Mo isotope fractionation during adsorption to Fe (oxyhydr)oxides

Molybdenum (Mo) isotopes have great potential as a paleoredox indicator, but this potential is currently restricted by an incomplete understanding of isotope fractionations occurring during key (bio)geochemical processes. To address one such uncertainty we have investigated the isotopic fractionation of Mo during adsorption to a range of Fe (oxyhydr)oxides, under variable Mo/Fe-mineral ratios and pH. Our data confirm that Fe (oxyhydr)oxides can readily adsorb Mo, highlighting the potential importance of this removal pathway for the global Mo cycle. Furthermore, adsorption of Mo to Fe (oxyhydr)oxides is associated with preferential uptake of the lighter Mo isotopes. Fractionations between the solid and dissolved phase (Δ⁹⁸Mo) increase at higher pH, and also vary with mineralogy, increasing in the order magnetite (Δ⁹⁸Mo = 0.83 ± 0.60‰) < ferrihydrite (Δ⁹⁸Mo = 1.11 ± 0.15‰) < goethite (Δ⁹⁸Mo = 1.40 ± 0.48‰) < hematite (Δ⁹⁸Mo = 2.19 ± 0.54‰). Small differences in isotopic fractionation are also seen at varying Mo/Fe-mineral ratios for individual minerals. The observed isotopic behaviour is consistent with both fractionation during adsorption to the mineral surface (a function of vibrational energy) and adsorption of different Mo species/structures from solution. The different fractionation factors determined for different Fe (oxyhydr)oxides suggests that these minerals likely exert a major control on observed natural Mo isotope compositions during sediment deposition beneath suboxic through to anoxic (but non-sulfidic) bottom waters. Our results confirm that Mo isotopes can provide important information on the spatial extent of different paleoredox conditions, providing they are used in combination with other techniques for evaluating the local redox environment and the mineralogy of the depositing sediments.     

The molecular mechanism of Mo isotope fractionation during adsorption to birnessite

Fractionation of Mo isotopes during adsorption to manganese oxides is a primary control on the global ocean Mo isotope budget. Previous attempts to explain what drives the surprisingly large isotope effect δ^97/95Mo_dissolved-δ^97/95Mo_adsorbed=1.8‰ have not successfully resolved the fractionation mechanism. New evidence from extended X-ray absorption fine structure analysis and density functional theory suggests that Mo forms a polymolybdate complex on the surfaces of experimental and natural samples. Mo in this polynuclear structure is in distorted octahedral coordination, while Mo remaining in solution is predominantly in tetrahedral coordination as . Our results indicate that the difference in coordination environment between dissolved Mo and adsorbed Mo is the cause of isotope fractionation. The molecular mechanism of metal isotope fractionation in this system should enable us to explain and possibly predict metal isotope effects in other systems where transition metals adsorb to mineral surfaces.     

Experimental investigation on the carbon isotope fractionation of methane during gas migration by diffusion through sedimentary rocks at elevated temperature and pressure

Molecular transport (diffusion) of methane in water-saturated sedimentary rocks results in carbon isotope fractionation. In order to quantify the diffusive isotope fractionation effect and its dependence on total organic carbon (TOC) content, experimental measurements have been performed on three natural shale samples with TOC values ranging from 0.3 to 5.74%. The experiments were conducted at 90°C and fluid pressures of 9 MPa (90 bar). Based on the instantaneous and cumulative composition of the diffused methane, effective diffusion coefficients of the ¹²CH₄ and ¹³CH₄ species, respectively, have been calculated.Compared with the carbon isotopic composition of the source methane (δ¹³C₁ = −39.1‰), a significant depletion of the heavier carbon isotope (¹³C) in the diffused methane was observed for all three shales. The degree of depletion is highest during the initial non-steady state of the diffusion process. It then gradually decreases and reaches a constant difference (Δ δ = δ¹³C_diff −δ¹³C_source) when approaching the steady-state. The degree of the isotopic fractionation of methane due to molecular diffusion increases with the TOC content of the shales. The carbon isotope fractionation of methane during molecular migration results practically exclusively from differences in molecular mobility (effective diffusion coefficients) of the ¹²CH₄ and ¹³CH₄ entities. No measurable solubility fractionation was observed.The experimental isotope-specific diffusion data were used in two hypothetical scenarios to illustrate the extent of isotopic fractionation to be expected as a result of molecular transport in geological systems with shales of different TOC contents. The first scenario considers the progression of a diffusion front from a constant source (gas reservoir) into a homogeneous “semi-infinite” shale caprock over a period of 10 Ma.In the second example, gas diffusion across a 100 m caprock sequence is analyzed in terms of absolute quantities and isotope fractionation effects. The examples demonstrate that methane losses by molecular diffusion are small in comparison with the contents of commercial size gas accumulations. The degree of isotopic fractionation is related inversely to the quantity of diffused gas so that strong fractionation effects are only observed for relatively small portions of gas.The experimental data can be readily used in numerical basin analysis to examine the effects of diffusion-related isotopic fractionation on the composition of natural gas reservoirs.     

