Si Isotope Fractionation Research Articles

Silicon isotope systematics of acidic weathering of fresh basalts, Kilauea Volcano, Hawai’i

Silicon stable isotopes are fractionated by a host of low-temperature aqueous processes, making them potentially useful as a weathering proxy. Here we characterize the silicon isotope signature of surficial chemical weathering of glassy basaltic lava flows at Kilauea Volcano, Hawaii. Fresh basalt flow surfaces (<40years old) frequently feature opaque amorphous silica surface coatings up to 80μm thick. These silica coatings and associated silica cements are enriched in the heavier isotopes of Si (δ30SiNBS-28=+0.92‰ to +1.36‰) relative to their basaltic substrate (δ30SiNBS-28=−0.3‰ to −0.2‰). Secondary clays and opals are typically depleted in 30Si relative to the dissolved reservoirs from which they precipitated, so this sense of isotopic fractionation is unusual. Mechanisms capable of producing isotopically heavy secondary minerals were explored by conducting batch alteration experiments on fresh basaltic glass. Batch acidic alteration of basalt glass in HCl, H2SO4, and HF produced silica-rich surface layers resembling the Hawaiian surface coatings. Differences in fluid chemical composition affected the direction and magnitude of Si isotope fractionation. Basalt leaching in HCl or H2SO4 produced 30Si-enriched fluids (1000 lnαprecip-Si(aq)≅−0.8‰) and 30Si-depleted secondary silica layers. In contrast, HF-bearing experiments produced highly 30Si-depleted fluid compositions (1000 lnαprecip-Si(aq) up to +8‰). Larger isotopic fractionations were observed in experiments with lower fluid–rock ratios. In Hawaii, where altering fluids contain H2SO4 and HCl but minimal HF, high δ30Si values for the silica coatings were likely achieved by Rayleigh fractionation. Aqueous 30Si-enriched silica was released during incongruent basalt dissolution then subsequently transported and deposited from an evaporating solution at the flow surface. Our results indicate that (1) altering fluid chemistry and fluid–rock ratio impact the Si isotope signature of chemical weathering and (2) δ30Si of solids produced by low temperature aqueous alteration may diverge sharply from watershed- or landscape-scale weathering trends.

Silicon isotopes: from cosmos to benthos

Silicon is the second most abundant element on the Earth and one of the more abundant elements in our Solar System. Variations in the relative abundance of the stable isotopes of Si (Si isotope fractionation) in different natural reservoirs, both terrestrial (surface and deep Earth) as well as extra-terrestrial (e.g. meteorites, lunar samples), are a powerful tracer of present and past processes involving abiotic as well as biotic systems. The versatility of the Si isotope tracer is reflected in its wide-ranging applications from understanding the origin of early Solar System objects, planetary differentiation, Moon formation, mantle melting and magma differentiation on the Earth, ancient sea-water composition, to modern-day weathering, clay formation and biological fractionation on land as well as in the oceans. The application of Si isotopes as tracers of natural processes started over six decades ago and its usage has seen a sudden increase over the last decade due to improvements in mass spectrometry, particularly the advent of multi-collector inductively coupled plasma mass spectrometers, which has made Si isotope measurements safe and relatively easy while simultaneously improving the accuracy and precision of measurements.

The effect of Al on Si isotope fractionation investigated by silica precipitation experiments

