Nanoscale effects of strontium on calcite growth: An in situ AFM study in the absence of vital effects

This experimental study presents in situ measurements of step migration rates for layer growth of calcite at various levels of superaturation and fluid Sr concentrations. Our results show that Sr has complex behavior as an impurity. At low concentrations, Sr promotes faster growth. This effect may be associated with slight shifts in calcite solubility when Sr is incorporated or may be due to as yet uncharacterized kinetic effects. At higher concentrations, Sr stops step advancement by pinning kink-sites or step edges. The threshold concentration of Sr needed to halt growth is positively correlated with supersaturation.Addition of Sr to the calcite growth system leads to significant changes in hillock morphology. Hillocks become elongate perpendicular to the projection of the c-glide plane, in contrast to the changes previously reported for Mg. Step edges also become scalloped, and the boundary between the obtuse-stepped flanks disappears and is replaced by a new step direction with edges parallel to [010].Incorporation of Sr was measured at two supersaturation levels and identical fluid [Sr]. The results indicate a strong positive correlation between fluid supersaturation and crystal Sr content. Further, Sr is strongly fractionated between obtuse- and acute-stepped flanks by a factor of approximately two. The sensitivity of Sr uptake to supersaturation may explain apparently contradictory results in the literature regarding whether Sr uptake in the calcite produced by one-celled marine organisms is controlled by temperature. In addition, Sr contents of natural calcite samples may be good indicators of the levels of supersaturation at which the crystals grew.Results of this investigation demonstrate the importance of understanding impurity-specific interactions with calcite growth surfaces at the microscopic scale. Despite similar chemical behavior in some systems, Mg and Sr clearly have very different effects on calcite growth. If Sr and other impurities are to be used as robust indicators of growth conditions in natural calcite samples, well grounded understanding of the mechanisms of recording trace element signatures in calcite is an essential step toward correctly deciphering paleoenvironmental signals from fossil calcite compositions.     

The impact of salinity on the Mg/Ca and Sr/Ca ratio in the benthic foraminifera Ammonia tepida: Results from culture experiments

Over the last decade, sea surface temperature (SST) reconstructed from the Mg/Ca ratio of foraminiferal calcite has increasingly been used, in combination with the δ¹⁸O signal measured on the same material, to calculate the δ¹⁸O_w, a proxy for sea surface salinity (SSS). A number of studies, however, have shown that the Mg/Ca ratio is also sensitive to other parameters, such as pH or , and salinity. To increase the reliability of foraminiferal Mg/Ca ratios as temperature proxies, these effects should be quantified in isolation. Individuals of the benthic foraminifera Ammonia tepida were cultured at three different salinities (20, 33 and 40 psu) and two temperatures (10-15 °C). The Mg/Ca and Sr/Ca ratios of newly formed calcite were analyzed by Laser Ablation ICP-MS and demonstrate that the Mg concentration in A. tepida is overall relatively low (mean value per experimental condition between 0.5 and 1.3 mmol/mol) when compared to other foraminiferal species, Sr being similar to other foraminiferal species. The Mg and Sr incorporation are both enhanced with increasing temperatures. However, the temperature dependency for Sr disappears when the distribution factor D_Sr is plotted as a function of calcite saturation state (Ω). This suggests that a kinetic process related to Ω is responsible for the observed dependency of Sr incorporation on sea water temperature. The inferred relative increase in D_Mg per unit salinity is 2.8% at 10 °C and 3.3% at 15 °C, for the salinity interval 20-40 psu. This implies that a salinity increase of 2 psu results in enhanced Mg incorporation equivalent to 1 °C temperature increase. The D_Sr increase per unit salinity is 0.8% at 10 °C and 1.3% at 15 °C, for the salinity interval 20-40 psu.     

Temperature effects on the calcite skeletal composition of deep-water gorgonians (Isididae)

We test for and calibrate a proxy for ocean temperature based on the skeletal composition of the widely distributed, deep-sea gorgonians in the family Isididae (bamboo corals), through use of three complementary methods: a short-term comparison of element/Ca ratios to a four-year temperature record, a long-term comparison with oceanographic records spanning forty years, and a geographic comparison of Isidids collected at sites ranging from the tropics to Antarctica. The assays consistently support a temperature-dependency for Mg/Ca ratios and suggest S/Ca is indirectly affected by temperature, but indicate little or no effect of temperature on P/Ca and Sr/Ca. The consensus relationship between Mg/Ca and temperature for Isidid calcite from the comparisons with the temperature time-series is T = −0.505 + 0.048 Mg/Ca, where T is in °C, Mg/Ca is in mmol/mol, and the applicable range is 3-6 °C. The results of the geographic assay, though imprecise, suggest the applicable range extends to temperatures below freezing. The scatter of data points around the regression of temperature and Mg/Ca is wide in all assays. This could reflect the effect of factors other than temperature on Mg/Ca ratios, but is also likely to reflect limitations of the field data, the effects of assumed constant growth rates in the corals and instrumental analytical error. The combined effects of micro-scale variability in growth rates and wide confidence intervals for each data point suggests that environmental reconstruction from Isidid internode calcite from sparse data or at time scales less than decades be done with caution. Comparisons within and among colonies do not indicate strong vital effects on ontogenetic variability in the corals, other than possibly close to the central pore of the coral. However, similar Mg/Ca ratios for Isidids from Antarctic and more temperate regions suggest adaptation to local conditions and hence a role for physiology at higher taxonomic levels, at least. Taxonomically higher level vital effects are also suggested by large differences between gorgonian families in their regressions between Mg/Ca and temperature, by Mg/Ca ratios that overlap over a wide temperature and habitat range, and for a non-linear relationship between temperature and the slope of the Mg/Ca-temperature relationship across the order.     

