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The Cedars ultramafic block hosts alkaline springs (pH > 11) in which calcium carbonate forms upon uptake of atmospheric CO2 and at times via mixing with surface water. These processes lead to distinct carbonate morphologies with “floes” forming at the atmosphere-water interface, “snow” of fine particles accumulating at the bottom of pools and terraced constructions of travertine. Floe material is mainly composed of aragonite needles despite CaCO3 precipitation occurring in waters with low Mg/Ca ( The calcium carbonates exhibit an extreme range and approximately 1:1 covariation in δ13C (−9 to −28‰ VPDB) and δ18O (0 to −20‰ VPDB) that is characteristic of travertine formed in high pH waters. The large isotopic fractionations have previously been attributed to kinetic isotope effects accompanying CO2 hydroxylation but the controls on the δ13C-δ18O endmembers and slope have not been fully resolved, limiting the use of travertine as a paleoenvironmental archive. The limited areal extent of the springs (∼0.5 km2) and the limited range of water sources and temperatures, combined with our sampling strategy, allow us to place tight constraints on the processes involved in generating the systematic C and O isotope variations. We develop an isotopic reaction–diffusion model and an isotopic box model for a CO2-fed solution that tracks the isotopic composition of each dissolved inorganic carbon (DIC) species and CaCO3. The box model includes four sources or sinks of DIC (atmospheric CO2, high pH spring water, fresh creek water, and CaCO3 precipitation). Model parameters are informed by new floe Δ44Ca data (−0.75 ± 0.07‰), direct mineral growth rate measurements (4.8 to 8 × 10−7 mol/m2/s) and by previously published elemental and isotopic data of local water and DIC sources. Model results suggest two processes control the extremes of the array: (1) the isotopically light end member is controlled by the isotopic composition of atmospheric CO2 and the kinetic isotope fractionation factor (KFF (‰) = (α − 1) × 1000) accompanying CO2 hydroxylation, estimated here to be −17.1 ± 0.8‰ (vs. CO2(aq)) for carbon and −7.1 ± 1.1‰ (vs. ‘CO2(aq) + H2O’) for oxygen at 17.4 ± 1.0 °C. Combining our results with revised CO2 hydroxylation KFF values based on previous work suggests consistent KFF values of −17.0 ± 0.3‰ (vs. CO2(aq)) for carbon and −6.8 ± 0.8‰ for oxygen (vs. ‘CO2(aq) + H2O’) over the 17–28 °C temperature range. (2) The isotopically heavy endmember of calcium carbonates at The Cedars reflects the composition of isotopically equilibrated DIC from creek or surface water (mostly HC O 3 - , pH = 7.8–8.7) that occasionally mixes with the high-pH spring water. The bulk carbonate δ13C and δ18O values of modern and ancient travertines therefore reflect the proportion of calcium carbonate formed by processes (1) and (2), with process (2) dominating the carbonate precipitation budget at The Cedars. These results show that recent advances in understanding kinetic isotope effects allow us to model complicated but common natural processes, and suggest ancient travertine may be used to retrieve past meteoric water δ18O and atmospheric δ13C values. There is evidence that older travertine at The Cedars recorded atmospheric δ13C that predates large-scale combustion of fossil fuels.

The solution stoichiometry dependence of calcium isotope fractionation during calcite precipitation was investigated as a direct test of the conceptual model of calcium isotope discrimination driven by Ca exchange at surface sites during growth. Classical ion-by-ion models of calcite growth predict a strong solution stoichiometry influence on Δ44/40Cacalcite-fluid: In low Ca2+:CO32– solutions, Δ44/40Ca is predicted to approach a kinetic limit (∼−2 to −4‰), while in high Ca2+:CO32– solutions, exchange at dominantly Ca-occupied kink sites drives Δ44/40Ca towards the equilibrium fractionation (near 0‰). To test this prediction, a series of seeded and unseeded constant composition calcite growth experiments were performed in which all aspects of solution chemistry were held constant and the Ca2+:CO32– activity ratio was varied. Experiments were performed at pH 8.5, ionic strength 0.1 M (adjusted with KCl), and calcite saturation index (SI = log10({Ca2+}{CO32–}/Ksp)) of either 0.5 or 0.8. Calcium isotope fractionation is found to be weakly stoichiometry dependent. The expected trend of larger magnitude fractionations at lower Ca2+:CO32– is observed, but the magnitude of change in Δ44/40Ca over the solution stoichiometries studied (Ca2+:CO32– = 1–250) is only ∼ 0.4‰. Similar trends in Δ44/40Ca with Ca2+:CO32– are observed at SI = 0.5 and 0.8, with smaller magnitude fractionations at lower supersaturation. This yields an inverse correlation between Δ44/40Ca and growth rate, confirming the Δ44/40Ca-rate relationship for inorganic calcite growth observed by Tang et al. (2008) . The ion-by-ion model framework captures measured Δ44/40Ca only when a surface complexation model is incorporated, highlighting the role of surface speciation in dictating Ca attachment/detachment dynamics. The model captures observed trends with Ca2+:CO32– using best-fit kinetic and equilibrium fractionations consistent with end-members observed in natural systems (αkinetic ∼ 0.9978, αeq ∼ 0.9998). This result implies a total possible range in Δ44/40Ca of 2‰ and suggests that for most carbonate precipitating environments, solution supersaturation will be a stronger determinant of Δ44/40Ca than stoichiometry. The demonstrated importance of surface speciation, however, implies a strong pH influence on Δ44/40Ca, independent of its influence on carbonate ion activity, that requires further investigation. The results of this study provide strong evidence supporting the model of kink-exchange driven Ca isotope fractionation and suggest that calcite grows by a dominantly classical mechanism over the solution conditions investigated. Model predictions regarding the relationship between Δ44/40Ca and growth inhibition in the presence of impurity ions lay the foundation for the use of Ca isotopes as molecular tracers of carbonate crystal growth pathways.

