Deep Earth Water Research Articles

Oxidized Sulfur Species in Slab Fluids as a Source of Enriched Sulfur Isotope Signatures in Arcs

AbstractRecycling of oxidized sulfur from subducting slabs to the mantle wedge provides simultaneous explanations for the elevated oxygen fugacity (fO2) in subduction zones, their high hydrothermal and magmatic sulfur outputs, and the enriched sulfur isotopic signatures (i.e., δ34S > 0‰) of these outputs. However, a quantitative understanding of the abundance and speciation of sulfur in slab fluids consistent with high pressure experiments is lacking. Here we analyze published experimental data for anhydrite solubility in H2O‐NaCl solutions to calibrate a high‐pressure aqueous speciation model of sulfur within the framework of the deep earth water model. We characterize aqueous complexes, required to account for the high experimental anhydrite solubilities. We then use this framework to predict the speciation and solubility of sulfur in chemically complex fluids in equilibrium with model subducting mafic and ultramafic lithologies, from 2 to 3 GPa and 400 to 800°C at log fO2 from FMQ‐2 to FMQ+4. We show that sulfate complexes of calcium and sodium markedly enhance the stability of sulfate in moderately oxidized fluids in equilibrium with pyrite at fO2 conditions of FMQ+1 to +2, causing large sulfur isotope fractionations up to 10‰ in the fluid relative to the slab. Such fluids could impart oxidized, sulfur‐rich and high δ34S signatures to the mantle wedge that are ultimately transferred to arc magmas, without the need to invoke 34S‐rich subducted lithologies.

Reaction path model of the formation of abiotic immiscible hydrocarbon fluids in subducted carbonated serpentinites, Lanzo Massif (Western Italian Alps)

Fluids generated from subducted slabs participate in the cycling of deep carbon in the crust and upper mantle. In these fluids, aqueous carbon species vary in oxidation state between +IV and -IV depending on whether the fluids are oxidizing or reducing, respectively. Most studies of subduction-zone fluids have focused on oxidized carbon species. However, recent studies of natural samples have demonstrated the occurrence of deep, reducing fluids, generated in both subducted oceanic upper mantle and crustal rocks. CH4-H2-rich fluid inclusions in subducted carbonated serpentinites have demonstrated the existence of abiotic, immiscible, hydrocarbon fluids at upper mantle conditions. To investigate the formation of such immiscible hydrocarbon fluids during the evolution of subducted carbonated serpentinites, we used equilibrium constants from the Deep Earth Water model to carry out predictive chemical mass transfer modeling to simulate the alteration reactions. A novel feature of the models was the inclusion of an immiscible hydrocarbon fluid containing six components (CH4,f,C2H6,f,C3H8,f,isoC4H10,f,CO2,f,H2,f). This feature enabled prediction of the formation of a separate immiscible fluid in equilibrium with aqueous species and minerals.We developed a predictive reaction path model of invasive H2,f reacting with carbonated serpentinites and interstitial aqueous fluids for comparison with the natural samples from the Lanzo Massif, western Italian Alps. Over a range of temperatures and pressures, immiscible hydrocarbon fluids formed in association with altered mineral assemblages. Reaction progress caused the transformation of carbonated serpentinites and the formation of clinopyroxene, brucite, graphite, and hydrocarbon fluids, along with changes of pH, logfO2, and aqueous species. CH4,f was the most abundant hydrocarbon species in all the models. The overall results at 2.0 GPa and 400 to 450 °C were consistent with the natural samples from the Lanzo Massif. Interestingly, large amounts of H2O formed due to oxidation of H2. More hydrocarbons and H2O formed in models with lower fluid/rock mass ratios or with more reactant H2. Models at different pressure and temperature conditions showed similar results with some variation in the relative stabilities of aragonite, graphite and olivine solid solution, and associated differences in mineral sequences, hydrocarbon fluids, values of aqueous species, and the final logfO2 and pH. Our models strongly support the laboratory and field evidence that reduction of carbonated serpentinites by infiltrating H2 fluids can cause the formation of immiscible, abiotic hydrocarbon fluids in subduction zones.

