Deep Water Cycle Research Articles

Hydrochemical Characteristics and Temporal Variations of Geothermal Water Quality in Tangtou, Shandong, China

Geothermal water resources are a kind of clean energy, which is a renewable resource to a certain extent and has a high value of development and utilization. To understand the hydrochemical characteristics, origins, and temporal variations of geothermal water quality in Tangtou, 13 geothermal water samples from 2007 to 2019 and geothermal geological conditions were collected. Cl− and Na+ are the major ions, which make the geothermal water belong Cl-Na type. The total dissolved solids values of geothermal waters were 1560–2512 mg/L and pH were in the range of 6.7–8.8. The development of faulted structures provides conditions for the formation of geothermal water. In addition, geothermal water is recharged by shallow underground cold water. Water-rock interaction, as well as mixing processes, in the process of long runoff path and the slower deep-water cycle is the main factor controlling the chemical composition of geothermal water. Calculated saturation index values indicated that the geothermal water was saturated with respect to silicate and carbonate minerals. Cation and silica chemical geothermometers indicated that reservoir temperatures ranged from 94.63 to 196.10 °C and from 69.13 to 123.75 °C, respectively. Based on the grey relational analysis, the main physicochemical components of geothermal water are obviously correlated with the geothermal water exploitation and precipitation. Overall, affected by exploitation and precipitation, main physicochemical components (such as the total dissolved solids, total hardness, temperature, Na+, Ca2+, Cl−, K+, and Mg2+) showed a certain stage.

Deep Water Cycling and Sea Level Change Since the Breakup of Pangea

AbstractFirst‐order variations in sea level exhibit amplitudes of ∼200 m over periods that coincide with those of supercontinental cycles (∼300–500 Myr). Proposed mechanisms for this sea level change include processes that change the container volume of the ocean basins and the relative elevation of continents. Here we investigate how unbalanced rates of water exchange between Earth's surface and mantle interior, resulting from fluctuations in tectonic rates, can cause sea level changes. Previous modeling studies of subduction water fluxes suggest that the amount of water that reaches sub‐arc depths is well correlated with the velocity and age of the subducting plate. We use these models to calibrate a parameterization of the deep subduction water flux, which we together with a parameterization of mid‐ocean ridge outgassing, then apply to reconstructions of Earth's tectonic history. This allows us to estimate the global water fluxes between the oceans and mantle for the past 230 Myr and compute the associated sea level change. Our model suggests that a sea level drop of up to 130 m is possible over this period and that it was partly caused by the ∼150Ma rift pulse that opened the Atlantic and forced rapid subduction of old oceanic lithosphere. This indicates that deep water cycling may be one of the more important sea level changing mechanisms on supercontinental time scales and provides a more complete picture of the dynamic interplay between tectonics and sea level change.

Project VoiLA: Volatile Recycling in the Lesser Antilles

Deep water cycle studies have largely focused on subduction of lithosphere formed at fast spreading ridges. However, oceanic plates are more likely to become hydrated as spreading rate decreases.

Intimate link between ammonium loss of phengite and the deep Earth's water cycle

Phengite, a high-pressure white mica, is stable down to ∼300 km depth in subduction environment and represents an important carrier of nitrogen and hydrogen to the deep Earth. Although the determination of nitrogen and hydrogen recycling efficiency to subduction zones is crucial for building global geodynamic models of volatile elements, no experimental data have so far evaluated the rate of ammonium release from phengite and its potential effect on phengite dehydration in a descending slab. Here, we investigated the kinetics of ammonium loss and dehydration of ammonium-bearing and ammonium-free phengite samples using FTIR spectroscopy. Single phengite grains were analyzed after annealing at high temperatures from 720 to 850°C for different periods of time. The results show that ammonium loss is one order of magnitude faster than dehydration at the same temperature (for instance, with diffusion coefficients of 1.5×10−15 m2/s and 1.0×10−16 m2/s at 800°C for ammonium loss and dehydration, respectively). Moreover, dehydration of ammonium-bearing phengite is one order of magnitude faster than that of ammonium-free phengite (with diffusion coefficients of 1.0×10−16 m2/s and 2.0×10−17 m2/s at 800°C for dehydration in ammonium-bearing and ammonium-free phengite, respectively). The activation energies of ammonium loss, dehydration in ammonium-bearing phengite and dehydration in ammonium-free phengite are around 150, 535, 548 kJ/mol, respectively. These results imply that ammonium release from phengite is not only easier than dehydration but also can promote dehydration. The geochemical behavior of ammonium in phengite is consistent with previous observations on natural samples from subduction zones of different geothermal gradients. Furthermore, since ammonium release can trigger dehydration, water may be released at depth locus shallower than theoretical phengite breakdown (i.e. <300 km). Thus, ammonium present in hydrous minerals may exert a strong control on the water flux released in subduction zones.