The use of coexisting calcite-ankerite solid solutions as a geothermometer

Limited solid solution of Mg and Fe²⁺ occurs in calcite coexisting with dolomiteankerite. This substitution is strongly temperature-dependent. Experimentally determined calcite compositions co-existing with a dolomite phase are available in the binary system CaCO₃-MgCO₃ between 500° C and 900° C (Harker and Tuttle, 1955). This information is extrapolated to lower temperatures and is combined with three synthetic calcite-ankerite pairs determined at 400° C, 450° C and 500° C (Rosenberg, 1967).The compositions of six naturally occurring calcites coexisting with ankerites from Sofala, N.S.W. are not accurately known, but X-ray determined compositional limits assuming firstly Mg substitution then Fe substitution yield maximum and minimum values for both possibilities. These limits are plotted on the ternary CaCO₃-MgCO₃-FeCO₃ together with the experimentally derived solvus isotherms. Assuming equilibration at constant temperature, actual compositions of these natural calcites plot along the 415° C isotherm.     

The effect of dissolved magnesium on creep of calcite II: transition from diffusion creep to dislocation creep

We extended a previous study on the influence of Mg solute impurity on diffusion creep in calcite to include deformation under a broader range of stress conditions and over a wider range of Mg contents. Synthetic marbles were produced by hot isostatic pressing (HIP) mixtures of calcite and dolomite powders for different intervals (2–30 h) at 850°C and 300 MPa confining pressure. The HIP treatment resulted in high-magnesian calcite aggregates with Mg content ranging from 0.5 to 17 mol%. Both back-scattered electron images and chemical analysis suggested that the dolomite phase was completely dissolved, and that Mg distribution was homogeneous throughout the samples at the scale of about two micrometers. The grain size after HIP varied from 8 to 31 μm, increased with time at temperature, and decreased with increasing Mg content (>3.0 mol%). Grain size and time were consistent with a normal grain growth equation, with exponents from 2.4 to 4.7, for samples containing 0.5–17.0 mol% Mg, respectively. We deformed samples after HIP at the same confining pressure with differential stresses between 20 and 200 MPa using either constant strain rate or stepping intervals of loading at constant stresses in a Paterson gas-medium deformation apparatus. The deformation tests took place at between 700 and 800°C and at strain rates between 10⁻⁶ and 10⁻³ s⁻¹. After deformation to strains of about 25%, a bimodal distribution of large protoblasts and small recrystallized neoblasts coexisted in some samples loaded at higher stresses. The deformation data indicated a transition in mechanism from diffusion creep to dislocation creep. At stresses below 40 MPa, the strength was directly proportional to grain size and decreased with increasing Mg content due to the reductions in grain size. At about 40 MPa, the sensitivity of log strain rate to log stress, (n), became greater than 1 and eventually exceeded 3 for stresses above 80 MPa. At a given strain rate and temperature, the stress at which that transition occurred was larger for samples with higher Mg content and smaller grain size. At given strain rates, constant temperature, and fixed grain size, the strength of calcite in the dislocation creep regime increased with solute content, while the strength in the diffusion creep regime was independent of Mg content. The results suggest that chemical composition will be an important element to consider when solid substitution can occur during natural deformation.     