OH incorporation and retention in eclogite-facies garnets from the Zermatt–Saas area (Switzerland) and their contribution to the deep water cycle

Abstract. The incorporation mechanisms of OH groups in garnet were investigated in a suite of high-pressure rocks from the Zermatt–Saas area (Switzerland) using a combination of Fourier transform infrared spectroscopy (FTIR) and electron probe micro-analysis (EPMA). Investigated garnet specimens include grossular–andradite–uvarovite solid solutions in serpentinite and rodingite and almandine–grossular–pyrope–spessartine solid solutions in eclogite, mafic fels and meta-sediment. All rocks experienced the same peak metamorphic conditions corresponding to a burial depth of ∼ 80 km (∼ 540 ∘C, 2.3 GPa), allowing determination of the OH content in garnet as a function of rock type. The capacity for OH incorporation into garnet strongly depends on its composition. Andradite-rich (400–5000 µg g−1 H2O) and grossular-rich garnet (200–1800 µg g−1 H2O) contain at least 1 order of magnitude more H2O than almandine-rich garnet (< 120 µg g−1 H2O). Microscale analyses using FTIR and EPMA profiles and maps reveal the preservation of OH zoning throughout the metamorphic history of the samples. The OH content correlates strongly with Mn, Ca and Ti zoning and produces distinct absorption bands that are characteristic of multiple nano-scale OH environments. The use of 2D diffusion modelling suggests that H diffusion rates in these rocks is as low as log(D[m2 s−1]) = −24.5 at 540 ∘C. Data were collected for the main garnet-bearing rock types of the Zermatt–Saas area allowing a mass balance model of H2O to be calculated. The result shows that ∼ 3360 kg H2O km−1 (section of oceanic crust) yr−1 could be transported by garnet in the subducting slab beyond 80 km depth and contributed to the deep-Earth water cycle during the Eocene subduction of the Piemonte–Liguria Ocean.

Numerical modeling on global-scale mantle water cycle and its impact on the sea-level change

Using numerical modeling of the global-scale mantle water cycle, I have clarified the mechanisms governing the deep Earth water content variations and evaluated their effects on sea-level changes. The following points are found: 1. The temporal variations of water content in the deep Earth are mainly determined by a balance between the ingassing flux from the ocean and the dehydration flux in the mantle wedge. 2. The primary control of ingassing flux comes from the water content of oceanic crust, rather than the style of plate subduction promoted by the free surface boundary condition. 3. The water content of oceanic crust ranges from 0.1 to 1 wt. percent to match the ingassing flux with the inferred one from the geological analysis.Based on the best-fit scenario for the ingassing flux from the ocean to the deep interior, I obtain an average rate of sea-level change of −1 to −1.5 m/Myrs over 1 billion year, which is nearly consistent with the long-term trend of sea-level change estimated from geological and paleontological data (∼−1 m/Myr). Note, however, that the detailed profile of sea-level change in the numerical modeling is difficult to explain the observation one. This underlines the importance of dehydration process in the mantle wedge in understanding the sea-level change resulting from the water budget in the Earth's deep interior over the Earth's history.

Water speciation and hydrogen isotopes in hydrous stishovite: implications for the deep Earth water cycle