Copper isotope fractionation during surface adsorption and intracellular incorporation by bacteria

Copper isotopes may prove to be a useful tool for investigating bacteria-metal interactions recorded in natural waters, soils, and rocks. However, experimental data which attempt to constrain Cu isotope fractionation in biologic systems are limited and unclear. In this study, we utilized Cu isotopes (δ⁶⁵Cu) to investigate Cu-bacteria interactions, including surface adsorption and intracellular incorporation. Experiments were conducted with individual representative species of Gram-positive (Bacillus subtilis) and Gram-negative (Escherichia coli) bacteria, as well as with wild-type consortia of microorganisms from several natural environments. Ph-dependent adsorption experiments were conducted with live and dead cells over the pH range 2.5-6. Surface adsorption experiments of Cu onto live bacterial cells resulted in apparent separation factors (Δ⁶⁵Cu_{solution-solid} = δ⁶⁵Cu_solution − δ⁶⁵Cu_solid) ranging from +0.3‰ to +1.4‰ for B. subtilis and +0.2‰ to +2.6‰ for E. coli. However, because heat-killed bacterial cells did not exhibit this behavior, the preference of the lighter Cu isotope by the cells is probably not related to reversible surface adsorption, but instead is a metabolically-driven phenomenon. Adsorption experiments with heat-killed cells yielded apparent separation factors ranging from +0.3‰ to −0.69‰ which likely reflects fractionation from complexation with organic acid surface functional group sites. For intracellular incorporation experiments the lab strains and natural consortia preferentially incorporated the lighter Cu isotope with an apparent Δ⁶⁵Cu_{solution-solid} ranging from ∼+1.0‰ to +4.4‰. Our results indicate that live bacterial cells preferentially sequester the lighter Cu isotope regardless of the experimental conditions. The fractionation mechanisms involved are likely related to active cellular transport and regulation, including the reduction of Cu(II) to Cu(I). Because similar intracellular Cu machinery is shared by fungi, plants, and higher organisms, the influence of biological processes on the δ⁶⁵Cu of natural waters and soils is probably considerable.     

Effect of temperature and mass transport on transition metal isotope fractionation during electroplating

Transition metal stable isotope signatures can be useful for tracing both natural and anthropogenic signals in the environment, but only if the mechanisms responsible for fractionation are understood. To investigate isotope fractionations due to electrochemistry (or redox processes), we examine the stable isotope behavior of iron and zinc during the reduction reaction  + 2e⁻ = M_metal as a function of electrochemical driving force, temperature, and time. In all cases light isotopes are preferentially electroplated, following a mass-dependent law. Generally, the extent of fractionation is larger for higher temperatures and lower driving forces, and is roughly insensitive to amount of charge delivered. The maximum fractionations are δ^56/54Fe = −4.0‰ and δ^66/64Zn = −5.5‰, larger than observed fractionations in the natural environment and larger than those predicted due to changes in speciation. All the observed fractionation trends are interpreted in terms of three distinct processes that occur during an electrochemical reaction: mass transport to the electrode, chemical speciation changes adjacent to the electrode, and electron transfer at the electrode. We show that a large isotope effect adjacent the electrode surface arises from the charge-transfer kinetics, but this effect is attenuated in cases where diffusion of ions to the electrode surface becomes the rate-limiting step. Thus while a general increase in fractionation is observed with increasing temperature, this appears to be a result of thermally enhanced mass transport to the reacting interface rather than an isotope effect associated with the charge-transfer kinetics. This study demonstrates that laboratory experiments can successfully distinguish isotopic signatures arising from mass transport, chemical speciation, and electron transfer. Understanding how these processes fractionate metal isotopes under laboratory conditions is the first step towards discovering what role these processes play in fractionating metal isotopes in natural systems.     