The coprecipitation of Sr into calcite precipitates induced by bacterial ureolysis in artificial groundwater: Temperature and kinetic dependence

A suite of experiments was performed to investigate the partitioning of Sr²⁺ (to mimic the radionuclide ⁹⁰Sr) between calcite and artificial groundwater in response to the hydrolysis of urea (ureolysis) by Bacillus pasteurii under simulated in situ aquifer conditions. Experiments were performed at 10, 15, and 20°C over 7 days in microcosms inoculated with B. pasteurii ATCC 11859, containing an artificial groundwater and urea (AGW) or an AGW including a Sr contaminant treatment. During the experiments, the concentration of ammonium generated by bacterial ureolysis increased asymptotically, and derived rate constants (k_urea) that were between 13 and 10 times greater at 20°C than at 15 and 10°C. Calcite precipitation was initiated after similar amounts of urea had been hydrolyzed (∼ 4.0 mmol L^-1) and a similar critical saturation state (mean S_critical = 53, variation = 20%) had been reached, independent of temperature and Sr treatment. Because of the positive relationship between the rate of ureolysis and temperature, precipitation began by the end of day 1 at 20°C, and between days 1 and 2 at 15 and 10°C. The rate of calcite precipitation increased with, and was fundamentally controlled by calcite saturation state (S), irrespective of temperature. The presence of Sr slightly slowed calcite precipitation rates at equivalent values of S, which may reflect the screening of active nucleation and crystal growth sites by Sr. Homogeneous partitioning coefficients (D_Sr) exhibited a positive association with calcite precipitation rates, but were greater at higher experimental temperatures at equivalent precipitation rates (20°C mean = 0.46; 15°C mean = 0.24; 10°C mean = 0.29).     

Polycrystalline calcite to aragonite transformation kinetics: experiments in synthetic systems

The kinetics of the calcite to aragonite transformation have been investigated using synthetic polycrystalline calcite aggregates, with and without additional minerals present. The reaction progresses as a function of time were measured at four temperature/pressure conditions: (1) 550 °C/1.86 GPa; (2) 600 °C/2.11 GPa; (3) 650 °C/2.11 GPa, and (4) 700 °C/2.29 GPa. Experiments reveal that Mg-calcite and Fe-calcite transforms to aragonite at considerably slower rates than pure calcite, and that Sr-bearing calcite and calcite + quartz aggregates transform at significantly higher rates than pure calcite. The reaction progresses vs. time data for pure calcite were fitted to Cahns grain-boundary nucleation and interface-controlled growth model. Evidence for interface-controlled growth is provided by petrographic observations of grain boundaries. The activation energy for aragonite growth from the synthetic polycrystalline calcite determined in this study is significantly lower than that previously determined from a natural marble. The discrepancy in rates and activation energy may be attributed to the nature of grain boundaries, to deformational strain or the presence of impurities in the studied samples, and likely to uncertainties in experimental conditions. The results of this study imply that the variation of local petrologic conditions, in addition to temperature, pressure and grain size, may play an important role in determining the rates for the calcite to aragonite transformation in nature.Editorial responsibility: W. Schreyer     

Effect of ionic strength on the Mg content of calcite: Toward a physical basis for minor element uptake during step growth

This experimental study determined the effect of ionic strength (IS) on the uptake of Mg into calcites that grew by the classical step propagation process. Using flow-through AFM and defined solution chemistry, calcite was grown in NaCl and KCl solutions of known supersaturation state while measuring the corresponding growth kinetics. Analysis of the resulting calcite compositions by SIMS shows that Mg content is inversely correlated with IS for both electrolytes. A sixfold increase in IS decreases the Mg-content by up to 40%. Overgrowths that developed in NaCl solutions contain more Mg than samples that grew in KCl solutions. The corresponding kinetic measurements reveal that step propagation rates are independent of IS within experimental error but are electrolyte-specific. In NaCl solutions, steps with the obtuse geometry move significantly faster than acute steps, but in KCl solutions, the acute and obtuse steps move at similar rates.Analysis of the data suggests that the decrease in Mg content with increasing IS arises from the interplay of ion-kink interactions between the background cations (Na⁺ or K⁺), the primary solute cation (Ca²⁺), and the impurity (Mg²⁺). A simple physical model proposes that increasing levels of electrolytes block the attachment of the strongly hydrated Mg²⁺ ion relative to Ca²⁺ but the effects are step-specific for each type of electrolyte. Whereas K⁺ interacts weakly with kink sites along both step directions, Na⁺ interacts preferentially with acute steps and, consequently, slows their rate of step propagation relative to obtuse steps. Because Na⁺ increases the fraction of the surface that develops from acute steps and because Mg is preferentially incorporated into the kink sites of acute steps, calcite overgrowths developed in NaCl solutions contain more Mg than those in grown in KCl. Thus, the salt-specific Mg contents measured in this study can be explained by shifts in the distribution of step types and the ability of each step type to incorporate Mg. The findings reconcile apparent discrepancies regarding the effect of IS on calcite kinetics and Mg incorporation observed in laboratory-based studies.     

