Grain boundaries as reservoirs of incompatible elements in the Earth's mantle

High-temperature deuterium tracks the thermal stability of hydroxyl in epidote and zoisite

Studying the thermal stability of the -OH groups in epidote-group minerals is of significant importance for understanding the deep-water cycle of the Earth. Epidote group minerals are among the common silicate minerals in the mafic oceanic crust of subducting lithospheric plates and are important water carriers in the deep mantle as well as in the Earth's deep-water cycle. Deuteration reactions offer significant advantages in tracing the thermal stability of the -OH groups in minerals, allowing for the labeling of hydrogen without affecting the mineral structure. The structural properties of epidote and zoisite with different Fe3+ contents at various temperatures were studied using X-ray diffraction, Raman spectroscopy, thermogravimetry, and deuterium tracing techniques to understand the influence of Fe3+/Al3+ substitution on the water-carrying capacity and hydroxyl group thermal stability of epidote group minerals. The results suggest that the axial thermal expansion coefficients of epidote are α0(a) = 1.42(8)*10-5/K-1, α0(b) = 1.77(9)*10-5/K-1, α0(c) = 2.7(6)*10-5/K-1, and zoisite are α0(a) = 0.85(5)*10-5/K-1, α0(b) = 2.13(8)*10-5/K-1, α0(c) = 4.16(8)*10-5/K-1. In addition, the activation temperatures of the epidote and zoisite hydroxyl groups are similar at approximately 573(1) K, and the decomposition temperatures of epidote and zoisite are 1170 K and 1198 K, respectively. The deuteration process of epidote and zoisite before decomposition is divided into two stages: 573-773 K and 823-1123 K, the deuteration degrees of both increased with increasing temperature in each stage. The integral area growth rate of -OD peak in infrared spectroscopy is found to be as follows: 0.0239 K-1 in 573-773 K and 0.1938 K-1 in 823-1123 K for epidote, and 0.0374 K-1 in 573-773 K and 0.1812 K-1 in 823-1123 K for zoisite. Moreover, owing to the Fe3+/Al3+ substitution, the structural characteristics of zoisite gradually evolve to epidote at high temperatures, and the stability of the hydroxyl group decreases. Therefore, in the geothermal environment of plate subduction, the hydroxyl groups in epidote and zoisite are first activated when the temperature rises to 573 K, followed by the exchange and transport of H+ from the surrounding environment or minerals, leading to the dehydration and decomposition of epidote and zoisite. Our results provide key insights into water storage and migration in subduction zones.

Potassium distribution and isotope composition in the lithospheric mantle in relation to global Earth's reservoirs

Recent analytical advances have provided means to measure potassium (K) isotopes in various terrestrial materials, but little is known about K distribution and stable isotope composition in the lithospheric mantle because of: (a) common low K abundances, (b) potential contamination and alteration, (c) diversity of mantle rocks and minerals hosting K in different tectonic settings. We report K abundances and δ41K values for well-studied whole-rock (WR) mantle xenoliths (spinel and garnet peridotites and pyroxenites) from mobile belts, a craton, a subduction zone, as well as for K-rich phases (mica, amphibole, silicate glass) and xenolith-bearing volcanic materials (67 samples). The xenolith materials show extremely broad ranges of K content (7 μg/g to 6.6 wt.%) and δ41K (-2.77‰ to 0.62‰). They contrast with the narrow δ41K range for host volcanic materials (-0.53‰ to -0.27‰) and literature data on oceanic basalts (melting products of upwelling asthenosphere: -0.43 ± 0.17‰, 2sd). Amphibole-bearing subduction zone peridotites show the highest WR δ41K values (0.40 to 0.62‰) likely inherited from fluids released into the mantle wedge from subducted oceanic crust. All other WR samples yield negative δ41K: -0.06‰ to -2.77‰ for peridotites and -0.17‰ to -0.52‰ for pyroxenites. The δ41K in K-rich mantle phases range from positive values (0.16 to 0.57‰) for phlogopite in strongly metasomatized peridotites to negative values (-0.27 to -0.94‰) for phlogopite, amphibole and glass pockets from other samples, which cannot be explained by equilibrium inter-mineral fractionation inferred from ab initio calculations. We attribute the broad δ41K variations to (a) isotope fractionation during fluid-rock interaction in the mantle, and (b) distinct sources of K-bearing fluids. Kinetic isotope fractionation during fluid percolation and diffusion is inferred for composite xenoliths (phlogopite and pyroxenite veins in peridotites) that have lower δ41K in the hosts than in the veins due to slower migration of 41K than 39K from the veins (former fluid channels) to host mantle. Overall, the K isotope fractionation in the lithospheric mantle appears to be greater than for magmatic fractionation in the crust. The average δ41K of normal off-craton continental lithospheric mantle calculated from the least altered fertile and lightly metasomatized lherzolites is -0.57 ± 0.28‰ (2sd). This value is within error (though a little lower) of estimates for both continental crust and MORB and OIB mantle sources indicating that these major silicate Earth reservoirs have similar bulk δ41K values, apparently due to low or negligible K isotopic fractionation during their formation by magmatic differentiation and melting. By contrast, K isotopes in modern and fossil subduction zones are fractionated via fluid-related equilibrium and kinetic processes.

