Displays

As the major component of garnet, the second most abundant phase in Earth’s transition zone, MgSiO₃-majorite (Mgmj) may be an important reservoir of water. Previous works at ambient conditions show that water is incorporated in Mgmj by substituting Si with H in the Si-O tetrahedra. Due to the challenges of experiments and simulations, there are still very limited data on hydrous MgSiO₃-majorite at simultaneously high temperatures and pressures.

In this study, we have carried out extensive first principles calculations to determine the properties of hydrous MgSiO₃-majorite up to 2000 K and 40 GPa. We systematically considered all possible incorporation mechanisms with seven substitutional reactions. By solving the equations of equilibrium constants of the reactions, we obtained the ratios of various polyhedral substitutions and their temperature and pressure dependences. Our results show that the occupations of water (hydrogen) in Mgmj change mainly with temperature and are generally pressure-independent. Almost all of hydrogen occupy in the Si-O tetrahedra at 300 K, but the ratio of Mg-O dodecahedra increases remarkably with temperature and is the highest at 2000 K, which suggests that water in Mgmj under mantle conditions would be different from that observed at ambient conditions and implies the importance of in-situ measurements of high-TP experiments.

We have also calculated the elastic velocities and anisotropies of two types of hydrous Mgmj which caused by substitutions of Mg and Si. Our results show that ~1 wt% water in Si vacancy would decrease wave velocities of Mgmj by ~3% in both V_P and V_S, while the same amount of water in Mg vacancy would more effectively decrease the velocities by ~6%. In addition, water in Mg vacancy would noticeably improve the seismic anisotropy and the dlnV_s/dlnV_P of Mgmj while water in Si vacancy shows much smaller effects.

How to cite: Zhang, Z., Lou, Y., Stackhouse, S., and Walker, A.: Water in MgSiO3-majorite at high temperatures and pressures: Incorporation mechanisms and thermoelastic properties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12297, https://doi.org/10.5194/egusphere-egu2020-12297, 2020.

The incorporation of H into minerals imposes profound effects on their physicochemical signatures of the solid Earth. The locations of hydrogen reservoirs are detected by seismology. However, the mineral responsible for storing large quantity of hydrogen, particularly in Earth’s lower mantle is still controversial. Combining a set of in-situ probes at high pressure-temperature and first principles simulation, we investigated the solubility and behaviors of H in silica and hydroxide up to the conditions found at the core-mantle boundary. The solubility of hydrogen keeps high in those minerals even along the mantle geotherm. Under deep lower mantle pressures, hydrogen atoms are free from the hydroxyl bonding and becomes highly diffusive. The swift diffusion of hydrogen ions induces soaring electrical conductivity when the sample is laser heated. Those exotic properties indicate novel transport mechanisms for both charge and mass at Earth’s deep lower mantle. The potential of hydrogen enriched volatile reservoirs may carry major impacts on the electrical and magnetic behaviors, as well as redox, H isotopic mixing, and other geochemical processes in the Earth’s deep interiors.

How to cite: Hu, Q. and Mao, H.: Mineral reservoirs and behaviors of hydrogen in Earth’s lower mantle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2559, https://doi.org/10.5194/egusphere-egu2020-2559, 2020.

The water content in the mantle transition zone exerts a controlling influence on the dynamical and chemical evolution of Earth, but is poorly known. In principle the water content at the top of the transition zone can be inferred by comparing the velocity and density contrasts across the 410-km seismic discontinuity with predictions based on the phase transition of olivine to wadsleyite. The high-quality elastic data of at pressure and temperature (PT) conditions of the transition zone are crucial but are very challenge for experiments to obtain. Calculating these elastic data at high PT conditions in conventional method are also very expensive. Instead, these elastic data were calculated using the method of Wu and Wentzcovitch (2011), which reduces the computational workload to tenth of the conventional method. All calculations for two phases were conducted using the same computational details as far as possible, which guarantees that the velocity and density differences between two phases have very high precise. All these calculated elastic data agree well with the available experimental data. The iron and water effect on the elasticity are also well described.

With these high-quality elastic data covered the PT condition of the transition zone, we analyze the water and wadsleyite content at the top of the transition zone. We found that the water content of wadsleyite at the top of the transition zone can be well constrained when density and velocities jumps are considered together. For a pyrolitic mantle composition with ~60% olivine, our best fit is ~ 0.5 wt% water at the top of the transition zone. If the transition zone is dry, as suggested by some electrical conductivity models, the upper mantle may only contain ~ 50% olivine (Wang et al., 2019).

Wang, W-Z., Walter, M.J., Peng, Y., Redfern, S., Wu, Z-Q., 2019a. Constraining olivine abundance and water content of the mantle at the 410-km discontinuity from the elasticity of olivine and wadsleyite. Earth Planet. Sci. Lett. 519, 1–11.

Wu, Z-Q., Wentzcovitch, R.M., 2011. Quasiharmonic thermal elasticity of crystals: An analytical approach. Phys. Rev. B - Condens. Matter Mater. Phys. 83, 1–8. doi:10.1103/PhysRevB.83.184115

How to cite: Wu, Z., Wang, W., Walter, M., Ye, P., and Redfern, S.: Constraining the olivine amount and water content at 410-km discontinuity with the elasticity of wadsleyite and olivine, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4082, https://doi.org/10.5194/egusphere-egu2020-4082, 2020.