Authigenic layer silicate minerals in borehole Elmore 1, Salton Sea Geothermal Field,California, USA

A combined petrographic/X-ray/electron microprobe and energy dispersive system investigation of sandstone cuttings from borehole Elmore # 1 near the center of the Salton Sea Geothermal Field has revealed numerous regular variations in the composition, texture, mineralogy and proportions of the authigenic layer silicate minerals in the temperature interval 185° C (411.5 m depth) to 361° C (2,169 m). At temperatures near 190° C, dolomite/ankerite+calcite-bearing sandstones contain an illite/mixed layer phase with 10% expandable layers (dolomite/ankerite zone). In shale, the percentage of expandable layers in the mixed layer phase changes from 10–15% at 185° C to 5% at 210° C (494 m). In the interval 250° C (620 m) to 325° C (1,135 m), the calcite+pyrite+epidote-bearing sandstones contain a layer silicate assemblage of chlorite and illite (chlorite-calcite zone). In the shallower portions of this metamorphic zone, the illite contains 0–5% expandable layers, while at depths greater than 725 m (275° C) it is completely free of expandable layers. On increasing temperature, the white mica shows regular decreases in Si^IV, Mg and Fe, and increase in Al^IV, Al^VI, and interlayer occupancy, as it changes gradually from fine-grained illite (=textural sericite) to coarse-grained recrystallized phengitic white mica. In the same interval, chlorite shows decreases in Al^VI and octahedral vacancies and an increase in total Mg+Fe. The sandstones range from relatively unmodified detrital-textured rocks with porosities up to 20% and high contents of illite near 250° C to relatively dense hornfelsic-textured rocks with trace amounts of chlorite and phengite and porosities near 5% at 325° C. Numerous complex reactions among detrital (allogenic) biotite, chlorite, and muscovite, and authigenic illite and chlorite, occur in the chlorite-calcite zone.Biotite appears, and calcite disappears, at a temperature near 325° C and a depth of 1,135m. The biotite zone so produced persists to 360° C in sandstone, at which temperature orthoclase disappears and andradite garnet appears at a depth near 2,155 m. Throughout the biotite zone and into the garnet zone, the biotite undergoes compositional changes that are very similar to those observed in illite/phengite in the chlorite-calcite zone, including increases in interlayer occupancy, Al^IV, Al^VI, and Ti, and decreases in F^–, Si^IV, and Mg/Fe_t+Mg, on increasing temperature. Biotite thus changes from a siliceous, K-deficient biotite at the biotite isograd to a typical low-grade metamorphic biotite at temperatures near 360° C. Minor amounts of talc appear with biotite at the biotite isograd in sandstone, while actinolite appears in both sandstone and shale at temperatures near 340° C (1,325 m). Chlorite completely disappears from sandstone at temperatures of approximately 350° C (1,500 m), and diminishes abruptly in amount in the more chloritic shales at the same depth.     

Origin, timing, and temperature of secondary calcite-silica mineral formation at Yucca Mountain, Nevada

The origin of secondary calcite-silica minerals in primary and secondary porosity of the host Miocene tuffs at Yucca Mountain has been hotly debated during the last decade. Proponents of a high-level nuclear waste repository beneath Yucca Mountain have interpreted the secondary minerals to have formed from cool, descending meteoric fluids in the vadose zone; critics, citing the presence of two-phase fluid inclusions, argued that the minerals could only have formed in the phreatic zone from ascending hydrothermal fluids. Understanding the origin, temperature, and timing of these minerals is critical in characterizing geologically recent fluid flux at the site, and has significant implications to whether waste should be stored at Yucca Mountain.Petrographic and paragenetic studies of 155 samples collected from the Exploratory Studies Facility (ESF) and repository block cross drift (ECRB) tunnels indicate that heterogeneously distributed calcite with lesser chalcedony, quartz, opal, and fluorite comprise the oldest secondary minerals. These are typically overgrown by intermediate-aged calcite, often exhibiting distinctive bladed habits. The youngest event recorded across the site is the deposition of Mg-enriched (up to ∼1 wt%) and depleted, growth-zoned calcite intergrown with U-enriched opal. The cyclical variation in Mg enrichment and depletion is probably related to climate changes that have occurred during the last few million years. The distribution of secondary minerals is consistent with precipitation in the vadose zone.Fluid inclusion petrography of sections from the 155 samples determined that 96% of the fluid inclusion assemblages (FIAs) contained liquid-only inclusions that formed at ambient temperatures (<35°C). However, 50% of the samples (n = 78) contained relatively rare FIA that contain both liquid-only and liquid plus vapor inclusions (herein termed two-phase FIAs) that formed at temperatures above 35°C. Virtually all of these two-phase FIAs occur in paragenetically old calcite; rare two-phase inclusion assemblages were also observed in early fluorite and quartz, and early-intermediate calcite. Homogenization temperatures (≡ trapping temperatures) across Yucca Mountain are generally 45 to 60°C, but higher temperatures reaching 83°C were recorded in calcite from the north portal and ramp of the ESF. Cooler temperatures of ∼35 to 45°C were recorded in the intensely fractured zone. Multiple populations of two-phase FIAs from lithophysal cavities in the ESF and ECRB cross drift indicate early fluid cooling with time from temperatures >45°C in early calcite, to <35 to 45°C in paragenetically younger calcite. Freezing point depressions range from −0.2 to −1.6°C, indicating trapping of a low salinity fluid. The majority of intermediate calcite and all outermost Mg-enriched calcite contains rare all-liquid inclusions and formed from ambient temperature (<35°C) fluids.Carbon and oxygen isotope data reveal a consistent trend of decreasing δ¹³C (from 9.5 to −8.5‰) and increasing δ¹⁸O (from 5.2 to 22.1‰) values from paragenetically early calcite to Mg-enriched growth-zoned calcite. Depleted δD values (−131 to −90‰) of inclusion fluids from intermediate and the youngest Mg-enriched calcite indicate derivation from surface meteoric fluids. Recalculation of δ¹⁸O_H2O values of −12 to −10‰ is consistent with derivation from paleometeoric fluids.Results of integrated U-Pb dating (opal and chalcedony) and fluid inclusion microthermometry indicate that two-phase FIAs that trapped fluids of >50°C are older than 6.29 ± 0.30 Ma. Two-phase FIAs in paragenetically later calcite, which formed from fluids of 35 to 45°C, are older than 5.32 ± 0.02 Ma. There is no evidence for trapping of fluids with elevated temperatures during the past 5.32 my. The youngest Mg-enriched calcite intergrown with opal began to precipitate between about 1.9 to 2.9 Ma and has continued to precipitate within the past half million years. The presence of liquid-only inclusions and the consistent occurrence of Mg-enriched calcite and opal as the youngest event indicate a minor, but chemically distinct, ambient temperature (<35°C) fluid flux during the past 2 to 3 my.     

