Reconciling the elemental and Sr isotope composition of Himalayan weathering fluxes: insights from the carbonate geochemistry of stream waters

Determining the relative proportions of silicate vs. carbonate weathering in the Himalaya is important for understanding atmospheric CO₂ consumption rates and the temporal evolution of seawater Sr. However, recent studies have shown that major element mass-balance equations attribute less CO₂ consumption to silicate weathering than methods utilizing Ca/Sr and ⁸⁷Sr/⁸⁶Sr mixing equations. To investigate this problem, we compiled literature data providing elemental and ⁸⁷Sr/⁸⁶Sr analyses for stream waters and bedrock from tributary watersheds throughout the Himalaya Mountains. In addition, carbonate system parameters (P_CO2, mineral saturation states) were evaluated for a selected suite of stream waters. The apparent discrepancy between the dominant weathering source of dissolved major elements vs. Sr can be reconciled in terms of carbonate mineral equilibria. Himalayan streams are predominantly Ca²⁺-Mg²⁺-HCO₃⁻ waters derived from calcite and dolomite dissolution, and mass-balance calculations demonstrate that carbonate weathering contributes ∼87% and ∼76% of the dissolved Ca²⁺ and Sr²⁺, respectively. However, calculated Ca/Sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock, suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and P_CO2 gradients in the Himalaya.The state of calcite and dolomite saturation was evaluated across these gradients, and the data show that upon descending through the Himalaya, ∼50% of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas CO₂. Stream water Ca/Mg and Ca/Sr ratios decrease as the degree of supersaturation with respect to calcite increases, and Mg²⁺, Ca²⁺, and HCO₃⁻ mass balances support interpretations of preferential Ca²⁺ removal by calcite precipitation. On the basis of patterns of saturation state and P_CO2 changes, calcite precipitation was estimated to remove up to ∼70% of the Ca²⁺ originally derived from carbonate weathering. Accounting for the nonconservative behavior of Ca²⁺ during riverine transport brings the Ca/Sr and ⁸⁷Sr/⁸⁶Sr composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock, thereby permitting consistency between elemental and Sr isotope approaches to partitioning stream water solute sources. These results resolve the dissolved Sr²⁺ budget and suggest that the conventional application of two-component Ca/Sr and ⁸⁷Sr/⁸⁶Sr mixing equations has overestimated silicate-derived Sr²⁺ and HCO₃⁻ fluxes from the Himalaya. In addition, these findings demonstrate that integrating stream water carbonate mineral equilibria, divalent cation compositional trends, and Sr isotope inventories provides a powerful approach for examining weathering fluxes.     

Effects of freshwater Synechococcus sp. cyanobacteria pH buffering on CaCO3 precipitation: Implications for CO2 sequestration

In the present study, a mixed-flow steady-state bio-reactor was designed to biomineralize CO₂ as a consequence of photosynthesis from active Synechococcus sp. Dissolved CO₂, generated by constant air bubbling of inorganic and cyanobacteria stock solutions, was the only source of inorganic carbon. The release of hydroxide ion by cyanobacteria from photosynthesis maintained highly alkaline pH conditions. In the presence of Ca²⁺ and carbonate species, this led to calcite supersaturation under steady state conditions. Ca²⁺ remained constant throughout the experiments showing the presence of steady state conditions. Similarly, the Synechococcus sp. biomass concentration remained stable within uncertainty. A gradual pH decrease was observed for the highest Ca²⁺ condition coinciding with the formation of CaCO₃. The high degree of supersaturation, under steady-state conditions, contributed to the stabilization of calcite and maintained a constant driving force for the mineral nucleation and growth. For the highest Ca²⁺ condition a fast crystal growth rate was consistent with rapid calcite precipitation as suggested further by affinity calculations. Although saturation state based kinetic precipitation models cannot accurately reflect the controls on crystal growth kinetics or reliably predict growth mechanisms, the relatively reaction orders obtained from modeling of calcite precipitation rates as function of decreasing carbonate concentration suggest that the precipitation occurred via surface-controlled rate determining reactions. These high reaction orders support in addition the hypothesis that crystal growth proceeded through complex surface controlled mechanisms. In conclusion, the steady state supersaturated conditions generated by a constant cyanobacteria biomass and metabolic activity strongly suggest that these microorganisms could be used for the development of efficient CO₂ sequestration methods in a controlled large-scale environment.     

