Water-rock Ratios Research Articles

Garnet major and trace element evidence of the alteration and mineralizing processes associated with genesis of the Qiaomaishan skarn deposit, Xuancheng ore district, eastern China

The Qiaomaishan deposit is a skarn-type Cu–W deposit located within the Xuancheng ore district of the Middle–Lower Yangtze River Metallogenic Belt of eastern China. The mineralization within the deposit is predominantly located along the contact between a porphyritic granodiorite intrusion and surrounding Carboniferous limestone units. The deposit is associated with a well-developed and zoned skarn system that includes both endoskarn and exoskarn zones. This study presents new garnet major, trace, and rare earth element (REE) geochemical data for these skarn zones, discusses the factors (e.g., crystal chemistry, oxygen fugacity and water/rock interaction) that control the incorporation of trace elements into garnet, and constrains the formation of skarn within the Qiaomaishan deposit. Grossular (Adr21–70Grs29–78; type 1) garnet is generally located within the endoskarn and is light REE (LREE) enriched but heavy REE (HREE) depleted with weakly negative Eu anomalies and higher total REE contents than the exoskarn garnet in the study area. Type 1a grossular garnet is further split from type 1b grossular garnet by the presence of positive Pr and Nd anomalies and high LREE concentrations. Exoskarn garnet has pure andradite compositions and is split into type 2 and 3 garnets, where type 2 andradite garnet has variably positive Eu anomalies and variable total REE contents that are indicative of formation under transitional or oscillating conditions (e.g., pH, oxygen fugacity). In comparison, the type 3 andradite garnet has low total REE contents and positive Eu anomalies. Endoskarn garnet Al2O3 and total REE contents positively correlate, indicating that REE is incorporated into this garnet through coupled YAG-type substitution. The differences between the total REE contents of the type 2 exoskarn and endoskarn garnets may reflect host rock buffering as well as variations in the crystal chemistry of the two garnets present within the different parts of the skarn system (grossular and andradite). In comparison, the differences in the total REE contents of the two types of exoskarn garnet may reflect variations in water–rock ratios and crystal growth rates. The different Eu anomalies and U contents present within the different types of garnets record variations in the pH and oxygen fugacity of the hydrothermal fluid over time. These changes may have promoted the crystallization of magnetite and scheelite and the formation of the tungsten mineralization within the deposit. This study demonstrates the usefulness of garnet in tracking variations in mineralizing conditions and processes and confirms the validity of these minerals as vectors or indicator minerals that can be used in combination with traditional exploration approaches to determine whether a system is mineralized or not during the early stages of exploration.

Short lifespans of serpentinization in the rocky core of Enceladus: Implications for hydrogen production

The discovery of a liquid ocean on Saturn's small moon Enceladus and evidence of modern hydrothermal activity provide an unexpected new environment in which to expand the search for life. However, as with the age of the moons themselves, the age of the liquid ocean and any hydrothermal activity therein remains an area of debate. Based on physical and chemical observations from the Cassini mission we can apply known mineral dissolution rates, estimated water-rock ratios from Enceladus' density, and variable water flow rates within the rocky core to constrain durations of active serpentinization. On this basis we developed a 1-D reactive transport model to compare the effect of initial olivine percentage, grain size, temperature, and flow rate on timespans of primary olivine alteration in a rocky core the size and density of Enceladus'. In most cases, olivine alteration and precipitation of hydrous secondary minerals results in a water-limited alteration regime. An alteration front that propagates in the direction of water flow then controls the overall rate of olivine alteration. Of the parameters explored, high initial olivine percentages and slow fluid flow rates were the strongest predictors of long serpentinization times, while temperature and grain size had a smaller effect. The annual global H2 production rate in all model cases (> 1 × 1012 mol yr−1) is several orders of magnitude greater than the minimum H2 release rate calculated from the observed H2 in Enceladus' plume (1 × 109 mol yr−1), suggesting that any ongoing active serpentinization processes in the core are likely nearing completion. The longest timescales indicate the potential for olivine alteration and H2 production for up to ~75 Myr, consistent with weathering rates of terrestrial peridotite massifs. If the H2 produced from Enceladus is sourced from primary mineral alteration, these results suggest that hydrothermal activity in the core of Enceladus may have developed only very recently – even as recent as within the past 100 Myr.

Chemical Composition and Genesis Implication of Garnet from the Laoshankou Fe-Cu-Au Deposit, the Northern Margin of East Junggar, NW China