Controls on ostracod valve geochemistry, Part 1: Variations of environmental parameters in ostracod (micro-)habitats

The variations of environmental conditions (T°, pH, δ¹³C_DIC, [DIC], δ¹⁸O, Mg/Ca, and Sr/Ca) of ostracod habitats were examined to determine the controls of environmental parameters on the chemical and isotopic composition of ostracod valves. Results of a one-year monitoring of environmental parameters at five sites, with depths of between 2 and 70 m, in Lake Geneva indicate that in littoral to sub-littoral zones (2, 5, and 13 m), the chemical composition of bottom water varies seasonally in concert with changes in temperature and photosynthetic activity. An increase of temperature and photosynthetic activity leads to an increase in δ¹³C values of DIC and to precipitation of authigenic calcite, which results in a concomitant increase of Mg/Ca and Sr/Ca ratios of water. In deeper sites (33 and 70 m), the composition of bottom water remains constant throughout the year and isotopic values and trace element contents are similar to those of deep water within the lake. The chemical composition of interstitial pore water also does not reflect seasonal variations but is controlled by calcite dissolution, aerobic respiration, anaerobic respiration with reduction of sulphate and/or nitrate, and methanogenesis that may occur in the sediment pores. Relative influence of each of these factors on the pore water geochemistry depends on sediment thickness and texture, oxygen content in bottom as well as pore water. Variations of chemical compositions of the ostracod valves of this study vary according to the specific ecology of the ostracod species analysed, that is its life-cycle and its (micro-)habitat. Littoral species have compositions that are related to the seasonal variations of temperature, δ¹³C values of DIC, and of Mg/Ca and Sr/Ca ratios of water. In contrast, the compositions of profundal species are largely controlled by variations of pore fluids along sediment depth profiles according to the specific depth preference of the species. The control on the geochemistry of sub-littoral species is a combination of controls for the littoral and profundal species as well as the specific ecology of the species.     

Experimental determination of growth rate effect on U and Mg partitioning between aragonite and fluid at elevated U concentration

Results are reported from an experimental study in which the partitioning of U and Mg between aragonite and an aqueous solution were determined as a function of crystal growth rate. Crystals, identified as aragonite by X-ray diffractometry and micro-Raman spectroscopy, were grown by diffusion of CO₂ from an ammonium carbonate source into a calcium-bearing solution at temperatures of 22 and 53 °C. Hemispherical bundles (spherulites) of aragonite crystals were produced, the growth rates of which decreased monotonically from the spherulite interiors to the edges and thus provide the opportunity to examine the influence of growth rate on crystal composition. Element concentration ratios were measured using electron microprobe (EMP) and fluid composition was determined by inductively coupled plasma-mass spectrometry (ICP-MS) and atomic absorption (AA). Growth rates were determined directly by addition of a Dy spike to the fluid during the experiment that was subsequently located in an experimentally precipitated spherulite using secondary ion mass spectrometry (SIMS). At 22 °C both U/Ca and Mg/Ca partition coefficients exhibited a strong growth rate dependence when crystal growth rates were low, and became independent of growth rate when crystal growth rates were high. The U/Ca ratios in aragonite increase between 22 and 53 °C; in contrast Mg/Ca ratios show inverse dependence on temperature.     