Microenvironmental controls on mineralogy and habit of CaCO3 precipitates: an example from an active travertine system

Analysis of water and associated carbonate precipitates from a small, warm-spring travertine system in SW Colorado, USA, provide an example of the: (i) great variability of the geochemical parameters within these dynamic systems, and (ii) significance of the microenvironment in controlling mineralogy and morphology of carbonate precipitates. Waters emerged from the springs highly charged in CO₂, with an initial p_CO2 of 1.2 × 10⁵ Pa. Degassing of the CO₂ from the waters decreased the pH from 6.1 to 8.0, resulting in an increase of 8%‰ in δ¹³C values downflow in the total CO₂ in solution and an increase in the I_SAT from 2.1 to as high as 63 times supersaturation with respect to calcite. Due to changes in the stable isotopic composition of the waters downflow as well as changes in the degree of supersaturation, stable isotopic analyses range greatly from locale to locale within this small system. Near the spring vents, at relatively low I_SAT levels, well-developed rhombohedra of calcite formed as biotically induced precipitates around diatom stalks and other algae as well as abiotic crusts. In contrast, near the distal end of the system, very high I_SAT levels were reached and resulted in the precipitation of skeletal-dendritic crystals of calcite on copper substrates, floating rafts of laterally linked hemispheres of aragonite crystals, and bimineralic carbonate-encrusted bubbles. Microenvironmental parameters control the mineralogy and habit of these precipitates.     

Cave air and hydrological controls on prior calcite precipitation and stalagmite growth rates: Implications for palaeoclimate reconstructions using speleothems

Hourly resolved cave air P_CO2 and cave drip water hydrochemical data illustrate that calcite deposition on stalagmites can be modulated by prior calcite precipitation (PCP) on extremely short timescales. A very clear second-order covariation between cave air P_CO2 and drip water Ca²⁺ concentrations during the winter months demonstrates the effects of degassing-induced PCP on drip water chemistry. Estimating the strength of the cave air P_CO2 control on PCP is possible because the PCP signal is so clear; at our drip site a one ppm shift in Ca²⁺ concentrations requires a P_CO2 shift of between 333 and 667 ppm. This value will undoubtedly vary from site to site, depending on drip water flow rate, residence time, drip water-cave air P_CO2 differential, and availability of low P_CO2 void spaces in the vadose zone above the cave. High-resolution cave environmental measurements were used to model calcite deposition on one stalagmite in Crag Cave, SW Ireland, and modelled growth over the study period (222 μm over 171 days) is extremely similar to the amount of actual calcite growth (240 μm) over the same time interval, strongly suggesting that equations used to estimate stalagmite growth rates are valid. Although cave air P_CO2 appears to control drip water hydrochemistry in the winter, drip water dilution caused by rain events may have played a larger role during the summer, as evidenced by a series of sudden drops in Ca²⁺ concentrations (dilution) followed by much more gradual increases in drip water Ca²⁺ concentrations (slow addition of diffuse water). This research demonstrates that PCP on stalactites, cave ceilings, and void spaces within the karst above the cave partially controls drip water chemistry, and that thorough characterisation of this process at individual caves is necessary to most accurately interpret climate records from those sites.     

Multi-proxy approach (H/H, O/O, C/C and Sr/Sr) for the evolution of carbonate-rich groundwater in basalt dominated aquifer of Axum area, northern Ethiopia