Water concentration in pyroxenes from mantle xenoliths is frequently used to trace water content in the lithospheric mantle. We do not understand yet how these pyroxenes can preserve a memory of their deep equilibrium during their transport to the surface. In an attempt to evaluate the role of grain boundaries in the exchange of hydrogen between the pyroxenes of the xenoliths and the host magma, we have launched a program of experiments of H exchange in blocks of mantle xenoliths of centimetre size. The blocks, all from the same xenolith, contain clinopyroxenes, orthopyroxenes and olivine of mm to sub-millimetre size. We present here the results of a series of H-D exchange performed at 600, 700 and 900 ^oC at room pressure in a deuterium enriched gas. OH-OD profiles recorded by micro-infrared spectroscopy in pyroxenes at the edge of the block are only slightly different from the ones recorded in pyroxenes at the centre of the block. These results show that the diffusion/solubility of hydrogen in grain boundaries is fast enough to equilibrate rapidly the grains at the center of the xenoliths. It proves that in nature the δD signature of xenoliths is very likely controlled by the equilibrium with the host magma even in the case of xenoliths with large grain size.

We will also present preliminary results on the role of grain boundary diffusion in the control of hydrogen exchange involving reactions activated at a higher temperature such as the oxidation-reduction of iron (1/2H₂ + Fe³⁺  =  H_i⁺ + Fe²⁺) and the formation/destruction of cation vacancies.

How to cite: Thomaidis, K. and Ingrin, J.: Role of grain boundary diffusion in H-D exchange in mantle xenoliths , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10930, https://doi.org/10.5194/egusphere-egu2020-10930, 2020.

Although the continental lower crust is often assumed to be dry and strong, water in nominally anhydrous minerals can significantly decrease viscosity of granulites and affect the mechanical coupling between the crust and upper mantle. Here we measured water content and fabrics of 16 granulite xenoliths from the Udachnaya and Komsomolskaya kimberlites in the central Siberian craton, which were erupted in the Late Silurian. The equilibrium pressure and temperature of the granulite samples are in the range of 0.9–1.3 GPa and 683–822 ºC. The mean water contents in clinopyroxene, garnet and plagioclase are 744±272 ppm, 100±64 ppm, 423±245 ppm, respectively, suggesting the water-rich lower crust. The bulk water contents in granulites are independent on pressure and composition, but show a negative correlation with temperature. Compared with previous studies on granulite xenoliths and terrane granulites, our granulite samples have much higher bulk water contents. The lattice-preferred orientation of clinopyroxene is characterized by activation of the dominant slip system (100)[001], whereas garnet is randomly orientated. Plagioclase developed two dominant slip systems (001)[010] and (001)[100]. Calculated seismic anisotropy indicates that the weak fabric strength of these granulite samples will result in weak seismic anisotropy of the lower crust beneath the Siberian craton. We propose that during eruption of the kimberlite pipes in the Late Silurian, the lower crust of the Siberian craton, at least beneath the kimberlite fields, had high water contents, relatively low strength, weak seismic anisotropy, and high electrical conductivity. Such status may be representative for the lower crust beneath a stable craton. The following Siberian Traps in the end of Permian was associated with the magma underplating, which probably dehydrated and strengthened the lower crust of the Siberian carton.

How to cite: Wang, Q., Jin, T., and Shatsky, V.: Water Content, Deformation and Seismic Properties of the Lower Crust Beneath the Siberian Craton: Evidence from Granulite Xenoliths, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22134, https://doi.org/10.5194/egusphere-egu2020-22134, 2020.

Nominally anhydrous minerals in the lithospheric mantle, such as olivine and pyroxenes can host a small amount (tens to hundreds of ppm) of structurally bound hydroxyl (‘water’). Numerous studies pointed out that water has a strong effect on the rheological properties of the lithospheric mantle, such as melting temperature, electrical conductivity, viscosity and seismic wave propagation speed. Water content of mantle xenoliths can thus be used to estimate such rheological properties which can then be compared with geophysical observations.

In this study we present effective viscosities and electrical resistivities calculated with the use of ‘water’ contents of upper mantle xenoliths from the Carpathian-Pannonian region (CPR). The CPR is a young extensional basin in Central Europe, where intraplate alkali basalts sampled the lithosphere in five areas, including locations from both the central and marginal regions. ‘Water’ contents are generally higher in xenoliths from the marginal areas compared with those from the central areas of the CPR, due to significant hydrogen loss during the extension in the Miocene (Patkó et al., 2019). It is demonstrated that due to the different ‘water’ contents, the lithospheric mantle in the central areas can be characterized with higher effective viscosity and electrical resistivity, and thus can be considered as more rigid than the marginal areas. This relative rigidity induced by lithospheric thinning may be a general feature of extensional basin systems worldwide, and can be regarded as a ‘self-healing’ mechanism of the extending lithosphere.

