We present a theoretical model of the stability and migration of carbonate-rich melts to test whether they can explain seismic low-velocity layers (LVLs) observed above stalled slabs in several convergent tectonic settings. The LVLs, located atop the mantle transition zone, contain small (similar to 1 vol%) amounts of partial melt, possibly derived from melting of subducted carbonate-bearing oceanic crust. Petrological and geochemical evidence from inclusions in superdeep diamonds supports the existence of slab-derived carbonate melt, which may potentially explain the origin of the observed melt in the LVL. However, the presumptive reducing nature of the ambient mantle can be an impediment to the stability of carbonated melt. To reconcile this apparent contradiction, we test the stability and migration rates of carbonate-rich melts atop a stalled slab as a function of melt percolation, redox freezing, amount of carbon supplied by subduction, and the metallic Fe concentration in the mantle. Our results demonstrate that carbonaterich melts in the LVL can potentially survive redox freezing over long geological time scales. We also show that the amount of subducted carbon exerts a stronger influence on the stability of carbonate melt than does the mantle redox condition. Concentration dependent melt density leads to rapid melt propagation through channels while a constant melt density causes melt to migrate as a planar front. Our calculations suggest that the LVLs can sequester significant fractions of carbon transported to the mantle by subduction. (C) 2019 Elsevier B.V. All rights reserved.

The day and nightside temperatures of hot Jupiters are diagnostics of heat transport processes in their atmospheres. Recent observations have shown that the nightsides of hot Jupiters are a nearly constant 1100 K for a wide range of equilibrium temperatures (T (eq)), lower than those predicted by 3D global circulation models. Here we investigate the impact of nightside clouds on the observed nightside temperatures of hot Jupiters using an aerosol microphysics model. We find that silicates dominate the cloud composition, forming an optically thick cloud deck on the nightsides of all hot Jupiters with T (eq) <= 2100 K. The observed nightside temperature is thus controlled by the optical depth profile of the silicate cloud with respect to the temperature-pressure profile. As nightside temperatures increase with T (eq), the silicate cloud is pushed upward, forcing observations to probe cooler altitudes. The cloud vertical extent remains fairly constant due to competing impacts of increasing vertical mixing strength with T (eq) and higher rates of sedimentation at higher altitudes. These effects, combined with the intrinsically subtle increase of the nightside temperature with T (eq) due to decreasing radiative timescale at higher instellation levels, lead to low, constant nightside photospheric temperatures consistent with observations. Our results suggest a drastic reduction in the day-night temperature contrast when nightside clouds dissipate, with the nightside emission spectra transitioning from featureless to feature-rich. We also predict that cloud absorption features in the nightside emission spectra of hot Jupiters should reach >= 100 ppm, potentially observable with the James Webb Space Telescope.

The "wet" silicate solidus of mantle peridotite defines the initial melting temperature of Earth's mantle under water-saturated conditions and the second critical endpoint (SCEP) marks the high P-T end of the wet solidus. However, the location of the wet solidus has remained an outstanding issue for over 50 years and the position of the SCEP is hotly debated. Published wet solidi show a difference of 200-600 degrees C at a given pressure while reported SCEPs range from <4 to >6 GPa. Using a large-volume multianvil apparatus, we investigated the water-saturated melting behavior of a fertile peridotite at 3-6 GPa, 950-1200 degrees C, and obtained well-preserved quenched materials. On the basis of textures and compositions of the quenched materials, we bracket the wet solidus to between 950 degrees C and 1000 degrees C at 3 GPa and the SCEP between 3 and 4 GPa. Combining our experimental results with seismologic and petrologic observations, we propose that the lithosphere-asthenosphere boundary in subduction zones should be constrained by the wet solidus and emphasize the role of a deep hydrous partial-melting zone (DHPMZ) on magma genesis within the mantle wedge. We suggest that the DHPMZ is a source of hydrous melts to the primary melting zone in the mantle wedge and that the position of the volcanic front and its magma production rate may largely be controlled by melting and melt segregation processes within the DHPMZ. Our experimental results also suggest that high-magnesian magmas (e.g., boninite, picrite, and komatiite) could be formed at conditions representative of subduction zones.

Optical secondary eclipse measurements made by Kepler reveal a diverse set of geometric albedos for hot Jupiters with equilibrium temperatures between 1550 and 1700 K. The presence or absence of high-altitude condensates, such as Mg2SiO4, Fe, Al2O3, and TiO2, can significantly alter optical albedos, but these clouds are expected to be confined to localized regions in the atmospheres of these tidally locked planets. Here, we present 3D general circulation models and corresponding cloud and albedo maps for six hot Jupiters with measured optical albedos in this temperature range. We find that the observed optical albedos of K2-31b and K2-107b are best matched by either cloud-free models or models with relatively compact cloud layers, while Kepler-8b's and Kepler-17b's optical albedos can be matched by moderately extended (f(sed) = 0.1) parametric cloud models. HATS-11b has a high optical albedo, corresponding to models with bright Mg2SiO4 clouds extending to very low pressures (f(sed) = 0.03). We are unable to reproduce Kepler-7b's high albedo, as our models predict that the dayside will be dominated by dark Al2O3 clouds at most longitudes. We compare our parametric cloud model with a microphysical cloud model. We find that even after accounting for the 3D thermal structure, no single cloud model can explain the full range of observed albedos within the sample. We conclude that a better knowledge of the vertical mixing profiles, cloud radiative feedback, cloud condensate properties, and atmospheric metallicities is needed in order to explain the unexpected diversity of albedos in this temperature range.

Water clouds are expected to form on Y dwarfs and giant planets with equilibrium temperatures near or below that of Earth, drastically altering their atmospheric compositions and their albedos and thermal emission spectra. Here we use the 1D Community Aerosol and Radiation Model for Atmospheres (CARMA) to investigate the microphysics of water clouds on cool substellar worlds to constrain their typical particle sizes and vertical extent, taking into consideration nucleation and condensation, which have not been considered in detail for water clouds in H/He atmospheres. We compute a small grid of Y-dwarf and temperate giant-exoplanet atmosphere models with water clouds forming through homogeneous nucleation and heterogeneous nucleation on cloud condensation nuclei composed of meteoritic dust, organic photochemical hazes, and upwelled potassium chloride cloud particles. We present comparisons with the Ackerman & Marley parameterization of cloud physics to extract the optimal sedimentation efficiency parameter (f (sed)) using Virga. We find that no Virga model replicates the CARMA water clouds exactly and that a transition in f (sed) occurs from the base of the cloud to the cloud top. Furthermore, we generate simulated thermal emission and geometric albedo spectra and find large, wavelength-dependent differences between the CARMA and Virga models, with different gas absorption bands reacting differently to the different cloud distributions and particularly large differences in the M band. Therefore, constraining the vertically dependent properties of water clouds will be essential to estimate the gas abundances in these atmospheres.