In order to reveal the formation mechanism of different garnets and its implications for the fluid evolution in the Laoshankou Fe-Cu-Au deposit in the northern margin of East Junggar (NW China), three types of garnet have been investigated in detail in this study. (1) Type 1 grossular, formed at Ca-silicate stage (stage I, the pre-mineralization stage), was replaced by Type 2 garnet and magnetite, and displays a compositional range of Grs44–53Adr44–53, which has relatively lower total REE (rare earth elements) contents (8.14–32.8 ppm) and markedly depleted LREE (light rare earth elements) with distinctive positive Eu anomaly (1.36–9.61). (2) Type 2 Al-rich andradite, formed at the early sub-stage of amphibole-epidote-magnetite stage (stage II, the main magnetite mineralization stage), can be divided into two sub-types, i.e., Type 2a and Type 2b. Type 2a garnets exhibit polysynthetic twinning and relatively narrow compositional variations of Adr63–66Grs31–34 with HREE-(heavy rare-earth elements) enrichment and positive Eu anomalies (3.22–3.69). Type 2b garnets own wide compositional variations of Adr55–77Grs21–43 with relatively higher REE contents (49.1–124 ppm), markedly depleted LREE and a distinctive positive Eu anomaly (2.11–4.61). (3) Type 3 andradite (Adr>91) associated with sulfide stage (stage III, the main copper-gold mineralization stage) is different from other types of garnets in Laoshankou, which are characterized by lowest total REE contents (1.66–91.1 ppm), flat HREE patterns, LREE-enrichment and the strongest positive Eu anomalies (3.31–45.48). Incorporation of REE into garnet is largely controlled by external factors, such as fluid chemistry, pH, ƒO2 and water-rock ratios as well as its crystal chemistry. Type 1 and 2 garnets mainly follow the creation of X2+ (e.g., Ca2+) site vacancy, e.g., [X2+]−3VIII[]+1VIII[REE3+]+2VIII. The REE3+ substitution mechanism for Type 3 garnet is the Na+-REE3+ coupled substitutions, e.g., [X2+]−2VIII[X+]+1VIII[REE3+]+1VIII, without the evaluation of the creation of site vacancy. The compositional variations from Type 1 to Type 3 garnet indicate significant differences of fluid compositions and physicochemical conditions, and can be used to trace the fluid–rock interaction and hydrothermal evolution of garnet. Type 1 grossular was formed by magmatic fluid under low water–rock ratios and ƒO2, and neutral pH environment by diffusion metasomatism in a nearly closed system with the preferential incorporation into the grossular of HREE. As the long fluid pore residence and continuing infiltration metasomatism under nearly closed-system conditions, fluids with high water/rock ratios were characterized by increased ƒO2, more active incorporation of Fe3+ and REE, and formed Type 2 Al-rich andradite. In contrast, Type 3 garnet formed by oxidizing magmatic fluid under a mildly acidic environment with highest ƒO2 and water–rock ratios, and was influenced by externally derived high salinity and Ca-rich fluids in an open system. Thus, the geochemical features of different types and generations of garnets in the Laoshankou deposit can provide important information of fluid evolution, revealing a transition from neutral magmatic fluid to oxidizing magmatic fluid with addition of external non-magmatic Ca-rich fluid from the Ca-silicate stage to the sulfide stage. The above proved the fluid evolution process further indicates that the Laoshankou deposit prefers to be an IOCG-like (iron oxide-copper-gold) deposit rather than a typical skarn deposit.

The role of low-temperature 18O exchange in the isotopic evolution of deep subsurface fluids

Ca-Na-Cl fluids with high concentrations of dissolved reduced gases reside within fractures in crystalline Precambrian rocks around the world, and have been most intensively studied within South Africa, Fennoscandia and the Canadian Shield. In contrast to surface waters, shallow groundwaters, sedimentary basin brines and metamorphic fluids, the δ18O and δ2H values for these Ca-Na-Cl fluids typically plot to the left/above of the Global Meteoric Water Line (GMWL). To date, most interpretive frameworks for these fracture fluids have focused on their production via water-rock alteration reactions that affect both δ18O and δ2H values, resulting in co-variation of water isotope values above the GMWL. Such alteration processes include silicate hydration coupled with formation of secondary minerals, radiolytic H2 formation, isotopic exchange with a H2-rich gas, or isotope exchange with O and/or H-bearing minerals. This study presents the first compiled global isotopic dataset for these types of fluids, integrating a large amount of unpublished data with the previously published literature in order to investigate these fracture fluid systems on a global scale. Importantly this global perspective allows differentiation between fluids impacted by late-stage mixing with meteoric waters, from fluids that reflect the most saline end-members stored in the host rocks in hydrogeologic isolation from the surface hydrologic cycle. The most saline fluids are shown to occupy a more restricted range of δ18O-δ2H space than previously recognised, with end-member fluids from all of the Precambrian rock settings investigated occupying a range of δ2H δ18O isotope space within which there is no co-variation in these values. These findings suggest a set of common processes may define the isotopic signatures of these most saline end-member fluids in Precambrian settings around the world – creating common signatures identifiable in these fluids, despite differences in geologic setting. This study identifies the important role of oxygen isotopic exchange between primary fluids (associated with hydrothermal/metamorphic activity) and the host rocks, taking place under low temperature, low-volume, water-rock ratios over long (Ma) geologic timescales. This process results in progressive 18O depletion in the fluids over time, while δ2H values remain less affected. For each site the specific isotopic signature of the fracture fluid end-member depends on initial hydrothermal/metamorphic fluid composition, rates of isotopic exchange, water-to-rock ratios, and in-situ residence times. We suggest the often-observed co-variation in both δ18O-δ2H above the GMWL primarily results from late stage mixing of the fracture fluid end-members with (paleo)-meteoric water, resulting in isotopic regression back towards the GMWL, with end-points defined by the local meteoric-climatic conditions for each site.