Isotopic and chemical composition of groundwater from wells and springs, and surface water from the basalt-dominated Axum area (northern Ethiopia) provides evidence for the origin of water and dissolved species. Shallow (depth < 40 m) and deep groundwater are distinguished by both chemical and isotopic composition. Deep groundwater is significantly enriched in dissolved inorganic carbon up to 40 mmol l⁻¹ and in concentrations of Ca²⁺, Mg²⁺, Na⁺ and Si(OH)₄ compared to the shallow type.The δ²H and δ¹⁸O values of all solutions clearly indicate meteoric origin. Shifts from the local meteoric water line are attributed to evaporation of surface and spring water, and to strong water–rock interaction. The δ¹³C_DIC values of shallow groundwater between −12 and −7‰ (VPDB) display the uptake of CO₂ from local soil horizons, whereas δ¹³C_DIC of deep groundwater ranges from −5 to +1‰. Considering open system conditions with respect to gaseous CO₂, δ¹³C_DIC = +1‰ of the deep groundwater with highest PCO₂ = 10^−0.9 atm yields δ¹³C_CO2(gas) ≈ −5‰, which is close to the stable carbon isotopic composition of magmatic CO₂. Accordingly, stable carbon isotope ratios within the above range are referred to individual proportions of CO₂ from soil and magmatic origin. The uptake of magmatic CO₂ results in elevated cations and Si(OH)₄ concentrations. Weathering of local basalts is documented by ⁸⁷Sr/⁸⁶Sr ratios of the groundwater from 0.7038 to 0.7059. Highest values indicate Sr release from the basement rocks. Besides weathering of silicates, neoformation of solids has to be considered, which results in the formation of, e.g., kaolinite and montmorillonite. In several solutions supersaturation with respect to calcite is reached by outgassing of CO₂ from the solution leading to secondary calcite formation.     

Discovery of galena in Tertiary pebbly coquina and its implication in mineralization

A suite of terrestrial-marine transitional carbonate beds occur in the Lower Tertiary of the Kuqa Basin, Xinjiang. The pebbly coquina, Paleocene in age, is distributed along the southern pediment of the Tianshan Mountains north of Baicheng, where Pb-mineralization has been observed for the first time. The pebbly coquina consists of well-rounded quartzitic pebbles, quartz sands, shell fossils, shell fragments and equigranular fecal pellets. Four types of carbonate minerals have been revealed: 1) euhedral dolomite (Mg₆₀Ca₄₀)CO₃, 2) anhedral dolomite (Mg₅₇Ca₄₃)CO₃, 3) dolomitic calcite (Mg₂₀Ca₈₀)CO₃ and magnesitic dolomite with a trace amount of Pb(Mg₇₃Ca₂₇)CO₃. Five types of galena filling the pebbly coquina have been found: 1) in the shell tower end, 2) on the inner shell wall, 3) on the outer shell wall, 4) around the shell wall, and 5) among shell fragments. The galena replacement took place mainly in the early stage of diagenesis. Stable isotope analyses show (δ¹³C values range from −0.8 to +2.3 and δ¹⁸O values from −0.1 to +0.5, indicating a marine environment under the influence of fresh water, which is consistent with the result of megascopic facies analysis in paleogeography. It is concluded that the discovery of galena in the Tertiary pebbly coquina in the north of Baicheng, Xinjiang may provide new clues to the searching for strata-bound Pb-Zn deposits in the vast area of northern Tarim Basin.     

Fast diffusion along mobile grain boundaries in calcite

Experimental measurements of grain boundary diffusion are usually conducted on static boundaries, despite the fact that grain boundaries deep in the Earth are frequently mobile. In order to explore the possible effect of boundary mobility on grain boundary diffusion rates we have measured the uptake of ⁴⁴Ca from a layer of ⁴⁴Ca-enriched calcite powder during the static recrystallization of a single crystal of calcite at 900°C. A region about 500 μm wide adjacent to the powder layer is heterogeneously enriched in ⁴⁴Ca, and complex zoning patterns, including sharp steps in composition and continuous increases and decreases in ⁴⁴Ca content, are developed. In metamorphic rocks, these would normally be interpreted in terms of changes in pressure or temperature, Rayleigh fractionation, or episodic fluid infiltration. These explanations cannot apply to our experiments, and instead the zoning patterns are interpreted as being due to variations in grain boundary migration rate. We have applied an analytical model which allows the product of grain boundary diffusion coefficient and grain boundary width (D _GB δ) to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer. The value of D _GB δ in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries under the same conditions. In order to allow the scale of chemical equilibrium (and hence textural evolution) to be predicted under both experimental and geological conditions, we present quantitative diffusion-regime maps for static and mobile boundaries in calcite, using both published values and our new values for grain boundary diffusion in mobile boundaries. Enhanced diffusion in mobile boundaries has wide implications for the high temperature rheology of Earth materials, for geochronology, and for interpretations of the length- and time-scales of chemical mass-transport. Moreover, zones of anomalously high electrical conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones, rather than regions of anomalous mineralogy, water- or melt-content as is generally suggested.     