Mass-dependent isotope fractionation occurring during precipitation of solids in low-temperature environments often depends on precipitation rate. Using a series of precipitation experiments in which continuous precipitation and dissolution of Si solids is forced by daily cyclic freezing (solid formation) and thawing (solid re-dissolution), we show this dependence. We conducted six Si precipitation experiments for about 120days with initial dissolved Si concentration of 1.6mmol/l Si, at pH values between 4.5 and 7, with additions of 0.1–1mM of dissolved aluminum (Al), and in the absence of Al. During all experiments increasing amounts of an X-ray amorphous silica-containing solid are formed. No Si isotope fractionation occurs during formation of almost pure Si solids, interpreted as an absence of Si isotope fractionation during polymerization of silicic acid. Si isotope fractionation occurs only in the high-Al concentration experiments, characterized by an enrichment of the light Si isotopes in the solids formed early. With ongoing duration of the experiments, a re-dissolution of these solids is indicated as the Si isotope value of the complementary solution shifts to lighter values and eventually reaches near-initial compositions. Hence, our high-Al experiments are characterized by a gradual shift from a regime that is dominated by unidirectional kinetic isotope fractionation with solids formed that are up to 5‰ lighter in their 30Si/28Si ratio than the corresponding solution, to one of steady-state between dissolution and precipitation with the 30Si/28Si ratio of the solid being almost identical to the solution (Δsolid‐solution≈0). This suggests that the enrichment of light Si isotopes found in natural environments is caused exclusively by a unidirectional kinetic isotope effect during fast precipitation of solids, aided by co-precipitation with Al phases or other carrier phases (e.g. Fe(III)). By contrast, during slow precipitation, or in the absence of a carrier phase like Al, no Si isotope fractionation is expected and solids obtain the composition of the ambient fluid.

Isotope fractionation induced by phase transformation: First-principles investigation for Mg2SiO4

With increasing pressure, olivine transforms to wadsleyite, ringwoodite, and finally dissociates into bridgmanite and ferropericlase, producing dramatic structure changes along these phase transformations. Here we investigated the equilibrium isotope fractionation of Mg, Si, and O among these phases using first-principles calculations. Both Mg and Si have measurable isotope fractionations among these phases, even at mantle's pressure–temperature conditions. Wadsleyite and ringwoodite are depleted in heavy Si but enriched in heavy Mg relative to olivine. Bridgmanite is depleted in heavy Si and Mg among all phases. Increasing pressure can slightly reduce the size of Si isotope fractionation but significantly depress Mg isotope fractionation among these phases. Mg and Si isotope fractionation among Mg2SiO4 polymorphs may provide a promising way to “probe” the depth and temperature of origin of mantle xenoliths.

Silicon isotope fractionation during abiotic silica precipitation at low temperatures: Inferences from flow-through experiments

Silicon isotopes have considerable potential as proxy for (near-) surface processes and environmental conditions. However, unambiguous interpretations of isotope signatures in natural silica deposits are often hampered by a lack of independent quantitative information on isotopic fractionations operating under the environmental conditions of interest. We performed seeded silica precipitation experiments using flow-through reactors in the 10–60°C temperature range to alleviate this problem. The principal objective was to quantify the silicon isotope fractionations during controlled precipitation of amorphous silica from a flowing aqueous solution. The experiments were designed to simulate silica deposition induced by a temperature drop, with particular relevance for (near-) surface hydrothermal systems associated with steep temperature gradients.Monitored differences in silicon isotope ratios (30Si/28Si and 29Si/28Si) between input and output solutions demonstrated a systematic sequence in behavior. During an initial time interval, that is, before the reaction system reached steady state, the observed isotope shifts were influenced by dissolution of the seed material, the saturation state of the solution and the specific surface area of the seeds. After reaching steady state, the selective incorporation of silicon isotopes by the solid phase exhibited an explicit temperature dependency: the lighter isotopes were preferentially incorporated, and apparent fractionation magnitudes increased with decreasing temperature.Calculated magnitudes of silicon isotope fractionations between precipitated and dissolved silica (Δ30Si=δ30Siprecipitate (calculated)−δ30Siinput solution) were −2.1‰ at 10°C, −1.2‰ at 20°C, −1.0‰ at 30°C, −0.5‰ at 40°C, 0.1‰ at 50°C, and 0.2‰ at 60°C (s.d.⩽0.6‰, based on replicate experiments). Hence, fractionation was nearly insignificant at temperatures ⩾50°C. Apart from this relationship with temperature, our results indicate that the effective Si isotope fractionation during precipitation from a solution is subject to changes in the saturation state, reactive surface area and flow regime. We therefore infer that, to a significant extent, solid–fluid fractionation in natural (near-) surface environments is system dependent.