Stishovite is a key mineral for understanding the deep Earth water cycle because of its potential as a main carrier for water into the transition zone and lower mantle. During subduction-related metamorphism of basaltic oceanic crust, stishovite stabilizes at 8–9 GPa and comprises 10–25 vol% of the bulk mineralogy, with some experimental studies indicating that stishovite can accommodate 3.5 wt% H2O or more in the transition zone and upper lower mantle. This large water solubility has been explained by a hydrogarnet substitution mechanism (1Si4+ ↔ 4H+) and/or the incorporation of interstitial molecular water. To investigate water speciation and hydrogen isotope behavior, we synthesized partially deuterated hydrous stishovite at 9 GPa and 450 °C in a multi-anvil press (MA). The hydrous stishovite contains on average 1.69 ± 0.05 wt% water, which is consistent with earlier MA studies but is significantly lower than the 3.5 wt% reported from in situ diamond anvil cell (DAC) studies made at higher pressures and temperatures. 1H MAS NMR spinning sideband characteristics suggest a high abundance of interstitial molecular water in hydrous stishovite, while the presence of a hydrogarnet defect cannot be ruled out. Unit-cell volumes and deuterium enrichment in the quenched hydrous stishovite indicate that ~ 45% of the water is lost from the stishovite upon quenching and decompression of the experiment, consistent with a higher solubility. This implies that the pristine water contents of a P–T–fO2 equilibrated hydrous stishovite cannot be quenched to 1 atm and room temperature from classical MA experiments. We further present a capillary-based recovery method for fluid from experimental capsules, allowing direct determination of the D/H ratio of the experimental fluid and indirect determination of the hydrous stishovite. Using Rayleigh modeling to account for the quench-related water loss, we find that, at 450 °C and 9 GPa, deuterium is 3.5–4.5 times enriched in hydrous stishovite relative to coexisting aqueous fluid. This is opposite of what is commonly observed for mineral–fluid pairs above 300 °C, rendering hydrous stishovite a potential sink for deuterium and decreasing the D/H ratio of coexisting aqueous fluids. Partial decomposition (30–60%) of hydrous stishovite during mantle upwelling and production of primary basaltic melts could be accompanied by high-temperature D/H fractionation, decreasing the hydrogen isotope composition of such melts towards “mantle-like” δD values between −75 and −220‰.

The importance of carbon to the formation and composition of silicates during mantle metasomatism

Mineral and fluid inclusions in mantle diamonds provide otherwise inaccessible information concerning the nature of mantle metasomatism and the role of fluids in the mass transfer of material through the Earth’s interior. We explore the role of carbon concentration during fluid-rock metasomatism in generating the range of garnet and clinopyroxene compositions observed in diamonds from the sub-continental lithospheric mantle. We use the Deep Earth Water model to predict the results of metasomatism between silicic, carbonatitic and peridotitic fluids and common mantle rocks (peridotites, eclogites and pyroxenites) at 5 GPa, 1000 °C, across a range of redox conditions (logfO2 = −2 to −4 ΔFMQ), and a wide range of initial carbon concentrations in the metasomatic fluids. Our results show that the predicted compositions of metasomatic garnets and clinopyroxenes are controlled by the initial geochemistry of fluids and rocks, with subsequent mineral-specific geochemical evolution following definable reaction pathways. Model carbon-rich metasomatic fluids that can form diamond (initial C-content >5.00 molal) result in Mg-rich garnets and clinopyroxenes typical of peridotitic, eclogitic, and websteritic inclusions in diamonds. However, model carbon-poor metasomatic fluids that do not form diamond result in Mg-poor, Ca-rich garnets and clinopyroxenes. Such garnets and clinopyroxenes do nevertheless occur as inclusions in diamonds. In our models, the abundance of carbon in the fluids controls the behaviour of the bivalent ions through the formation of aqueous Mg-Ca-Fe-C complexes, which directly govern the composition of garnets and clinopyroxenes precipitated during the metasomatic processes. As the C-rich initial fluids can form the higher Mg-eclogitic, peridotitic, and websteritic inclusions in diamonds, these inclusions can be syngenetic (metasomatic) or possibly protogenetic. However, in our models, the relatively Mg-poor, Ca- and Fe-rich eclogitic garnet and clinopyroxene inclusions found in mantle diamonds formed from C-poor fluids that do not precipitate diamond. These inclusions most likely reflect a metasomatic event prior to being incorporated into their host diamonds, or they could represent protolith-based protogenetic geochemistry. Therefore, the paragenetic groups used to classify diamonds should not be considered a genetic classification, as the role of fluid geochemistry appears to be more important than the one played by host rock geochemistry.

Massive abiotic methane production in eclogite during cold subduction.