Kinetics of calcite precipitation induced by ureolytic bacteria at 10 to 20°C in artificial groundwater

The kinetics of calcite precipitation induced in response to the hydrolysis of urea by Bacillus pasteurii at different temperatures in artificial groundwater (AGW) was investigated. The hydrolysis of urea by B. pasteurii exhibited a temperature dependence with first order rate constants of 0.91 d⁻¹ at 20°C, 0.18 d⁻¹ at 15°C, and 0.09 d⁻¹ at 10°C. At all temperatures, the pH of the AGW increased from 6.5 to 9.3 in less than 1 d. Dissolved Ca²⁺ concentrations decreased in an asymptotic fashion after 1 d at 20°C and 15°C, and 2 d at 10°C. The loss of Ca²⁺ from solution was accompanied by the development of solid phase precipitates that were identified as calcite by X-ray diffraction. The onset of calcite precipitation at each temperature occurred after similar amounts of urea were hydrolyzed, corresponding to 8.0 mM NH₄⁺. Specific rate constants for calcite precipitation and critical saturation state were derived from time course data following a second-order chemical affinity-based rate law. The calcite precipitation rate constants and critical saturation states varied by less than 10% between the temperatures with mean values of 0.16 ± 0.01 μmoles L⁻¹ d⁻¹ and 73 ±3, respectively. The highest calcite precipitation rates (ca. 0.8 mmol L⁻¹ d⁻¹) occurred near the point of critical saturation. While unique time course trajectories of dissolved Ca²⁺ concentrations and saturation state values were observed at different temperatures, calcite precipitation rates all followed the same asymptotic profile decreasing with saturation state regardless of temperature. This emphasizes the fundamental kinetic dependence of calcite precipitation on saturation state, which connects the otherwise dissimilar temporal patterns of calcite precipitation that evolved under the different temperature and biogeochemical regimes of the experiments.     

Formation and evolution of carbonate chimneys at the Lost City Hydrothermal Field

The Lost City Hydrothermal Field at 30°N, near the Mid-Atlantic Ridge, is an off-axis, moderate temperature, high-pH (9-10.8), serpentinite-hosted vent system. The field is hosted on ∼1.5 Ma crust, near the summit of the Atlantis Massif. Within the field, actively venting carbonate chimneys tower up to 60 m above the seafloor, making them the tallest vent structures known. The chemistry of the chimneys and vent fluids is controlled by serpentinization reactions between seawater and underlying peridotite. Mixing of <40-91 °C calcium-rich vent fluids with seawater results in the precipitation of variable mixtures of aragonite, calcite, and brucite. The resultant deposits range from tall, graceful pinnacles to fragile flanges and delicate precipitates that grow outward from fissures in the bedrock. In this study, mineralogy, petrographic analyses, major and trace element concentrations, and Sr isotopic compositions are used to propose a model for the growth and chemical evolution of carbonate chimneys in a serpentinite-hosted environment. Our results show that nascent chimneys are characterized by a porous, interlacing network of aragonite, and brucite minerals that form extremely fragile structures. The chemistry of these young deposits is characterized by ∼10 wt% Ca and up to 27 wt% Mg, extremely low trace metal concentrations, and ⁸⁷Sr/⁸⁶Sr isotope ratios near 0.70760. During aging of the chimneys, progressive reactions with seawater result in the dissolution of brucite, the conversion of aragonite to calcite, and infilling of pore spaces with calcite. The oldest chimneys are dominated by calcite, with bulk rock values of up to 36 wt% Ca and <1 wt% Mg. These older structures contain higher concentrations of trace metals (e.g., Mn and Ti), and have Sr isotope ratios near seawater values (0.70908). Exposed ultramafic rocks are prevalent along the Mid-Atlantic, Arctic, and Indian Ocean ridge networks and it is likely that other Lost City-type systems exist.     

Ca-Mg inter-diffusion in synthetic polycrystalline dolomite-calcite aggregate at elevated temperatures and pressure