The equilibrium partition coefficient of strontium ( K Sr eq ) between aqueous solutions and calcite is still poorly known, even though it is a valuable parameter for studies involving the use of calcite trace element geochemistry for reconstructing paleoenvironments and fluid chemistry. In this paper we use pore fluid data from deep sea carbonate sediments to constrain K Sr eq at low temperature (5–17 °C) and show that the derived values are consistent with laboratory calcite precipitation experiments at 25 °C when the latter are corrected for kinetic effects. Using these low-temperature values, and experimental data available at higher temperature based on replacement reactions, we show that all of the data can be accounted for by a single formulation based on the thermodynamics of the relevant components of seawater-like fluids and the SrCO3–CaCO3 solid solution. The value of pore fluid data is that the fluids and carbonate sediment have been in contact for millions of years so local equilibrium is approached, the overall system can be treated with one-dimensional models, and the temperature is constrained. These natural systems provide an opportunity to investigate low-temperature equilibrium in the carbonate system that is difficult to probe in the laboratory because of sluggish exchange kinetics. We estimate values of K Sr eq using measured Sr and Ca concentrations in pore fluid and sediment solid calcite and a numerical model of sediment deposition, reaction and transport. The model is used to fit the Sr, Ca, and sulfate concentrations observed in pore fluids of several calcite-dominated sites that we believe are optimal for understanding Sr partitioning. Combining the pore fluid results with experimental measurements at higher temperature results in the following expression which applies for the temperature range 0–200 °C: K S r eq T = 0.025 exp Δ G r , 0 R 1 298.15 - 1 T where ΔGr,0 is the free energy change associated with the exchange reaction and temperature is in Kelvin. The uncertainty is approximately ±20%. Recently summarized thermodynamic data yield ΔGr,0 = 1.2 kcal/mol (5.0 kJ/mol) which fits well the lower limit of the high temperature data. The corresponding activity coefficient for the SrCO3 component in the calcite crystal structure is 5.4 for the lower value of ΔGr,0, and 3.17 for the higher value. The derived low-temperature values of K Sr eq of 0.020 to 0.025 (for 0 °C to 25 °C) have implications for models of marine carbonate diagenesis, and the interpretation of vein carbonate Sr/Ca in oceanic crust. Published data showing much higher KSr values in Mg-bearing solutions are not representative of equilibrium values for either Mg or Sr.

We present results from 25 hydrothermal quartz growth experiments, all conducted at 800 °C and 1 kbar but with varying starting materials and run times, to address discrepancies between calibrations of the titanium-in-quartz (TitaniQ) thermobarometer. In our experiments, a gold capsule is loaded with silica glass, water, and either rutile or anatase as the TiO2 source. In most experiments, there is also a large quartz seed crystal contained in an open inner capsule. The use of rutile versus anatase has a significant influence on the (re)crystallization pathways of the SiO2 and TiO2 components. When rutile is used, quartz overgrowths have abundant open cavities and complex zonations. The rutile does not completely dissolve because rutile is the stable TiO2 polymorph, and yet, new rutile forms at the quartz seed-overgrowth interface and on the outer surface of quartz crystals. This suggests crystallization of quartz near Ωrut ∼ 1, but wide-ranging Ti concentrations and zonations in quartz are indicative of kinetic effects. When powdered anatase is used, the quartz overgrowths look markedly different, lacking the open cavities and instead exhibiting step edges and terraces. The Ti concentrations in quartz from these experiments are also wide-ranging but reach larger values. Our results span the range of previous calibrations and indicate that Ti concentrations in quartz are sensitive to the TiO2/SiO2 ratio of the fluid as opposed to the absolute concentration (or activity) of dissolved TiO2. We present a kinetic model for quartz and rutile growth from a fluid where the input parameters are the initial degrees of supersaturation with respect to quartz and rutile, the total reactive surface area, and rate constants that link the degree of supersaturation to net precipitation rates. The model can explain many of the salient features of our experimental results, as well as those from previous studies, but requires that the rate constant multiplied by the reactive surface area for rutile is less than that of quartz, and that rutile solubility depends on the SiO2 concentration of the fluid, as documented in the recent literature. Complete quartz-rutile equilibrium may not have been established in any of the experimental studies, but low-pressure experiments with slowly grown quartz seem to be more reliable than extrapolations from high-pressure experiments for thermobarometry of shallow natural systems.

In order to reconstruct tectonic evolution history of the southern margin of Asia (i.e., Lhasa terrane) before the India-Asia collision, here we present a comprehensive study on the clastic rocks in the southern Lhasa terrane with new perspectives from sedimentary geochemistry, detrital zircon geochronology and Hf isotope. Clasts from the Jurassic-Early Cretaceous sedimentary sequences (i.e., Yeba and Chumulong Formations) display high compositional maturity and experienced moderate to high degree of chemical weathering, whereas those from the late Early-Late Cretaceous sequences (Ngamring and Shexing Formations) are characterized by low compositional maturity with insignificant chemical weathering. Our results lead to a coherent scenario for the evolution history of the Lhasa terrane. During the Early-Middle Jurassic (∼192-168Ma), the Lhasa terrane was speculated to be an isolated geological block. The Yeba Formation is best understood as being deposited in a back-arc basin induced by northward subduction of the Neo-Tethys ocean with sediments coming from the interiors of the Lhasa terrane. The Middle Jurassic-Early Cretaceous Lhasa-Qiangtang collision resulted in the formation of a composite foreland basin with southward-flowing rivers carrying clastic materials from the uplifted northern Lhasa and/or Qiangtang terranes. During the late Early-Late Cretaceous (∼104-72Ma), the Gangdese magmatic arc was uplifted rapidly above the sea level, forming turbidites (Ngamring Formation) in the Xigaze forearc basin and fluvial red beds (Shexing Formation) on the retro-arc side. At the end of Late Cretaceous, the Lhasa terrane was likely to have been uplifted to high elevation forming an Andean-type margin resembling the modern South America before the India-Asia collision.

Studies of Sr isotopic composition of thousands of samples of marine sediments and fossils have yielded a curve of 87Sr/86Sr versus age for seawater Sr that extends back to 1 billion years. The ratio has fluctuated with large amplitude during this time period, and because the ratio is always uniform in the oceans globally at any one time, it is useful as a stratigraphic correlation and age-dating tool. The ratio also appears to reflect major tectonic and climatic events in Earth history and hence provides clues as to the causes, timing, and consequences of those events. The seawater 87Sr/86Sr ratio is generally high during periods marked by continent-continent collisions, and lower when continental topography is subdued, and seafloor generation rates are high. There is evidence that major shifts in the seawater ratio can be ascribed to specific orogenic events and correlate with large shifts in global climate.