Methane (CH4) is a critical but overlooked component in the study of the deep carbon cycle. Abiotic CH4 produced by serpentinization of ultramafic rocks has received extensive attention, but its formation and flux in mafic rocks during subduction remain poorly understood. Here, we report massive CH4-rich fluid inclusions in well-zoned garnet from eclogites in Western Tianshan, China. Petrological characteristics and carbon-hydrogen isotopic compositions confirm the abiotic origin of this CH4. Reconstructed P-T-fO2-fluid trajectories and Deep Earth Water modeling imply that massive abiotic CH4 was generated during cold subduction at depths of 50-120km, whereas CO2 was produced during exhumation. The massive production of abiotic CH4 in eclogites may result from multiple mechanisms during prograde high pressure-ultrahigh pressure metamorphism. Our flux calculation proposes that abiotic CH4 that has been formed in HP-UHP eclogites in cold subduction zones may represent one of the largest, yet overlooked, sources of abiotic CH4 on Earth.

A relatively dry mantle transition zone revealed by geomagnetic diurnal variations.

The distribution of water within the mantle transition zone (MTZ) has important implications for the material circulation and partial melting of the mantle. Although solubility of hydrogen is very high, leading to speculations that the MTZ plays a key role in the deep-Earth water cycle, the actual water content remains an open question. Electrical conductivity of mantle minerals is very sensitive to water content, so reliable estimates of this physical parameter in the MTZ would provide valuable constraints. Here, we use recently developed joint inversion of geomagnetic diurnal variation for realistic source structure and one-dimensional mantle conductivity profile. Synthetic tests show that the resulting profile is a reasonable proxy for the electrical conductivity distribution of continental mantle over depths where model resolution is best (200 to 600 kilometer), even in the presence of lateral heterogeneity. The inferred water concentration in the MTZ is 0.03 weight %, one to two orders of magnitude below the solubility of wadsleyite and ringwoodite.

PyGeochemCalc: A Python package for geochemical thermodynamic calculations from ambient to deep Earth conditions

Over the past half century, techniques for evaluating the thermodynamics of water-rock interactions from ambient to deep Earth conditions have advanced incredibly and in myriad directions. As these tools for analyzing the thermodynamic states of geochemical species as a function of temperature, pressure, and composition have multiplied, so too have the possibilities for tracing water-rock interaction from ambient to deep conditions on Earth and beyond. Yet, the aqueous geochemical community still lacks a centralized platform for incorporating this constantly updating thermodynamic data into aqueous geochemical models. Here, we introduce PyGeochemCalc (PyGCC), a community-driven, open-source Python package that meets this need by providing a consolidated set of functions for calculating the thermodynamic properties of gas, aqueous, and mineral (including solid solutions and variable-formula clays) species, as well as reactions amongst these species, over a broad range of temperature and pressure conditions. The PyGCC package utilizes the revised Helgeson-Kirkham-Flowers (HKF) equation of state, and newly proposed density-based extrapolations based upon it, to calculate the thermodynamic properties of aqueous species; a choice of equations of state and electrostatic models (including the Deep Earth Water (DEW) model) to calculate thermodynamic and dielectric properties of water; and heat capacity functions to calculate thermodynamic properties of minerals and gases. Additionally, PyGCC integrates these functions to generate thermodynamic databases for various geochemical programs, including the Geochemist's Workbench (GWB), EQ3/6, TOUGHREACT, and PFLOTRAN, with straightforward possibilities for extension to other simulators. The various functions in the package can also be modularly utilized, and introduced into other modeling tools, as desired. In this paper, we detail the capabilities of PyGCC and the equations it relies on for calculating thermodynamic properties of water, aqueous species, and gases. Although the fundamental thermodynamic data necessary for state-of-the-science PyGCC calculations will necessarily evolve as our collective geochemical knowledge base expands, PyGCC's open source, community-driven design will allow for users to keep pace via rapid implementation of these advancements in this modern geochemical tool.

Electrical Conductivities and Association Constants in Dilute Aqueous NdCl3Solutions from 298 to 523 K along an Isobar of 25 MPa

Electrical measurements were performed in dilute aqueous NdCl3 solutions from 298 to 523 K along the 25 MPa isobar to obtain limiting conductances and association constants. The specific conductance data were estimated using a continuous flow cell and a Markov Chain Monte Carlo (MCMC) correction algorithm. The limiting conductances for the salts in water were derived by regressing the mean spherical approximation (MSA) conductance model and speciation analyses based on the MCMC algorithm and the Deep Earth Water (DEW) model. The limiting conductances derived from the experimental data agree well with a predictive correlation proposed by Smolyakov, Anderko, and Lencka. Only the first association constant between neodymium and chloride could be derived at low temperatures (< 373 K) due to the apparent large statistical uncertainty of the second association constant. Above 373 K, both association constants could be derived and show a reasonable agreement with Migdisov and Williams-Jones and Gammons et al.