First-principles calculations of equilibrium silicon isotope fractionation among mantle minerals

Silicon isotope fractionation factors for mantle silicate minerals, including olivine, wadsleyite, ringwoodite, pyroxenes, garnet (pyrope), majorite, and Mg-perovskite, are calculated using density functional theory. Our results show that equilibrium fractionations of Si isotopes are negligible among pyroxenes, olivine, and pyrope, but are significant between olivine and its polymorphs (wadsleyite and ringwoodite). There is also significant Si isotope fractionations between mantle minerals with different Si coordination numbers (CN), such as Mg-perovskite (CN=6) and olivine polymorphs (CN=4). When in equilibrium with each other, 30Si/28Si decreases in the order of olivine>pyroxenes>wadsleyite>majorite>ringwoodite>Mg-perovskite.Our calculation predicts significant Si isotope fractionation between mantle minerals, e.g., perovskite vs. ringwoodite, majorite vs. pyroxene, and olivine vs. its polymorphs even at high pressure and temperature conditions of deep mantle. The Si CN in silicate melt increases with increasing pressure, implying that Si isotope fractionation between silicate and metal could be a function of pressure. Our results suggest that Si isotopic fractionation factor between silicate and metal may decrease with increasing pressure; consequently, Si isotopic fractionation factor obtained from low pressure experiments may not be applicable to Si isotope fraction during core formation which occurred at high pressure. Finally, Si isotopes could also be fractionated between perovskite-rich mantle and residual melt during magma-ocean cooling in the lower mantle because of their different Si CNs. If such primordial signature is not destroyed and partially preserved through the Earth’s history, significant Si isotope heterogeneity could still exist between the upper and lower mantle.

Si stable isotope fractionation during adsorption and the competition between kinetic and equilibrium isotope fractionation: Implications for weathering systems

The adsorption of Si onto amorphous Al-hydroxides is the cause for the light Si isotope signature that secondary crystalline clay minerals in weathering systems carry. We propose this hypothesis from a series of adsorption experiments in which the light isotopes are being favored during Si adsorption onto crystalline gibbsite and in which the associated fractionation factor depends on the solution's initial Si concentration.Three adsorption experiments were carried out at pH7 with different initial Si concentrations of 0.36, 0.71 and 1.42mmol/l Si start concentrations. As Al-hydroxide adsorbent, 30g/l crystalline gibbsite were used to provide equal surface area in all experiments. Adsorption rates are higher with higher initial Si concentration. At the same time, calculated apparent isotope fractionation factors 103lnαadsorbed/solution decrease from −1.8 to −3‰ with increasing initial Si concentration. As care was taken to avoid isotope fractionation during transport of dissolved Si to the gibbsite surface, the mass dependence of the activation energy barrier at the interface is causing the kinetic isotope fractionation. Within the mass balance framework of DePaolo (2011) the shift in Si isotope fractionation with initial Si concentration is interpreted to be induced by different kinetic isotope fractionation factors associated with the forward reaction. Only after ca. two months do the isotope ratios begin to adjust to an equilibrium isotope fractionation factor that is close to 0‰. With such slow re-equilibration Si adsorption differs fundamentally from transition metals that re-equilibrate isotopically within hours after adsorption onto Fe and Mn oxide surfaces.These observations may provide an explanation for the light Si isotope signature clay minerals formed during weathering carry: the light Si isotope composition is being inherited early on during Si adsorption onto amorphous Al-hydroxides and is potentially carried over during all further stages of transformation.