Experimental determination of magnesium isotope fractionation during higher plant growth

Two higher plant species (rye grass and clover) were cultivated under laboratory conditions on two substrates (solution, phlogopite) in order to constrain the corresponding Mg isotope fractionations during plant growth and Mg uptake. We show that bulk plants are systematically enriched in heavy isotopes relative to their nutrient source. The Δ²⁶Mg_plant-source range from 0.72‰ to 0.26‰ for rye grass and from 1.05‰ to 0.41‰ for clover. Plants grown on phlogopite display Mg isotope signatures (relative to the Mg source) ∼0.3‰ lower than hydroponic plants. For a given substrate, rye grass display lower δ²⁶Mg (by ∼0.3‰) relative to clover. Magnesium desorbed from rye grass roots display a δ²⁶Mg greater than the nutrient solution. Adsorption experiments on dead and living rye grass roots also indicate a significant enrichment in heavy isotopes of the Mg adsorbed on the root surface. Our results indicate that the key processes responsible for heavy isotope enrichment in plants are located at the root level. Both species also exhibit an enrichment in light isotopes from roots to shoots (Δ²⁶Mg_leaf-root = −0.65‰ and −0.34‰ for rye grass and clover grown on phlogopite respectively, and Δ²⁶Mg_leaf-root of −0.06‰ and −0.22‰ for the same species grown hydroponically). This heavy isotope depletion in leaves can be explained by biological processes that affect leaves and roots differently: (1) organo-Mg complex (including chlorophyll) formation, and (2) Mg transport within plant. For both species, a positive correlation between δ²⁶Mg and K/Mg was observed among the various organs. This correlation is consistent with the link between K and Mg internal cycles, as well as with formation of organo-magnesium compounds associated with enrichment in heavy isotopes. Considering our results together with the published range for δ²⁶Mg of natural plants and rivers, we estimate that a significant change in continental vegetation would induce a change of the mean river δ²⁶Mg that is comparable to analytical uncertainties.     

Carbon isotope fractionation during low temperature carbonization of foxtail and common millets

Stable carbon isotopes of organic matter and fossilized plant remains can be used to effectively reconstruct local palaeoclimate changes, especially from plants using a single photosynthetic mode. The charred grains of foxtail and common millet are chemically stable in the environment and have been preserved widely and continuously throughout the Holocene in North China. The charred remains of these species are ideal materials for reconstructing the palaeoclimate based on δ¹³C of foxtail and common millets heated to temperatures up to around 250 °C. This study reports δ¹³C values of modern millets carbonized at different temperatures. The results indicate that there are no significant changes in δ¹³C of intact and charred samples of foxtail millet (?0.46‰) and common millet (?0.49‰) for temperatures below 300 °C. The δ¹³C of charred foxtail millet formed at 250 °C were 0.2‰ higher in δ¹³C than the source samples. In contrast, the δ¹³C of charred common millet formed at 250 °C were 0.2‰ lighter in δ¹³C than the source samples. The δ¹³C values of grains were determined in part by the carbon content (i.e., starches, lignins and lipids) and the variable thermal tolerances of these compounds to heating. However, the observed ¹³C carbonization associated with fractionation of only 0.2‰ in grains is much less than the natural variation typically found in wood. We therefore suggest that δ¹³C measured in carbonized grains can serve as an effective indicator for paleoenvironmental and archaeological reconstructions.     

Experimental determination of the role of diffusion on Li isotope fractionation during basaltic glass weathering