Geochemical data on widely distributed igneous rocks of southern Tibet are used to reconstruct paleo-crustal thickness during the 50+ million years that have elapsed since the onset of the India-Asia collision. We use two approaches, one based on Nd isotopes and an assimilation-recharge model for granitic magma genesis and another empirical method based on trace element geochemistry (La/Yb). The focus is on granitic rocks of two age ranges in a segment of the southern Lhasa Block between approximately 89.5° and 92.5°E longitude. One age range, 45 to 62 Ma, spans the time of the onset of collision and for which we infer the geochemistry of granitic rocks reflects mainly pre-collision structure. The other age range is 21 to 9 Ma for the Nd isotopic approach, and 32 to 9 Ma for La/Yb, where the geochemistry must reflect post-collision structure. Our results suggest that the pre- and syn-collision southern margin of the Lhasa block, that portion now located within 50–60 km of the Indus-Yarlung suture (IYS) and south of 29.8°N latitude was relatively thin, about 25–35 km thick until 45 Ma. At approximately 29.8°-29.9°N latitude there was a pronounced crustal discontinuity, and north of that latitude (for a distance that we cannot constrain), the inferred crustal thickness was greater, at least 50–55 km, as indicated by latest Cretaceous and Early Tertiary granitoids and ignimbrites that have large fractions of assimilated continental crust and high La/Yb ratios. Post-collision Nd isotopic and La/Yb data from granitoids younger than 32 Ma suggest that the southern margin south of 29.8°N was thickened substantially to at least 55–60 km (based on Nd isotopes) and possibly as much as 70–75 km (based on La/Yb) by Early to mid-Miocene time. These observations require that thickening of the southern Lhasa Block margin in the period 45–32 Ma was non-uniform; the crust now within 60 km of the suture was thickened by approximately 40 km whereas the crust north of 29.9°N latitude was thickened much less, or not at all. The region currently between 29.8°N and the YTS may have been the highest elevation mountain terrane in the period from roughly 30 to 20 Ma. The amount of Miocene denudation reflects this difference, as there is evidence of substantially more denudation near the IYS than in the region north of 29.9°N. Some of the difference in thickening could be due to magmatic additions from the mantle in the region south of 29.8°N, but there is need for at least 30 km of tectonic thickening between 45 and 32 Ma. The non-uniform thickening suggests that the high elevations at the southern margin of the Himalaya-Tibet orogen propagated southward by about 200 km, from north of Lhasa to their present position, during the period from 50 to 20 Ma. Present crustal thickness requires an additional 10–15 km of more uniform post-Miocene thickening.

Calcium isotope data ( δ 44 Ca , μ 42 / 44 Ca , and μ 48 / 44 Ca ) are reported for high-temperature metamorphic rocks and minerals, and compared with density-functional theory (DFT) estimates of equilibrium Ca isotope fractionation factors for plagioclase, garnet, clinopyroxene, orthopyroxene, olivine, and apatite. The data and calculations are used to evaluate equilibrium and kinetic fractionation effects that apply to high-temperature metamorphism, where extended residence at high temperature should promote equilibration, but where centimeter-to-meter scale Ca transport could produce diffusive kinetic effects. At upper-granulite facies conditions (T ≥ ∼900 °C), DFT-predicted equilibrium fractionations between minerals are ≤0.8‰, decreasing to ca. 0.6‰ at 1100 °C. We find much larger δ 44 Ca variations in both whole-rock samples (range of ∼4‰) and individual minerals (range of ∼8‰), and large variations in the Ca isotope fractionation between mineral pairs (e.g. Δ 44 Ca grt-plag from −1.5 to +1.5‰). Deviations from equilibrium tend to be larger in concert with indications of higher temperature, such as increasing whole-rock Mg#, plagioclase anorthite content, orthopyroxene Ca/Mg, and garnet Mg#. These large variations are inferred to be due to intragranular or grain-boundary diffusion during metamorphism, as this is the only mechanism that can produce such large isotopic variations. We can confirm the kinetic origin of the variations using measurements of μ 48 / 44 Ca by MC-ICP-MS to distinguish kinetic from equilibrium fractionation processes using a triple-isotope approach. A new variable ( Δ 48 Ca ′ ) quantifies deviations from the Ca-isotope equilibrium slope on a plot of 48Ca/44Ca vs. 42Ca/44Ca. Available geochronological constraints and numerical modeling indicate that observed kinetic isotope fractionations between adjacent high and low Ca rock layers require effective Ca diffusivities of 10−10 to 10−7 m2/yr, and a ratio of Ca isotope diffusivities of D 44 / D 40 ≈ 0.99. The diffusivities are consistent with Ca transport by volume- and grain-boundary diffusion. The apparent contrast in isotopic diffusivities is large and more consistent with silicate liquids than aqueous fluids. This study confirms that kinetic Ca isotope effects are abundant in nature and can overwrite small equilibrium effects, even at high temperatures and even when other techniques (such as Fe-Mg exchange and Ca-in-Opx thermometry) suggest the establishment of chemical equilibrium. Our results imply that kinetic fractionation effects may complicate the use of δ 44 Ca measurements for geothermometry or as a tracer of carbonate recycling into the mantle.