References:

Patkó, L., Liptai, N., Kovács, I. J., Aradi, L. E., Xia, Q.-K., Ingrin, J., Mihály, J., O’Reilly, S. Y., Griffin, W. L., Wesztergom, V., Szabó, C., 2019. Extremely low structural hydroxyl contents in upper mantle xenoliths from the Nógrád-Gömör Volcanic Field (northern Pannonian Basin): Geodynamic implications and the role of post-eruptive re-equilibration. Chemical Geology, 507, 23-41.

How to cite: Liptai, N., Lange, T. P., Patkó, L., Berkesi, M., Szabó, C., and Kovács, I. J.: ‘Water’ content as a tool to estimate rheological differences in the lithosphere of young extensional basins, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19001, https://doi.org/10.5194/egusphere-egu2020-19001, 2020.

Eclogite is an important constituent of subduction slabs and plays a critical role in transporting surface materials (e.g., water) into the deep Earth. Eclogite consists mainly of omphacite and garnet. Although nominally anhydrous, omphacite and garnet contain some amount of structural water (OH) in the lattice, which is up to >1500 ppm wt. H₂O. This is virtually the highest content in nominally anhydrous minerals (NAMs) derived from the crust and upper mantle (Ingrin and Skogby, 2000). The electrical property of NAMs is very sensitive to water content and a small amount of water could dramatically enhance the conductivity. Thus, laboratory measured conductivity data of omphacite and garnet may help to understand the deep water recycling by eclogitized slab.

In this study, we have systemically determined the conductivity of omphacite and garnet with different water contents. The experiments were carried out at 350-800 °C, 1 GPa (note that the effect of pressure itself on conductivity is very small) and Ni-NiO buffered conditions. The data show that the conductivity of both omphacite and garnet increases with water content or temperature. The bulk conductivity is then modeled for different mineral compositions and water contents over a range of conditions (Liu et al., 2019). In combination with the geophysically documented high resistivity of the crustal part in deep subducted slabs, we suggest that the water content in omphacite and garnet in the deep-subducted eclogites should not be high at mantle depths. This provides new insights into the deep water recycling by subducted eclogites.

References:

Ingrin, J., and Skogby, H., 2000, Hydrogen in nominally anhydrous upper-mantle minerals: Concentration levels and implications: European Journal of Mineralogy, 12, 543–570.

Liu, H., Zhu, Q., and Yang, X., 2019, Electrical conductivity of OH-bearing omphacite and garnet in eclogite: the quantitative dependence on water content: Contributions to Mineralogy and Petrology, 174, doi:10.1007/s00410-019-1593-3.

How to cite: Liu, H. and Yang, X.: Electrical conductivity of omphacite and garnet in eclogite: implications for water recycling in the mantle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6359, https://doi.org/10.5194/egusphere-egu2020-6359, 2020.

Recent astronomy studies observed the existence of hundreds of ice giants in the universe. Ice is one of the major components of these ice giants. Experimental studies on the physical properties of ice at high pressures are thus important for understanding the composition and evolution of these ice giants. Here, we have synthesized high-quality single-crystal H₂O-ice with 0.5 m NaCl. Single-crystal elasticity of ice was measured by Brillouin spectroscopy combined with X-ray diffraction up to 93 GPa at 300 K using diamond anvil cells. All the elastic moduli of ice-VII exhibit a nearly linear increase with pressure up to 43 GPa at 300 K, although the off-diagonal modulus C₁₂ and shear modulus C₄₄ slightly deviate from the Cauchy relation between 10 and 20 GPa. The longitudinal modulus, C₁₁, and C₁₂ show a clear softening when ice-VII changes to the pre-transition ice-VII state. Meanwhile, we also observed a weak drop in the unit cell volume across this change in our high-quality single-crystal X-ray diffraction measurements. We also present first experimental measurements on the single-crystal elasticity of ice-X. Our experimental results were also used to model the anisotropy of ice at high pressures.

How to cite: Mao, Z. and Shi, W.: Single-crystal elasticity of Ice at high pressures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4316, https://doi.org/10.5194/egusphere-egu2020-4316, 2020.

Enhanced mutual solubility between silicate and water at elevated temperature and pressure in Earth’s interior (subduction zones in particular) allows the formation of supercritical geofluids with composition intermediate between silicate melts and aqueous fluids. The forming conditions of supercritical fluids are controlled by the critical curves, the wet solidi curves and the second critical endpoints of rock-H₂O systems. With unusual physicochemical properties, supercritical fluids have the potential to play a crucial role in mediating material recycling at subduction zones, mobilizing and enriching ore-forming elements, inducing intermediate to deep focus earthquakes, and modulating Earth’s habitability. Challenges in the study of supercritical fluids using experimental and computational simulations as well as natural rocks and mineral deposits demand breakthroughs in future development of transformative technologies.

How to cite: Ni, H.: Properties and effects of supercritical fluids, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1648, https://doi.org/10.5194/egusphere-egu2020-1648, 2020.