In order to use lithium isotopes as tracers of silicate weathering, it is of primary importance to determine the processes responsible for Li isotope fractionation and to constrain the isotope fractionation factors caused by each process as a function of environmental parameters (e.g. temperature, pH). The aim of this study is to assess Li isotope fractionation during the dissolution of basalt and particularly during leaching of Li into solution by diffusion or ion exchange. To this end, we performed dissolution experiments on a Li-enriched synthetic basaltic glass at low ratios of mineral surface area/volume of solution (S/V), over short timescales, at various temperatures (50 and 90 °C) and pH (3, 7, and 10). Analyses of the Li isotope composition of the resulting solutions show that the leachates are enriched in ⁶Li (δ⁷Li = +4.9 to +10.5‰) compared to the fresh basaltic glass (δ⁷Li = +10.3 ± 0.4‰). The δ⁷Li value of the leachate is lower during the early stages of the leaching process, increasing to values close to the fresh basaltic glass as leaching progresses. These low δ⁷Li values can be explained in terms of diffusion-driven isotope fractionation. In order to quantify the fractionation caused by diffusion, we have developed a model that couples Li diffusion with dissolution of the glassy silicate network. This model calculates the ratio of the diffusion coefficients of both isotopes (a = D₇/D₆), as well as its dependence on temperature, pH, and S/V. a is mainly dependent on temperature, which can be explained by a small difference in activation energy (0.10 ± 0.02 kJ/mol) between ⁶Li⁺ and ⁷Li⁺. This temperature dependence reveals that Li isotope fractionation during diffusion is low at low temperatures (T < 20 °C), but can be significant at high temperatures. However, concerning hydrothermal fluids (T > 120 °C), the dissolution rate of basaltic glass is also high and masks the effects of diffusion. These results indicate that the high δ⁷Li values of river waters, in particular in basaltic catchments, and the fractionated values of hydrothermal fluids are mainly controlled by precipitation of secondary phases.     

Molecular dynamics simulations of kinetic isotope fractionation during the diffusion of ionic species in liquid water

Interpretation of isotope ratios, a powerful tool in geochemical investigations of fluid-rock systems, requires an understanding of all relevant processes that fractionate isotopes. One such process, diffusion in liquid water, has remained problematic despite its potential significance as a major cause of kinetic isotope fractionation. Recent laboratory experiments published by [Richter, F. M., Mendybaev, R. A., Christensen, J. N., Hutcheon, I. D., Williams, R. W., Sturchio, N. C., and Beloso Jr., A. D. (2006) Kinetic isotopic fractionation during diffusion of ionic species in water. Geochim. Cosmochim. Acta70, 277-289.] have shown clearly for the first time that lithium and chloride isotopes are fractionated by diffusion in liquid water, whereas magnesium isotopes are not. In the present paper, we present the results of molecular dynamics simulations of lithium, chloride, and magnesium diffusion in liquid water that were designed to provide molecular-scale insight into the experimental findings of Richter et al. (2006). Our results indicate that the self-diffusion coefficients of lithium, chloride, and magnesium isotopes follow an inverse power-law dependence on ion mass (, where D_i is the self-diffusion coefficient of a solute with isotopic mass m_i). The power-law exponents (β) deduced for lithium, chloride, and magnesium from the diffusivity data of Richter et al. (2006) are consistent with the mass dependencies found in our simulations. Further analysis of our simulation results showed that the experimental β-values are inversely related to the residence times of water molecules in the first solvation shells of the diffusing ions, as expected from mode-coupling and renormalized kinetic theories.     

Ni isotope fractionation during coprecipitation of Fe(III)(oxyhydr)oxides in Si solutions

The dramatic decline in aqueous Ni concentrations in the Archean oceans during the Great Oxygenation Event is evident in declining solid phase Ni concentrations in Banded Iron Formations (BIFs) at the time. Several experiments have been performed to identify the main removal mechanisms of Ni from seawater into BIFs, whereby adsorption of Ni onto ferrihydrites has shown to be an efficient process. Ni isotopic measurements have shown limited isotopic fraction during this process, however, most experiments have been conducted in simple solutions containing varying proportions of dissolved Fe and Ni as NO₃ salts, as opposed to Cl salts which are dominant in seawater. Further, Archean oceans were, before the advent of siliceous eukaryotes, likely saturated with amorphous Si as seen in the interlayered chert layers within BIFs. Despite Si being shown to greatly affect the Ni elemental partitioning onto ferrihydrite solids, no studies have been made on the effects of Si on the Ni isotope fractionation. Here we report results of multiple coprecipitation experiments where ferrihydrite precipitated in mixed solutions with Ni and Si. Ni concentrations in the experiments ranged between 200 and 4000 nM for fixed concentrations of Si at either 0, 0.67 or 2.2 mM. The results show that Si at these concentrations has a limited effect on the Ni isotope fractionation during coprecipitation of ferrihydrite. At 0.67 mM, the saturation concentration of cristobalite, the isotopic fractionation factors between the precipitating solid and experimental fluid are identical to experiments not containing Si (0.34 ± 0.17‰). At 2.2 mM Si, and the saturation concentration of amorphous silica, however, the Ni isotopic composition of the ferrihydrite solids deviate to more negative values and show a larger variation than at low or no Si, and some samples show fractionation of up to 0.5‰. Despite this seemingly more unstable fractionation behaviour, the combined results indicate that even at as high concentrations of Si as 2.2 mM, the δ⁶⁰Ni values of the forming ferrihydrites does not change much. The results of our study implicate that Si may not be a major factor in fractionating stable Ni isotopes, which would make it easier to interpret future BIF record and reconstruct Archean ocean chemistry.     