Interatomic potentials for CaCO3 polymorphs (calcite and aragonite), fitted to elastic and vibrational data

Calcite and aragonite have been modeled using rigid-ion, two-body Born-type potentials, supplemented by O-C-O angular terms inside the CO₃ groups. A shell model has also been developed for calcite. Atomic charges, repulsive parameters and force constants have been optimized to reproduce the equilibrium crystal structures, the elastic constants and the Raman and infrared vibrational frequencies. The rigid-ion potential RIM (atomic charges:z _O= -0.995e,z _C = 0.985e,z _Ca = 2.0e) fitted to calcite properties is able to account for those of aragonite as well. Experimental unit-cell edges, elastic constants, internal and lattice frequencies are reproduced with average relative errors of 2.1, 5.5, 2.4, 15.1% for calcite and of 0.2, 19.4, 2.5, 11.8% for aragonite, respectively. The RIM potential is suitable for thermodynamic and phase diagram simulations in the CaCO₃ system, and is discussed and compared to other potentials.     

Geochemical controls on a calcite precipitating spring

A small spring fed stream was found to precipitate calcite by mainly inorganic processes and in a nonuniform manner. The spring water originated by rainwater falling in a 0.8 km² basin, infiltrating, and dissolving calcite and dolomite followed by dissolution of gypsum or anhydrite. The Ca²⁺/Mg²⁺ indicates that calcite is probably precipitated in the subsurface from a supersaturated solution. This water emerges from the spring still about 5 times supersaturated with respect to calcite and continues calcite precipitation. When 10 times supersaturation is reached, due to CO₂ degassing the precipitation is more rapid. The calcite accumulation from the stream with a flow of 5 l/s is calculated to be 12600 kg/yr with the highest rates in areas where CO₂ degassing is the greatest. The non-equilibrium, as shown by the high calcite supersaturation, is also reflected in a variable partitioning pattern for Sr²⁺ between the water and calcite.     

Hydrogeochemical and Stable Isotope Characteristics of the River Idrijca (Slovenia), the Boundary Watershed Between the Adriatic and Black Seas

The hydrogeochemical and isotope characteristics of the River Idrijca, Slovenia, where the world’s second largest mercury (Hg) mine is located, were investigated. The River Idrijca, a typical steep mountain river, has an HCO₃ ⁻–Ca²⁺–Mg²⁺ chemical composition. Its Ca²⁺/Mg²⁺ molar ratio indicates that dolomite weathering prevails in the watershed. The River Idrijca and its tributaries are oversaturated with respect to calcite and dolomite. The pCO₂ pressure is up to 13 times over atmospheric pressure and represents a source of CO₂ to the atmosphere. δ¹⁸O values in river water indicate primary control from precipitation and enrichment of the heavy oxygen isotope of infiltrating water recharging the River Idrijca from its slopes. The δ¹³C_DIC values range from −10.8 to −6.6‰ and are controlled by biogeochemical processes in terrestrial environments and in the stream: (1) exchange with atmospheric CO₂, (2) degradation of organic matter, (3) dissolution of carbonates, and (4) tributaries. The contributions of these inputs were calculated according to steady state equations and are estimated to be—11%:19%:30%:61% in the autumn and 0%:26%:39%:35% in the spring sampling seasons.     