Earth’s mantle is predicted to contain as much or more water as its hydrosphere, which is important because the presence of water lowers the viscosity of mantle rocks. How water is distributed within mantle rocks is therefore fundamental to understanding Earth’s geodynamic behaviour, but the picture currently remains unclear. On the grain scale, previous analyses have revealed incompatible element partitioning in grain boundaries using EPMA, and the presence of H-enriched regions close (tens of µm) to grain boundaries using synchrotron-based FT-IR. The results of such studies have been used to suggest that grain boundaries may store water in concentrations hundreds of times higher than in grain interiors. Chemical segregation at grain boundaries is generally accepted to influence grain boundary diffusivity, which in turn affects the bulk viscosity of materials deforming by diffusion creep, a mechanism which is predicted to dominate the deformation of large parts of the mantle.

This study was designed to directly image the distribution of heavy water (D₂O) at the nanoscale in a synthetic peridotite sample using high-resolution secondary ion mass spectrometry (NanoSIMS), for the first time. The sample was annealed at temperature and pressure conditions typical of Earth’s upper mantle (1250 °C, 0.3 GPa) for three hours, to facilitate diffusion of ²H into olivine and pyroxene grains. Preliminary NanoSIMS results suggested that the partitioning of ²H into grain boundary regions, where observed, was at least an order of magnitude lower (partition coefficient of ~10¹) than has previously been predicted, indicating that, for typical mantle grain sizes, grain boundaries do not act as a significant storage reservoir for water in Earth’s mantle (or those of other terrestrial planets). The initial data were limited to a relatively small number of boundaries per sample. In this phase of the study, electron backscatter diffraction data has been collected from a single sample to characterise grain (mis)orientations at multiple sites suitable for NanoSIMS analyses. This is necessary because the strength of the chemical signature collected within grains and at grain boundaries using NanoSIMS is dependent on the orientations of those grains with respect to the NanoSIMS beam (matrix effects), the angle of the grain boundary with respect to the sample surface, and the misorientation angle between the two grains that comprise the boundary of interest. ²H, ¹⁶O, ¹⁶O²H and ²⁸Si measurements will be collected from multiple boundaries by NanoSIMS using a Cs⁺ beam with a 100 nm diameter to quantify grain boundary partitioning. The NanoSIMS isotope profiles will be presented as ²H/²⁸Si ratios to account for variation in measured isotope concentrations due to matrix effects. The results of the analysis will help quantify the degree to which water undergoes grain boundary segregation in mantle rocks under equilibrium partitioning conditions.

How to cite: Gardner, J., Tielke, J., Mecklenburgh, J., Mariani, E., and Wheeler, J.: Do grain boundaries act as a water reservoir in Earth's mantle? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9091, https://doi.org/10.5194/egusphere-egu2020-9091, 2020.

Garnet is a common mineral in crustal metamorphic rocks and a primary constituent of the Earth’s upper mantle. Natural silicate garnets are stable over a wide range of pressure and temperature conditions and have very complex chemical compositions. The chemical composition of a natural garnet reflects its growth environment making garnet a critical indicator mineral for many geological processes. Garnet, like olivine and many other nominally anhydrous minerals, can incorporate water as hydroxyl (OH^-) into its crystal structure. The water content in natural silicate garnets ranges from almost dry to thousands of parts per million. The compositional effect on the water content and the mechanisms of OH^- incorporation in silicate garnets are not well understood, majorly impeded by their complex compositions. Here we reported the influence of chemical compositions on the water content and OH^- substitution mechanisms in silicate garnets by studying synthetic garnet samples with compositions in the pyrope-grossular solid solution. We synthesized a series of hydrous pyrope-grossular garnets with various magnesium to calcium ratios by a multi-anvil press. Fourier transform infrared spectroscopy and Raman spectroscopy were applied to characterize the OH^- content and identify OH^- substitution mechanisms in the synthetic samples. The influence of hydration on the crystal structure of samples was investigated by the single-crystal X-ray diffraction technique. Our experimental results will help to constrain the bulk water content in the Earth’s interior and to interpret seismic data of a hydrous mantle. 

How to cite: Chang, Y.-Y., Huang, Y.-C., and Kung, J.: A systematic study of OH in hydrous pyrope-grossular garnets , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4680, https://doi.org/10.5194/egusphere-egu2020-4680, 2020.

Coesite, a high-pressure SiO2 polymorph, has drawn extensive interest from the mineralogical community for a long time. In this study, we synthesized hydrous coesite samples with different B and Al concentrations at 5 and 7.5 GPa (1273 K). The B concentration could be more than 400 B/10⁶Si with about 300 ppmw. H2O, while the Al content can be as much as 1200 ~ 1300 Al/10⁶Si with CH2O restrained to be less than 10 ppmw. Hence, B-substitution may prefer the mechanism of Si⁴⁺ = B³⁺ + H⁺, whereas Al-substitution could be dominated by 2Si⁴⁺ = 2Al³⁺ + O_V. The doped B³⁺ and Al³⁺ cations may be concentrated in the Si1 and Si2 tetrahedra, respectively, and make noticeable changes in the Si-O4 and Si-O5 bond lengths. In-situ high-temperature Raman and Fourier Transformation Infrared (FTIR) spectra were collected at ambient pressure. The single crystals of coesite were observed to be stable up to 1500 K. The isobaric Grüneisen parameters (ϒ_iP) of the external modes (< 350 cm^-1) are systematically smaller in the Al-doped samples, as compared with those for the Al-free ones, while most of the OH-stretching bands shift to higher frequencies in the high temperature range up to ~ 1100 K

How to cite: Ye, Y., Miao, Y., Smyth, J. R., and Zhang, J.: Crystal structures and high-temperature vibrational spectra for synthetic boron and aluminum doped hydrous coesite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1492, https://doi.org/10.5194/egusphere-egu2020-1492, 2020.