The lateral and vertical growth of laterite weathering profiles, Hawaiian Islands, USA

AbstractThe Hawaiian Islands permit investigation of tropical chemical weathering rates and processes on a single rock type, basalt. Chronosequences are investigated as a function of rainfall due to the varying age of each island, including Kauai (~4 Ma), Oahu (~2 Ma), and Hawaii's Kohala Peninsula (~0.3 to 0.17 Ma). Understanding tropical critical zone (CZ) development is vital given the large populations in developing countries that rely on it.HVSR (horizontal‐to‐vertical spectral ratio) seismic soundings on Kauai indicate that ~60% of the variability in laterite thickness is due to gradients in precipitation, with errors in erosion corrections and variability in the original permeability structure of the volcanic sequence playing important roles. Basalts have higher horizontal than vertical hydraulic conductivity (Kh > Kv), and local variability in likely drives much of the remaining differences in laterite thickness.HVSR is well suited for estimating laterite thickness as it has been shown to reliably detect the base of the weathering profile, is rapid (20 min/sounding), highly portable, and occupies a very small footprint.Comparison of Kauai and Oahu weathering profiles suggests that the Oahu laterites are fully or nearly fully formed, despite being half the age of Kauai. By contrast, the young laterites on Kohala (~170 to ~300 ka) exhibit greatly contrasting thicknesses, where coastal laterites are thick and interior laterites are thin, suggesting that early weathering on shield volcanoes produces wedge‐shaped laterites near the coast.With time, the thick (coastal) end of the wedge propagates upslope such that a fully developed, constant‐thickness laterite carapace can form in ~2 Ma or less. The development of thickened coastal laterites on young substrates depends on greater water–rock ratios as vertically infiltrating water upslope is diverted laterally. This view of laterite development is very different compared to endmember models of continental weathering and CZ development. © 2020 John Wiley & Sons, Ltd.

The Surtsey volcano geothermal system: An analogue for seawater-oceanic crust interaction with implications for the elemental budget of the oceanic crust

Surtsey is a young volcanic island in the offshore extension of Iceland's southeast rift zone that grew from the seafloor during explosive and effusive eruptions in 1963–1967. In 1979, a cored borehole (SE-1) was drilled to 181 m depth and in 2017 three cored boreholes (SE-2a, SE-2b and SE-3) were drilled to successively greater depths. The basaltic deposits host a low-temperature (40–141 °C) seawater-dominated geothermal system. Surtsey provides an ideal environment to study water-rock interaction processes in a young seawater geothermal system. Elemental concentrations (SiO2, B, Na, Ca, Mg, F, dissolved inorganic carbon, SO4, Cl) and isotope contents (δD, δ18O) in borehole fluids indicate that associated geothermal waters in submarine deposits originated from seawater modified by reactions with the surrounding basalt. These processes produce authigenic minerals in the basaltic lapilli tuff and a corresponding depletion of certain elements in the residual waters. Coupling of measured and modelled concentrations investigates the effect of temperature and associated abundance of authigenic minerals on chemical fluxes from and to the igneous oceanic crust during low-temperature alteration. The annual chemical fluxes calculated at 50–150 °C range from −0.01 to +0.1 × 1012 mol yr−1 for SiO2, +0.2 to +129 × 1012 mol yr−1 for Ca, −129 to −0.8 × 1012 mol yr−1 for Mg and −21 to +0.4 × 1012 mol yr−1 for SO4 where negative values indicate chemical fluxes from the ocean into the oceanic crust and positive values indicate fluxes from the oceanic crust to the oceans. These flux calculations reveal that water-rock interaction at varying water-rock ratios and temperatures produces authigenic minerals that serve as important sinks of seawater-derived SiO2, Mg and SO4. In contrast, water-rock interaction accompanied by dissolution of basaltic glass and primary crystal fragments provides a significant source of Ca. Such low-temperature alteration could effectively influence the elemental budget of the oceanic igneous crust and ocean waters. The modeling provides insights into water chemistries and chemical fluxes in low-temperature MOR recharge zones. Surtsey also provides a valuable young analogue for assessing the chemical evolution of fluid discharge over the life cycles of seamounts in ridge flank systems.