Experimental and theoretical study of pressure effects on hydrogen isotope fractionation in the system brucite-water at elevated temperatures

A detailed, systematic experimental and theoretical study was conducted to investigate the effect of pressure on equilibrium D/H fractionation between brucite (Mg(OH)₂) and water at temperatures from 200 to 600°C and pressures up to 800 MPa. A fine-grained brucite was isotopically exchanged with excess amounts of water, and equilibrium D/H fractionation factors were calculated by means of the partial isotope exchange method. Our experiments unambiguously demonstrated that the D/H fractionation factor between brucite and water increased by 4.4 to 12.4‰ with increasing pressure to 300 or 800 MPa at all the temperatures investigated. The observed increases are linear with the density of water under experimental conditions. We calculated the pressure effects on the reduced partition function ratios (β-factor) of brucite (300-800 K and P ≤ 800 MPa) and water (400-600°C and P ≤ 100 MPa), employing a statistical-mechanical method similar to that developed by Kieffer (1982) and a simple thermodynamic method based on the molar volumes of normal and heavy waters, respectively. Our theoretical calculations showed that the reduced partition function ratio of brucite increases linearly with pressure at a given temperature (as much as 12.6‰ at 300 K and 800 MPa). The magnitude of the pressure effects rapidly decreases with increasing temperature. On the other hand, the β-factor of water decreases 4 to 5‰ with increasing pressure to 100 MPa at 400 to 600°C. Overall D/H isotope pressure effects combined from the separate calculations on brucite and water are in excellent agreement with the experimental results under the same temperature-pressure range. Our calculations also suggest that under the current experimental conditions, the magnitude of the isotope pressure effects is much larger on water than brucite. Thus, the observed pressure effects on D/H fractionation are common to other systems involving water. It is very likely that under some geologic conditions, pressure is an important variable in controlling D/H partitioning.     

Experimental constraints on Fe isotope fractionation during magnetite and Fe carbonate formation coupled to dissimilatory hydrous ferric oxide reduction

Iron isotope fractionation between aqueous Fe(II) and biogenic magnetite and Fe carbonates produced during reduction of hydrous ferric oxide (HFO) by Shewanella putrefaciens, Shewanella algae, and Geobacter sulfurreducens in laboratory experiments is a function of Fe(III) reduction rates and pathways by which biogenic minerals are formed. High Fe(III) reduction rates produced ⁵⁶Fe/⁵⁴Fe ratios for Fe(II)_aq that are 2-3‰ lower than the HFO substrate, reflecting a kinetic isotope fractionation that was associated with rapid sorption of Fe(II) to HFO. In long-term experiments at low Fe(III) reduction rates, the Fe(II)_aq-magnetite fractionation is −1.3‰, and this is interpreted to be the equilibrium fractionation factor at 22°C in the biologic reduction systems studied here. In experiments where Fe carbonate was the major ferrous product of HFO reduction, the estimated equilibrium Fe(II)_aq-Fe carbonate fractionations were ca. 0.0‰ for siderite (FeCO₃) and ca. +0.9‰ for Ca-substituted siderite (Ca_0.15Fe_0.85CO₃) at 22°C. Formation of precursor phases such as amorphous nonmagnetic, noncarbonate Fe(II) solids are important in the pathways to formation of biogenic magnetite or siderite, particularly at high Fe(III) reduction rates, and these solids may have ⁵⁶Fe/⁵⁴Fe ratios that are up to 1‰ lower than Fe(II)_aq. Under low Fe(III) reduction rates, where equilibrium is likely to be attained, it appears that both sorbed Fe(II) and amorphous Fe(II)(s) components have isotopic compositions that are similar to those of Fe(II)_aq.The relative order of δ⁵⁶Fe values for these biogenic minerals and aqueous Fe(II) is: magnetite > siderite ≈ Fe(II)_aq > Ca-bearing Fe carbonate, and this is similar to that observed for minerals from natural samples such as Banded Iron Formations (BIFs). Where magnetite from BIFs has δ⁵⁶Fe >0‰, the calculated δ⁵⁶Fe value for aqueous Fe(II) suggests a source from midocean ridge (MOR) hydrothermal fluids. In contrast, magnetite from BIFs that has δ⁵⁶Fe ≤0‰ apparently requires formation from aqueous Fe(II) that had very low δ⁵⁶Fe values. Based on this experimental study, formation of low-δ⁵⁶Fe Fe(II)_aq in nonsulfidic systems seems most likely to have been produced by dissimilatory reduction of ferric oxides by Fe(III)-reducing bacteria.     