At equilibrium, the D/H isotope fractionation factor between H2 and H2O (αH2O-H2(eq)) is a sensitive indicator of temperature, and has been used as a geothermometer for natural springs and gas discharges. However, δDH2 measured in spring waters may underestimate subsurface temperatures of origin due to partial isotopic re-equilibration during ascent and cooling. We present new experimental data on the kinetics of D–H exchange for H2 dissolved in liquid water at temperatures below 100 °C. Comparing these results with published exchange rates obtained from gas phase experiments (100–400 °C), we derive a consistent activation energy of 52 kJ/mol, and the following rate expressions; ln k = 9.186 - 6298 / T and k 1 = 9764.61 [ H 2 O ] e - 6298 / T where T is absolute temperature (K), k is the universal rate constant ([L/mol]/hr), and k1 is a pseudo-first-order constant (hr−1) applicable to water-dominated terrestrial systems by constraining [H2O] as the density of H2O (in mol/L) at the P-T of interest. The density-dependent rate constant accounts for the kinetic disparity of D–H exchange with H2 when dissolved in liquid H2O relative to a gas/steam phase, exemplifed by 1/k1 at 100 °C of ∼2 days in liquid, versus ∼7 yrs in saturated steam. This difference may explain the high variability of δDH2 observed in fumarolic gases. Fluids convecting in the crust frequently reach T > 225 °C, where isotopic equilibrium is rapidly attained (

Heavy sulfur isotopic compositions of arc lavas suggest a seawater component in the sulfur budget of arc lavas, but whether the seawater signature derives from the subducting slab or from magma interactions with lithologies in the upper plate is unclear. To see through the effects of degassing or crustal processing, a study was conducted on the S isotopic composition of deep arc cumulates from 45–90 km beneath the Sierra Nevada batholith in California, a Cretaceous continental arc. These cumulates represent the crystal line of descent from magmatic differentiation of hydrous arc basalts. The deepest (up to 60–90 km) and most primitive cumulates are low in Fe and have high molar Mg/(Mg + Fe), whereas the shallow and more evolved cumulates are high in Fe and have low Mg/(Mg + Fe). Bulk rock δ 34 S correlates with Fe and negatively with Mg/(Mg + Fe). The most primitive cumulates are isotopically similar to the Earth's mantle whereas the more evolved cumulates are heavier by 6‰ in the direction of seawater sulfate. The mantle-like S isotopic signatures of the primitive cumulates indicate that the contribution of slab-derived sulfate to arc lavas may not be as large as widely thought. Heavy S isotopic signatures are seen only in the evolved arc cumulates, which suggests that the seawater signature of arc lavas may not all derive directly from the slab, but perhaps during magma interaction with pre-arc crust. In continental arcs, pre-arc crust is dominated by accreted marine metasediments and metabasalts, and in island arcs, by seawater altered oceanic crust in the upper plate. The limited contribution of slab sulfate to the mantle source of Sierran arc magmas, if generalizable, suggests that sulfate in the subducting slab is efficiently released well before the arc magmatic front. Such a scenario would be consistent with the higher solubility of sulfate in aqueous fluids compared to that of sulfide. In summary, it is suggested here that the upper plate, in the form of seawater altered crust and sediments, may be as or more important for the sulfur budget in arc magmas than the subducting slab. Early loss of sulfate from the slab during subduction suggests that the dominant S species transported to the deep mantle is in the reduced form – sulfide.

The roughness of solid surfaces influences mineral dissolution rates by affecting flow and transport in the near-surface regions and by increasing the surface area available for reaction. The impact of surface area is commonly accounted for by using the surface roughness factor (SRF), which is the ratio between the total surface area and the nominal or geometric surface area. The coupled impacts of hydrodynamics and transport, however, are rarely considered. In this study, we performed pore-scale reactive transport simulations in a series of synthetic 2D rough fractures to investigate the compound effects of surface roughness on the reaction rates in fractures. Simulation results show that while reaction rates increase with SRF, the increase is not linearly proportional to that of the surface area. Rather, local concentration gradients resulting from flow and transport processes limit the increase in the rate. In addition, surface roughness gives rise to concentration gradients that do not otherwise develop in the flat-surface geometries typically considered in modeling studies. To describe the impacts of the surface area increase on reaction rate at different roughness and flow velocities, three distinct regimes were identified. A unified mathematical relationship was also developed that allows the reaction rate in a rough fracture to be approximated by the well-mixed reactor reaction rate and a correction factor. The correction factor follows a power-law function of SRF, with the multiplying factor and exponent expressed as exponential functions of the Peclet and Damkohler number. This mathematical formulation provides a valuable upscaling approach for effective integration of sub-grid scale surface roughness in larger scale continuum models.

Carbon isotope ( δ 13 C) variations measured in carbonates have been attributed to large-scale phenomena throughout Earth history, such as changes in atmospheric oxygen or global glaciations. These interpretations follow from a model wherein the δ 13 C of marine dissolved inorganic carbon (DIC) is controlled by the relative sedimentary burial rates of biogenic carbonate (BC) and organic carbon (OC). A new model proposes authigenic carbonate (AC) as a third major sedimentary C pool, implying that δ 13 C anomalies are not necessarily indicative of extreme changes in the global carbon cycle and/or atmospheric oxygen. Two conditions are required for AC formation to significantly alter bulk carbonate δ 13 C: the AC isotopic composition must be at least ∼3‰ different from that of BC and the AC/BC ratio must be >0.1. We use pore fluid Ca and Sr concentrations to estimate rates of AC formation in Late Cenozoic marine sediments, then calculate relative fractions of AC, OC, and BC. Today AC is not expected to constitute a significant fraction of total sedimentary carbon (AC+OC+BC) globally; however, there are modern sites where local conditions promote elevated AC/BC and anaerobic metabolisms can alter the δ 13 C of pore fluids. We investigate these sites to determine what conditions might enable AC to alter δ 13 C of marine DIC. We find there is very little net addition of AC relative to BC, but large quantities of AC form today across many settings via recrystallization. In settings where remineralization of organic matter causes recrystallized carbonate to form with modified δ 13 C, AC/BC is generally too low for this recrystallization to significantly shift the δ 13 C of the bulk carbonate. However, exceptions are found in sites with very low BC and extensive methane oxidation, suggesting that this environment type would need to be globally extensive in the past in order for AC formation to change the δ 13 C of marine DIC.