Specifying recharge zones and mechanisms of the transitional geothermal field through hydrogen and oxygen isotope analyses with consideration of water-rock interaction

Recharge mechanism and water–rock interaction (WRI) along the recharge flows in a transitional geothermal system that characterize the chemical and physical properties of vapor and liquid phases, has not yet been fully understood due to complexities of fluid origin and geologic structure. To clarify the fluid origin and WRI processes in the system, this study applied hydrogen and oxygen isotope, B, Cl, and rare alkali metals analyses by considering fractionation characteristics of 18O in rocks and liquid phases, and correction of δ2H and δ18O values in the steam composition. The investigation were done by using 20 samples collected from liquid-dominated, vapor-dominated, and two-phase wells in one of transitional reservoir fields in West Java, Indonesia. The isotopic fractionation factors calculated from a single-step steam separation clearly divided the samples into four zones: boiling parent fluid, vapor, condensate fluid, and diluted steam-heated fluid. The parent fluid that has initial Cl− concentration of 10,000 mg/kg and low water-rock ratio of W/R ≤ 0.2 compositions was found to be the most essential part of the recharge system. Recharge mechanism involves meteoric water from the elevation of 1200 m – 1300 m a.s.l. infiltrates deeply through NE-SW regional faults, and becomes the parent fluid in the reservoir. The residual fluid after boiling of the parent fluid remains in the liquid reservoir with W/R ≤ 0.2, while the vapor phase forms a parasitic steam cap above the liquid reservoir with 0.2 ≤ W/R ≤ 0.7.

Triple oxygen isotope systematics as a tracer of fluids in the crust: A study from modern geothermal systems of Iceland

Triple oxygen isotope systematics of ancient hydrothermally altered rocks has been previously used to constrain environmental conditions of the Precambrian. To validate those studies, we report high-precision triple oxygen isotope measurements (expressed as Δ′17O with reference slope 0.528) of quartz, epidote and well fluids from modern geothermal areas of Iceland, Krafla and Reykjanes, as well as measurements from the extinct 6 Ma Geitafell hydrothermal system. At these systems, basalts reacted with distinct fluid sources at temperatures ranging from 250 to 400°C. Resolvable difference between isotope compositions of surface waters and rocks enables novel insights into boiling, isotope exchange at variable water-rock ratios, and remelting of altered rock. Our measurements of δD, Δ′17O, and δ′18O in well fluids show that the reactions proceeded at water-rock ratios of 0.1 to 2, and reveal the addition of meteoric water in the Reykjanes system, and near-surface boiling and steam-liquid separation at Krafla. The δ′18O and Δ′17O values of fluids shift due to exchange with rocks at high temperature following the slope 0.51 in the triple isotope space. Near-surface boiling and steam-liquid separation cause shifts in δD and δ′18O values of the fluids. Their Δ′17O values do not change significantly owning to the shallow slope of liquid-vapor equilibrium fractionation relative to the reference line slope. Epidote δ′18O and Δ′17O values in all three localities closely resemble the isotope composition of local fluid sources. The measured slope of triple oxygen isotope fractionation between quartz and epidote at 250–400°C is 0.526±0.001. We suggest that triple oxygen isotope composition of epidote can be used as a direct first-order proxy for the equilibrium fluid δ′18O and Δ′17O values. The calibrated quartz-water equilibrium fractionation for triple oxygen isotopes yields general agreement with the local fluid sources, within ±1.5‰ of their δ18O values, while the Δ′17O agree within ±0.02‰. We present in situ δ18O measurements in a quartz crystal from Krafla that show several ‰ heterogeneities which may affect the reconstructed equilibrium fluid values. We tested the effect of shallow crustal contamination on the Δ′17O values of rhyolitic glasses from Krafla, including those quenched and extracted by drilling, that likely formed by assimilation of low-δ18O hydrothermally altered crust. Our Δ′17O measurements constrain the degree of crustal assimilation to 10–20 %. Our study shows that the Δ′17O values measured in geothermal fluids, secondary minerals and low δ18O contaminated magmas can provide key information on the conditions of water-rock reaction and magma genesis, and contain additional details that were not accessible through conventional analyses of δD and δ18O.

Reconstructing carbonate alteration histories in orogenic sedimentary basins: Xigaze forearc, southern Tibet

Abstract Carbonate clumped isotope thermometry (T( Δ 47 )) and the oxygen stable isotope record ( δ 18 O ) provide critical constraints on the temperature of carbonate mineral formation and isotopic composition of ancient waters used in reconstructions of past climate, tectonics, and ecological and environmental change. The robust use of these proxies requires that carbonate minerals in the rock record retain primary isotopic compositions through often complex post-depositional thermal histories. New carbonate clumped isotope thermometry and sedimentology data are paired with existing thermochronology and isotope exchange reaction modeling to provide unique constraints on the alteration history of the carbon and oxygen isotopes in Tethyan marine carbonates (Jialazi Fm, Tso Jianding Group) of the Xigaze forearc, southern Tibet. Within the framework of the textural and isotopic data, the effects of early and late burial alteration on Δ 47 and δ 18 O values of marine rocks that were buried to 4 km above sea level are modeled. The alteration products are considered within two well-studied mechanistic frameworks: water-rock recrystallization and solid-state reordering of 13 C 18 O bonds at high temperature. The proposed alteration model is as follows: (1) shallow, early diagenesis with water of similar composition and temperature to formation waters infills primary porespace with microspar; (2) high-temperature, rock-buffered water-rock exchange and partial solid-state reordering at >3–4 km depth; and (3) near-surface, low-temperature water-rock exchange on the exhumation pathway. The Δ 47 heterogeneity within individual samples (0.456 to 0.721) requires significant differential alteration on the retrograde path (high to low temperature, low water-rock ratio) in which primary compositions and compositions inherited during early burial diagenesis are overprinted with modern meteoric values. A low T( Δ 47 ) value is often assumed to record the primary temperature of the depositional environment. However, samples yielding low T( Δ 47 ) values in the Jialazi Fm have undergone extensive high-temperature water-rock alteration, and therefore, require a mechanism for increasing 13 C 18 O bond ordering on the retrograde pathway while preserving many primary biogenic structures and micritic textures. This study highlights the potential significance of late-stage, low-temperature alteration of carbonates derived from orogenic sedimentary basins for studies of terrestrial paleoenvironments and tectonics. This study also provides an example framework for combining observational and analytical data to reconstruct a carbonate alteration history of a buried sedimentary package.