Postmelting hydrogen enrichment in the oceanic lithosphere

The large range of H₂O contents recorded in minerals from exhumed mantle rocks has been challenging to interpret, as it often records a combination of melting, metasomatism, and diffusional processes in spatially isolated samples. Here, we determine the temporal variations of H₂O contents in pyroxenes from a 24-Ma time series of abyssal peridotites exposed along the Vema fracture zone (Atlantic Ocean). The H₂O contents of pyroxenes correlate with both crustal ages and pyroxene chemistry and increase toward younger and more refractory peridotites. These variations are inconsistent with residual values after melting and opposite to trends often observed in mantle xenoliths. Postmelting hydrogen enrichment occurred by ionic diffusion during cryptic metasomatism of peridotite residues by low-degree, volatile-rich melts and was particularly effective in the most depleted peridotites. The presence of hydrous melts under ridges leads to widespread hydrogen incorporation in the oceanic lithosphere, likely lowering mantle viscosity compared to dry models.

Selective leaching of vanadium over iron from vanadium slag

A new comprehensive utilization of vanadium slag (VS) method focusing on inhibiting leaching of iron (Fe) during the leaching of vanadium (V) using sulfuric acid (SA) was proposed. In this process, Cr₂O₃ was added to VS to conjugate with Fe in the VS to form (Fe_x,Cr_1-x)₂O₃ which is insoluble in SA, resulting in the decrease of leaching ratio (LR) of Fe to avoid the subsequent separation difficulty of V in leachate. The phase evolutions of VS during the roasting and SA leaching process, and the influences of roasting temperature, roasting time, and addition of Cr₂O₃ on the LR of V, Fe and chromium (Cr) from VS were studied. When the addition of Cr₂O₃ is 12 wt.%, the mass concentration of V in the leachate is 1 order magnitude higher than Fe and the mass ratio of V to Fe reaches 18.34. The LR of V, Fe and Cr are 91% 1.39% and 0.28%, respectively. The leaching residue can be reused as ironmaking raw material. More importantly, the (Fe_x,Cr_1-x)₂O₃ and Fe₂TiO₅ can be separated from the leaching residue and recycled as raw materials for black ceramic pigments and titanium dioxide production by mineral processing technology, respectively.

Tiny droplets of ocean island basalts unveil Earth's deep chlorine cycle

Fully characterising the exchange of volatile elements between the Earth’s interior and surface layers has been a longstanding challenge. Volatiles scavenged from seawater by hydrothermally altered oceanic crust have been transferred to the upper mantle during subduction of the oceanic crust, but whether these volatiles are carried deeper into the lower mantle is poorly understood. Here we present evidence of the deep-mantle Cl cycle recorded in melt inclusions in olivine crystals in ocean island basalts sourced from the lower mantle. We show that Cl-rich melt inclusions are associated with radiogenic Pb isotopes, indicating ancient subducted oceanic crust in basalt sources, together with lithophile elements characteristic of melts from a carbonated source. These signatures collectively indicate that seawater-altered and carbonated oceanic crust conveyed surface Cl downward to the lower mantle, forming a Cl-rich reservoir that accounts for 13–26% or an even greater proportion of the total Cl in the mantle.

Volatile exchange between the Earth’s interior and surface layers is one of the central issues in mantle geochemistry. Here the authors present evidence that chlorine is transferred from the surface to the deep mantle by subducted oceanic crust, forming a chlorine-rich mantle reservoir.

A revised thermodynamic model for crystal surfaces. I. Theoretical aspects

A revised thermodynamic model to study surface segregation.

Grain boundary mobility of carbon in Earth's mantle: a possible carbon flux from the core

The importance of carbon in Earth's mantle greatly exceeds its modest abundance of approximately 1,000-4,000 ppm. Carbon is a constituent of key terrestrial volatiles (CO, CO(2), CH(4)), it forms diamonds, and it may also contribute to the bulk electrical properties of the silicate Earth. In contrast to that of the mantle, the carbon content of Earth's metallic core may be quite high ( approximately 5 wt %), raising the possibility that the core has supplied carbon to the mantle over geologic time. The plausibility of this process depends in part upon the mobility of carbon atoms in the solid mantle. Grain boundaries of mantle minerals could represent fast pathways for transport as well as localized sites for enrichment and storage of carbon. Here, we report the results of an experimental study of grain-boundary diffusion of carbon through polycrystalline periclase (MgO) and olivine ([Mg,Fe](2)SiO(4)) that were obtained by determining the extent of solid solution formation between a graphite source and a metal sink (Ni or Fe) separated by the polycrystalline materials. Experimental materials were annealed at 1,373-1,773 K and 1.5-2.5 GPa pressure. Calculated diffusivities, which range up to 10(-11) m(2).s(-1), are fast enough to allow transport over geologically significant length scales ( approximately 10 km) over the age of the Earth. Mobility and enrichment of carbon on grain boundaries may also explain the high electrical conductivity of upper mantle rocks, and could result in the formation of C-H-O volatiles through interactions of core-derived C with recycled H(2)O in subduction zones.

