An official website of the United States government
Abstract Density of silicate melt dictates melt migration and establishes the gross structure of Earth's interior. However, due to technical challenges, the melt density of relevant compositions is poorly known at deep mantle conditions. Particularly, water may be dissolved in such melts in large amounts and can potentially affect their density at extreme pressure and temperature conditions. Here we perform first‐principles molecular dynamics simulations to evaluate the density of Fe‐rich, eutectic‐like silicate melt (Emelt) with varying water content up to about 12 wt %. Our results show that water mixes nearly ideally with the nonvolatile component in silicate melt and can decrease the melt density significantly. They also suggest that hydrous melts can be gravitationally stable in the lowermost mantle given its likely high iron content, providing a mechanism to explain seismically slow and dense layers near the core‐mantle boundary.
more »
« less
Ultrafast x-ray detection of low-spin iron in molten silicate under deep planetary interior conditions
The spin state of Fe can alter the key physical properties of silicate melts, affecting the early differentiation and the dynamic stability of the melts in the deep rocky planets. The low-spin state of Fe can increase the affinity of Fe for the melt over the solid phases and the electrical conductivity of melt at high pressures. However, the spin state of Fe has never been measured in dense silicate melts due to experimental challenges. We report detection of dominantly low-spin Fe in dynamically compressed olivine melt at 150 to 256 gigapascals and 3000 to 6000 kelvin using laser-driven shock wave compression combined with femtosecond x-ray diffraction and x-ray emission spectroscopy using an x-ray free electron laser. The observation of dominantly low-spin Fe supports gravitationally stable melt in the deep mantle and generation of a dynamo from the silicate melt portion of rocky planets.
more »
« less
- Award ID(s):
- 2153918
- PAR ID:
- 10525897
- Publisher / Repository:
- NSF-PAR
- Date Published:
- Journal Name:
- Science Advances
- Volume:
- 9
- Issue:
- 42
- ISSN:
- 2375-2548
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract A comprehensive analysis of experimental data and theoretical simulations on the partial molar volume of water in silicate melt indicates that finite strain theory successfully describes the compression of the H2O component dissolved in silicate melt at high pressures and temperatures. However, because of the high compressibility of the water component, a fourth order equation of state fit is required to accurately simulate experimental results on water's volume in silicate melts at a deep upper mantle, transition zone, and lower mantle pressures. Data from previous shock compression experiments on hydrous minerals in which melting occurs along the Hugoniot are used to provide an experimental constraint on the partial molar volume of water in silicate melt at deep mantle temperatures and pressures. The equation of state of the water component indicates that, depending on elastic averaging technique, the amount of water that could be present in neutrally or negatively buoyant mafic/ultramafic melts above the 410 km seismic discontinuity is upper‐bounded at 5.6 wt%: smaller than previously inferred, and consistent with melt being confined to a narrow depth range above the 410 km discontinuity. If melt is predominantly distributed along grain boundaries in low aspect ratio films, extents of melting as low as 2% could produce observed seismic velocity reductions. The ability of the lowermost mantle to contain negatively buoyant hydrous liquids hinges on the trade‐off between iron content and hydration: at these depths, substantially higher degrees of hydration could be present within partial melts.more » « less
-
null (Ed.)Water (H2O) as one of the most abundant fluids present in Earth plays crucial role in the generation and transport of magmas in the interior. Though hydrous silicate melts have been studied extensively, the experimental data are confined to relatively low pressures and the computational results are still rare. Moreover, these studies imply large differences in the way water influences the physical properties of silicate magmas, such as density and electrical conductivity. Here, we investigate the equation of state, speciation, and transport properties of water dissolved in Mg1-xFexSiO3 and Mg2(1-x)Fe2xSiO4 melts (for x = 0 and 0.25) as well as in its bulk (pure) fluid state over the entire mantle pressure regime at 2000 to 4000 K using first-principles molecular dynamics. The simulation results allow us to constrain the partial molar volume of the water component in melts along with the molar volume of pure water. The predicted volume of silicate melt+water solution is negative at low pressures and becomes zero above 15 GPa. Consequently, the hydrous component tends to