidebearing orthopyroxenite and calcium carbonate were conducted in a multi-anvil cubic apparatus and a piston-cylinder press in this study.The experimental result shows that oldhamite can exist in the mantle and during the mantle-crust interaction process.Furthermore, through thermodynamic calculations and comparing the oxygen-fugacity values of the atmosphere and magma of different planets in the solar system, we infer that whether a large amount of CaS or CaSO 4 appears on the surface of a planet is closely related to the oxygen fugacity of the planetary magma and atmospheric composition.This paper could shed light on the formation of alkaline-earth metal sulfides, the origin of enstatite chondrites, C-O-S-Ca cycles, the mantle metasomatism mechanism, the crustal contamination process of mantlederived magma and planetary habitability.

High P -T experiments
Here we show that oldhamite forms under conditions compatible with Earth's upper mantle and lower crust through the reaction between sulfide-bearing orthopyroxenite and CaCO 3 at 1.5 GPa/1510 K, 0.5 GPa/1320 K, and 0.3 GPa/1273 K in a graphite-lined Au 75 Pd 25 capsule (Fig. 1 ).Oldhamite was observed in the central reaction zone of recovered samples (Fig. 1 D and E and Supplementary Fig. S5).Hereafter we refer to this formation process as the sulfidemagma-calcite (SMC) interaction.In the absence of CaCO 3 , the produced partial melt from orthopyroxenite under these three P-T conditions are basaltic melts, among which the melts produced under 1.5 GPa/1510 K and 0.5 GPa/1320 K are high-Mg basaltic melts (Fig. 1 B) (with SiO 2 = 5 4.5 −5 4.9 wt%, MgO = 9.54 −10.19 wt%; Supplementary section 3 and Supplementary Table S3).

Determination of oxygen-fugacity environment for the stable existence of oldhamite
In natural terrestrial samples, the occurrence of CaS is generally constrained by the following reactions: 2 Ca S (s) As seen from these equations, CaS can be easily oxidized to form CaSO 4 or CaSO 4 • 2H 2 O. Thus, the stable existence of CaS depends mainly on oxygen fugacity.In order to quantitatively calculate the oxygen-fugacity boundary where CaS can exist stably, this paper defines two oxygen buffers for the first time: OA buffer and OLS buffer.The oxygen fugacity at the oldhamite-anhydrite equilibrium (Equation ( 1), named the OA buffer) and the oldhamite-lime-sulfur equilibrium (Equation ( 2), named the OLS buffer) can be determined by Equations ( 4) and ( 5 where P is the pressure in bar and T is the temperature in K.The detailed process for the quantitative formula calculation of these two buffers is listed in Supplementary sections 5.  2 ).If the oxygen-fugacity value is lower than OLS, oldhamite is stable.On the contrary, the oxygen-fugacity value of the anhydrite stable field is higher than OA.

New formation mechanism of oldhamite
The partial melts from orthopyroxenite in our experiments are basaltic melts or high-Mg basaltic melts.This is consistent with the partial melting process of mantle pyroxenites that in part produced midocean ridge basalts (MORBs) and Hawaiian shield basalts [ 35 , 36 ].It is worth noting that similar high-Mg basaltic melt is the parent magma of some magmatic Cu-Ni-Pt deposits in orogenic belts or related to mantle plumes [ 37 ].These melts are derived from the mantle but usually interacted with crustal carbonate [ 38 ].The oxygen fugacity in the graphitelined noble metal capsule used in this study is about FMQ-2.1 [ 39 ] (FMQ = fayalite-magnetite-quartz redox buffer, in lg f o 2 ).Hence, our experiments simulate the reaction between basaltic and carbonate magma in the mantle, the metasomatism of the mantle by carbonate melts, and the reaction process between basaltic magma and crustal carbonate.Based on our findings, oldhamite can exist in the mantle under these conditions.In our run products, limes (CaO), which were formed by the decomposition   [ 8 ], [ 27 ], [ 28 ], [ 29 ], [ 30 ] and [ 31 ], respectively.The general lg f o 2 and temperature range of the upper mantle are from Refs [ 32 ] and [ 33 ], respectively.The upper limit of lg f o 2 range of Earth's atmosphere before GOE is from Ref. [ 34 ].
of CaCO 3 , were observed in the central reaction zone (Fig. 1 E).Pentlandite or pyrrhotite is present around oldhamite, and cavities formed by bubbles are present close to oldhamite (Fig. 1 D and Supplementary Fig. S5).Hence, the most probable route for CaS formation is: Ca O ( melt ) + Fe S ( melt ) = Ca S ( solid ) + Fe O ( melt ) (7) Here, we name this process the sulfide-magmacalcite interaction model (SMC model).
The SMC model is different from the previous genetic model in three ways.The first difference is the formation location.Previous researchers believed that oldhamite exists in the enstatite meteorite, on the surface of Mercury, and in the lunar regolith (Fig. 3 ) [ 3 , 5 , 40 ], but we proved that it could be stable in the upper mantle.It is reasonable that it could be formed in the interior of other terrestrial planets with magmatic activities.This expands the distribution range of oldhamite in the solar system (Fig. 3 ).The second difference is that the formation process by the SMC interaction in this study is totally different from the nebula gas mechanism.Besides, oldhamite in lunar regolith is considered to be formed by the amalgamation of vapor phases of Ca and S produced during meteorite impact [ 40 ].This is also completely different from the SMC model.The third difference is about oxygen fugacity.Former researchers thought that an oxygen fugacity below IW-2.7 is necessary for the stable existence of oldhamite in the solar nebula model [ 8 -10 ].However, our experiment and calculation results show that the oldhamite can be stable below IW + 3.3 (Fig. 2 ), which is six orders of magnitude higher than the limit in the solar nebula mechanism.