We conducted hydroponic experiments growing soybean (Glycine max), rice (Oryza sativa), and wheat (Triticum aestivum) under K and Ca replete conditions to establish the degree of K isotopic fractionation by plants, and compare the isotopic fractionation of Ca and K. Each of the test plants displays fractionation relative to the growth solution favoring the light isotopes of K and Ca. The average δ41K values of the roots from the three plant species were similar, and have an overall average of −0.55 ± 0.24‰ 2s, while the overall average δ44Ca for roots is −0.67 ± 0.44. For leaves, the overall average of δ41K is −0.97 ± 0.4‰, compared to an overall average leaf δ44Ca of −0.83 ± 0.09‰. In the case of the soybean plants, the lightest K and Ca occurs in the stems with average δ41K of −1.31 ± 0.40‰ 2s and average δ44Ca of −1.20 ± 0.19 ‰ 2s. We present a simple box model involving the relative fluxes of K and its isotopic fractionation that reproduces our K isotopic observations and suggests a fractionation of ∼...

To understand the characteristics of Ca isotope composition and fractionation in silicate-dominated Quaternary aquifer system, hydrochemical and isotope studies (87Sr/86Sr, 13CDIC and 44/40Ca) were conducted on groundwater, sediment and rock samples from the Datong basin, China. Along the groundwater flow path from the basin margin to the center, groundwater hydrochemical type evolves from Ca-HCO3 to Na-HCO3/Na-Cl type, which results from aluminosilicate hydrolysis, vertical mixing, cation exchange between CaX2 and NaX, and calcite/dolomite precipitation. These processes cause the decrease in groundwater Ca concentration and the associated modest fractionation of groundwater Ca isotopes along the flowpath. The groundwater δ44/40Ca value varies from −0.11 to 0.49‰. The elevated δ44/40Ca ratios in shallow groundwater are attributed to vertical mixing involving addition of irrigation water, which had the average δ44/40Ca ratio of 0.595‰. Chemical weathering of silicate minerals and carbonate generates depleted δ44/40Ca signatures in groundwater from Heng Mountain (east area) and Huanghua Uplift (west area), respectively. Along the groundwater flow path from Heng Mountain to central area of east area, cation exchange between CaX2 and NaX on clay mineral results in the enrichment of heavier Ca isotope in groundwater. All groundwater samples are oversaturated with respect to calcite and dolomite. The groundwater environment rich in organic matter promotes the precipitation of carbonate minerals via the biodegradation of organic carbon, thereby further promoting the elevation of groundwater δ44/40Ca ratios.

A negative shift in the calcium isotopic composition of marine carbonate rocks spanning the end-Permian extinction horizon in South China has been used to argue for an ocean acidification event coincident with mass extinction. This interpretation has proven controversial, both because the excursion has not been demonstrated across multiple, widely separated localities, and because modeling results of coupled carbon and calcium isotope records illustrate that calcium cycle imbalances alone cannot account for the full magnitude of the isotope excursion. Here, we further test potential controls on the Permian-Triassic calcium isotope record by measuring calcium isotope ratios from shallow-marine carbonate successions spanning the Permian-Triassic boundary in Turkey, Italy, and Oman. All measured sections display negative shifts in δ44/40Ca of up to 0.6‰. Consistency in the direction, magnitude, and timing of the calcium isotope excursion across these widely separated localities implies a primary and global δ44/40Ca signature. Based on the results of a coupled box model of the geological carbon and calcium cycles, we interpret the excursion to reflect a series of consequences arising from volcanic CO2 release, including a temporary decrease in seawater δ44/40Ca due to short-lived ocean acidification and a more protracted increase in calcium isotope fractionation associated with a shift toward more primary aragonite in the sediment and, potentially, subsequently elevated carbonate saturation states caused by the persistence of elevated CO2 delivery from volcanism. Locally, changing balances between aragonite and calcite production are sufficient to account for the calcium isotope excursions, but this effect alone does not explain the globally observed negative excursion in the δ13C values of carbonate sediments and organic matter as well. Only a carbon release event and related geochemical consequences are consistent both with calcium and carbon isotope data. The carbon release scenario can also account for oxygen isotope evidence for dramatic and protracted global warming as well as paleontological evidence for the preferential extinction of marine animals most susceptible to acidification, warming, and anoxia.

Geochemical reactions add complexity to the characterization and prediction of fracture hydraulic properties because they depend on factors that are highly heterogeneous, such as mineral composition. However, systematic analyses of fracture evolution in mineralogically heterogeneous systems are still limited. In this study, we investigated fracture evolution in multimineral systems using a reduced dimension reactive transport model. The model was developed and tested based on experimental studies and addresses the complex morphological and geochemical changes that arise from the presence of multiple minerals of different reactivities. Numerical experiments were performed using randomly generated initial fracture geometries based on representative geostatistics, different categories of mineral composition, and a range of flow rates that are relevant to geologic carbon storage systems. The simulation results showed distinct dissolution regimes at different flow rates, each of which produced characteristic d...

Interactions between supercritical (sc) CO2 and minerals are important when CO2 is injected into geologic formations for storage and as working fluids for enhanced oil recovery, hydraulic fracturing, and geothermal energy extraction. It has previously been shown that at the elevated pressures and temperatures of the deep subsurface, scCO2 alters smectites (typical swelling phyllosilicates). However, less is known about the effects of scCO2 on nonswelling phyllosilicates (illite and muscovite), despite the fact that the latter are the dominant clay minerals in deep subsurface shales and mudstones. Our studies conducted by using single crystals, combining reaction (incubation with scCO2), visualization [atomic force microscopy (AFM)], and quantifications (AFM, X-ray photoelectron spectroscopy, X-ray diffraction, and off-gassing measurements) revealed unexpectedly high CO2 uptake that far exceeded its macroscopic surface area. Results from different methods collectively suggest that CO2 partially entered the muscovite interlayers, although the pathways remain to be determined. We hypothesize that preferential dissolution at weaker surface defects and frayed edges allows CO2 to enter the interlayers under elevated pressure and temperature, rather than by diffusing solely from edges deeply into interlayers. This unexpected uptake of CO2, can increase CO2 storage capacity by up to ∼30% relative to the capacity associated with residual trapping in a 0.2-porosity sandstone reservoir containing up to 18 mass % of illite/muscovite. This excess CO2 uptake constitutes a previously unrecognized potential trapping mechanism.