On the implications of the coupled evolution of the deep planetary interior and the presence of surface ocean water in hydrous mantle convection

We investigate the influence of the deep mantle water cycle incorporating dehydration reactions with subduction fluxes and degassing events on the thermal evolution of the Earth as a consequence of core–mantle thermal coupling. Since, in our numerical modeling, the mantle can have ocean masses ∼12 times larger than the present-day surface ocean, it seems that more than 13 ocean masses of water are at the maximum required within the planetary system overall to partition one ocean mass at the surface of the present-day Earth. This is caused by effects of water-dependent viscosity, which works at cooling down the mantle temperature significantly so that the water can be absorbed into the mantle transition zone and the uppermost lower mantle. This is a result similar to that without the effects of the thermal evolution of the Earth's core (Nakagawa et al., 2018). For the core's evolution, it seems to be expected for a partially molten state in the deep mantle over 2 billion years. Hence, the metal–silicate partitioning of hydrogen might have occurred at least 2 billion years ago. This suggests that the hydrogen generated from the phase transformation of hydrous-silicate-hosted water may have contributed to the partitioning of hydrogen into the metallic core, but it is still quite uncertain because the partitioning mechanism of hydrogen in metal–silicate partitioning is still controversial. In spite of many uncertainties for water circulation in the deep mantle, through this modeling investigation, it is possible to integrate the co-evolution of the deep planetary interior within that of the surface environment.

Modeling Slab Temperature: A Reevaluation of the Thermal Parameter

AbstractWe reevaluate the effects of slab age, speed, and dip on slab temperature with numerical models. The thermal parameter Φ = t v sin θ, where t is age, v is speed, and θ is angle, is traditionally used as an indicator of slab temperature. However, we find that an empirically derived quantity, in which slab temperature T ∝ log (t−av−b) , is more accurate at depths &lt;120 km, with the constants a and b depending on position within the slab. Shallower than the decoupling depth (~70–80 km), a~1 and b~0, that is, temperature is dependent on slab age alone. This has important implications for the early devolatilization of slabs in the hottest (youngest) cases and for shallow slab seismicity. At subarc depths (~100 km), within the slab mantle, a~1 and b~0 again. However, for the slab crust, now a~0.5 and b~1, that is, speed has the dominant effect. This is important when considering the generation of arc magmatism, and in particular, slab melting and the generation of slab‐derived melange diapirs. Moving deeper into the Earth, the original thermal parameter performs well as a temperature indicator, initially in the core of the slab (the region of interest for deep water cycling). Finally, varying the decoupling depth between 40 and 100 km has a dominant effect on slab temperatures down to 140‐km depth, but only within the slab crust. Slab mantle temperature remains primarily dependent on age.

Aquatic, deep water peak VO2 testing for individuals with spinal cord injury

Objective: To determine the reliability of peak VO2 testing for individuals with spinal cord injury (SCI) in deep water and on land; and to examine the relationship between these two testing conditions.Design: Reliability study.Setting: Comprehensive rehabilitation center in Baltimore, MD, USA.Participants: 17 participants (13 men, 4 women) with motor complete and incomplete SCI. Participants were randomized into either aquatic or arm cycle ergometer first measurements.Intervention: Pilot study to assess peak VO2.Outcome measures: Peak VO2 measured with metabolic cart in supported deep water with the addition of Aquatrainer® connection, and on land with arm cycle ergometer. Two trials were conducted for each condition with 48 h separating each test.Results: Peak oxygen consumption reliability was statistically significant for both conditions, aquatic (r = 0.93, P < 0.001) and arm cycle ergometry (r = 0.96, P < 0.001). Additionally, aquatic and arm cycle peak VO2 correlation existed (r = 0.72, P < 0.001). For these 17 participants, lower extremity motor score influenced supported, deep water peak VO2, B = 0.57, P < 0.02, whereas age, sex, and weight did not impact deep water or ergometer values.Conclusion: Determining peak VO2 for individuals with SCI is highly reproducible for arm cycle ergometry and in deep water assessment. Additionally, aquatic, deep water peak VO2 testing is valid when compared to arm cycle ergometry. Although the peak VO2 relationship between deep water and arm cycle ergometry is high, variance in the two conditions does exist. Therefore, it is important to assess peak VO2 via the same exercise modality utilized in the treatment intervention.