Integrated thermodynamic and thermomechanical numerical modeling: A useful method for studying deep Earth water and carbon cycling

The plate tectonic evolution controls the mass and energy circulation between the Earth's surface and interior, during which the volatiles (e.g., water and carbon) could also accompany the tectonic up- and down-wellings. The deep Earth injection of volatiles (ingassing mainly via subduction) can modify the physical and chemical properties of rocks and thus strongly affect the processes and dynamics of Earth's interior. On the other hand, the emission of volatiles to the Earth's surface (outgassing mainly through volcanism and rifting) can influence the evolution of atmosphere, climate and habitability. Thus, the deep water and carbon cycling plays important roles in not only the solid Earth dynamics, but also the surficial environments. The time-dependent evolution of deep Earth condition and large-scale plate tectonic reconstruction have generally been used to infer the fluxes and pathways of water and carbon cycling, as well as the resulting ocean water and atmospheric carbon evolution in the Earth's surficial reservoirs. Besides multiple observations and experiments, the integrated thermodynamic and thermomechanical numerical modeling provides as a useful way for studying deep Earth water and carbon cycling in variable tectonic settings and with different conditions. In the review, I firstly introduce the numerical method integrating thermodynamic data into thermomechanical principles. Then, several typical numerical models are introduced, which highlight, respectively, the (de)hydration in shallow lithospheric subduction channel, the deep water cycling in the upper mantle and transition zone, as well as the subduction-induced carbon cycling. Finally, the general issues about water and carbon fluxes between the Earth's interior and surface are discussed, which are less constrained issues and require further studies with possible contributions from the integrated thermodynamic and thermomechanical numerical modeling.

Machine Learning for Identification of Primary Water Concentrations in Mantle Pyroxene

AbstractThe approach of estimating the H2O content of basaltic magmas via clinopyroxene (cpx) phenocrysts is a potentially effective way to glimpse the deep Earth water cycle. However, it is difficult to ascertain using traditional geochemical methods whether hydrogen (H) measured in cpx phenocrysts represents a primary signature that can ultimately inform estimates of the mantle water content. In this study, we conducted machine learning on the major element compositions and H2O content of cpx phenocrysts (1904 samples in total). Using the support vector machine (SVM), we defined a classifier (overall accuracy &gt;92%) that can separate cpx that have undergone H diffusion, and thus modification of their original water content, from those that have not experienced H diffusion. Our trained SVM model has broad implications for understanding the primary water content of magma, the variations in water content during magma evolution, and the water cycle in the deep Earth.

Retrograde carbon sequestration in orogenic complexes: A case study from the Chinese southwestern Tianshan

The interaction between ascending carbonic fluids and rocks at shallow depths in orogenic systems plays a crucial role in carbon distribution and fluxes regulation. Here we present field, microstructural, petrological and geochemical results of carbonic fluid–rock interactions affecting exhumed mafic eclogites from the Chinese southwestern Tianshan ultrahigh-pressure (UHP) metamorphic belt. The mafic eclogites recorded two successive, superimposed, metamorphic–metasomatic stages: graphite precipitation along fractures and veins at eclogite facies (Stage#1) and pervasive rock carbonation (i.e., silicate dissolution and carbonate precipitation) at retrograde amphibolite to greenschist facies (Stage#2). This work focuses on Stage#2 carbonation, which consists of the transformation of Stage#1 graphite-bearing eclogites by carbonate + paragonite (± zoisite) + quartz. Carbon and oxygen isotopic data and Deep Earth Water (DEW) thermodynamic modeling suggest an external metasedimentary source for the Stage#2 carbonation. This deep carbon sequestration event can be referred to retrograde, greenschist-facies conditions at about 10 kbar and 450 °C, and redox conditions equal or more oxidized than the quartz–fayalite–magnetite (QFM) buffer. Modeling results also show that the saturation of carbonic fluid with respect to graphite can boost retrograde carbonation processes. Our findings provide new insights into the reactivity of metastable, exhumed metamafic rocks with ascending carbonic fluids in orogenic systems. We conclude that retrograde, fluid-mediated rock carbonation can significantly impact on carbon fluxes from active collisional belts.