This paper presents an initial evaluation and concept description of an approach to CO2 storage where the reservoir rocks are volcanic terrains that have been built up from the seafloor and consist of several kilometers of stacked lava, pyroclastic, and volcano-sedimentary rocks, and where CO2 could be injected in large quantities in the supercritical or liquid state. These coastal “Saline Volcanic Basins” (SVB) have massive volume, heterogeneous internal structure, and relatively low temperatures. SVB's occur in island arcs and so-called hot spots such as Hawaii and Iceland. In both settings, the volcanic formations are exceedingly thick, border the ocean, are below sea level, and are saturated with seawater at storage depths. Many SVB reservoirs are accessible with onshore wells within which intercalated high and low-permeability layers and low temperatures can keep supercritical CO2 in a relatively high-density state and promote solution and capillary trapping, in addition to mineralization. Some regions of the subsurface may be at low enough temperature to allow for CO2 hydrate formation as an additional trapping mechanism. Our initial assessment of storage potential focuses on the northeast portion of the island of Hawaii, where there is direct information about the subsurface volcanic stratigraphy and hydrology based on observations made during two scientific drilling projects that penetrated to 3.5 km with continuous coring. Initial analysis, including simulations of a 50 Megaton injection of supercritical CO2, suggests that this region could be effective for permanent storage, and potentially for gigatons of CO2.

The geological calcium cycle is linked to the geological carbon cycle through the weathering and burial of carbonate rocks. As a result, calcium (Ca) isotope ratios (44Ca/40Ca, expressed as δ44/40Ca) can help to constrain ancient carbon cycle dynamics if Ca cycle behavior can be reconstructed. However, the δ44/40Ca of carbonate rocks is influenced not only by the δ44/40Ca of seawater but also by diagenetic processes and fractionation associated with carbonate precipitation. In this study, we investigate the dominant controls on carbonate δ44/40Ca in Upper Permian to Middle Triassic limestones (ca. 253 to 244 Ma) from south China and Turkey. This time interval is ideal for assessing controls on Ca isotope ratios in carbonate rocks because fluctuations in seawater δ44/40Ca may be expected based on several large carbon isotope (δ13C) excursions ranging from − 2 to + 8‰. Parallel negative δ13C and δ44/40Ca excursions were previously identified across the end-Permian extinction horizon. Here, we find a second negative excursion in δ44/40Ca of ~ 0.2‰ within Lower Triassic strata in both south China and Turkey; however, this excursion is not synchronous between regions and thus cannot be interpreted to reflect secular change in the δ44/40Ca of global seawater. Additionally, δ44/40Ca values from Turkey are consistently 0.3‰ lower than contemporaneous samples from south China, providing further support for local or regional influences. By measuring δ44/40Ca and Sr concentrations ([Sr]) in two stratigraphic sections located at opposite margins of the Paleo-Tethys Ocean, we can determine whether the data represent global conditions (e.g., secular variations in the δ44/40Ca of seawater) versus local controls (e.g., original mineralogy or diagenetic alteration). The [Sr] and δ44/40Ca data from this study are best described statistically by a log-linear correlation that also exists in many previously published datasets of various geological ages. Using a model of early marine diagenetic water-rock interaction, we illustrate that this general correlation can be explained by the chemical evolution of bulk carbonate sediment samples with different initial mineralogical compositions that subsequently underwent recrystallization. Although early diagenetic resetting and carbonate mineralogy strongly influence the carbonate δ44/40Ca values, the relationship between [Sr] and δ44/40Ca holds potential for reconstructing first-order secular changes in seawater δ44/40Ca composition.

ConspectusGeologic carbon storage (GCS) involves capture and purification of CO2 at industrial emission sources, compression into a supercritical state, and subsequent injection into geologic formations. This process reverses the flow of carbon to the atmosphere with the intention of returning the carbon to long-term geologic storage. Models suggest that most of the injected CO2 will be “trapped” in the subsurface by physical means, but the most risk-free and permanent form of carbon storage is as carbonate minerals (Ca,Mg,Fe)CO3. The transformation of CO2 to carbonate minerals requires supply of the necessary divalent cations by dissolution of silicate minerals. Available data suggest that rates of transformation are highly uncertain and difficult to predict by standard approaches. Here we show that the chemical kinetic observations and experimental results, when they can be reduced to a single cation-release time scale that describes the fractional rate at which cations are released to solution by miner...

© 2017 American Chemical Society. A novel reactive transport model has been developed to examine the processes that affect fracture evolution in a carbonate-rich shale. An in situ synchrotron X-ray microtomography experiment, flowing CO2saturated water through a single fracture mini-core of Niobrara Shale provided the experimental observations for the development and testing of the model. The phenomena observed included the development of a porous altered layer, flow channeling, and increasingly limited calcite dissolution. The experimental observations cannot be explained by models that consider only mineral dissolution and development of an altered layer. The difference between the fracture volume change recorded by the microtomography images and what would be expected from mineral dissolution alone suggest that there is erosion of the altered layer as it develops. The numerical model includes this additional mechanism, with the erosion rate based on the thickness of the altered layer, and successfully captures the evolution of the geochemical reactions and morphology of the fracture. The findings imply that the abundance (with a threshold of approximately 35%) and reactivity of the rapidly reacting mineral control the development and erodibility of the altered layer on the fracture surfaces, and therefore fracture opening.