lower the melt density to similar extent over much of the mantle pressure regime irrespective of composition. Our results also show that hydrogen diffuses fast in silicate melts and enhances the melt electrical conductivity in a way that differs from electrical conduction in the bulk water. The speciation of the water component varies considerably from the bulk water structure as well. Water is dissolved in melts mostly as hydroxyls at low pressure and as -O-H-O-, -O-H-O-H- and other extended species with increasing pressure. On the other hand, the pure water behaves as a molecular fluid below 15 GPa, gradually becoming a dissociated fluid with further compression. On the basis of modeled density and conductivity results, we suggest that partial melts containing a few percent of water may be gravitationally trapped both above and below the upper mantle-transition region. Moreover, such hydrous melts can give rise to detectable electrical conductance by means of electromagnetic sounding observations.more » « less
-
This study evaluates changes in copper (Cu) speciation that occur in sulfate-dominated basaltic and andesitic magmas equilibrated at oxygen fugacities (fO2’s) above the nickel-nickel oxide (NNO) buffer. Cu K-edge microfocused X-ray absorption fine structure spectroscopy (XAFS) data are presented from both natural and synthetic silicate glasses. Natural samples analyzed include olivine-hosted melt inclusions from tephra of mafic cinder cones in the Lassen segment of the Cascade arc (USA) and from the Michoacan-Guanajuato volcanic field (Mexico) as representative samples from melts equilibrated at fO2 > NNO. A comparison with melts equilibrated at fO2 < NNO is provided by analysis of olivine-hosted melt inclusions from Kīlauea Volcano. Data are also presented from copper- and sulfur-bearing synthetic hydrous andesitic glasses synthesized over a range of fO2, from roughly NNO-2 to NNO+2. The Cu spectroscopy data from the natural and synthetic glasses show two dominant Cu species, Cu1+ oxides (referred to here as Cu-O) and Cu1+ sulfides (referred to here broadly as Cu-S, but not precluding Cu-Fe-S species). The relative proportion of each species present correlates with the relative concentration of dissolved sulfide in the melt. Synthetic sulfur-bearing glasses equilibrated at NNO-1.2 were found to contain exclusively Cu-S species. Sulfur-bearing experimental glasses equilibrated at NNO-0.5 give calculated Cu-O/(Cu-O + Cu-S), defined here as the “Cu-O fraction”, of < 0.10, whereas sulfur-bearing glasses synthesized at NNO+0.6 and NNO+1.8 give calculated Cu-O fraction > 0.96. Natural melt inclusions from Lassen and Kīlauea show a bimodal distribution in Cu-O fraction, with overlapping ranges, of 0.14-0.77 for Lassen and 0.18- 0.78 for Kīlauea. Michoacan-Guanajuato inclusions yield Cu-O fractions of 0.68-0.91. The difference in the calculated proportions of Cu-O to Cu-S species appear correlated with available sulfide in the melt. As relative S2- concentrations decrease, the dissolved Cu species in the melt evolves from dominantly Cu-S to Cu-O. This includes melts equilibrated at fO2’s where S6+ is the dominant S species. At intermediate sulfide abundances both species appear to coexist. Thermodynamic modeling of the Cu speciation in these silicate melts suggests that speciation of Cu as a CuFeS2 melt species (akin to chalcopyrite or intermediate solid solution) most accurately predicts the measured Cu species. The modeling suggests that aFeO in the silicate melt, fO2 and melt S2- (expressed as fS2) are the most important parameters controlling the proportions of Cu-O vs. Cu-S species. Our results provide a new perspective for understanding Cu solubility, transport, and partitioning in magmatic systems.more » « less
-
Abstract We investigate the consequences of nonideal chemical interaction between silicate and overlying hydrogen-rich envelopes for rocky planets using basic tenets of phase equilibria. Based on our current understanding of the temperature and pressure conditions for complete miscibility of silicate and hydrogen, we find that the silicate-hydrogen binary solvus will dictate the nature of atmospheres and internal layering in rocky planets that garnered H2-rich primary atmospheres. The temperatures at the surfaces of supercritical magma oceans will correspond to the silicate-hydrogen solvus. As a result, the radial positions of supercritical magma ocean–atmosphere interfaces, rather than their temperatures and pressures, should reflect the thermal states of these planets. The conditions prescribed by the solvus influence the structure of the atmosphere, and thus the transit radii of sub-Neptunes. Separation of iron-rich metal to form metal cores in sub-Neptunes and super-Earths is not assured due to prospects for neutral buoyancy of metal in silicate melt induced by dissolution of H, Si, and O in the metal at high temperatures.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1126/sciadv.adi6153