Water in the form of hydrogen defects in olivine strongly influences the physical properties of olivine, thereby being responsible for physical/chemical processes in the deep Earth. Knowledge of hydrogen defects in olivine is fundamental to understand water distribution and its impact on the upper mantle. However, the current explanations of water effects on processes in the deep Earth are mainly based on hydrogen defects observed at ambient conditions. Since hydrogen is highly mobile, the migration of hydrogen between lattice sites at high temperature and high pressure may not be quenchable. Therefore, there arises a question: whether the hydrogen defects in olivine obtained from infrared spectra at ambient conditions are the same as those at the temperature and pressure conditions of the upper mantle? Here, we carry out in situ high-temperature and high-pressure infrared spectroscopic investigations on hydrogen defects in the natural olivine and synthetic Fe-free forsterite. We find that hydrogen defects exhibit disordering at temperature-pressure conditions of the upper mantle, and hydrogen defects corresponding to pure Si vacancies display re-configuration under compression. Interestingly, dehydrogenation experiments of the natural olivine indicate interactions of hydrogen defects. The lost hydrogen of the titanium-clinohumite defects does not completely release out of the crystal. It can migrate to pure Si vacancies and, also, can move to Mg vacancies coupling with trivalent cations. Thus, dehydrogenation and interactions of hydrogen storage sites may be very complex. There may be other reactions among storage sites during dehydrogenation, depending on the chemical compositions, hydrogen storage sites, and the annealing conditions. In conclusion, we report disordering and reconfiguration of hydrogen storage sites at high temperature and high pressure, and also interactions of hydrogen storage sites during dehydrogenation. These are vital for understanding water distribution and processes in the deep Earth.

How to cite: Yang, Y. and Xia, Q.: Behavior of hydrogen defects in olivine at high temperature and high pressure: disordering, re-configuration and interation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2109, https://doi.org/10.5194/egusphere-egu2020-2109, 2020.

Aqueous fluid derived from the dehydration of subducting slab can dissolve and transfer carbon to mantle wedge, and thus plays an important role in the globe deep carbon cycle. Carbonates are major phases of carbon in the subducting slab, however their solubilities in the subduction zone fluid are poorly constrained. This heavily hinder our understanding of the  deep carbon cycle. Magnesite is one of the carbonates in the subducting slab, and can be stabilized to sub-arc depth. We determined the solubility of magnesite in pure water and saline fluids buffered by silicate by in situ observation of quantitative magnesite totally dissolved in quantitative fluid under high temperature and pressure in Hydrothermal Diamond Anvil Cell (HDAC). The results demonstrated that the solubility of magnesite in pure water is 0.010-0.026 mol/kg H₂O at 1.0-3.3 GPa and 600-900 ℃, and that it increases as increasing temperature, but has no obvious pressure effect. This data is close to the experimental measurement of calcite solubility in literature, but slightly higher than the theoretical results calculated using DEW model. The solubility of magnesite in 5 wt % NaCl solution equilibrium with quartz is 0.22 mol/ kg, at 700 ℃ and 1.5 GPa，an order of magnitudes higher than that in the pure water. Since the formation of new silicate minerals, such as olivine or talc, depends on silicon activity in the fluid, the dissolution of silicate would boost the solubility of magnesite. This mechanism has been previously reported in the Alps metasedimentary rocks. Therefore, the aqueous fluid, rich in saline and silicon in fore-arc and sub-arc depths, has the ability to dissolve and transfer almost all the carbonates in the subducting slab to the overlying mantle wedge.

How to cite: Li, W.-C., Wang, Q., and Ni, H.: Experimental determination of magnesite solubility in water and silicate saturated saline solutions under high temperature and pressure, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6178, https://doi.org/10.5194/egusphere-egu2020-6178, 2020.

Antigorite dehydration is well known as a key process in convergent boundaries for the genesis of mantle wedge partial melting and intermediate-depth earthquakes. However, the crystallographic preferred orientations (CPOs) of prograde minerals from antigorite dehydration and its effects on seismic anisotropy of subducting slabs remain ambiguous and controversial. Here we report hydrostatic dehydration experiments on foliated serpentinized peridotite at pressures of 0.3-6 GPa and temperatures of 700-900 °C. Our results show that the orientations of prograde olivine inherit orientations from adjacent olivine grains in the olivine-rich layer by epitaxial growth. In contrast, olivine CPOs evolved with the grain size from fiber-[001] featuring clear [100] point maxima and [001] girdles for fine-grained olivine to orthorhombic patterns characterized by clear [100] and [001] point maxima for coarse-grained olivine, i.e., type-C CPO. We propose that the fine-grained fiber-[001] CPO is developed by topotactic growth at the onset of dehydration, while the orthorhombic type-C CPO for the coarse-grained olivine, especially the [001] point maximum along the lineation, is mainly developed by anisotropic growth resulting from anisotropic fluid flow during the dehydration. The developed olivine type-C CPO in the antigorite-rich layer after antigorite dehydration could explain the trench or strike parallel seismic anisotropy observed at convergent plate boundaries.