The rates of mineral dissolution reactions in porous media are difficult to predict, in part because of a lack of understanding of mineral reactive surface area in natural porous media. Common estimates of mineral reactive surface area used in reactive transport models for porous media are typically ad hoc and often based on average grain size, increased to account for surface roughness or decreased by several orders of magnitude to account for reduced surface reactivity of field as opposed to laboratory samples. In this study, accessible mineral surface areas are determined for a sample from the reservoir formation at the Nagaoka pilot CO2 injection site (Japan) using a multi-scale image analysis based on synchrotron X-ray microCT, SEM QEMSCAN, XRD, SANS, and FIB-SEM. This analysis not only accounts for accessibility of mineral surfaces to macro-pores, but also accessibility through connected micro-pores in smectite, the most abundant clay mineral in this sample. While the imaging analysis reveals that most of the micro- and macro-pores are well connected, some pore regions are unconnected and thus inaccessible to fluid flow and diffusion. To evaluate whether mineral accessible surface area accurately reflects reactive surface area a flow-through core experiment is performed and modeled at the continuum scale. The core experiment is performed under conditions replicating the pilot site and the evolution of effluent solutes in the aqueous phase is tracked. Various reactive surface area models are evaluated for their ability to capture the observed effluent chemistry, beginning with parameter values determined as a best fit to a disaggregated sediment experiment (Beckingham et al., 2016) described previously. Simulations that assume that all mineral surfaces are accessible (as in the disaggregated sediment experiment) over-predict the observed mineral reaction rates, suggesting that a reduction of RSA by a factor of 10–20 is required to match the core flood experimental data. While the fit of the effluent chemistry (and inferred mineral dissolution rates) greatly improve when the pore-accessible mineral surface areas are used, it was also necessary to include highly reactive glass phases to match the experimental observations, in agreement with conclusions from the disaggregated sediment experiment. It is hypothesized here that the 10–20 reduction in reactive surface areas based on the limited pore accessibility of reactive phases in core flood experiment may be reasonable for poorly sorted and cemented sediments like those at the Nagaoka site, although this reflects pore rather than larger scale heterogeneity.

The strontium/calcium (Sr/Ca) ratio in paleoseawater is important for evaluating the global fluxes of continental weathering products and carbonate deposition in the oceans. Existing reconstructions of the paleoseawater Sr/Ca ratio using different methods give conflicting results. We use a modeling approach to estimate paleo-Sr/Ca for the past 50 million years that uses pore fluid chemistry and bulk sediment measurements of carbonate ooze and chalk from the Ontong Java Plateau as input. The effects of diagenesis in modifying the Sr/Ca ratio recorded in bulk carbonate sediments are estimated using a reactive transport model, and those effects are accounted for in determining the initial Sr/Ca ratio of the biogenic carbonate sediments. The Sr/Ca of the paleo-oceans is calculated based on an effective partition coefficient ( K Sr bio = 0.198 ) inferred from Sr/Ca in modern sediments. The model simulates deposition of sediments on the seafloor, reactions between pore fluid and solid (dissolution and precipitation of calcite), and diffusion within the pore fluid. Measured pore fluid Sr, Ca and sulfate concentrations provide input for the Sr model. This approach, applied to four Ocean Drilling Program (ODP) sites (803, 805, 806, 807) that have long Cenozoic records of carbonate deposition, indicates that the Sr/Ca ratio of seawater has increased slowly over the past 50 million years from a value of about 8 mmol/mol to the modern value of 9.4, with fluctuations of about ±1 over 10 Myr periods. This result is consistent with previous estimates based on foraminifera and coral aragonite, but differs from recent estimates obtained with other approaches. The near-constant seawater Sr/Ca over the past 50 million years is consistent with likely weathering sources to the oceans. The fluctuations suggest that there were modest shifts in the aragonite/calcite ratio of marine carbonate.

[Image: see text] We present the results of experiments involving the isotopic fractionation by diffusion of K(+) cations in methanol at 298 K along with supporting molecular dynamic simulations. The experiments, using glass Rayleigh fractionators filled with a methanol solution of KCl, constrain the ratio of the individual diffusivities of (41)K and (39)K (D((41)K)/D((39)K)) in methanol, and so the isotopic fractionation of K(+) due to diffusion in methanol, to 0.9995 ± 0.0001. This isotopic fractionation is 25% of the fractionation resulting from K(+) diffusion in water. This is consistent with published molecular dynamics simulations indicating greater hydrodynamic coupling between K(+) ions and solvating methanol molecules compared to K(+) ions and solvating water molecules.

Elucidating geodynamic processes at depth relies on a correct interpretation of petrological and geochemical features in magmatic records. In southern Tibet, both potassic volcanic rocks and adakitic intrusions exhibit high Sr/Y and La/Yb, and low Y and Yb concentrations. But these two rock types have contrasting temporal–spatial distributions and isotopic variations. Here we present a systematic study on the postcollisional potassic and adakitic rocks in order to investigate their petrogenetic links with the coeval mantle-derived ultrapotassic rocks and shed light on the potential input from underthrusted Indian continental crust. We found that adakitic intrusions with higher K2O/Na2O tend to display lower Y and higher SiO2, suggesting that the mantle-derived ultrapotassic melts, showing relatively high Y and Yb concentrations, only played a minor role in adakitic magmatism. Therefore, the unradiogenic 143Nd/144Nd and the dramatic decrease of zircon eHf(t) values since ~ 35 Ma shown by postcollisional adakites should be interpreted as reflecting the crustal input from Indian plate. Unlike adakitic intrusions in southern Lhasa subterrane, potassic volcanic rocks share similar spatial distributions with ultrapotassic rocks, and their isotopic discrepancy is diminishing with volcanic activity becomes younger and migrates eastward. Evidence from whole-rock Pb and zircon Hf isotopes further indicates that potassic volcanic rocks are more likely to originate from partial melting of the overthickened and isotopically heterogeneous Lhasa terrane crust rather than the underthrusted Indian continental crust. The elevated Rb/Sr and varying Sr/CaO in potassic volcanic rocks provide an argument for sanidine + plagioclase + clinopyroxene as the major fractionating phases during magmatic differentiation. These findings not only highlight the significance of potassic and adakitic rocks in providing constraints on the geodynamic processes beneath southern Tibet, but also imply that special caution is needed if we attempt to probe into the nature of mantle lithosphere using isotopic tracers of the Tibetan ultrapotassic rocks.

The end-Triassic mass extinction coincided with a negative δ13C excursion, consistent with release of 13C-depleted CO2 from the Central Atlantic Magmatic Province. However, the amount of carbon released and its effects on ocean chemistry are poorly constrained. The coupled nature of the carbon and calcium cycles allows calcium isotopes to be used for constraining carbon cycle dynamics and vice versa. We present a high-resolution calcium isotope (δ44/40Ca) record from 100 m of marine limestone spanning the Triassic/Jurassic boundary in two stratigraphic sections from northern Italy. Immediately above the extinction horizon and the associated negative excursion in δ13C, δ44/40Ca decreases by ∼0.8‰ in 20 m of section and then recovers to preexcursion values. Coupled numerical models of the geological carbon and calcium cycles demonstrate that this δ44/40Ca excursion is too large to be explained by changes to seawater δ44/40Ca alone, regardless of CO2 injection volume and duration. Less than 20% of the δ44/40Ca excursion can be attributed to acidification. The remaining 80% likely reflects a higher proportion of aragonite in the original sediment, based largely on high concentrations of Sr in the samples. Our study demonstrates that coupled models of the carbon and calcium cycles have the potential to help distinguish contributions of primary seawater isotopic changes from local or diagenetic effects on the δ44/40Ca of carbonate sediments. Differentiating between these effects is critical for constraining the impact of ocean acidification during the end-Triassic mass extinction, as well as for interpreting other environmental events in the geologic past.