Carbon-oxygen isotopic covariation in hydrothermal calcite during degassing of CO2

The effect of the outgassing of CO₂ from a hydrothermal fluid on the C- and O-isotopic compositions of calcite, which is precipitated from this fluid, is quantitatively modelled in terms of batch and Rayleigh distillation equations. Both CO₂ degassing and calcite precipitation are considered to be the removal mechanisms for dissolved carbon species from the fluid. Combined degassing-precipitation models are then developed by taking H₂CO₃ and HCO ₃ ^– , respectively, as the dominant dissolved carbon species. A positive correlation array between ¹³C and ¹³O values of calcite can be yielded by the precipitation of calcite from a H₂CO ₃ ^– -dominant fluid, accompanied by a progressive decrease in temperature during CO₂ degassing, whereas calcite precipitated from a HCO ₃ ^– -dominant fluid under the same conditions tends to display much smaller variation in ¹³C values than in ¹⁸O values. The combined processes of CO₂ degassing and calcite precipitation result in lowering the ¹³C value of calcites with respect to those precipitated in a closed system simply due to temperature effect. Carbon and oxygen isotopic data for calcite from the Kushikino gold-mining area in Japan illustrate the application of quantitative modelling, and degassing of CO₂ is suggested as a more likely cause for the precipitation of the calcite and quartz in this mining area.     

Carbonate chemistry of groundwater from chalk,Givendale, East Yorkshire

Precipitation, soil and spring waters from an outlier of Chalk were analysed over a one year period for field pH, and contents of Ca⁺², Mg⁺², HCO₃^? and other dissolved solids. Measured soil log PCO₂ (atm) varied between a minimum of ?2.60 and maximum of ?1.46, and could be predicted from measurements of soil temperature. Soil waters evolved under open system conditions with respect to soil CO₂, and were undersaturated with calcite during the winter recharge period.The chemistry of the springs is related to their topographic position. Group 1 springs, located below a feather edge of chalk, had both their minimum and maximum PCO₂s predicted by the soil CO₂ data, suggesting open system CO₂ evolution. Group 2 springs, located along the scarp slope had minimum PCO₂s predicted by the soil data, but maximum PCO₂s which could only be explained by a closed system evolution from the maximum soil CO₂ observed. Group 1 springs were close to calcite saturation, whereas Group 2 springs were significantly undersaturated with calcite. The two groups could be identified by linear discriminant analysis of measured Ca²⁺, pH and HCO₃^? concentrations.     

接触变质条件下变碳酸岩CO2释放与定量计算——以郯庐断裂南段肥东县双山为例

为了探讨接触变质带内变碳酸盐岩变质过程CO₂释放的数量和排放CO₂的物理、化学及地质条件,根据递进变质反应和时间积分流体通量模型,定量分析和定量计算了双山地区变碳酸盐岩在接触变质作用中释放CO₂的通量。计算结果得到CO₂的通量值为0.729×104~2.446×104 mol/cm2,CO₂的来源以接触变质反应释放为主;CO₂的生成释放与变质程度呈正相关关系。自白云石带至方解石带变质流体中X_CO2不断升高,但钙铝榴石带由于岩浆水影响,流体通量最高而X_CO2急剧下降。      

Stable carbon and oxygen isotope investigation in historical lime mortar and plaster – Results from field and experimental study