How to cite: Zhang, J., Liu, W., and Wang, Y.: Experimental investigation of antigorite dehydration fabrics at high pressure and high temperature, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2015, https://doi.org/10.5194/egusphere-egu2020-2015, 2020.

Experiments have been conducted to assess the effects of temperature, oxygen fugacity, crystallographic orientation, silica activity and chemical composition on the diffusivity and substitution mechanisms of hydrogen in orthopyroxene (opx). Axially oriented ~cuboids of natural Tanzanian opx were dehydrated at 1 bar in a gas mixing furnace (H₂-CO₂ mix) at three different oxygen fugacities (~QFM-1,~QFM+1, ~QFM-7), and two different silica activity buffers (olivine+pyroxene or pyroxene+quartz) between 700°C and 1000°C. Profiles of hydrogen content versus distance were extracted from experimental samples using Fourier-Transform Infrared (FTIR) spectroscopy, with diffusion coefficients extracted using relevant analytical solutions and numerical approximations of Fick’s second law. Diffusion is the fastest along [001] ( D_[001]>D_[010]>D_[100]). Fitting the diffusion coefficients to the isobaric Arrhenius relationship (logD=logD₀+(-Q/(2.303RT)) gives activation energies (Q) and pre-exponential factors (logD₀) between 127 to 162 kJmol^-1 and –4.29 to -5.42  m²s^-1 , respectively, for ~QFM-1.

The extracted hydrogen diffusivities are faster than previously measured by 0.5 to 5 orders of magnitude at ~1000 °C and ~700°C, respectively (Carpenter (2003), Stalder and Skogby (2003), Stalder and Behrens (2006), Stalder and al. (2007)) and are slightly slower, but strikingly close, to those of the fastest experimentally-determined diffusivity of H in olivine (Kohlstedt and Mackwell, 1998), suggesting a mechanism akin to proton-polaron exchange. This presents a paradoxical decoupling between natural and experimental observations. In most cases for mantle xenoliths, natural olivine has low water contents (<35 ppm), or are dry, and show H diffusive loss of water, where natural opx contains between 10 and 460 ppm and rarely show H diffusive loss (Demouchy and Bolfan-Casanova (2016), suggesting opx is more capable of recording the mantle water signature. With hydrogen diffusivities of olivine and opx being quite similar, however, both minerals should suffer from the same rate of dehydration during ascent, thus show low or zero water content in natural settings, which is not the case. Therefore, the inference that pyroxenes are better recorder of water in the mantle (e.g. Warren et Hauri (2014), Peslier (2010)) cannot be a simple function of diffusivities. A case study on an opx crystal showing a dehydration profile from a spinel-peridotite xenolith, hosted in an alkaline magma, from Patagonia supports this. Using the H diffusion coefficients from this study, the calculated rates of ascent of the mantle xenolith in alkaline magma are comparable to those associated with kimberlite magmas. The two suggestions we present are the following: i) Changing the boundary conditions may modify the hydrogen diffusive flux through the xenolith history and ii) The measured diffusivities would be apparent diffusivities as there might be different pathways or mechanisms of diffusion.

How to cite: Demers-Roberge, A., Jollands, M., Tollan, P., and Müntener, O.: Is orthopyroxene really a reliable recorder of mantle water signatures? Insights from an experimental study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15024, https://doi.org/10.5194/egusphere-egu2020-15024, 2020.

Water plays an important role in lowering melting temperature of rocks. The water-saturated solidus of rock is critical for understanding the magma generation and the dynamics of the Earth. There have been a lot of water-saturated solidi of rocks constrained by traditional quench method in literature. However, since both of the hydrous silicate melt and aqueous fluid can be quenched to glasses at high pressure, it is difficult to discriminate whether the quenched glasses were from melt or not. As a result, the water-saturated solidi of rocks from different studies may show significant discrepancy. One way to solve this problem is to detect the characteristics change of the rock system in situ, and electrical conductivity measurement is one of the good options. It is known that hydrous melt has much higher electrical conductivity than solid rock, and temperature is much more effective in enhancing melt electrical conductivity than that for aqueous fluid. Once the partial melting is triggered, the electrical conductivity of the water-saturated rock system may have remarkable increase if the hydrous melt is interconnected in the system. Accordingly, the abrupt change of electrical conductivity may mark the solidus temperature. In this study, we performed electrical conductivity measurement for the determination of water-saturated solidus of albite. We adopted albite as the starting material because its water-saturated solidus is well known, which can help to verify the accuracy our method, and its quenched products are not so controversial. The electrical conductivity measurements were carried out at four different pressures ranging from 0.35 GPa to 1.7 GPa in a 3/4″ piston cylinder apparatus with impedance spectroscopy. The obvious change of electrical conductivity was observed at solidus temperature within error, with increase of 1.8-0.18 log unit at 0.35-1.7 GPa. The results showed a stronger increase of conductivity at lower pressures, and fitted well with the water-saturated solidus of albite in literature. One defect of this method is the loss of water during experiment. The final water content in the system is about 1-2 wt%, comparing to the initial 10-15 wt% H₂O. Nevertheless, the whole system is still water saturated, since water solubility in albite is fairly low. Therefore, if such a method can be improved to keep more water, it may be applied to other rocks to better constrain the water-saturated solidi in the future.