Triple oxygen and hydrogen isotopic study of hydrothermally altered rocks from the 2.43–2.41 Ga Vetreny belt, Russia: An insight into the early Paleoproterozoic seawater

Abstract The early Paleoproterozoic represents a period of rapid changes in Earth systems that could have affected the stable isotopic composition of seawater. The well-preserved pillow structures, hyaloclastites and komatiitic basalts of the 2.43–2.41 Ga Vetreny belt, Baltic Shield provide a record of high-temperature water-rock interaction induced by contemporaneous seawater. Here we present results of mineralogical, fluid inclusion, hydrogen, and triple oxygen isotopic analysis of hydrothermal alteration products. Emphasis is given to vein-filling quartz and epidote as they likely formed at high water-rock ratios. Ten minerals pairs, quartz-epidote and quartz-calcite, returned temperatures of isotopic equilibrium between 286 and 387 °C, which compares well to the homogenization temperatures measured for saline fluid inclusions hosted in vein quartz. The computed δD and δ18O values of equilibrium fluids range between −31 and +12‰, and −1.36 and +3.20‰, respectively, which overlap with the isotopic composition of ice-free world seawater and fluids generated at submarine hydrothermal systems. This is the earliest piece of evidence suggesting that early Paleoproterozoic seawater had a δD value close to 0‰. We also present triple oxygen isotopic composition of quartz and epidote that formed in similar facies of hydrothermal alteration from the relatively young (6–7 Ma) oceanic crust as sampled by the ODP Hole 504B in the eastern Pacific Ocean. These data show similarity to the triple oxygen isotope analyses of the Vetreny belt rocks indicating that the 2.43–2.41 Ga seawater had the Δ17O value close to that of modern-day seawater. Due to small fractionation at 300–390 °C (αepidote-water ≈ 1), epidotes present a strong evidence that equilibrium fluids had Δ′17O values close to 0‰. Based on the previously published quartz-water calibration, the computed Δ17O values of equilibrium fluids range between −0.11 and −0.03‰, significantly lower than that of seawater or inferred seawater-derived fluids at low water-rock ratios. This can be explained by multiple factors including phase separation of fluids or/and presence of low-temperature quartz overgrowths. Both are reflected in the fluid inclusion data and in situ δ18O measurements by ion microprobe (SIMS) presented here. Overall, our study suggests that the δ18O, Δ17O and δD values of the 2.43–2.41 Ga seawater were −1.7 ± 1.1, −0.001 ± 0.011 and 0 ± 20‰ respectively, similar to the modern values, which reflects the dominant role of submarine hydrothermal alteration in the stable isotopic budget of seawater throughout Earth’s history.

Post-mining groundwater geochemistry evolution in on the Levikha sulfide deposits of the Middle Urals (Russia)

Mining of volcanogenic copper pyrite deposits of the Middle Urals leads to the rock disintegration, their grinding, increasing the interaction surface. Sulfide minerals are oxidized by interaction with oxygen-enriched meteoric water and secondary minerals are formed (crystallo-hydrates of sulphates, hydroxides). Here technogenic zone hypergene (sulfuric acid weathering crust) is being formed. Once the water pumping activities are finished, the piezometric level recovers its position decreasing the unsaturated zone created during the mining activity, this increases the water-rock ratio increasing the amount of suppliers of sulphates, iron, non-ferrous metals and other elements to groundwater. It defines a low rate of rehabilitation of mining areas after the completion of mining and cessation of dewatering. The regularities of acidic water formation after filling the depression cone are investigated on the abandoned copper mines of the Middle Urals (Russia). The longevity of formation of extremely acidic and polluted waters is determined by the degree of saturation of the supergene zone with secondary minerals and the intensity of water exchange. Seasonal decrease in the component’s concentrations t in the period of winter and summer low water was recorded. The unstable character of groundwater components changes with a half-life of more than 5 thousand days is established. The duration of rehabilitation and restoration of groundwater quality will be at least a hundred years.