Oxygen isotope fractionation during the dolomitization of calcium carbonate

The oxygen isotope fractionation accompanying the hydrothermal dolomitization of CaCO₃ between 252 and 295°C has been investigated. Dolomitization (which occurs via the crystallization of one or more intermediate magnesian calcite phases) is characterised by a progressive lowering in δ⁸O, which smoothly correlates with the change in the Mg/(Mg + Ca) and the $\text{Sr}\text{(Mg + Ca)}$ ratios and with the sequential phase formation. The data support the proposals of Katz and Matthews (1977) that (a) all reaction occurs by solution and reprecipitation, (b) intermediate phases and dolomite form sequentially and (c) the intermediate phases form within limited solution zones surrounding the dissolving precursor. Calculated volumes of the solution zone for the aragonite → low magnesian calcite transformation are within the range 3.7–6.7 × 10^?5 liters (out of 5 × 10^?3 liters, the volume of the bulk solution used in the present study), and agree well with those calculated from strontium and magnesium partitioning data. Dolomite precipitates in apparent isotopic equilibrium with the bulk solution. The temperature dependence of the fractionation is defined by the equation 1000 Inα_D-H2O = 3.06 × 10⁶T^?2 ? 3.24 Dolomite-water fractionations from this equation are significantly lower than those obtained by extrapolation of the Northrop And Clayton (1966) calibration. The reaction zone model can be applied to explain near zero dolomite-calcite oxygen isotope fractionations reported by Epsteinet al. (1964).     

养护温度对水泥固化淤泥强度影响试验研究

采用水泥固化海泥（CSMC）作为围海垦地填料正在被广泛关注。由于水化放热,当环境温度较高时大体积填筑的CSMC内部实际养护温度可能明显高于室内标准养护温度（≈ 20 ℃）,这必然会对CSMC的强度发展产生影响,但目前工程实践中并未考虑这一温度差异的影响。因此,基于室内试验,测试了不同配合比CSMC试样在不同养护温度下的强度发展过程,并重点探讨了养护温度对CSMC强度发展的影响规律及其内在机制。结果表明：养护温度对CSMC强度发展影响显著,高的养护温度不仅会显著提高CSMC的早期强度,而且会比较明显地提高CSMC的晚期强度,这就意味着在CSMC垦地工程实践中考虑养护温度的影响是十分必要的。鉴于此,文中最后给出了如何在CSMC垦地工程实践中考虑养护温度对强度发展影响的建议和设计流程。     

Experimental calibration of oxygen isotope fractionation between quartz and zircon

We report the results of an experimental calibration of oxygen isotope fractionation between quartz and zircon. Data were collected from 700 to 1000 °C, 10–20 kbar, and in some experiments the oxygen fugacity was buffered at the fayalite–magnetite–quartz equilibrium. Oxygen isotope fractionation shows no clear dependence on oxygen fugacity or pressure. Unexpectedly, some high-temperature data (900–1000 °C) show evidence for disequilibrium oxygen isotope partitioning. This is based in part on ion microprobe data from these samples that indicate some high-temperature quartz grains may be isotopically zoned. Excluding data that probably represent non-equilibrium conditions, our preferred calibration for oxygen isotope fractionation between quartz and zircon can be described by:

This relationship can be used to calculate fractionation factors between zircon and other minerals. In addition, results have been used to calculate WR/melt–zircon fractionations during magma differentiation. Modeling demonstrates that silicic magmas show relatively small changes in δ¹⁸O values during differentiation, though late-stage mafic residuals capable of zircon saturation contain elevated δ¹⁸O values. However, residuals also have larger predicted melt–zircon fractionations meaning zircons will not record enriched δ¹⁸O values generally attributed to a granitic protolith. These results agree with data from natural samples if the zircon fractionation factor presented here or from natural studies is applied.     

