Capturing the oxidation of silicon carbide in rocky exoplanetary interiors

Theoretical models predict the condensation of silicon carbide around host stars with C/O ratios higher than 0.65 (cf. C/O$_{\mathrm{Sun}}$ = 0.54), in addition to its observations in meteorites, interstellar medium and protoplanetary disks. Consequently, the interiors of rocky exoplanets born from carbon-enriched refractory material are often assumed to contain large amounts of silicon carbide. Here we aim to investigate the stability of silicon carbide in the interior of carbon-enriched rocky exoplanets and to derive the reaction leading to its transformation. We performed a high-pressure high-temperature experiment to investigate the reaction between a silicon carbide layer and a layer representative of the bulk composition of a carbon-enriched rocky exoplanet. We report the reaction leading to oxidation of silicon carbide producing quartz, graphite, and molten iron silicide. Combined with previous studies, we show that in order to stabilize silicon carbide, carbon saturation is not sufficient, and a complete reduction of Fe$^{2+}$ to Fe$^{0}$ in a planetary mantle is required, suggesting that future spectroscopic detection of Fe$^{2+}$ or Fe$^{3+}$ on the surface of rocky exoplanets would imply the absence of silicon carbide in their interiors.Comment: Accepted for publication in Astronomy & Astrophysic

Forming the Moon from terrestrial silicate-rich material

Recent high-precision measurements of the isotopic composition of lunar rocks demonstrate that the bulk silicate Earth and the Moon show an unexpectedly high degree of similarity. This is inconsistent with one of the primary results of classic dynamical simulations of the widely accepted giant impact model for the formation of the Moon, namely that most of the mass of the Moon originates from the impactor, not Earth. Resolution of this discrepancy without changing the main premises of the giant impact model requires total isotopic homogenisation of Earth and impactor material after the impact for a wide range of elements including O, Si, K, Ti, Nd and W. Even if this process could explain the O isotope similarity, it is unlikely to work for the much heavier, refractory elements. Given the increasing uncertainty surrounding the giant impact model in light of these geochemical data, alternative hypotheses for lunar formation should be explored. In this paper, we revisit the hypothesis that the Moon was formed directly from terrestrial mantle material. We show that the dynamics of this scenario requires a large amount of energy, almost instantaneously generated additional energy. The only known source for this additional energy is nuclear fission. We show that it is feasible to form the Moon through the ejection of terrestrial silicate material triggered by a nuclear explosion at Earths core-mantle boundary (CMB), causing a shock wave propagating through the Earth. Hydrodynamic modelling of this scenario shows that a shock wave created by rapidly expanding plasma resulting from the explosion disrupts and expels overlying mantle and crust material.Comment: 26 pages, 5 figures, 1 tabl

Quantifying garnet-melt trace element partitioning using lattice-strain theory: Assessment of statistically significant controls and a new predictive model

As a complement to our efforts to update and revise the thermodynamic basis for predicting garnet-melt trace element partitioning using lattice-strain theory (van Westrenen and Draper in Contrib Mineral Petrol, this issue), we have performed detailed statistical evaluations of possible correlations between intensive and extensive variables and experimentally determined garnet-melt partitioning values for trivalent cations (rare earth elements, Y, and Sc) entering the dodecahedral garnet X-site. We applied these evaluations to a database containing over 300 partition coefficient determinations, compiled both from literature values and from our own work designed in part to expand that database. Available data include partitioning measurements in ultramafic to basaltic to intermediate bulk compositions, and recent studies in Fe-rich systems relevant to extraterrestrial petrogenesis, at pressures sufficiently high such that a significant component of majorite, the high-pressure form of garnet, is present. Through the application of lattice-strain theory, we obtained best-fit values for the ideal ionic radius of the dodecahedral garnet X-site,

Characterization of mesostasis areas in mare basalts: constraining melt compositions from which apatite crystallizes