Solvation of simple ions in water at extreme conditions.

The interaction of ions and water at high pressure and temperature plays a critical role in Earth and planetary science yet remains poorly understood. Aqueous fluids affect geochemical properties ranging from water phase stability to mineral solubility and reactivity. Here, we report first-principles molecular dynamics simulations of mono-valent ions (Li+, K+, Cl-) as well as NaCl in liquid water at temperatures and pressures relevant to the Earth's upper mantle (11 GPa, 1000 K) and concentrations in the dilute limit (0.44-0.88 m), in the regime of ocean salinity. We find that, at extreme conditions, the average structural and vibrational properties of water are weakly affected by the presence of ions, beyond the first solvation shell, similar to what was observed at ambient conditions. We also find that the ionic conductivity of the liquid increases in the presence of ions by less than an order of magnitude and that the dielectric constant is moderately reduced by at most ∼10% at these conditions. Our findings may aid in the parameterization of deep earth water models developed to describe water-rock reactions.

Origin of megacrysts by carbonate-bearing metasomatism: a case study for the Muskox kimberlite, Slave craton, Canada

Low-Cr and high-Cr clinopyroxene, garnet, olivine and ilmenite megacrysts from the Muskox kimberlite (Canada) were analysed for major and trace elements, as well as Sr, Nd and Pb isotopes. Samples display compositional overlap with respective phases in websterites, and clinopyroxene isotope systematics reveal similarities to both websteritic and metasomatic clinopyroxene in peridotites from the same kimberlite, in addition to Muskox and Jericho kimberlites. All lithologies may represent the products of mixing between EM1 mantle, relict Proterozoic enriched mantle and HIMU carbonatitic fluid. Equilibrium melts calculated from clinopyroxene trace element data using experimental distribution coefficients for feasible proto-kimberlitic melts yield a range of metasomatic agents. Conclusion on the carbonate-bearing nature of the metasomatism is based on the presence of a HIMU isotopic signature and results obtained from thermodynamic modelling using the Deep Earth Water model. Modelling was carried out with three fluid compositions; asthenospheric, eclogitic and kimberlitic. Our results suggest mineral compositions analogous to megacrysts cannot be produced by metasomatism of mantle peridotite by carbonate-free hydrous fluids. Isotope systematics argue against a strictly cognate relationship between megacrysts and their host kimberlite, instead suggesting that megacrysts and websterites may represent products of regional metasomatism by carbonatitic HIMU fluids shortly predating kimberlite magmatism. Supplementary material: A table containing parameters used in isotopic mixing models is available at https://doi.org/10.6084/m9.figshare.c.5255825

Deserpentinization in Subduction Zones as a Source of Oxidation in Arcs: a Reality Check

AbstractPrevious studies have concluded that dehydration of serpentinites in subduction zones produces oxidizing fluids that are the cause of oxidized arc magmas. Here, observations of natural samples and settings are combined with thermodynamic models to explore some of the factors that complicate interpretation of the observations that form the basis of this conclusion. These factors include the variability of serpentinite protoliths, the roles of carbon and sulfur in serpentinite evolution, variability in serpentinization in different tectonic settings, changes in the bulk compositions of ultramafic rocks during serpentinization, fundamental differences between serpentinization and deserpentinization, and the absence of precise geothermobarometers for ultramafic rocks. The capacity of serpentinite-derived fluids to oxidize sub-arc magma is also examined. These fluids can transport redox budget as carbon-, sulfur-, and iron-bearing species. Iron- and carbon-bearing species might be present in sufficient concentrations to transport redox budget deep within subduction zones, but are not viable transporters of redox budget at the temperatures of antigorite breakdown, which produces the largest proportion of fluid released by serpentinite dehydration. Sulfur-bearing species can carry significant redox budget, and calculations using the Deep Earth Water (DEW) model show that these species might be stable during antigorite breakdown. However, oxygen fugacities of ∼ΔFMQ + 3 (where FMQ refers to the fayalite–magnetite–quartz buffer, and ΔFMQ is log fO2 – log fO2, FMQ), which is close to, or above, the hematite–magnetite buffer at the conditions of interest, are required to stabilize oxidized sulfur-bearing species. Pseudosection calculations indicate that these conditions might be attained at the conditions of antigorite breakdown if the starting serpentinites are sufficiently oxidized, but further work is required to assess the variability of serpentinite protoliths, metamorphic pressures and temperatures, and to confirm the relative positions of the mineral buffers with relation to changes in fluid speciation.