This study measures the reaction rate of dolomite and aragonite (calcite) into Mg-calcite at 800, 850, and 900°C and 1.6 GPa. The dry synthetic dolomite-aragonite aggregate transformed very rapidly into dolomite-calcite polycrystalline aggregate while Mg-calcites formed at a relatively slow rate, becoming progressively richer in Mg with run time. We modeled the reaction progress semi-empirically by the first-order rate law. The temperature dependence of the overall transport rate of MgCO₃ into calcite can be described by the kinetic parameters (E?=?231.7 kJ/mol and A _o ?=?22.69 h^?1). Extrapolation using the Arrhenius equation to the conditions during exhumation of UHPM rocks indicates that the reaction of dolomite with aragonite into Mg-saturated calcite can be completed as the P-T path enters the Mg-calcite stability field in a geologically short time period (<1 Ky). On the other hand, the extrapolation of the rate to prograde metamorphic conditions reveals that the Mg-calcite formed from dolomitic marble in the absence of metamorphic fluid may not reach Mg-saturation until temperatures corresponding to high-grade metamorphism (e.g., >340°C and >10 My). SEM-EDS analysis of individual calcite grains shows compositional gradients of Mg in the calcite grains. The Mg-Ca inter-diffusion coefficient at 850°C is around 1.68?×?10^?14 m²/sec if diffusion is the major control of the reaction. The calculated closure temperatures for Ca-Mg inter-diffusion as a function of cooling rate and grain size reveal that Ca/Mg resetting in calcite in a dry polycrystalline carbonate aggregate (with grain size around 1 mm) may not occur at temperatures below 480°C at a geological cooling rate around 10°C/My, unless other processes, such as short-circuit interdiffusion along grain boundaries and dislocations, are involved.     

Rate and mechanism in prograde metamorphism

For a given rate of heat input into a prograde metamorphic sequence the extent of overstep of reaction temperature (disequilibrium) depends on the slowest of three sequential steps: (a) surface detachment of reactant minerals, (b) transport of material to the site of mineral growth, and (c) nucleation and growth of the product mineral. We have developed analytical expressions which enable determination of the rates of mineral dissolution and growth and of advective and diffusive mass transport during metamorphism. The dissolution and growth steps are linear functions of the driving force (– G) of the overall reaction while diffusion may take place either through a grain boundary fluid film or through the disorganized grain boundary itself.While little is known about heterogeneous nucleation, we argue from field observations that the rate of nucleation is not in general rate limiting. Additionally, if a fluid phase is present true grain boundary diffusion cannot be the mechanism which transports material over the mm to cm distances observed between reactant and product minerals.Simple models of contact (200° C temperature rise in 10,000 years) and regional (10° C per million years) metamorphic events lead to several conclusions concerning the rate determining step. Firstly, growth and dissolution are extremely rapid, dehydration reactions at 500° C going to completion in 2×10² years (contact) and 1×10⁴ years (regional), if all solutes are readily transported. Secondly, the effect of substantial fracture flow of fluid is to divert the transporting medium away from the grain boundary region and hence to retard the transport step. Under most such circumstances it appears that diffusive transport of aqueous SiO₂ or Mg species will be rate controlling. Despite this retardation of reaction rates, the extent of disequilibrium is rarely more than a few degrees C. Extensive disequilibrium (40° C) can only occur for reactions such as the andalusite sillimanite transformation which have very small entropy changes and which occur in rapid metamorphic events.     

Benthic foraminiferal Mg/Ca-paleothermometry: a revised core-top calibration

Core-top samples from different ocean basins have been analyzed to refine our current understanding of the sensitivity of benthic foraminiferal calcite magnesium/calcium (Mg/Ca) to bottom water temperatures (BWT). Benthic foraminifera collected from Hawaii, Little Bahama Bank, Sea of Okhotsk, Gulf of California, NE Atlantic, Ceara Rise, Sierra Leone Rise, the Ontong Java Plateau, and the Southern Ocean covering a temperature range of 0.8 to 18°C were used to revise the Cibicidoides Mg/Ca-temperature calibration. The Mg/Ca-BWT relationship of three common Cibicidoides species is described by an exponential equation: Mg/Ca = 0.867 ± 0.049 exp (0.109 ± 0.007 × BWT) (stated errors are 95% CI). The temperature sensitivity is very similar to a previously published calibration. However, the revised calibration has a significantly different preexponential constant, resulting in different predicted absolute temperatures. We attribute this difference in the preexponential constant to an analytical issue of accuracy. Some genera, notably Uvigerina, show apparently lower temperature sensitivity than others, suggesting that the use of constant offsets to account for vital effects in Mg/Ca may not be appropriate. Downcore Mg/Ca reproducibility, as determined on replicate foraminiferal samples, is typically better than 0.1 mmol mol⁻¹ (2 S.E.). Thus, considering the errors associated with the Cibicidoides calibration and the downcore reproducibility, BWT may be estimated to within ±1°C. Application of the revised core-top Mg/Ca-BWT data to Cenozoic foraminiferal Mg/Ca suggests that seawater Mg/Ca was not more than 35% lower than today in the ice-free ocean at 50 Ma.     