Lime mortar and plaster were sampled from Roman, medieval and early modern buildings in Styria. The historical lime mortar and plaster consist of calcite formed in the matrix during setting and various aggregates. The stable C and O isotopic composition of the calcite matrix was analyzed to get knowledge about the environmental conditions during calcite formation. The δ¹³C_matrix and δ¹⁸O_matrix values range from −31 to 0‰ and −26 to −3‰(VPDB), respectively. Obviously, such a range of isotope values does not represent the local natural limestone assumed to be used for producing the mortar and plaster. In an ideal case, the calcite matrix in lime mortar and plaster is isotopically lighter in the exterior vs. the interior mortar layer according to the relationship δ¹⁸O_matrix = 0.61 · δ¹³C_matrix − 3.3 (VPDB). Calcite precipitation by uptake of gaseous CO₂ into alkaline Ca(OH)₂ solutions shows a similar relationship, δ¹⁸O_calcite = 0.67 · δ¹³C_calcite − 6.4 (VPDB). Both relationships indicate that the ¹³C/¹²C and ¹⁸O/¹⁶O values of the calcite reflect the setting behaviour of the lime mortar and plaster. Initially, CO₂ from the atmosphere is fixed as calcite, which is accompanied by kinetic isotope fractionation mostly due to the hydroxylation of CO₂ (δ¹³C_matrix ≈  −25‰ and δ¹⁸O_matrix ≈ −20‰). As calcite formation continued the remaining gaseous CO₂ is subsequently enriched in ¹³C and ¹⁸O causing later formed calcite to be isotopically heavier along the setting path in the matrix. Deviations from such an ideal isotopic behaviour may be due to the evolution of H₂O, e.g. evaporation, the source of CO₂, e.g. from biogenic origin, relicts of the natural limestone, and secondary effects, such as recrystallization of calcite. The results of the field and experimental study suggest that isotope values can be used as overall proxies to decipher the origin of carbonate and the formation conditions of calcite in the matrix of ancient and recent lime mortar and plaster. Moreover, these proxies can be used to select calcite matrix from historical lime mortar and plaster for ¹⁴C dating.     

Centennial-scale oscillations in oxygen and carbon isotopes of endogenic calcite from a 15,500 varve year record of the Piànico interglacial

The palaeolake record from Piànico (Southern Alps) comprises a sequence of 15,500 continuous calcite varves formed during peak interglacial conditions around 400 ka ago. The varved nature of these deposits allows precise sub-sampling of five varve year intervals for stable isotope analyses. All samples consist of calcite precipitated in the epilimnion of the lake, with contents of detrital carbonate below 4%. Four significant negative δ¹⁸O oscillations occurred during the upper half of the interglacial. The most prominent of these oscillations has an amplitude of −1.1‰ and lasted 780 varve years. The three other oscillations are shorter (125–195 varve years) and of lower amplitude (0.4–0.9‰). An additional major drop in δ¹⁸O occurs 315 varve years before the end of continuous calcite precipitation in the lake. This shift marks the end of long interglacial conditions and the beginning of harsher climate conditions and glacier advances in the Southern Alps. In contrast, the four δ¹⁸O oscillations within the period of continuous formation of calcite varves reflect natural intra–interglacial climate dynamics.     

Microbial dissolution of calcite at T = 28 °C and ambient pCO2

This study used batch reactors to quantify the mechanisms and rates of calcite dissolution in the presence and absence of a single heterotrophic bacterial species (Burkholderia fungorum). Experiments were conducted at T = 28°C and ambient pCO₂ over time periods spanning either 21 or 35 days. Bacteria were supplied with minimal growth media containing either glucose or lactate as a C source, NH₄⁺ as an N source, and H₂PO₄⁻ as a P source. Combining stoichiometric equations for microbial growth with an equilibrium mass-balance model of the H₂O-CO₂-CaCO₃ system demonstrates that B. fungorum affected calcite dissolution by modifying pH and alkalinity during utilization of ionic N and C species. Uptake of NH₄⁺ decreased pH and alkalinity, whereas utilization of lactate, a negatively charged organic anion, increased pH and alkalinity. Calcite in biotic glucose-bearing reactors dissolved by simultaneous reaction with H₂CO₃ generated by dissolution of atmospheric CO₂ (H₂CO₃ + CaCO₃ → Ca²⁺ + 2HCO₃⁻) and H⁺ released during NH₄⁺ uptake (H⁺ + CaCO₃ → Ca²⁺ + HCO₃⁻). Reaction with H₂CO₃ and H⁺ supplied ∼45% and 55% of the total Ca²⁺ and ∼60% and 40% of the total HCO₃⁻, respectively. The net rate of microbial calcite dissolution in the presence of glucose and NH₄⁺ was ∼2-fold higher than that observed for abiotic control experiments where calcite dissolved only by reaction with H₂CO₃. In lactate bearing reactors, most H⁺ generated by NH₄⁺ uptake reacted with HCO₃⁻ produced by lactate oxidation to yield CO₂ and H₂O. Hence, calcite in biotic lactate-bearing reactors dissolved by reaction with H₂CO₃ at a net rate equivalent to that calculated for abiotic control experiments. This study suggests that conventional carbonate equilibria models can satisfactorily predict the bulk fluid chemistry resulting from microbe-calcite interactions, provided that the ionic forms and extent of utilization of N and C sources can be constrained. Because the solubility and dissolution rate of calcite inversely correlate with pH, heterotrophic microbial growth in the presence of nonionic organic matter and NH₄⁺ appears to have the greatest potential for enhancing calcite weathering relative to abiotic conditions.     