Low temperature stable mineral recrystallization of foraminiferal tests and implications for the fidelity of geochemical proxies

Fossilized foraminiferal tests are widely used as proxies to constrain paleoenvironments, including past sea surface and bottom water temperatures and the chemical composition of ancient seawater. However, relatively little is known about the extent to which calcareous foraminiferal tests exchange with seawater prior to burial. In this study, we used a 45Ca radiotracer approach to quantify the extent and rate of foraminiferal calcite–fluid exchange over a three-month period. Modern foraminiferal tests exhibited faster exchange with a 45Ca-spiked solution (2.6⋅10−4 to 2.0⋅10−5 mol/m2/d) than fossil foraminiferal tests (7.6⋅10−5 to 5.3⋅10−6 mol/m2/d) from ODP Site 807. The impact of dissolved Si on the exchange rate of modern foraminifera was minimal (2.6⋅10−4 to 9.2⋅10−6 mol/m2/d). Unlike foraminiferal calcite, inorganic calcite exchanged at remarkably slow rates (6.6⋅10−6 to 5.9⋅10−7 mol/m2/d) suggesting that biogenic calcite is more reactive than inorganic calcite. Increased Ca2+, Mg2+, and Sr2+ concentrations in fluids from parallel tracer-free reactors indicated preferential dissolution of tests, though there was no overt change in test morphology during experiments (as imaged by scanning electron microscopy and micro-CT scanning). Time-dependent box model simulations of 45Ca, as well as aqueous elemental chemistry, support the hypothesis that calcite–fluid exchange occurred via dissolution–reprecipitation of foraminiferal tests that was induced by intra-test chemical heterogeneity. The short-term exchange reactions investigated in this study are comparable to the reactions that can occur in the seawater column and at the sediment–seawater interface. This work suggests that dissolution–reprecipitation in such settings has the potential to overprint the chemistry of foraminiferal tests without overt physical alteration, which can lead to a potential bias in paleoenvironmental reconstructions. Model-based estimates suggest a maximum of 2–3°C alteration of Mg/Ca-based temperatures over experimental time scales. Model efforts also suggest that the diagenetic trajectories in Mg/Ca–δ18O space, for instance, are impacted by the partition coefficient of Mg in calcite (λMg) as well as by the mechanism of alteration. This is demonstrated for (i) the ratio of exchange to dissolution and (ii) the relative rate of high-Mg-calcite to low-Mg-calcite reaction. We have illustrated that the mechanism of alteration, in addition to water-rock ratios, determine diagenetic trajectories. Finally, surface reaction has implications for our ability to constrain fractionation factors in experimental and natural systems. The experimental observations suggest that exchange between tests and solution is nominal over ∼100 days. Experiments aimed at deducing fractionation factors rely on the equilibration of bulk solid and fluid, since these are the readily analyzed reservoirs. We quantify the fraction of carbonate (>5%) that must react so that surface reaction does not impact the interpretation of Ca isotopic fractionation factors, and discuss the implications for more “fluid-dominated” elements such as Mg and O.

In-situ oxygen isotope analyses in serpentine minerals: Constraints on serpentinization during tectonic exhumation at slow- and ultraslow-spreading ridges

Peridotites exhumed along detachment faults at slow- and ultraslow-spreading ridges undergo variable and complex serpentinization. Here, we present in-situ oxygen isotope analyses in serpentine minerals from a textural sequence identified in samples drilled at the Atlantis Massif (Mid-Atlantic ridge, MAR, 30°N) and dredged along the easternmost Southwest Indian ridge (SWIR, 62–65°E). The textural sequence is similar at both locations and involves: mesh texture and bastite formation after olivine and orthopyroxene, mesh texture recrystallization into chrysotile- and antigorite-dominated textures, and banded and fibrous veins. The δ18O in serpentine decreases with the textural sequence, which we interpret to record an increase in the time-integrated water-rock ratio under a nearly constant temperature. While mesh texture development starts at stoichiometric water-rock ratios and creates an isotopic variability at a scale of ~100 μm, recrystallized textures indicate serpentinization temperatures on the order of 260–290 °C (seawater-dominated fluid hypothesis) or 320–360 °C (hydrothermally altered seawater hypothesis). The subsequent banded veins record the evolution towards colder and seawater-dominated fluids potentially present in shallower levels of the detachment footwall. Serpentinization leading to the mesh textures appears faster and more heterogeneous in Atlantis Massif samples than in SWIR samples. We propose that such features are influenced by the amount of gabbros intruded in the peridotite. In addition to influencing fluid pathways, gabbros also provide a source for observed Si-enrichments in some Atlantis Massif samples and control the pH, which in turn influences the mobility of Al.

Rapid desorption of radium isotopes from black shale during hydraulic fracturing. 2. A model reconciling radium extraction with Marcellus wastewater production

Radium in hydraulic fracturing wastewaters derives from two isotopically distinct end-members in the shale, labile 228Ra hosted by mineral surfaces (226Ra/228Ra atom ratio ~250) and exchangeable 226Ra hosted by organic surfaces (226Ra/228Ra ~10,000). Here we use mass balance and isotope mixing models to reconcile extraction of Ra from these phases with mechanisms of Marcellus wastewater production. Radium isotopic mass balance requires that the characteristic water-rock ratio between wastewater and shale is exceedingly low, on the order of 0.04, and that this ratio decreases with time during wastewater production. An evolving water-rock interaction drives increasing Ra concentrations (=[Ra]) and 226Ra/228Ra ratios during wastewater production, all mediated by increasing [Ca2+] that favors desorption of 226Ra from organics. Our observations and models of Ra isotope geochemistry are best reconciled with observations of water and salinity mass balance, δ18O, Na-Br-Cl, and 87Sr/86Sr if wastewater is produced by mixing of injected fluids with a limited volume of pore brine (on the order of 13% by volume), accompanied by contemporaneous extraction of excess alkaline earth elements by water-rock exchange. Validated using Ra isotope data, this model attributes the extreme salinity and [Ra] in wastewaters to the progressive, hydrologic enrichment of injected fluids during hydraulic fracturing.

