Oxygen controls on magmatism in rocky exoplanets.

Refractory oxygen bound to cations is a key component of the interior of rocky exoplanets. Its abundance controls planetary properties including metallic core fraction, core composition, and mantle and crust mineralogy. Interior oxygen abundance, quantified with the oxygen fugacity (fO2), also determines the speciation of volatile species during planetary outgassing, affecting the composition of the atmosphere. Although melting drives planetary differentiation into core, mantle, crust, and atmosphere, the effect of fO2 on rock melting has not been studied directly to date, with prior efforts focusing on fO2-induced changes in the valence ratio of transition metals (particularly iron) in minerals and magma. Here, melting experiments were performed using a synthetic iron-free basalt at oxygen levels representing reducing (log fO2 = -11.5 and -7) and oxidizing (log fO2 = -0.7) interior conditions observed in our solar system. Results show that the liquidus of iron-free basalt at a pressure of 1 atm is lowered by 105 ± 10 °C over an 11 log fO2 units increase in oxygen abundance. This effect is comparable in size to the well-known enhanced melting of rocks by the addition of H2O or CO2 This implies that refractory oxygen abundance can directly control exoplanetary differentiation dynamics by affecting the conditions under which magmatism occurs, even in the absence of iron or volatiles. Exoplanets with a high refractory oxygen abundance exhibit more extensive and longer duration magmatic activity, leading to more efficient and more massive volcanic outgassing of more oxidized gas species than comparable exoplanets with a lower rock fO2.

Natural Fe-bearing aluminous bridgmanite in the Katol L6 chondrite.

Bridgmanite, the most abundant mineral of the Earth's lower mantle, has been reported in only a few shocked chondritic meteorites; however, the compositions of these instances differ from that expected in the terrestrial bridgmanite. Here, we report the first natural occurrence of Fe-bearing aluminous bridgmanite in shock-induced melt veins within the Katol L6 chondrite with a composition that closely matches those synthesized in high-pressure and temperature experiments over the last three decades. The Katol bridgmanite coexists with majorite and metal-sulfide intergrowths. We found that the natural Fe-bearing aluminous bridgmanite in the Katol L6 chondrite has a significantly higher Fe3+/ΣFe ratio (0.69 ± 0.08) than coexisting majorite (0.37 ± 0.10), which agrees with experimental studies. The Katol bridgmanite is arguably the closest natural analog for the bridgmanite composition expected to be present in the Earth's lower mantle. Textural observations and comparison with laboratory experiments suggest that the Katol bridgmanite formed at pressures of ∼23 to 25 gigapascals directly from the chondritic melt generated by the shock event. Thus, the Katol L6 sample may also serve as a unique analog for crystallization of bridgmanite during the final stages of magma ocean crystallization during Earth's formation.

Neoproterozoic to early Phanerozoic rise in island arc redox state due to deep ocean oxygenation and increased marine sulfate levels.

A rise in atmospheric O2 levels between 800 and 400 Ma is thought to have oxygenated the deep oceans, ushered in modern biogeochemical cycles, and led to the diversification of animals. Over the same time interval, marine sulfate concentrations are also thought to have increased to near-modern levels. We present compiled data that indicate Phanerozoic island arc igneous rocks are more oxidized (Fe3+/ΣFe ratios are elevated by 0.12) vs. Precambrian equivalents. We propose this elevation is due to increases in deep-ocean O2 and marine sulfate concentrations between 800 and 400 Ma, which oxidized oceanic crust on the seafloor. Once subducted, this material oxidized the subarc mantle, increasing the redox state of island arc parental melts, and thus igneous island arc rocks. We test this using independently compiled V/Sc ratios, which are also an igneous oxybarometer. Average V/Sc ratios of Phanerozoic island arc rocks are elevated (by +1.1) compared with Precambrian equivalents, consistent with our proposal for an increase in the redox state of the subarc mantle between 800 and 400 Ma based on Fe3+/ΣFe ratios. This work provides evidence that the more oxidized nature of island arc vs. midocean-ridge basalts is related to the subduction of material oxidized at the Earth's surface to the subarc mantle. It also indicates that the rise of atmospheric O2 and marine sulfate to near-modern levels by the late Paleozoic influenced not only surface biogeochemical cycles and animal diversification but also influenced the redox state of island arc rocks, which are building blocks of continental crust.

Redox variations in Mauna Kea lavas, the oxygen fugacity of the Hawaiian plume, and the role of volcanic gases in Earth's oxygenation.

The behavior of C, H, and S in the solid Earth depends on their oxidation states, which are related to oxygen fugacity (fO2). Volcanic degassing is a source of these elements to Earth's surface; therefore, variations in mantle fO2 may influence the fO2 at Earth's surface. However, degassing can impact magmatic fO2 before or during eruption, potentially obscuring relationships between the fO2 of the solid Earth and of emitted gases and their impact on surface fO2 We show that low-pressure degassing resulted in reduction of the fO2 of Mauna Kea magmas by more than an order of magnitude. The least degassed magmas from Mauna Kea are more oxidized than midocean ridge basalt (MORB) magmas, suggesting that the upper mantle sources of Hawaiian magmas have higher fO2 than MORB sources. One explanation for this difference is recycling of material from the oxidized surface to the deep mantle, which is then returned to the surface as a component of buoyant plumes. It has been proposed that a decreasing pressure of volcanic eruptions led to the oxygenation of the atmosphere. Extension of our findings via modeling of degassing trends suggests that a decrease in eruption pressure would not produce this effect. If degassing of basalts were responsible for the rise in oxygen, it requires that Archean magmas had at least two orders of magnitude lower fO2 than modern magmas. Estimates of fO2 of Archean magmas are not this low, arguing for alternative explanations for the oxygenation of the atmosphere.

Template Removal via Boudouard Equilibrium Allows for Synthesis of Mesostructured Molybdenum Compounds.

Oxidative thermal removal of the polymeric templates is not trivial for molybdenum oxides and hampers mesostructuring of this material. At ambient oxygen fugacity, MoVI is the thermodynamically stable oxidation state and sublimation of MoO3 leads to a quick loss of the mesostructure through Oswald ripening. Taking advantage of the Boudouard equilibrium allows to fix the oxygen fugacity at a level where non-volatile MoO2-x is stable while carbonaceous material may be oxidized by CO2 . Mesostructured MoO2-x can be chemically converted into MoO3 or MoN under retention of the mesostructure.

Redox condition changes caused by impacts: Insights from Chang'e-5 lunar glass beads.

Lunar materials are overall more reducing compared with their terrestrial counterparts, but the mechanism remains to be elucidated. In this study, we present a possible explanation for the changes in redox state of the lunar regolith caused by impact events, based on our investigations of the impact glass beads from Chang'e-5 mission. These glass beads contain iron metal grains and show concentration gradients of FeO and K2O (with or without Na2O) from their rims to centers. The compositional profiles exhibit error-function-like shapes, which indicates a diffusion-limited mechanism. Our numerical modeling results suggest that the iron metal grains on the surface of the glass beads were generated through the reduction of FeO by elemental K and (or) Na produced during the impact events. Meanwhile, the iron metal grains inside the bead may have formed due to oxygen diffusion driven by redox potential gradients. Furthermore, our study suggests that impact processes intensify the local reducing conditions, as evidenced by the presence of calcium sulfide particles within troilite grains that coexist with iron metal grains on the surface of the glass beads. This study provides insights into the oxygen diffusion kinetics during the formation of iron metal spherules and sheds light on the changes in redox conditions of lunar materials caused by impact events.

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

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.