Natural variations in the isotopic composition of some 50 chemical elements are now being used in geochemistry for studying transport processes, estimating temperature, reconstructing ocean chemistry, identifying biological signatures, and classifying planets and meteorites. Within the past decade, there has been growing interest in measuring isotopic variations in a wider variety of elements, and improved techniques make it possible to measure very small effects. Many of the observations have raised questions concerning when and where the attainment of equilibrium is a valid assumption. In situations where the distribution of isotopes within and among phases is not representative of the equilibrium distribution, the isotopic compositions can be used to access information on mechanisms of chemical reactions and rates of geological processes. In a general sense, the fractionation of stable isotopes between any two phases, or between any two compounds within a phase, can be ascribed to some combination of the mass dependence of thermodynamic (equilibrium) partition coefficients, the mass dependence of diffusion coefficients, and the mass dependence of reaction rate constants. Many documentations of kinetic isotope effects (KIEs), and their practical applications, are described in this volume and are therefore not reviewed here. Instead, the focus of this chapter is on the measurement and interpretation of mass dependent diffusivities and reactivities, and how these parameters are implemented in models of crystal growth within a fluid phase. There are, of course, processes aside from crystal growth that give rise to KIEs among non-traditional isotopes, such as evaporation (Young et al. 2002; Knight et al. 2009; Richter et al. 2009a), vapor exsolution (Aubaud et al. 2004), thermal diffusion (Richter et al. 2009a, 2014b; Huang et al. 2010; Dominguez et al. 2011), mineral dissolution (e.g., Brantley et al. 2004; Wall et al. 2011; Pearce et al. 2012 …

High iodine concentrations in groundwater have caused serious health problems to the local residents in the Datong basin, northern China. To determine the impact of water-sediment interaction and irrigation practices on iodine mobilization in aquifers, isotope ( 2 H, 18 O and 87 Sr/ 86 Sr) and hydrogeochemical studies were conducted. The results show that groundwater iodine concentrations vary from 14.4 to 2180 μg/L, and high iodine groundwater (>150 μg/L) mainly occurs in the central area of the Datong basin. Sediment iodine content is between 87 Sr/ 86 Sr values and groundwater chemistry suggest that aluminosilicate hydrolysis is the dominant process controlling hydrochemical evolution along groundwater flowpath, and the degradation of TOC/iodine-rich sediment mediated by microbes potentially triggers the iodine release from the sediment into groundwater in the discharge area. The vertical stratification of groundwater 18 O and 2 H isotope reflects the occurrence of a vertical mixing process driven by periodic surface irrigation. The vertical mixing could change the redox potential of shallow groundwater from sub-reducing to oxidizing condition, thereby affecting the iodine mobilization in shallow groundwater. It is postulated that the extra introduction of organic matter and O 2 /NO 3 /SO 4 could accelerate the microbial activity due to the supplement of high ranking electron acceptors and promote the iodine release from the sediment into shallow groundwater.

Our limited understanding of mineral reactive surface area contributes to significant uncertainties in quantitative simulations of reactive chemical transport in subsurface processes. Continuum formulations for reactive transport typically use a number of different approximations for reactive surface area, including geometric, specific, and effective surface area. In this study, reactive surface area estimates are developed and evaluated for their ability to predict dissolution rates in a well-stirred flow-through reactor experiment using disaggregated samples from the Nagaoka pilot CO2 injection site (Japan). The disaggregated samples are reacted with CO2 acidified synthetic brine under conditions approximating the field conditions and the evolution of solute concentrations in the reactor effluent is tracked over time. The experiments, carried out in fluid-dominated conditions at a pH of 3.2 for 650 h, resulted in substantial dissolution of the sample and release of a disproportionately large fraction of the divalent cations. Traditional reactive surface area estimation methods, including an adjusted geometric surface area and a BET-based surface area, are compared to a newly developed image-based method. Continuum reactive transport modeling is used to determine which of the reactive surface area models provides the best match with the effluent chemistry from the well-stirred reactor. The modeling incorporates laboratory derived mineral dissolution rates reported in the literature and the initial modal mineralogy of the Nagaoka sediment was determined from scanning electron microscopy (SEM) characterization. The closest match with the observed steady-state effluent concentrations was obtained using specific surface area estimates from the image-based approach supplemented by literature-derived BET measurements. To capture the evolving effluent chemistry, particularly over the first 300 h of the experiment, it was also necessary to account for the grain size distribution in the sediment and the presence of a highly reactive volcanic glass phase that shows preferential cation leaching.

Understanding fracture alteration resulting from geochemical reactions is critical in predicting fluid migration in the subsurface and is relevant to multiple environmental challenges. Here, we present a novel 2.5D continuum reactive transport model that captures and predicts the spatial pattern of fracture aperture change and the development of an altered layer in the near-fracture region. The model considers permeability heterogeneity in the fracture plane and updates fracture apertures and flow fields based on local reactions. It tracks the reaction front of each mineral phase and calculates the thickness of the altered layer. Given this treatment, the model is able to account for the diffusion limitation on reaction rates associated with the altered layer. The model results are in good agreement with an experimental study in which a CO2-acidified brine was injected into a fracture in the Duperow Dolomite, causing dissolution of calcite and dolomite that result in the formation of a preferential flow channel and an altered layer. With an effective diffusion coefficient consistent with the experimentally observed porosity of the altered layer, the model captures the progressive decrease in the dissolution rate of the fast-reacting mineral in the altered layer.