The preservation conditions of oldhamite on the surface of a celestial body
Our results support the presence of oldhamite in the mantle, but why is it hard to be found on Earth's surface?As the thermal expansion coefficient of oldhamite is high with a value of 4.03 × 10 −5 K −1 , the temperature is an important factor affecting whether oldhamite can maintain its crystal form [ 41 ].But the oldhamite crystal is stable at Mercury's surface with a high surface temperature of ≤723.15K [ 41 ], which is much higher than that of Earth's surface.Thus, the surface-temperature factor is not the reason why it is hard to find oldhamite on Earth's surface.A closer look at the redox conditions of the hosting magma and atmosphere is required.Oldhamite (including crystal and melt) is stable when lg f o 2 is below the OLS buffer, but cannot exist when lg f o 2 is above the OA buffer (Fig. 2 ).The lg f o 2 values of Mercury's sur- ).These oxygen fugacities are lower than the OLS buffer, which meets the conditions for the presence of oldhamite.On the other hand, as the lg f o 2 value of −4.32 of the Martian atmosphere is higher than OLS, no oldhamite but some sulfates have been found on Mars, despite the lg f o 2 of Martian magma being lower than OLS (Fig. 2 ).It means that the extremely low oxygen content in the atmosphere is necessary for the preservation of oldhamite, which is unsuitable for most known organisms to survive.Interestingly, Earth takes an intermediate position between Mercury and Mars (though closer to Mars).The average lg f o 2 values for arc basalts (FMQ + 0.96), ocean island magma (FMQ + 0.82), and most basalt related to mantle plumes on Earth (FMQ + 0.1) are higher than OLS [ 42 ].Moreover, the lg f o 2 value of −0.67 of Earth's atmosphere is much higher than OA (Fig. 2 ).The habitable surface conditions for humans on Earth do not support the preservation of oldhamite.Moreover, in aqueous solution, CaS can be hydrolysed to form calcium hydroxide.The abundant presence of liquid water on Earth's surface is also detrimental to the preservation of CaS.On the con-trary, although there is evidence to suggest the presence of water ice in permanent shadow areas within polar craters of Mercury and the Moon [ 43 , 44 ], liquid water has not been reported to occur on the surfaces of these two celestial bodies.Thus, the lack of large amount of liquid water on the surface perhaps is necessary in the preservation of oldhamites on a celestial body's surface.