Effects of melting, subduction-related metasomatism, and sub-solidus equilibration on the distribution of water contents in the mantle beneath the Rio Grande Rift

The distribution of water in the upper mantle plays a crucial role in the Earth's deep water cycle, magmatism, and plate tectonics. To better constrain how these large-scale geochemical systems operate, peridotite and pyroxenite mantle xenoliths from Kilbourne Hole (KH) and Rio Puerco (RP) along the Rio Grande Rift (NM, USA) were analyzed for water, and major and trace element contents. These xenoliths sample a lithosphere whose composition was influenced by subduction and rifting, and can be used to examine the effects of melting, metasomatism, and sub-solidus equilibration on the behavior of water. The first result is that in KH xenoliths, olivines underwent negligible H loss during xenolith ascent, i.e. preserved their mantle water contents. These olivine water contents are used to calculate mantle viscosities of 0.5–184 · 1021 Pa·s. These viscosity values are more than 40 times higher than those of the asthenosphere and show that KH peridotites represent samples from the lithosphere. The preservation of olivine water contents is exceptional for off-cratonic xenoliths, and the KH peridotites provide the first estimate of the average concentration of water in Phanerozoic continental mantle lithosphere at 81 ± 30 ppm H2O. The mantle lithosphere beneath the Rio Grande rift is nevertheless heterogeneous with water contents ranging from <0.5 to 120 ppm H2O in peridotites and from 227 to 400 ppm H2O in pyroxenites. A composite KH xenolith of a harzburgite cross-cut by a clinopyroxenite vein shows this heterogeneity at the cm scale. The second contribution of this study stems from the majority of the KH peridotites and two of the RP peridotites having major and trace elements that can be explained by partial melting without any need to invoke metasomatic processes. This allows to show that, prior to modelling the water content variation of each peridotite mineral during melting, a correction for sub-solidus equilibration has to be applied to the water contents of the minerals. Sub-solidus equilibration also provides an explanation for the discrepancy between the clinopyroxene/orthopyroxene ratio of water contents in natural peridotites worldwide and in laboratory experiments on water partitioning in peridotite minerals. Finally, the cryptically metasomatized peridotites, rare at KH and abundant at RP, as well as the pyroxenites, permit to decipher the origin and water contents of the metasomatic melts that affected the continental lithosphere beneath the Rio Grande Rift. Trace element modelling of the metasomatized KH and RP peridotites are consistent with metasomatism via melts that are of subduction origin. Melts in equilibrium with peridotites contain more water at RP (∼1 wt.% H2O) than at KH (∼0.5 wt.% H2O), although this did not result in a more water-rich mantle lithosphere at RP. Rio Puerco lies within the northern Rio Grande rift, proposed to have been affected by a flat slab subduction, which may explain the more hydrous and extensive metasomatism compared to the south, where KH is located.