The major-ion composition of silurian seawater

One-hundred fluid inclusions in Silurian marine halite were analyzed in order to determine the major-ion composition of Silurian seawater. The samples analyzed were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and one formation in the Early Silurian Canning Basin (Australia), the Mallowa Salt of the Carribuddy Group. The results indicate that the major-ion composition of Silurian seawater was not the same as present-day seawater. The Silurian ocean had lower concentrations of Mg²⁺, Na⁺, and SO₄²⁻, and much higher concentrations of Ca²⁺ relative to the ocean’s present-day composition. Furthermore, Silurian seawater had Ca²⁺ in excess of SO₄²⁻. Evaporation of Silurian seawater of the composition determined in this study produces KCl-type potash minerals that lack the MgSO₄-type late stage salts formed during the evaporation of present-day seawater. The relatively low Na⁺ concentrations in Silurian seawater support the hypothesis that oscillations in the major-ion composition of the oceans are primarily controlled by changes in the flux of mid-ocean ridge brine and riverine inputs and not global or basin-scale, seawater-driven dolomitization. The Mg²⁺/Ca²⁺ ratio of Silurian seawater was ∼1.4, and the K⁺/Ca²⁺ ratio was ∼0.3, both of which differ from the present-day counterparts of 5 and 1, respectively. Seawaters with Mg²⁺/Ca²⁺ <2 facilitate the precipitation of low-magnesian calcite (mol % Mg < 4) marine ooids and submarine carbonate cements whereas seawaters with Mg²⁺/Ca²⁺ >2 (e.g., modern seawater) facilitate the precipitation of aragonite and high-magnesian calcite. Therefore, the early Paleozoic calcite seas were likely due to the low Mg²⁺/Ca²⁺ ratio of seawater, not the pCO₂ of the Silurian atmosphere.     

Mineralogy and petrology of a piemontite-bearing schist, western Otago, New Zealand

Abstract. Pink piemontite-spessartine-bearing and grey-green spessartine-bearing manganiferous quartzose schists derived from siliceous pelagites, and green quartzofeldspathic schists, are described from the greenschist facies of the Haast Schist terrane, near Arrow Junction, western Otago. Electron microprobe data are reported for sphene, spessartine-rich garnet, manganoan epidote, piemontite, tourmaline, phengitic muscovite, chlorite, albite, haematite, rutile, manganoan calcite and chalcopyrite. Metamorphism occurred at about 6.4kbar, 400°C. X_co2 was above the quartz-rutile-calcite-sphene buffer (X_co2± 0.02) throughout the recorded metamorphic history of the piemontite schists. It dropped from above to below this critical buffering value in a spessartine-rich schist and it was close to or below the buffering value in the quartzofeldspathic schists. Production of piemontite required high f_O2, believed to be inherited from MnO_x in the parent pelagite. Substantial loss of O₂ (e.g. minimum of 0.19% by weight in one rock) during diagenesis and/or metamorphism is inferred. In the grey-green schists this inhibited piemontite formation. Slight loss of O₂ and Ca²⁺ accompanied minor late-stage replacement of piemontite by second generation spessartine. Observed zoning and mineral replacements indicate rise of temperature, drop in pressure, or invasion by solutions of lower f_O2 and X_CO2 equilibrated with surrounding schists. The detailed chemistry of the minerals studied correlates with available Mn and with bulk-rock (Fe³⁺ x 100)/(Fe²⁺+ Fe³⁺). The oxidation ratio ranges from 24 in average green quartzofeldspathic schist, through 78 in average grey-green manganiferous quartzose schist, to almost 100 in some piemontite-bearing schists. As Fe²⁺ gives way to Fe³⁺, Mg/Fe ratios tend to rise in chlorite, phengite, tourmaline, spessartine, and calcite, Mn increases and Ti decreases in haematite, Mn increases in spessartine and calcite, and Fe increases in rutile. Available divalent cations are depleted relative to Al; chlorite is more aluminous, and phengite more paragonitic than in typical Haast schists.     