The possible link between oldhamite and anhydrite in black smokers
Most of the oxygen fugacities of the upper mantle are located in the oldhamite stable field (Fig. 2 ), but it is hard to be found on Earth's surface.A reasonable explanation is the existence of an interface where CaS is converted to CaSO 4 between the upper mantle and the surface.On Earth, the MORBs are characterized by a redox state of FMQ −0.41 ± 0.43 (Supplementary Fig. S6) [ 45 ], which is close to FMQ −0.52, the upper-limit oxygen fugacity for the stable existence of oldhamite.There is a large amount of anhydrite and gypsum in the mid-ocean ridges black smoker system.Thus, the interface where CaS is converted into CaSO 4 may exist near or at the solidified MORBs (Supplementary Fig. S6).In the mantle underneath mid-ocean ridges, the conditions for the formation of CaS including carbonate magma and sulfide-bearing magma sometimes can be met.The super-deep diamond research revealed that the oxygen fugacity of the bottom of the mantle transition zone can be IW −6.7 [ 46 ], which is much lower than OLS.Generally, at a depth of ∼160 −170 km, the diamond is expected to convert into graphite at an oxygen fugacity of > FMQ-2 (Supplementary Fig. S6) [ 47 , 48 ].Redox melting [C (graphite) + 2Fe 2 O 3 (melt) + O 2-(melt) = 4FeO + CO 3 2 − (both in the melt)] takes place at a depth of ∼120 −150 km with an oxygen fugacity of ∼FMQ-1.6 (Supplementary Fig. S6) [ 48 ].The carbonate melt produced by the redox melting wi l l then ascend as a flux into the overlying mantle [ 49 ].The interaction between carbonate melt and sulfide-bearing basaltic magma could happen, providing there are conditions for the formation of oldhamite.
The δ 34 S V-CDT value of anhydrite in the midocean ridges black smoker system gradually decreases from the seawater's value of 20 ± 1 to 3.4 ‰ , as observed in the 1.8-km-deep dri l l hole in a middle-ocean ridge [ 50 ].The δ 34 S V-CDT value of 3.4 ‰ , which is much lower than that of modern seawater, is considered to have resulted from the oxidation of low-δ 34 S sulfide to sulfate in the MORB [ 51 ].Oldhamite is easily oxidized due to the large negative H 298 K value for Equation ( 1 ).Thus, the oxidation of CaS to CaSO 4 could be a viable route for the formation of sulfate.On the other hand, oldhamite is more easily enriched in light Ca isotopes than other Ca-bearing minerals [ 18 , 52 ].The dissolution of CaSO 4 that has experienced the CaS-CaSO 4 isotopic fractionation into the hydrothermal fluids at the mid-ocean ridges is expected to increase the δ 44/40 Ca value of the fluids relative to the host-rocks, which, indeed, has been observed [ 53 ].Thus, the oxidation of CaS to CaSO 4 offers a viable alternative for the origin of anhydrite in the mid-ocean ridges black smoker system, though a lot of gypsum in the midocean ridge is due to the decrease in the solubility of Ca and SO 4 2 − in seawater with increasing temperature [ 54 ].The new sulfate-formation mechanism, the CaS oxidation model in this paper, can explain some special Ca-S isotope characteristics, which is a supplement to the formation process of sulfate on planets.

The influence of oldhamite on atmospheric composition before the Great Oxidation Event
Earth's deep interior holds the key to habitability [ 55 -57 ].Oldhamites in Earth may affect the atmospheric composition.Many pieces of evidence show that Earth's atmosphere before the Great Oxidation Event (GOE) was initially free of O 2 [ 58 , 59 ].Around 2.46 −1.85 bi l lion years ago (Ga), oxygen levels rose from < 10 -7.1 -10 -5.1 that of present atmospheric levels (PAL) (lg f o 2 < −7.6 to −6.0, Fig. 2 ) to 10 -4.6 -10 -2.0 PAL (lg f o 2 = −5.3 to −2.7) [ 34 ], known as the GOE.To explore the reasons for the anoxic or oxygen-free feature of early Earth's atmosphere, we need to clarify the characteristics of the initial materials that were used to build Earth.Enstatite chondrites, one of the most important repositories of oldhamite (Table 1 ), are the most-reduced meteorites and have similar isotopic composition to terrestrial rocks, so are often considered to be representative of the material that formed Earth [ 60 , 61 ].That is, early Earth probably contains a significant amount of oldhamite.The Earth-Moon precursor materials with oldhamite enrichment could well explain the Sm/Nd ratio and Nd isotope features of Earth and the Moon [ 62 ].The upper limit of the lg f o 2 range of Earth's atmosphere before GOE is from −7.6 to −6.0 (Fig. 2 ) [ 34 ], which partially with the oldhamite stable field.Besides, the lg f o 2 value of magma ocean of early Earth is IW + 0.5 at 2173 K [ 31 ] (Fig. 2 ).Thus, both the atmospheric oxygen fugacity and the magmatic oxygen fugacity met the conditions for the existence of oldhamite, which would inhibit the generation of free oxygen due to the oxidation of oldhamite to CaSO 4 (Equation ( 1)).
The mass of Earth is ∼5.97 × 10 24 kg [ 63 ].The mass fraction of oldhamite in early Earth is estimated to be 0.072% −0.127% (Supplementary section 8).Thus, we can infer that the mass of oldhamite in early Earth is ∼4.30× 10 21 −7.58 × 10 21 kg.If all these oldhamites are converted into CaSO 4 , they wi l l consume 3.82 × 10 21 −6.74 × 10 21 kg O 2. The total O 2 amount of modern Earth's atmosphere is only ∼1.246 × 10 18 kg, 23% of the modern atmosphere's mass, where the atmosphere's mass is equal to 5.148 × 10 18 kg [ 64 ].It is clear that the potential influence of oldhamite on atmospheric oxygen content is enormous.After GOE, the lg f o 2 of Earth's atmosphere at ∼1.85 Ga ranges from −5.3 to −2.7 [ 34 ], which is higher than OLS.In this oxygenfugacity condition, some CaSO 4 are expected to appear on Earth after GOE.Calcium sulfate (CaSO 4 ) layer has indeed been observed in the 2.2-Ga sedimentary rocks in the Yerrida rift basin of Western Australia [ 65 ].Thus, oldhamite could have played a role in Earth's anoxic or oxygen-free atmosphere before GOE.