Rates of dehydration of olivines from San Carlos and Kilauea Iki

The rate at which H+ moves into, through, and out of olivine is of great interest for understanding the deep water cycle, assessing the fidelity of olivine-hosted melt inclusions as recorders of pre-eruptive melt compositions, and estimating the decompression rate of magma during volcanic eruptions. Here we conduct a series of experiments and observations on natural Fe-bearing olivine to better understand and quantify this H+ mobility, with a particular focus on the rate at which H+ diffuses out of olivine. We have carried out sequential dehydration experiments on single crystals of natural, Fe-bearing olivine polished and oriented along three crystallographic orientations. These efforts include four separate parts: (1) the partial hydration of polished blocks of xenolithic San Carlos olivine at 800 °C and 1000 °C, 1 GPa, and oxygen fugacity (fO2) at the Ni-NiO buffer (NNO); (2) sequential dehydration of a homogeneous block of San Carlos olivine at 800 °C, 1-atm, and fO2 of NNO-2.6; (3) sequential dehydration of a polished block of an olivine phenocryst from Kilauea Iki at 800 and 1000 °C, 1 atm, and fO2 held at NNO-2.6 until the final step, which was conducted at NNO + 1.9; and (4) the measurement by secondary ion mass spectrometry of hydrogen zonation profiles in an olivine phenocryst from Kilauea Iki coupled with Monte Carlo modeling to determine total H diffusivities in a natural setting. Hydrogen zonation profiles were measured in all three crystallographic directions a, b, and c through the uncut blocks before and after each time step for all experiments by Fourier transform infrared spectroscopy using light polarized || a and modeled in 3 dimensions to determine both total and peak-specific dehydration, producing over 50 sets of 3 apparent diffusivities.The results show that the apparent diffusivity of total H+ varies both over time and for olivines with different defect populations, and thus H+ does not diffuse at inherently site-specific rates. These variations are due to rapid reactions that may occur between sites as a function of changing defect concentrations. Our experiments reveal three new phenomena: (1) H+ that is associated with the Ti-clinohumite defect, [Ti-2H], can enter and exit olivine at the fastest rate measured. (2) The rate of H+ loss from specific infrared absorption peaks, and in total, may change with progressive dehydration due to the buildup of Fe3+ and redistribution of defects, particularly the rehydration of [Ti-2H]. (3) The following Arrhenius laws appear to apply to total H+ diffusion out of most natural olivines that have been studied experimentally and naturally during dehydration: Da = 10−5.4exp(−130/RT); Db = 10−6.9exp(−130/RT); and Dc = 10−6.6exp(−130/RT), where R is the gas constant 8.314 J/mol K; T is the temperature, the units of the diffusivities (D) are in m2/s; and the activation energy is 130 kJ/mol. This rate is slower than proton-polaron redox rate but faster than proton-metal vacancy diffusion and appears to characterize olivine of Fo ∼90 composition in a wide range of conditions, from self-diffusion, to melt inclusion re-equilibration, to mantle xenolith and volcanic phenocryst zonation during ascent.

Relative diffusivities of hydrous defects from a partially dehydrated natural olivine

The mechanisms of hydrogen mobility in nominally anhydrous minerals (NAMs) impact both the water storage in Earth’s mantle and its effective modification induced by mantle processes. Hydrogen can be stored in point defects in NAMs, and recent experimental studies in simple systems have shown that the diffusivity varies significantly from point defect to point defect. Here, a natural mantle-derived olivine from a peridotite xenolith (Pali-Aike in Chile) was analyzed by Fourier transform infrared spectroscopy (FTIR) to investigate if natural olivine can hold the signature of such site-specific diffusivity. The deconvolution of the spectra allows identifying 10 OH absorption bands; each band is attributed to a specific type of defect. The relative diffusivities associated with different defects were estimated and lead to the following results: (1) the OH bands from Group-I (3650–3500 cm−1) yield similar hydrogen diffusivities, except one band at 3598 cm−1, which yields hydrogen diffusivity five times slower than the others; (2) the OH bands from Group-II (3500–3100 cm−1) reveal heterogeneous hydrogen distribution, which is not compatible with a simple mechanism of ionic diffusion; (3) hydrogen diffusion along the [100] direction is five times faster than along [001] for all defects. The use of hydrogen concentration in olivine as a proxy for the deep water cycle requires detailed FTIR profiles to properly characterize ionic diffusion and its consequences on the bulk hydrogen concentration.