Calcite dissolution kinetics in saline waters

The effect of ionic strength (I), pCO₂, and temperature on the dissolution rate of calcite was investigated in magnesium-free, phosphate-free, low calcium (m_Ca2+ ≈ 0.01 m) simple KCl and NaCl solutions over the undersaturation range of 0.4 ≤ Ω_calcite ≤ 0.8. First-order kinetics were found sufficient to describe the rate data where the rate constant (k) is dependent on the solution composition. Rates decreased with increasing I and were faster in KCl than NaCl solutions at the same I indicating that Na⁺ interacts more strongly with the calcite surface than K⁺ or that water is less available in NaCl solutions. Rates increased with increasing pCO₂ and temperature, and their influences diminished at high I. Arrhenius plots yielded a relatively high activation energy (E_a ≈ 20 ± 2 kJ mol^− 1) which indicated that dissolution was dominated by surface-controlled processes. The multiple regression model (MR) of Gledhill and Morse (2006a) was found to adequately describe the results at high I in NaCl solutions, but caution must be used when extrapolating to low I or pCO₂ values. These results are consistent with the hypothesis that the mole fraction of “free” solvent (X_{“free”H2O}) plays a significant role in the dissolution kinetics of calcite with a minimum value of  45–55% required for dissolution to proceed in undersaturated solutions at 25–55 °C and pCO₂ = 0.1–1 atm. This hypothesis has been incorporated into a modified version of the MR model of Gledhill and Morse (2006a) where X_{“free”H2O} has replaced I and the Ca²⁺ and Mg²⁺ terms have been dropped:

     

Oscillatory zoning in garnet from the Willsboro Wollastonite Skarn, Adirondack Mts, New York: a record of shallow hydrothermal processes preserved in a granulite facies terrane

Oscillatory zoning in low δ¹⁸O skarn garnet from the Willsboro wollastonite deposit, NE Adirondack Mts, NY, USA, preserves a record of the temporal evolution of mixing hydrothermal fluids from different sources. Garnet with oscillatory zoning are large (1–3 cm diameter) euhedral crystals that grew in formerly fluid filled cavities. They contain millimetre‐scale oscillatory zoning of varying grossular–andradite composition (X_Adr = 0.13–0.36). The δ¹⁸O values of the garnet zones vary from 0.80 to 6.26‰ VSMOW and correlate with X_Adr. The shape, pattern and number of garnet zones varies from crystal to crystal, as does the magnitude of the correlated chemistry changes, suggesting fluid system variability, temporal and/or spatial, over the time of garnet growth. The zones of correlated Fe content and δ¹⁸O indicate that a high Fe³⁺/Al, high δ¹⁸O fluid mixed with a lower Fe³⁺/Al and δ¹⁸O fluid. The high δ¹⁸O, Fe enriched fluids were likely magmatic fluids expelled from crystallizing anorthosite. The low δ¹⁸O fluids were meteoric in origin. These are the first skarn garnet with oscillatory zoning reported from granulite facies rocks. Geochronologic, stable isotope, petrologic and field evidence indicates that the Adirondacks are a polymetamorphic terrane, where localized contact metamorphism around shallowly intruded anorthosite was followed by a regional granulite facies overprint. The growth of these garnet in equilibrium with meteoric and magmatic fluids indicates an origin in the shallow contact aureole of the anorthosite prior to regional metamorphism. The zoning was preserved due to the slow diffusion of oxygen and cations in the large garnet and protection from deformation and recrystallization in zones of low strain in thick, rigid, garnetite layers. The garnet provide new information about the hydrothermal system adjacent to the shallowly intruded massif anorthosite that predates regional metamorphism in this geologically complex, polymetamorphic terrane.