Mixing of carbonatitic into saline fluid during panda diamond formation

Diamonds containing fluid inclusions provide invaluable samples of upper mantle fluids, the study of which illuminates not only diamond formation but also the long-term evolution of the subcratonic, lithospheric mantle. The very large range of inclusion compositions worldwide has been interpreted to represent four end-member fluids: saline (rich in Na + K + Cl); silicic (rich in Si + Al); and carbonatitic (rich in Ca + Mg + Fe, with low-Mg and high-Mg end members). However, the sources and evolution of these fluids and the processes involved in diamond formation are still unclear.We used an unusual study of diamonds from the Panda kimberlite (Ekati Mine, Northwest Territories, Canada) in which both mineral and fluid inclusions in the diamonds were analyzed (Tomlinson et al., 2006) to develop models of the saline, silicic, and low-Mg carbonatitic fluids present in the Panda fluid inclusions. The models used aqueous speciation and solubility calculations to link the solid and fluid inclusion chemistry with model upper mantle rock types. We used the extended Deep Earth Water model to calculate equilibrium constants previously calibrated with experimental rock solubilities referring to upper mantle temperatures and pressures (Huang and Sverjensky, 2019). Our results at 950 °C and 4.5 GPa suggest that the saline fluid could originate from peridotite, the silicic fluid from eclogite, and the low-Mg carbonatitic fluid from carbonated dunite.The fluid models were then used to predict the irreversible, chemical mass transfer when the carbonatitic fluid infiltrated a harzburgite containing a saline fluid. Simultaneous reduction of formate and bicarbonate in the carbonatitic fluid and oxidation of aqueous hydrocarbons from the peridotitic fluid during mixing and reaction with harzburgite resulted in the formation of diamond, olivine, garnet, and clinopyroxene, and increases in the logfO2 and pH. Olivine was predicted to become more Fe-rich and garnet more Ca and Fe-rich with reaction progress, in agreement with reported temporal trends (core-to-rim) in the Panda mineral inclusions. The fluid at the site of diamond formation became more saline with reaction progress and the predicted aqueous phase concentrations of all elements changed consistent with trends in Panda fluid inclusions. In contrast, a prediction for a saline fluid infiltrating a harzburgite containing a carbonatitic fluid resulted in trends of the silicate minerals and the salinity with reaction progress that were in the opposite direction to data from the Panda diamonds. Overall, our study strongly supports the notion that fluids from subducting slabs could mix and precipitate diamonds containing carbon from both oxidized and reduced sources, while adding Ca and Fe to the sub-lithospheric cratonic mantle through metasomatic reactions.

HighPGibbs, a Practical Tool for Fluid‐Rock Thermodynamic Simulation in Deep Earth and its Application on Calculating Nitrogen Speciation in Subduction Zone Fluids