Crystallization of major silicate and oxide phases from basaltic melts produces late-stage liquids whose chemical compositions differ from the initial melt. These chemically evolved liquids crystallize phases in the interstitial mesostasis regions in lunar basaltic rocks. Enrichment of incompatible elements, including volatiles such as OH, F, Cl, is characteristic of these late-stage liquids and encourages growth of accessory phases including apatite [Ca5(PO4)2(F,Cl,OH)]. Apatite is the main volatile bearing crystalline phase in lunar rocks. It starts crystallizing after ~95% melt solidification in typical mare basalts, but could crystallize earlier, after ~85-90% solidification in KREEP basalts. Using the OH contents of apatites, several researchers have calculated water contents for parental magmas. These calculated parental magma water contents can then be used to estimate a range of values for water in the mantle source regions of mare basalts [e.g.,2-6]. Therefore, a better characterization of the mesostasis areas, and of the melts in which apatite forms, is paramount to gain further insights and constraints on water in the lunar interior, especially because important parameters such as partitioning of volatiles between late-stage melts and apatite remain poorly constrained

Using lunar apatite to assess the volatile inventory of the Moon

Lunar petrology, most notably the absense of hydrous minerals (such as micas and amphiboles) and the lack of Fe2O3, imply a low oxygen activity for the Moon. The anhydrous nature of the Moon is consistent with observed depletions in volatile elements compared to the Earth. Recent analytical developments have led to the re-investigation of lunar samples. In volcanic products, heterogeneous water contents in volcanic glass beads olivine-hosted melt inclusions and in the accessory phase apatite indicate a wetter lunar interior than previously thought. Analysis of lunar apatite has produced OH contents as high as ~12000 ppm and volatile contents of olivine-hosted melt inclusions appear to be similar to terrestrial mid-ocean ridge basalts values. However, analysis of Cl isotope compositions from a range of lunar rocks (basalts, glasses, apatite grains) identified a Cl fractionation 25 times larger than on Earth. This has been interpreted as reflecting a relatively dry lunar interior. The coupled nature of Cl and H, together with this high fractionation of Cl has been used to suggest the Moon’s mantle has H values as low as 10 ppb. To calculate the volatile contents of lunar melts, the partitioning behaviour of volatiles into apatite must be considered. Very little work has been done on the partition of volatiles under lunar conditions, however to fully constrain the H content of the magmatic source regions based on apatite grain measurements, determination of accurate partition coefficients is required. Experimental work using a piston-cylinder assembly at VU, University Amsterdam, is being carried out to derive these partition coefficients for volatiles (F, OH, Cl) between apatite and melts. Measurements of the volatile contents in experimental synthesised apatites are being carried out using a Cameca NanoSIMS 50L ion probe at the Open University. Primary experiments have looked at the temperature effect of F partitioning into apatite. This experimental work will be combined with measurements of Cl, F, and OH concentrations as well as Cl and H isotope compositions in mare basalts. This will provide better constraints on the volatile budget of the lunar magmatic source regions

The thermal equation of state of FeTiO_3 ilmenite based on in situ X-ray diffraction at high pressures and temperatures

We present in situ measurements of the unit-cell volume of a natural terrestrial ilmenite (Jagersfontein mine, South Africa) and a synthetic reduced ilmenite (FeTiO_3) at simultaneous high pressure and high temperature up to 16 GPa and 1273 K. Unit-cell volumes were determined using energy-dispersive synchrotron X-ray diffraction in a multi-anvil press. Mössbauer analyses show that the synthetic sample contained insignificant amounts of Fe^(3+) both before and after the experiment. Results were fit to Birch-Murnaghan thermal equations of state, which reproduce the experimental data to within 0.5 and 0.7 GPa for the synthetic and natural samples, respectively. At ambient conditions, the unit-cell volume of the natural sample [V_0 = 314.75 ± 0.23 (1 ) Å^3] is significantly smaller than that of the synthetic sample [V_0 = 319.12 ± 0.26 Å^3]. The difference can be attributed to the presence of impurities and Fe^(3+) in the natural sample. The 1 bar isothermal bulk moduli K_(T0) for the reduced ilmenite is slightly larger than for the natural ilmenite (181 ± 7 and 165 ± 6 GPa, respectively), with pressure derivatives K_0' = 3 ± 1. Our results, combined with literature data, suggest that the unit-cell volume of reduced ilmenite is significantly larger than that of oxidized ilmenite, whereas their thermoelastic parameters are similar. Our data provide more appropriate input parameters for thermo-chemical models of lunar interior evolution, in which reduced ilmenite plays a critical role

Modeling volatile species in magma ocean-atmosphere interactions on hot rockyexoplanets

Stars and planetary system