High-pressure single-crystal elasticity of wadsleyite and the seismic signature of water in the shallow transition zone

Earth's transition zone at depths between 410 km and 660 km plays a key role in Earth's deep water cycle since large amounts of hydrogen can be stored in the nominally anhydrous minerals wadsleyite and ringwoodite, (Mg,Fe)2SiO4. Previous mineral physics experiments on iron-free wadsleyite proposed low seismic velocities as an indicative feature for hydration in the transition zone. Here we report simultaneous sound wave velocity and density measurements on iron-bearing wadsleyite single crystals with 0.24 wt-% H2O. By comparison with earlier studies, we show that pressure suppresses the velocity reduction caused by higher degrees of hydration in iron-bearing wadsleyite, ultimately leading to a velocity cross-over for both P-waves and S-waves. Modeling based on our experimental results shows that wave speed variations within the transition zone as well as velocity jumps at the 410-km seismic discontinuity, both of which have been used in previous work to detect mantle hydration, are poor water sensors. Instead, the impedance contrast across the 410-km seismic discontinuity that is reduced in the presence of water can serve as a more robust indicator for hydrated parts of the transition zone.

Seismically invisible water in Earth's transition zone?

Ringwoodite, the dominant mineral at depths between 520 km and 660 km, can store up to 2–3 wt.% of water in its crystal structure, making the Earth's transition zone a plausible water reservoir that plays a central role in Earth's deep water cycle. Experiments show that hydration of ringwoodite significantly reduces elastic wave velocities at room pressure, but the effect of pressure remains poorly constrained. Here, a novel experimental setup enables a direct quantification of the effect of hydration on ringwoodite single-crystal elasticity and density at pressures of the Earth's transition zone and high temperatures. Our data show that the hydration-induced reduction of seismic velocities almost vanishes at conditions of the transition zone. Seismic data thus agree with a wide range of water contents in the transition zone.

Hydrogen incorporation and retention in metamorphic olivine during subduction: Implications for the deep water cycle

Incorporation of hydrogen into metamorphic olivine during dehydration reactions in the subducting oceanic lithosphere provides a mechanism to replenish the deep mantle with water. Fourier transform infrared spectroscopy of metamorphic olivines formed at 2.5 GPa and 550 °C through the reaction antigorite + brucite = olivine + chlorite + water shows water contents between 100 and 140 ppm H2O associated exclusively with silicon vacancies, similar to the highest values found in peridotite xenoliths. Brucite involvement in the olivine-forming reaction ensures H2O saturation and a low Si activity, favoring hydrogen incorporation into Si vacancies. The mapped water distribution in olivine is consistent with growth zoning and there is no evidence of water gain or loss. Thus, even for metamorphic timescales of several million years at 550 °C, no ionic diffusion modification is observed, in agreement with recent experimental findings. Metamorphic olivines formed by this dehydration reaction may contribute considerable amounts of water to the deep water cycle. Additionally, olivine with abundant H in Si vacancies are expected to be rheologically weaker than anyhdrous mantle olivine, and might provide a weak interface between slabs and mantle wedges in subduction zones at conditions beyond the stability of hydrous phases.

A role for subducted super-hydrated kaolinite in Earth’s deep water cycle

Water is the most abundant volatile component in the Earth. It continuously enters the mantle through subduction zones, where it reduces the melting temperature of rocks to generate magmas. The dehydration process in subduction zones, which determines whether water is released from the slab or transported into the deeper mantle, is an essential component of the deep water cycle. Here we use in situ and time-resolved high-pressure/high-temperature synchrotron X-ray diffraction and infrared spectra to characterize the structural and chemical changes of the clay mineral kaolinite. At conditions corresponding to a depth of about 75 km in a cold subducting slab (2.7 GPa and 200 °C), and in the presence of water, we observe the pressure-induced insertion of water into kaolinite. This super-hydrated phase has a unit cell volume that is about 31% larger, a density that is about 8.4% lower than the original kaolinite and, with 29 wt% H2O, the highest water content of any known aluminosilicate mineral in the Earth. As pressure and temperature approach 19 GPa and about 800 °C, we observe the sequential breakdown of super-hydrated kaolinite. The formation and subsequent breakdown of super-hydrated kaolinite in cold slabs subducted below 200 km leads to the release of water that may affect seismicity and help fuel arc volcanism at the surface. A super-hydrated clay mineral may play an important role in the solid Earth’s water cycle, according to laboratory experiments. The mineral kaolinite can carry and release large amounts of water during subduction.