AbstractThe HighPGibbs program is designed to calculate thermodynamic equilibrium of fluid‐rock systems up to depths of lithospheric mantle. It uses the Gibbs free energy minimization function of the HCh package to calculate mineral‐fluid equilibrium. Chemical potentials of minerals are calculated using the equations of states included in HCh; free energy of aqueous species is calculated using the Deep Earth Water model; and activity coefficients of charged species are estimated using the Davies variant of the Debye‐Hückel equation. HighPGibbs was applied to calculate nitrogen speciation in eclogite‐buffered fluids from 400 to 790 °C and 30 to 54 kbar, to evaluate the mobility of nitrogen in subducting oceanic crust. Regardless of whether the protolith was altered (and oxidized) or not, N2(aq) or NH3(aq) is the predominant form of nitrogen in the slab fluids at subarc temperatures, especially in cases of moderate or hot geotherms. Given that molecular nitrogen is highly incompatible in silicate minerals, the simulation indicates that nitrogen (as NH4+) in silicate minerals can be liberated during metamorphic devolatilization. The majority of nitrogen in subducting crusts can be unlocked during slab devolatilization and eventually expelled to the atmosphere via degassing of arc magmas. Therefore, oceanic crusts recycled to deep Earth will be depleted in nitrogen compared to the newly formed crust at spreading centers. As a result of the long‐term mantle convection, large proportions of the bulk silicate Earth may have suffered nitrogen extraction via subduction, and this may account for the nitrogen enrichment in the Earth's atmosphere.

CHNOSZ: Thermodynamic Calculations and Diagrams for Geochemistry

Thermodynamic calculations are an essential tool for many areas of geochemistry. Thermodynamics provides a framework for the quantitative description and prediction of the solubilities and relative stabilities of different minerals, metal transport in hydrothermal fluids, and geobiochemical reactions that drive microbial metabolism and contribute to the compositional variation of proteins. Accessible and up-to-date software and databases are important for the development and reproducible application of thermodynamic models. CHNOSZ is a free package for R that has been frequently updated since its first release in 2008. The package provides an integrated set of functions to calculate the standard molal thermodynamic properties and chemical affinities of reactions. It uses the graphical capabilities of R to produce high-quality chemical activity diagrams for aqueous species and predominance diagrams including Eh-pH (Pourbaix) and logfO2-T diagrams. The extensive database utilizes the well-known revised Helgeson-Kirkham-Flowers (HKF) equations for aqueous species. Recent additions to the database include the Berman equations for minerals, the Deep Earth Water model, which extends the applicability of the HKF equations to higher pressures, and the Akinfiev-Diamond model for aqueous nonelectrolytes. The package comes with many types of documentation, including technical help pages with short code examples, longer code demos, and in-depth vignettes combining code, text and graphics, giving users a wide array of starting points for their own research. This paper provides a concise overview of the package and illustrates the new features using examples selected from the literature. Although the package does not provide a complete chemical speciation model, numerous examples from the package demonstrate the ease of reproducing selected published calculations and sometimes identifying issues with existing datasets and models.

Extended Deep Earth Water Model for predicting major element mantle metasomatism

Abstract Fluids in the deep crust and upper mantle appear to have played roles in the long-term evolution of the subcratonic lithospheric mantle and the stabilities of the continents, in the geochemical cycles of the elements from subduction zones to Earth’s surface environment, and in the formation of diamonds. Much evidence of the chemistry of deep fluids has accumulated from studies of fluid inclusions in diamonds and xenoliths. But the origins of the fluids and their behavior are still unclear. In part, this is due to the lack of a comprehensive theoretical model of aqueous, high-pressure fluids. Traditional models have used a C-O-H-type of model, which contains no major rock-forming elements or aqueous ions or metal-complexes. In the present study, we use experimentally measured solubility data for multicomponent K-free eclogite, K-free peridotite and K-bearing peridotite rocks at upper mantle conditions from the literature to construct aqueous speciation solubility models that enabled calibration of the thermodynamic properties of ions and metal-complex species involving the elements Na, K, Mg, Ca, Fe, Al, Si, and C in an extended Deep Earth Water (DEW) model. New equilibrium constants were retrieved for the aqueous bisilicate anion, a silica trimer, silicate complexes of Ca, Fe, and Al, a silicate complex of Mg and bicarbonate, and formate complexes of Fe and Ca. The aqueous speciation and solubility model also took account of decreases in the activity of water and aqueous activity coefficients of neutral dissolved gases and included consideration of H 2 CO 3 0 . Based on the temperature and pressure dependences of the equilibrium constants, and supporting data covering a wide range of conditions, we then developed aqueous equation of state characterizations of the ions and metal-complex species. Overall, the results form a basis for modeling fluid-rock interactions under upper mantle conditions consistent with experimental solubility measurements.