Intra-shell oxygen isotope ratios in the benthic foraminifera genus Amphistegina and the influence of seawater carbonate chemistry and temperature on this ratio

Using secondary ion mass spectrometry (SIMS) we looked at the natural variability in the oxygen isotope ratio of the shallow water, symbionts-bearing foraminiferan Amphistegina lobifera. Live foraminifera were collected in February 2005 in the Gulf of Eilat, Israel. Vertical section exposing the knob area of this species represents the growth history of this species from August 2004 to February 2005. SIMS profile at a resolution of ∼15 μm (representing about 2 weeks considering the size of the knob area and the life span of ≈6 months of this foraminifera species) yielded δ¹⁸O changes of ∼1.5‰ that are compatible with the known temperature changes for the Gulf of Eilat for this period (21-27 °C). Natural variability between primary and secondary calcite at the knob area were obtained on horizontal section of the upper knob area. This section is semi-tangential to the growth lines and exposes relatively wide belts of the primary calcite which could be analysed using the SIMS (beam size of 10 × 20 μm). The primary calcite δ¹⁸O value is on average more than 3‰ lower than the secondary calcite that represents the bulk of the skeleton (more than 95% by weight). A vertical profile at the knob was obtained by rastering an area of 50 × 50 μm at vertical steps of roughly 1 μm. The profile revealed a narrow zone of lower δ¹⁸O compared to the higher values above and below it. The difference between the lowest δ¹⁸O and the highest one was also close to 2‰. The δ¹⁸O in the margin - keel area of A. lobifera is also lower compared to the bulk secondary calcite. Specimens that were cultured in the laboratory at a constant temperature and inorganic carbon but at different pH have increased their CaCO₃ weight by roughly a factor of 8. Single specimen from each pH (ranging between 7.90 and 8.45) were investigated with the SIMS at the knob area. While there is some variability within each specimen (perhaps related to the primary calcite), the general trend was a decrease in δ¹⁸O with increasing pH (or CO₃²⁻ concentration), in agreement with previous studies on planktonic foraminifera. Some other specimens grown at different temperatures (between 21 and 33 °C) were also measured with the SIMS at the knob area. For each temperature, we observed also some variability, nevertheless the trend of −0.2‰/°C in δ¹⁸O is observed.     

Site-specific incorporation of uranyl carbonate species at the calcite surface

Spatially resolved luminescence spectra from U(VI) co-precipitated at the (101?4) growth surface of synthetic calcite single crystals confirm heterogeneous incorporation corresponding to the distribution of structurally non-equivalent steps composing the vicinal surfaces of spiral growth hillocks. Spectral structure from U(VI) luminescence at the “-” vicinal regions and featureless, weak luminescence at the “+” vicinal regions are consistent with previously reported observations of enrichment at the former sites during calcite growth. Luminescence spectra differ between the non-equivalent regions of the crystal, with the spectral features from the “-” vicinal region corresponding to those observed in bulk calcite samples. Subtle spectral shifts are observed from U(VI) co-precipitated with microcrystalline calcite synthesized by a different method, and all of the U(VI)-calcite sample spectra differ significantly from that of U(VI) co-precipitated with aragonite.The step-selective incorporation of U(VI) can be explained by a proposed model in which the allowed orientation for adsorption of the dominant calcium uranyl triscarbonate species is controlled by the atomic arrangement at step edges. Differences in the tilt angles of carbonate groups between non-equivalent growth steps favor adsorption of the calcium uranyl triscarbonate species at “-” steps, as observed in experiments.     

Calcium isotope fractionation in calcite and aragonite

Calcium isotope fractionation was measured on skeletal aragonite and calcite from different marine biota and on inorganic calcite. Precipitation temperatures ranged from 0 to 28°C. Calcium isotope fractionation shows a temperature dependence in accordance with previous observations: 1000 · ln(α_cc) = −1.4 + 0.021 · T (°C) for calcite and 1000 · ln(α_ar) = −1.9 + 0.017 · T (°C) for aragonite. Within uncertainty the temperature slopes are identical for the two polymorphs. However, at all temperatures calcium isotopes are more fractionated in aragonite than in calcite. The offset in δ^44/40Ca is about 0.6‰. The underlying mechanism for this offset may be related to the different coordination numbers and bond strengths of the calcium ions in calcite and aragonite crystals, or to different Ca reaction behavior at the solid-liquid interface. Recently, the observed temperature dependence of the Ca isotope fractionation was explained quantitatively by the temperature control on precipitation rates of calcium carbonates in an experimental setting (Lemarchand et al., 2004). We show that this mechanism can in principle also be applied to CaCO₃ precipitation in natural environments in normal marine settings. Following this model, Ca isotope fractionation in marine Ca carbonates is primarily controlled by precipitation rates. On the other hand the larger Ca isotope fractionation of aragonite compared to calcite can not be explained by different precipitation rates. The rate control model of Ca isotope fractionation predicts a strong dependence of the Ca isotopic composition of carbonates on ambient CO₃²⁻ concentration. While this model is in general accordance with our observations in marine carbonates, cultured specimens of the planktic foraminifer Orbulina universa show no dependence of Ca-isotope fractionation on the ambient CO₃²⁻ concentration. The latter observation implies that the carbonate chemistry in the calcifying vesicles of the foraminifer is independent from the ambient carbonate ion concentration of the surrounding water.