Distribution, cycling and impact of water in the Earth's interior

AbstractThe Earth's deep interior is a hidden water reservoir on a par with the hydrosphere that is crucial for keeping the Earth as a habitable planet. In particular, nominally anhydrous minerals (NAMs) in the silicate Earth host a significant amount of water by accommodating H point defects in their crystal lattices. Water distribution in the silicate Earth is highly heterogeneous, and the mantle transition zone may contain more water than the upper and lower mantles. Plate subduction transports surface water to various depths, with a series of hydrous minerals and NAMs serving as water carriers. Dehydration of the subducting slab produces liquid phases such as aqueous solutions and hydrous melts as a metasomatic agent of the mantle. Partial melting of the metasomatic mantle domains sparks off arc volcanism, which, along with the volcanism at mid-ocean ridges and hotspots, returns water to the surface and completes the deep water cycle. There appears to have been a steady balance between hydration and dehydration of the mantle at least since the Phanerozoic. Earth's water probably originates from a primordial portion that survived the Moon-forming giant impact, with later delivery by asteroids and comets. Water could play a critical role in initiating plate tectonics. In the modern Earth, the storage and cycling of water profoundly modulates a variety of properties and processes of the Earth's interior, with impacts on surface environments. Notable examples include the hydrolytic weakening effect on mantle convection and plate motion, influences on phase transitions (on the solidus of mantle peridotite in particular) and dehydration embrittlement triggering intermediate- to deep-focus earthquakes. Water can reduce seismic velocity and enhance electrical conductivity, providing remote sensing methods for water distribution in the Earth's interior. Many unresolved issues around the deep water cycle require an integrated approach and concerted efforts from multiple disciplines.

An imbalance in the deep water cycle at subduction zones: The potential importance of the fore-arc mantle

The depth of slab dehydration is thought to be controlled by the thermal state of the downgoing slab: cold slabs are thought to mostly dehydrate beneath the arc front while warmer slabs should mostly dehydrate beneath the fore-arc. Cold subduction zone lavas are thus predicted to have interacted with greater extent of water-rich fluids released from the downgoing slab, and should thus display higher water content and be elevated in slab-fluid proxies (i.e., high Ba/Th, H2O/Ce, Rb/Th, etc.) compared to hot subduction zone lavas. Arc lavas, however, display similar slab-fluid signatures regardless of the thermal state of the slab, suggesting more complexity to volatile cycling in subduction zones. Here, we explore whether the serpentinized fore-arc mantle may be an important fluid reservoir in subduction zones and whether it can contribute to arc magma generation by being dragged down with the slab. Using simple mass balance and fluid dynamics calculations, we show that the dragged-down fore-arc mantle could provide enough water (∼7–78% of the total water injected at the trenches) to account for the water outfluxes released beneath the volcanic arc. Hence, we propose that the water captured by arc magmas may not all derive directly from the slab, but a significant component may be indirectly slab-derived via dehydration of dragged-down fore-arc serpentinites. Fore-arc serpentinite dehydration, if universal, could be a process that explains the similar geochemical fingerprint (i.e., in slab fluid proxies) of arc magmas.

Reliability and Validity Aquatic Peak VO2 for Individuals with Spinal Cord Injury

To understand peak VO2 testing reliability for individuals with SCI in 1) deep water and arm cycle ergometer conditions; and to determine the relationship between these two scenarios.

Fluorine follows water: Effect on electrical conductivity of silicate minerals by experimental constraints from phlogopite

