Fe-carbonyl is a key player in planetary magmas.

Introducing Fe-Carbonyl Although the volatile elements C and H are minor constituents of magmas on terrestrial planets (Earth-like rocky bodies, including Earth, Mercury, Venus, Mars, and the Moon), they play an outsized role in planetary processes, having crucial influence on planetary atmospheres and climates. For many planetary environments, including perhaps those of the earliest Earth, we understand relatively little about magmatic outgassing of volatiles because conditions are—or might have been—reducing. Whereas under oxidizing conditions the principle volcanic gases are H2O and CO2, the behavior of which in magmas is reasonably well understood, the identities and concentrations of similar species under reduced conditions has not been known. In PNAS, Wetzel et al. (1) present experimental results that suggest that the C could be carried chiefly in a surprising compound: Fe-carbonyl [Fe(CO)5]. On Earth, volcanic venting of H2O and CO2 are crucial factors in the origin and long-term maintenance of our oceans, atmosphere, climate, and plate tectonics (2–4). Similarly, early in the history of Mars, volcanic outgassing accompanying the vast outpourings of lava that built the giant Tharsis province may have created the greenhouse climate that stabilized liquid surface water (5). But what kinds of volcanic vapors are produced when magmas are oxygen-deficient? And what kinds of volatile fluxes can be created under these conditions? It is well understood that such magmas carry orders-of-magnitude lessdissolved CO2 than oxidized equivalents (6), so does that mean that reduced magmas have commensurately diminished influence on planetary surface conditions? Early efforts to establish whether magmas can dissolve CO came up empty (7). More recent studies have shown that CH4 is sparingly soluble (8), but the concentrations in melts similar to natural magmas appear to be very low (9). Now, Wetzel et al. (1) present experiments on an analog of lunar basalt, quenching a glass from high temperature and pressure and then examining the products spectroscopically. The authors identified the principle carbonaceous component in the glass as Fe carbonyl, Fe(CO)5. Metal carbonyl complexes are well-known in industrial chemistry and are present in biologically important compounds, such as hemoglobin, but are nearly unknown in geological materials.