A diffusion mechanism for core-mantle interaction

Understanding the geochemical behaviour of the siderophile elements--those tending to form alloys with iron in natural environments--is important in the search for a deep-mantle chemical 'fingerprint' in upper mantle rocks, and also in the evaluation of models of large-scale differentiation of the Earth and terrestrial planets. These elements are highly concentrated in the core relative to the silicate mantle, but their concentrations in upper mantle rocks are higher than predicted by most core-formation models. It has been suggested that mixing of outer-core material back into the mantle following core formation may be responsible for the siderophile element ratios observed in upper mantle rocks. Such re-mixing has been attributed to an unspecified metal-silicate interaction in the reactive D'' layer just above the core-mantle boundary. The siderophile elements are excellent candidates as indicators of an outer-core contribution to the mantle, but the nature and existence of possible core-mantle interactions is controversial. In light of the recent findings that grain-boundary diffusion of oxygen through a dry intergranular medium may be effective over geologically significant length scales and that grain boundaries can be primary storage sites for incompatible lithophile elements, the question arises as to whether siderophile elements might exhibit similar (or greater) grain-boundary mobility. Here we report experimental results from a study of grain-boundary diffusion of siderophile elements through polycrystalline MgO that were obtained by quantifying the extent of alloy formation between initially pure metals separated by approximately 1 mm of polycrystalline MgO. Grain-boundary diffusion resulted in significant alloying of sink and source particles, enabling calculation of grain-boundary fluxes. Our computed diffusivities were high enough to allow transport of a number of siderophile elements over geologically significant length scales (tens of kilometres) over the age of the Earth. This finding establishes grain-boundary diffusion as a potential fast pathway for chemical communication between the core and mantle.

Engineering metal-impurity nanodefects for low-cost solar cells

As the demand for high-quality solar-cell feedstock exceeds supply and drives prices upwards, cheaper but dirtier alternative feedstock materials are being developed. Successful use of these alternative feedstocks requires that one rigorously control the deleterious effects of the more abundant metallic impurities. In this study, we demonstrate how metal nanodefect engineering can be used to reduce the electrical activity of metallic impurities, resulting in dramatic enhancements of performance even in heavily contaminated solar-cell material. Highly sensitive synchrotron-based measurements directly confirm that the spatial and size distributions of metal nanodefects regulate the minority-carrier diffusion length, a key parameter for determining the actual performance of solar-cell devices. By engineering the distributions of metal-impurity nanodefects in a controlled fashion, the minority-carrier diffusion length can be increased by up to a factor of four, indicating that the use of lower-quality feedstocks with proper controls may be a viable alternative to producing cost-effective solar cells.

Phases, periphases, and interphases equilibrium by molecular modeling. I. Mass equilibrium by the semianalytical stochastic perturbations method and application to a solution between (120) gypsum faces

The SASP (semianalytical stochastic perturbations) method is an original mixed macro-nano-approach dedicated to the mass equilibrium of multispecies phases, periphases, and interphases. This general method, applied here to the reflexive relation C(k)<=>mu(k) between the concentrations C(k) and the chemical potentials mu(k) of k species within a fluid in equilibrium, leads to the distribution of the particles at the atomic scale. The macroaspects of the method, based on analytical Taylor's developments of chemical potentials, are intimately mixed with the nanoaspects of molecular mechanics computations on stochastically perturbed states. This numerical approach, directly linked to definitions, is universal by comparison with current approaches, DLVO Derjaguin-Landau-Verwey-Overbeek, grand canonical Monte Carlo, etc., without any restriction on the number of species, concentrations, or boundary conditions. The determination of the relation C(k)<=>mu(k) implies in fact two problems: a direct problem C(k)=>mu(k) and an inverse problem mu(k)=>C(k). Validation of the method is demonstrated in case studies A and B which treat, respectively, a direct problem and an inverse problem within a free saturated gypsum solution. The flexibility of the method is illustrated in case study C dealing with an inverse problem within a solution interphase, confined between two (120) gypsum faces, remaining in connection with a reference solution. This last inverse problem leads to the mass equilibrium of ions and water molecules within a 3 A thick gypsum interface. The major unexpected observation is the repulsion of SO(4) (2-) ions towards the reference solution and the attraction of Ca(2+) ions from the reference solution, the concentration being 50 times higher within the interphase as compared to the free solution. The SASP method is today the unique approach able to tackle the simulation of the number and distribution of ions plus water molecules in such extreme confined conditions. This result is of prime importance for all coupled chemical-mechanical problems dealing with interfaces, and more generally for a wide variety of applications such as phase changes, osmotic equilibrium, surface energy, etc., in complex chemical-physics situations.