Factors influencing the sorption and migration behavior of uranium in shale

The sorption of U(VI) on shale has been experimentally investigated as a function of pH, initial uranium concentration, rock particle size and water–rock ratio. The results showed the sorption equilibrium time was about 10 days, and the adsorption capacity presented a negative relationship with rock particle size and positive correlation with the liquid–solid ratio, also influenced by pH. The sorption process was fitted well with Freundlich isotherm. Experimental column was fully penetrated by tritium at about 230 min with a velocity of 0.13 cm/min while uranium was not penetrated the rock column in 180 days.

Boiling vapour-type fluids from the Nifonea vent field (New Hebrides Back-Arc, Vanuatu, SW Pacific): Geochemistry of an early-stage, post-eruptive hydrothermal system

In 2013, high-temperature vent fluids were sampled in the Nifonea vent field. This field is located within the caldera of a large shield-type volcano of the Vate Trough, a young extensional rift in the New Hebrides back-arc. Hydrothermal venting occurs as clear and black smoker fluids with temperatures up to 368°C, the hottest temperatures measured so far in the western Pacific.The physico-chemical conditions place the fluids within the two-phase field of NaCl–H2O, and venting is dominated by vapour phase fluids with Cl concentrations as low as 25mM. The fluid composition, which differs between the individual vent sites, is interpreted to reflect the specific geochemical fluid signature of a hydrothermal system in its initial, post-eruptive stage. The strong Cl depletion is accompanied by low alkali/Cl ratios compared to more evolved hydrothermal systems, and very high Fe/Cl ratios. The concentrations of REY (180nM) and As (21μM) in the most Cl-depleted fluid are among the highest reported so far for submarine hydrothermal fluids, whereas the inter-element REY fractionation is only minor.The fluid signature, which has been described here for the first time in a back-arc setting, is controlled by fast fluid passage through basaltic volcanic rocks, with extremely high water-rock ratios and only limited water-rock exchange, phase separation and segregation, and (at least) two-component fluid mixing. Metals and metalloids are unexpectedly mobile in the vapour phase fluids, and the strong enrichments of Fe, REY, and As highlight the metal transport capacity of low-salinity, low-density vapours at the specific physico-chemical conditions at Nifonea. One possible scenario is that the fluids boiled before the separated vapour phase continued to react with fresh glassy lavas. The mobilization of metals is likely to occur by leaching from fresh glass and grain boundaries and is supported by the high water/rock ratios. The enrichment of B and As is further controlled by their high volatility, whereas the strong enrichment of REY is also a consequence of the elevated concentrations in the host rocks. However, a direct contribution of metals such as As from magmatic degassing cannot be ruled out. The different fluid end-member composition of individual vent sites could be explained by mixing of vapour phase fluids with another fluid phase of different water/rock interaction history.

The calculation of water-rock ratios using trace element (Li, B) stable isotopes

The amount of aqueous fluids circulating into the oceanic crust can be estimated using mass balance equations based on stable isotope exchange between rock and water. Unlike oxygen and strontium, isotopic exchange of trace elements (such as B or Li) between fluids and rocks, operates along with a chemical evolution of the rocks (e.g. a large enrichment of B or Li) that must be integrated into any model of water-rock interaction. We propose a general dimensionless mass balance equation for single-pass open systems that describes the equilibrium elemental distribution and the isotopic composition of reacting rocks as a function of the amount of circulating water. Water-rock ratios calculated from B compositions of hydrothermally-altered basalts range from 8 to 100. They are lower than those previously published (most W/R > 300) but comparable to those inferred from Sr isotope ratios measured in the same samples (3 < W/R < 30). Similar low water-rock ratios from 2 to 20 are calculated from Li isotope compositions of altered basalts and serpentinized peridotites.