How to cite: Guo, X. and Ni, H.: In situ determination of water-saturated solidus by electrical discontinuity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8026, https://doi.org/10.5194/egusphere-egu2020-8026, 2020.

    A new continuous flow method, by combining high vacuum stepwise-heating (HVST) device with the thermal conversion elemental analyzer and gas isotope mass spectrometer (TC/EA-MS), is presented for determination of water contents and H isotope compositions for both structural hydroxyl and molecular water in garnet. By using the HVST device, molecular water and structural hydroxyl can be liberated step by step from garnet at different heating temperatures. By using the on-line quadrupole mass spectrometer in the HVST device, heating temperatures were determined for releasing the two forms of water from garnet from ultrahigh-pressure metamorphic eclogite in the Dabie orogen. Releasing temperatures of molecular water and structural hydroxyl from the garnet are 400°C and 1400°C, respectively. The garnet gives water of 228±39 ppm and a dD value of -110±10‰ for molecular water at dehydration temperature of 400°C for 1 hour, and water of 301±27 ppm and a dD value of -81±4‰ for structural OH at dehydration temperature of 1400°C for 1 hour. Therefore, the HVST-TC/EA-MS method can be used to analyze both water content and H isotope composition of the two forms of water in nominally anhydrous minerals.

How to cite: Gong, B., Chen, R.-X., and Zheng, Y.-F.: A new method to determine both water content and hydrogen isotope composition of two forms of water in nominally anhydrous minerals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12674, https://doi.org/10.5194/egusphere-egu2020-12674, 2020.

Transmission electron microscopy (TEM) is a powerful, yet scarcely used technique when it comes to investigating mantle minerals and fluid inclusions. It is capable to collect structural information of the studied mineral, its precise chemical composition, and makes nanofeatures visible, such as dislocations and nano-inclusions.

In this study TEM and STEM (scanning transmission electron microscopy) measurements were carried out on a set of ortho- and clinopyroxene samples from central and marginal localities of Carpathian Pannonian region (CPR), where Plio-Pleistocene alkaline basalt volcanism sampled the lithospheric mantle retrieving lithospheric mantle xenoliths. Objective of the study was to constrain the presence and formation mechanisms of sub-microscopic occurrence of pargasitic amphibole.

The detailed investigation of pargasite in the upper mantle is rather timely, because its presence may be the major cause for the rheologic contrast experienced between the lithosphere and the asthenosphere [1], [2]. The nominally anhydrous minerals’ (NAMs, as ortho- and clinopyroxene) structural hydroxyl [3] content or volatiles in fluid inclusions could lead to formation of pargasite [4]. In addition, pargasite could form interstitially during metasomatic intereactions.

Our observations so far suggest that hydrous silicate formation as sub-solidus exsolution in the central CPR may not have taken place. Ordering of the Ca forming Ca-rich and Ca-poor domains in an orthopyroxene grain was identified. Precursors of H⁺ diffusion were also recorded, such as dislocations and nanosized fluid inclusions. Diffusion of H⁺ could be active in the lattice scale through the disclinations along subgrain boundaries [3], [5] or dislocations in the host mineral along the boundary of nanoscale fluid inclusions [6], [7]. Clinopyroxene-amphibole phase boundary has been prepared by focused ion beam (FIB) milling technique from the marginal area of CPR. The chemical composition of the amphibole lamella provides evidence that the H₂O content of the nearby fluid inclusion migrated into the host clinopyroxene producing an amphibole lamella growing along the ‘c’ crystallographic axis [4].

Observations of the boundary of clinopyroxene and amphibole confirm that the amphibole octahedral layers penetrate the clinopyroxene structure. The precise nanoscale measurements (STEM mapping) of chemical composition of both the host and the lamellae can lead to profound implications on the original composition of the studied fluid inclusions.

[1] Green, D. H., Hibberson, W. O., Kovács, I. J., & Rosenthal, A. (2010). Nature, 467(7314), 448–451.

[2] Kovács, I. J., Lenkey, L., Green, D. H., Fancsik, T., Falus, G., Kiss, J., Orosz, L., Angyal, J., Vikor, Zs. (2017). Acta Geodaetica et Geophysica, 52, 183–204.

[3] Liptai, N., Kovács, I.J., Lange, T.P., Pálos, Zs., Berkesi, M., Szabó, Cs., Wesztergom, V. (2019). Goldschmidt Abstracts, 2019 1981.

[4] Lange, T.P., Liptai, N., Patkó, L., Berkesi, M., Kesjár, D., Szabó, Cs., Kovács, I. J. (2019). 25th European Current Research on Fluid Inclusions (ECROFI) , Abstract Series, 68.

[5] Demouchy, S., & Bolfan-Casanova, N. (2016). Lithos, 240–243, 402–425.

[6] Bakker, R. J., & Jansen, J. B. H. (1994). Contributions to Mineralogy and Petrology, 116, 7–20.