Impact of soil weathering degree on silicon isotopic fractionation during adsorption onto iron oxides in basaltic ash soils, Cameroon

The sequestration of silicon in soil clay-sized iron oxides may affect the terrestrial cycle of Si. Iron oxides indeed specifically adsorb aqueous monosilicic acid (H₄SiO₄⁰), thereby influencing Si concentration in soil solution. Here we study the impact of H₄SiO₄⁰ adsorption on the fractionation of Si isotopes in basaltic ash soils differing in weathering degree (from two weathering sequences, Cameroon), hence in clay and Fe-oxide contents, and evaluate the potential isotopic impact on dissolved Si in surrounding Cameroon rivers. Adsorption was measured in batch experiment series designed as function of time (0-72 h) and initial concentration (ic) of Si in solution (0.61-1.18 mM) at 20 °C, constant pH (5.5) and ionic strength (1 mM). After various soil-solution contact times, the δ³⁰Si vs. NBS28 compositions were determined in selected solutions by MC-ICP-MS (Nu Plasma) in medium resolution, operating in dry plasma with Mg doping with an average precision of ±0.15‰ (±2σ_SEM). The quantitative adsorption of H₄SiO₄⁰ by soil Fe-oxides left a solution depleted in light Si isotopes, which confirms previous study on synthetic Fe-oxides. Measured against its initial composition (δ³⁰Si = +0.02 ± 0.07‰ (±2σ_SD)), the solutions were systematically enriched in ³⁰Si reaching maximum δ³⁰Si values ranging between +0.16‰ and +0.95‰ after 72 h contact time. The enrichment of the solution in heavy isotopes increased with increasing values of three parameters: soil weathering degree, iron oxide content, and proportion of short-range ordered Fe-oxide. The Si-isotopic signature of the solution was partly influenced by Si release, possibly through mineral dissolution and Si desorption from oxide surfaces, depending on soil type, highlighting the complex pattern of natural soils. Surrounding Cameroon rivers displayed a mean Si-isotopic signature of +1.19‰. Our data imply that in natural environments, H₄SiO₄⁰ adsorption by soil clay-sized Fe-oxides at least partly impacts the Si-isotopic signature of the soil solution exported to water streams.     

硅同位素动力学分馏的实验研究及地质应用

由于硅在自然界中没有化学阶态变化，主要呈硅氧四面体形式存在，因此硅同位素的热力学分馏很小，由此产生的硅同位素变化不大。溶液中ＳｉＯ２沉淀过程的同位素动力学分馏是引起自然界硅同位素明显变化的主要原因。本文实验测定了溶液中ＳｉＯ２沉淀过程的硅同位素动力学分馏系数α，并结合所测定的硅同位素动力学分馏系数对现代海底黑烟囱、太古代条带状磁铁石英岩、风化成因粘土矿物的δ＾３０Ｓｉ值明显偏低；浅海碳酸盐台地中硅     

Experimental investigation on tensile strength and its loading rate effect of sandstone after high temperature treatment

High temperature damage of rock is a critical problem that must be well known for underground coal gasification, underground storage of nuclear waste, repairment of underground buildings after fire disaster. In order to study the influences of loading rate and high temperature on the tensile strength of sandstone, Brazilian splitting tests were conducted on sandstone disk samples treated with five different temperature levels between 25 and 800 °C at six different loading rates between 0.01 and 10 mm/min. Test results showed that tensile strength of disk samples increases gradually and reaches to the maximum at the temperature level of 400 °C, then drop sharply. The tensile strength of sandstone samples is characterized by obvious rate effect and it increases continuously with the increase of loading rate, meeting a Logarithmic Function. The lower limit tensile strength of sandstone ignoring the influence of rate effect was calculated, and it is helpful for providing some basis for the design of rock engineering.     

Hydrogen and oxygen isotope fractionation during crystallization of mirabilite and ice

The equilibrium fractionation factors between mirabilite (Na₂SO₄·10H₂O) and saturated sodium sulphate solution at 25°C and 0°C and between ice and 2·5 molal sodium chloride solution at ?10°C have been measured. For mirabilite, the deuterium factors are 1·017 and 1·019, and the oxygen-18 factors are 1·0014 and 1·0020 at 25°C and 0°C, respectively. For ice, the factors are 1·024 for deuterium and 1·0022 for oxygen-18 at ?10°C. These fractionation factors are used to estimate the fractionation factors between ice and mirabilite and concentrated sea water at ?10°C. It is concluded that the average binding strengths of hydrogen in ice and mirabilite are very similar.