A two-stage fluid history for the Orocopia Schist and associated rocks related to flat subduction and exhumation, southeastern California

ABSTRACTStable isotopes combined with pre-existing 40Ar/39Ar thermochronology at the Gavilan Hills and Orocopia Mountains in southeastern California record two stages of fluid–rock interaction: (1) Stage 1 is related to prograde metamorphism as Orocopia Schist was accreted to the base of the crust during late Cretaceous–early Cenozoic Laramide flat subduction. (2) Stage 2 affected the Orocopia Schist and is related to middle Cenozoic exhumation along detachment faults. There is no local evidence that schist-derived fluids infiltrated structurally overlying continental rocks. Mineral δ18O values from Orocopia Schist in the lower plate of the Chocolate Mountains fault and Gatuna normal fault in the Gavilan Hills are in equilibrium at 490–580°C with metamorphic water (δ18O = 7–11‰). Phengite and biotite δD values from the Orocopia Schist and upper plate suggest metamorphic fluids (δD ~ –40‰). In contrast, final exhumation of the schist along the Orocopia Mountains detachment fault (OMDF) in the Orocopia Mountains was associated with alteration of prograde biotite and amphibole to chlorite (T ~ 350–400°C) and the influx of meteoric-hydrothermal fluids at 24–20 Ma. Phengites from a thin mylonite zone at the top of the Orocopia Schist and alteration chlorites have the lowest fluid δD values, suggesting that these faults were an enhanced zone of meteoric fluid (δD < –70‰) circulation. Variable δD values in Orocopia Schist from structurally lower chlorite and biotite zones indicate a lesser degree of interaction with meteoric-hydrothermal fluids. High fluid δ18O values (6–12‰) indicate low water–rock ratios for the OMDF. A steep thermal gradient developed across the OMDF at the onset of middle Cenozoic slip likely drove a more vigorous hydrothermal system within the Orocopia Mountains relative to the equivalent age Gatuna fault in the Gavilan Hills.

High temperature glacial meltwater–rock reaction in the Neoproterozoic: Evidence from zircon in-situ oxygen isotopes in granitic gneiss from the Sulu orogen

To seek the occurrence of negative δ18O magmas in the Neoproterozoic, we conducted in-situ zircon O isotope analysis and U–Pb dating for granitic gneiss from the northeastern Sulu orogen, east-central China. Zircon U–Pb dating yields protolith ages of 753±15Ma to 780±13Ma and metamorphic ages of 209±3 to 244±7Ma. The Neoproterozoic cores with concordant U–Pb ages exhibit a wide δ18O range from −11.0‰ to 5.8‰, which is nearly the same as those for cores with discordant U–Pb ages. The Triassic rims of some samples have homogeneous δ18O values of around −10‰ whereas the rims of the other samples show a wider range from −9.8‰ to 5.0‰. The δ18O values as negative as −11.0‰ for zircons with concordant Neoproterozoic U–Pb ages are reported for the first time, representing the primary record of negative δ18O magma in the Neoproterozoic. The continental subduction-zone metamorphism in the Triassic did not erase the abnormal δ18O record in the protolith zircon cores despite metamorphic dehydration and partial melting under high-pressure to ultrahigh-pressure conditions. A conservative estimate suggests that the hydrothermal fluid reacted with the rocks should have δ18O values lower than −9.2‰, corresponding to the meteoric water in cold paleoclimate or the meltwater of local continental glaciation. The spatial variation in the O isotope compositions of Neoproterozoic zircons is a manifestation of the O isotope heterogeneity in the extinct hydrothermal-magmatic system. The hydrothermal alteration during the Neoproterozoic was incongruent, which was lately recorded by the wide range of δ18O values in the metamorphic zircons of Triassic age. The extensive O isotope exchange between the surface water and the deep rock requires high temperature and high water–rock ratios in continental rifting zones. This is ascribed to Neoproterozoic splitting of the South China Block from the Rodinia supercontinent.

Geochemical characteristics of the Arabshah kaolin deposit, Takab geothermal field, NW Iran

The Arabshah kaolin deposit (Takab geothermal field, NW Iran) is the product of alteration of Miocene dioritic rocks. According to mineralogical data, the rock-forming minerals in this deposit include kaolinite, quartz, muscovite-illite, pyrophyllite, accompanied by lesser amounts of rutile, chlorite, anatase, albite, gypsum, nontronite, and pyrite. Consideration of elemental ratios and geochemical indices such as TiO2, Nb + Cr, Ti + Fe, Sr + Ba, and La + Ce + Y demonstrated that both hypogene and supergene processes played a significant role in the development of this deposit. The mass change calculations revealed that elements like Zr, Ga, Hf, REEs, and Th which are normally immobile in ordinary alteration processes had both incremental and decremental trends during the development of this deposit. The Eu and Ce anomaly values (normalized to chondrite) in kaolinized samples vary within the range of 0.65–1.13 and 0.91–1.05, respectively. It seems that the variation of negative Eu anomaly values was controlled by kaolinization of feldspars by hypogene solutions and by scavenging of this element by Fe oxides and hydroxides (formed during oxidation of hypogene pyrite by supergene solutions). Variation of Ce anomalies also unravels the effective role of reducing hypogene fluids and to some extent of supergene solutions during kaolinization. Combination of the results obtained from mineralization considerations, mass change calculations of elements, and correlation coefficients illustrate that distribution and concentration of major, minor, and rare earth elements during kaolinization at Arabshah were affected by the function of factors such as changes in physico-chemical conditions of altering solutions (e.g., Eh and pH), adsorption, accessibility to complex-forming ligands, water-rock ratios, existing in resistant (to alteration) mineral phases, and scavenging by Fe and Mn oxides.