The influence of oldhamite on atmospheric composition during the Permian-Triassic boundary
The Permian-Triassic boundary (PTB) mass extinction was the most severe biotic crisis in the past 500 mi l lion years [ 66 , 67 ].During the PTB mass extinction period, a sharp increase in atmospheric CO 2 content and a decrease in atmospheric O 2 content occurred.The atmospheric CO 2 concentration at the PTB is estimated to have been 3314 ± 1097 ppm, which is more than double the Permian average and is 12 ± 4 times that of the current atmosphere [ 68 ].The atmospheric oxygen underwent a very sharp drop from 30% to 15% (volume fraction) at the PTB [ 69 ].The formation of 1 mole of CaS is accompanied by the production of 1 mole of CO 2 (Equations ( 6) and ( 7)) and the oxidation of 1 mole of CaS to sulfate wi l l consume 2 moles of O 2 (Equation ( 1)).The reaction between mantle magma and crustal rocks in the Siberian large igneous province (SLIP) is believed to be an important trigger for the above atmospheric composition change [ 70 ].The SLIP is characterized by tholeiitic basalts [ 71 ], whose composition is similar to the composition of the initial melt of the partial melting of orthopyroxenite in this study (Supplementary Table S3).The magma of SLIP is characterized by a low oxygen fugacity with a value of FMQ-1 .5 [ 71 ].Moreover, some famous magmatic Cu-Ni sulfide ore deposits include the world's largest one formed at the SLIP, indicating the presence of some sulfiderich magma.The above features meet the conditions for the formation of CaS.Thus, the intermediate effect of CaS, accompanied by CO 2 generation and O 2 consumption, can hardly be excluded during the interaction process between the upwelling of largescale reducing S-bearing magma from mantle and the crustal carbonates in the SLIP.

CONCLUSIONS
Two geological processes are likely to involve oldhamite as a transient phase, including mantle metasomatism by carbonate melts beneath the MORB region and crustal calcite contamination of mantlederived magma during the formation of some magmatic Cu-Ni-PGE sulfide deposits at the Siberian large igneous provinces.Oldhamite is a plausible precursor for igneous Ca-sulfate in MORB.The formation and oxidation of oldhamite are accompanied by the production of CO 2 and the consumption of O 2 , which have more or less influence on the atmospheric composition.Widespread existence of oldhamite on a planet's surface indicates a low oxygen fugacity for the magma of the planet, extremely low oxygen content in its atmosphere, and the lack of a large amount of liquid water on the planet's surface, which is not habitable.

METHODS High P -T experiments
Experimental petrological methods are used to simulate the formation process of oldhamite in the mantle.The initial material is pyrrhotite-pentlanditebearing orthopyroxenite and CaCO 3 powder.The mineral composition and chemical composition of starting materials are described in Supplementary section 1. Experiments were conducted at 0.5 GPa/1320 K and 1.5 GPa/1510 K, using a 20 0 0ton multi-anvil cubic apparatus at the University of Nevada, Las Vegas.Besides, the experiment under 0.3 GPa/1273 K was performed by using a piston-cylinder press at the Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences.Sample powder was packed in a graphite crucible placed in an Au 75 Pd 25 outer capsule with an outer diameter of 3 mm (Supplementary Figs S3A and S4).The mineral and melt composition analysis after the experiment was completed by using an electron microprobe.Please refer to Supplementary section 4 for the parameter design of the electron probe.