Fluorine and hydroxyl groups are minor constituents of silicate minerals, and share a lot of similarities concerning their physical and chemical properties. Hydroxyl groups significantly enhance the electrical conductivity of many silicate minerals, and it is expected that fluorine would have a comparable effect. This, however, has never been documented quantitatively. Here we present experimental approaches on this issue, by investigating the electrical conductivity of phlogopite with a wide range of fluorine content (but with broadly similar contents for other major elements). Electrical conductivities of gem-quality single crystal phlogopites, with samples prepared along the same orientation (normal to the (001) plane), were determined at 1GPa and 200–650°C using an end-loaded piston cylinder apparatus and a Solartron-1260 Impedance/Gain Phase Analyzer over the frequency range of 106 to 0.1Hz. The complex spectra usually show an arc in the high frequency range and a short tail in the low frequency range, which are caused by lattice conduction and electrode effects, respectively. The electrical conductivity increases with increasing fluorine content, and the main charge carriers are fluorine. The activation enthalpies are ∼180 to 200kJ/mol, nearly independent of fluorine content. The conductivity is linearly proportional to the content of fluorine, with an exponent factor of ∼1. The results demonstrate that conduction by fluorine leads to very high electrical conductivity at high temperatures. The influence of fluorine on electrical conductivity may be compared to that of hydrogen in nominally anhydrous minerals. This, along with the close association of fluorine and hydroxyl groups in silicate minerals and their similar crystal-chemical behaviors, suggests a more general role of fluorine in enhancing the electrical conductivity of many silicate minerals. Fluorine-rich assemblages, e.g., phlogopite and amphibole, could be locally enriched in the upper mantle, and if they form connected networks as observed for some natural samples, regionally high electrical conductivities could be produced. It has been recently proposed that the transition zone is probably a major reservoir for fluorine in the mantle, due to the significant dissolution of fluorine in wadsleyite and ringwoodite and the coupled incorporation with hydroxyl groups. As such, geophysically-resolved high electrical conductivities in the transition zone may be accounted for by fluorine in the dominant minerals, rather than by hydroxyl groups. The results of this work would stimulate a wide scope of future studies on the deep fluorine cycle, the deep water cycle and the geodynamical properties of the mantle.

On the numerical modeling of the deep mantle water cycle in global-scale mantle dynamics: The effects of the water solubility limit of lower mantle minerals

Water is the most important component in Earth system evolution. Here, I review the current understanding of the fate of water in the mantle dynamics system based on high-pressure and temperature experiments, geochemical analyses, seismological and geomagnetic observations, and numerical modeling of both regional- and global-scale mantle dynamics. In addition, as a numerical example, I show that the water solubility of the deep mantle is strongly sensitive to global-scale water circulation in the mantle. In a numerical example shown here, water solubility maps as functions of temperature and pressure are extremely important for revealing the hydrous structures in both the mantle transition zone and the deep mantle. Particularly, the water solubility limit of lower mantle minerals should be not so large as ~100 ppm for the mantle transition zone to get the largest hydrous reservoir in the global-scale mantle dynamics system. This result is consistent with the current view of mantle water circulation provided by mineral physics, which is also found as a hydrous basaltic crust in the deep mantle and the water enhancement of the mantle transition zone simultaneously. In this paper, I also discuss some unresolved issues associated with mantle water circulation, its influence on the onset and stability of plate motion, and the requirements for developing Earth system evolution in mantle dynamics simulations.

Characteristics of the geothermal water in Changbai Mountain volcanic region, northeast of China

Geothermal water is plentiful in Changbai Mountain region, northeastern China, due to the volcanic activities and widespread faults. For the exploration of geothermal resources, this study uses quartz and cation geothermometer to estimate the temperatures of the geothermal reservoir and uses the tubular models to evaluate the thermal gradient. The hydrogeochemical characteristics of the geothermal resources were also evaluated by hydrogeochemical analysis. The results showed that the geothermal reservoir temperatures of the four major thermal springs in Changbai Mountain region range from 72 to 169 °C. The average geothermal reservoir temperatures of Jinjiang hot springs, Changbai hot springs I, Xianrenqiao hot springs, and Changbai hot springs II are 129.25, 169, 89, and 73.67 °C, respectively. The geothermal gradient values of the four major thermal springs have different characteristics. The geothermal gradient values of Jinjiang hot springs and Changbai hot springs I are 4.6 and 3.1 °C/100 m, respectively. The geothermal gradient values of Xianrenqiao thermal springs and Changbai thermal springs II are both lower than 1.5 °C/100 m, with the values of 1.1 and 1.4 °C/100 m. And the geothermal gradients are influenced by Changbai Mountain Tianchi volcano. In addition, the water chemical analyses showed that the geothermal water types are HCO3-Na with higher concentrations of Na+, Cl−, SO4 2−, TDS, and HCO3 − than the non-thermal waters, which suggested a deep and long water cycle of the thermal water, and therefore a sufficient water-rock interaction.