[7] Viti, C., & Frezzotti, M. L. (2000). American Mineralogist, 85(10), 1390–1396.

How to cite: Pálos, Z., Pekker, P., Pósfai, M., Lange, T. P., Liptai, N., Berkesi, M., Szabó, C., and Kovács, I.: Transmission electron microscopy investigations of (hydrous) chain silicates from the lithospheric mantle beneath the Carpathian Pannonian Region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19844, https://doi.org/10.5194/egusphere-egu2020-19844, 2020.

Nominally anhydrous minerals are major components of the subducted continental slab and thus regard as important water reservoir in continental subduction zone. The water contents of NAMs are critical for understanding of fluid action and geodynamics of subduction zones. Fourier Transform Infrared Spectroscopy as well as major and trace element analyses were carried out on garnets in Jinheqiao eclogites from the Dabie orogen. The results demonstrate that garnet grains contain both molecular water and hydroxyl (OH). Contents of both hydroxyl and molecular water show rough correlation with Si, Ca, Al and Na, suggesting their incorporation in garnet is related to the formation of garnet. Molecular water is primary or transformed from hydroxyl during exhumation, implying molecular water an internal origin in eclogite. Garnet has varying total water contents up to thousands, with the highest water content corresponding to the garnet’s capacity for water storage under subduction zone condition. Water can be saturated in peak metamorphic garnet. The variable water contents in garnet was affected by several factors such as protolith nature, fluid availability, pressure and temperature, but dominated by decompression dehydration during exhumation. The high water contents of garnet suggest that garnet is not only an important media for subducted slab to transport water into deep mantle but also an important source for retrograde fluid during exhumation of deeply subducted continental slab.

How to cite: Chen, R.-X., Wang, Z.-M., Zheng, Y.-F., and Gong, B.: Water of garnet in Dabie UHP eclogite: implication for fluid action in continental subduction zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17424, https://doi.org/10.5194/egusphere-egu2020-17424, 2020.

We studied volcanic rocks from the Oas-Gutai Mts. (Transylvania, Romania) to measure the ‘structural hydroxyl’ content of the nominally anhydrous minerals (NAMs such as clinopyroxene, plagioclase, quartz), from which water content of the parental magma can be estimated.  The Neogene volcanic chain of the Carpathian-Pannonian region (CPR), due to petrologic variability, is an excellent area for such investigation.

Recent FTIR studies on the calc-alkaline rocks from CPR, showed that the ‘structural hydroxyl’ content of NAMs could be modified during and after volcanic eruptions [1], [2], [3]. However, transmission FTIR-microscopy is an adequate technique for recognizing this these changes because FTIR spectra of the NAMs indicate signs in the case of hydroxyl loss [4].

For studying the pre-eruptive water contents clinopyroxenes are the most promising mineral because it has one of the lowest diffusion rates for hydroxyl in NAMs [5]. With the detailed study of the clinopyroxenes FTIR spectra, conclusions can be drawn concerning the potential post-eruptive loss of hydroxyl [4].

We have examined 8 volcanic rock samples, four dacite samples from Oas and one basalt two andesite and one rhyolite sample from the Gutai Mts. The samples show diverse volcanic facies such as lava, ignimbrite and debris avalanche. The diversity of samples is important for future research because it will help to choose the most adequate volcanic facies to estimate the magmatic equilibrium water contents.

The studied clinopyroxenes contain 83-371 ppm ‘structural hydroxyl’ content,which can be considered as normal values compared to the work of [6] where ‘structural hydroxyl’ content in clinopyroxenes show a range from 75 to 390 ppm in the mafic calc-alkaline lavas from Salina, Italy.

[1] Lloyd, A.S., Ferriss, E., Ruprecht, P., Hauri, E.H., Jicha, B.R., & Plank, T. (2016): Journal of Petrology, 57, pp. 1865-1886

[2] Biró, T., I. Kovács, D. Karátson, R. Stalder, E. Király, G. Falus, T. Fancsik, J. & Sándorné Kovács (2017): American Mineralogist, 102, pp.

[3] Pálos, Z., Kovács, I. J., Karátson, D., Biró, T., Sándorné Kovács, J., Bertalan, É., & Wesztergom, V. (2019): Central European Geology, 62(1)

[4] Patkó, L., Liptai, N., Kovács, I., Aradi, L., Xia, Q.K., Ingrin, J., Mihály, J., O'Reilly, S.Y., Griffin, W.L., Wesztergom, V., & Szabó, C. (2019): Chemical Geology, 507, pp. 23-41.

[5] Farver, J.R. (2010): Reviews in Mineralogy and Geochemistry, 72 (1), pp. 447–507.

[6] Nazzareni, S., Skogby H., & Zanazzi, P.F. (2011): Contributions to Mineralogy and Petrology, 162, pp. 275–288.

How to cite: Kővágó, Á., Kovacs, M., Kesjár, D., Szabó, C., and Kovács, I.: New FTIR data of calc-alkaline volcanic rocks from the Oas-Gutai Mts. and post eruption effects on the water content of phenocrysts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18034, https://doi.org/10.5194/egusphere-egu2020-18034, 2020.