The determination of oxygen fugacity
The oxygen fugacities at the oldhamite-lime-sulfur equilibrium (OLS buffer) and at oldhamiteanhydrite equilibrium (OA buffer) were obtained by using thermodynamic calculation.The detailed calculation processes are listed in Supplementary sections 5.2 and 5.3.
The concept of the oxygen fugacity of rocks has been deeply rooted in people's minds, but some researchers are not familiar with the oxygen fugacity of the atmosphere.Here, we explain the calculation process of the oxygen fugacity of modern Earth's atmosphere.Oxygen accounts for 21% of modern Earth's atmospheric volume, so the partial pressure of oxygen is 0.21 bars.Then, log (0.21) = −0.67.That is, the lg f o 2 of modern Earth's atmosphere is −0.67 (Fig. 2 ).Similarly, the oxygen fugacities of Mercury's atmosphere, the Martian atmosphere, and the lunar atmosphere are also calculated in Supplementary section 5.1.

The determination of isotope fractionation
Past researchers have calculated the polynomial fitting of the ratio of the reduced partition function for 44 Ca/ 40 Ca of oldhamite and anhydrite by using the static first-principles calculation [ 18 , 52 ].Based on these results, the equilibrium Ca isotope fractionation between CaSO 4 and CaS is inferred.The inferred equation is listed in Supplementary section 6.

Figure 1 .
Figure 1.State and location of CaS generated by the interaction between sulfidebearing magma and calcite (SMC).(A) Reaction chamber for the partial melting experiments of the Po-Pn-bearing orthopyroxenite; (B) the partial melting of the Po-Pnbearing orthopyroxenite at 1.5 GPa/1510 K under scanning electron microscope (SEM); (C) reaction chamber for the contamination experiments between the Po-Pn-bearing orthopyroxenite and CaCO 3 ; (D) drop-shaped oldhamite in the inner part of the central reaction zone and disseminated Fe-Ni sulfide (bright white) at 0.5 GPa/1320 K; (E) oldhamite around lime at 0.3 GPa/1273 K; (F) the EDX/SEM spectrum of oldhamite; (G) the EDX/SEM spectrum of lime.Opx, orthopyroxene; Po, pyrrhotite; Pn, pentlandite; Od, oldhamite.

SunFigure 3 .
Figure 3.The known main distribution of oldhamite (CaS) in the solar system.facemagma range from IW −5.4 to −10.5 (lg f o 2 = −13.7 to −20.8) at 160 0 −180 0 K (Fig.2), and the lg f o 2 value of Mercury's atmosphere is −13.7 (Supplementary section 5.1).These values are much below the OLS buffer (Fig.2), permitting the conservation of oldhamite.That is, the widespread presence of oldhamite on the surface of a celestial body indicates an extremely anoxic environment.Oldhamite has also been found in the lunar regolith, which is a weathering product of rocks formed by meteorite impacts[ 40 ].The oxygen fugacity of the lunar basaltic glass and regolith are ∼IW −1.7 (lg f o 2 = −11.7)and IW + 1 (lg f o 2 = −9.1)at 1673 K, respectively[ 29 , 30 ], and the upper-limit lg f o 2 value of the lunar atmosphere is −15.6 (Supplementary section 5.1).These oxygen fugacities are lower than the OLS buffer, which meets the conditions for the presence of oldhamite.On the other hand, as the lg f o 2 value of −4.32 of the Martian atmosphere is higher than OLS, no oldhamite but some sulfates have been found on Mars, despite the lg f o 2 of Martian magma being lower than OLS (Fig.2).It means that the extremely low oxygen content in the atmosphere is necessary for the preservation of oldhamite, which is unsuitable for most known organisms to survive.Interestingly, Earth takes an intermediate position between Mercury and Mars (though closer to Mars).The average lg f o 2 values for arc basalts (FMQ + 0.96), ocean island magma (FMQ + 0.82), and most basalt related to mantle plumes on Earth (FMQ + 0.1) are higher than OLS[ 42 ].Moreover, the lg f o 2 value of −0.67 of Earth's atmosphere is much higher than OA (Fig.2).The habitable surface conditions for humans on Earth do not support the preservation of oldhamite.Moreover, in aqueous solution, CaS can be hydrolysed to form calcium hydroxide.The abundant presence of liquid water on Earth's surface is also detrimental to the preservation of CaS.On the con-

Table 1 .
Proportional fractions of oldhamite in different enstatite meteorites., low Fe and siderophile group enstatite chondrite meteorite; EH, high Fe and siderophile group enstatite chondrite meteorite.Phases listed as n.d. were not detected in that sample. EL