Silicon isotope fractionation in silicate minerals: Insights from first-principles models of phyllosilicates, albite and pyrope

Isotopic fractionation factors for oxygen and silicon in phyllosilicates (pyrophyllite, talc), albite and pyrope have been calculated using first-principles methods based on density functional theory. Based on exhaustive analysis of numerical convergence, we also update our previous calculations on enstatite and forsterite silicon fractionation properties. Calculated oxygen isotope fractionations agree well with existing estimates for talc and albite. In the case of silicon, qualitative agreement is found with natural data. For phyllosilicates (kaolinite, lizardite, pyrophyllite, talc), Si isotope fractionation properties appear to be correlated with stoichiometry:(1)1000lnα30Siphyllosilicate–quartz=aMg(T)·Mgeq.+aAl(T)·Aleq.Sieq.,where Sieq.=#Si, Aleq.=34#Al and Mgeq.=12#Mg (cation equivalents) are the charge-weighed stoichiometric coefficients of each cation, normalized to the charge of the silicon atom, and aX(T) are proportionality coefficients depending on temperature. It is suggested that the effect of cation X on Si isotope fractionation (i.e. aX(T)) will increase with decreasing electronegativity of X. Si isotope fractionation is further correlated with Si–O distances, suggesting a crystal chemical explanation for relation (1) in terms of electron donation effects. This relationship appears valid for quartz, pyrope and enstatite (R2=0.99,n=7), but forsterite is strongly anomalous (error of 0.7‰ at 600°C). These models indicate that attention should be given to chemical compositions in Si isotope studies. Relation (1) would explain the enrichment in heavy silicon isotopes accompanying magmatic differentiation.

Effects of growth and dissolution on the fractionation of silicon isotopes by estuarine diatoms

Studies of silicon (Si) isotope fractionation during diatom growth in open ocean systems have documented lower Si isotopic values (δ30Si) in the biogenic silica of diatom frustules compared to dissolved silicon. Recent findings also indicate that Si isotope fractionation occurs during dissolution of diatom frustules, producing higher δ30Si values in the remaining biogenic silica. This study focuses on diatoms from high production areas in estuarine and coastal areas that represent approximately 30–50% of the global marine primary production. Two species of diatoms, Thalassiosira baltica and Skeletonema marinoi, were isolated from the brackish Baltic Sea, one of the largest estuarine systems in the world. These species were used for laboratory investigations of Si isotope fractionation during diatom growth and the subsequent dissolution of the diatom frustules. Both species of diatoms give an identical Si isotope fractionation factor during growth of −1.50±0.36‰ (2σ) for 30Si, which falls in the range of −2.09‰ to −0.55‰ of published data. Our results also suggest a dissolution-induced Si isotope fractionation factor of −0.86‰ at early stage of dissolution, but this effect was observed only in DSi and no significant Si isotope change was observed for BSi. The growth and dissolution results are applied to a Baltic Sea sediment core to reconstruct DSi utilization by diatoms, and found to be in agreement with the observed DSi uptake rates in the overlying water column during diatom growth.

Silicon Isotopes as a New Method of Measuring Silicate Mineral Reaction Rates at Ambient Temperature

Experiments of albite dissolution under ambient temperature at pH 3 and 5 demonstrated the feasibility of using 29Si doping method to precisely measure silicate mineral dissolution rates. Initial solutions were doped with 29Si to result in an isotopic composition of 0.04% 28Si, 99.9% 29Si, and 0.06% 30Si, which compares with the Amelia and Evja albite with Si isotopic composition of 92.23% 28Si, 4.67% 29Si, and 3.10% 30Si. The isotopic contrast allows the detection of dissolution of a tiny amount of albite into the aqueous solution. Experimental results show rapid increase of 28Si abundance at the initial stage of fresh albite dissolution, reflecting reaction with high energy sites on the surface. Steady state dissolution rates were reached after 20-30 days at 22 oC. The evolution of 28Si and 29Si abundances (%) with time tightly constrains the albite dissolution rates, which are consistent with literature data. In experiments seeded with kaolinite, both albite dissolution rates and kaolinite precipitation rates were determined with a combination of Si isotopes and isotope dilution method of determining Si concentrations.The new experiments have also verified the assumptions that are necessary for the isotope doping method to be applicable. Experimental data showed no Si isotope fractionation during albite dissolution. The ion-exchange method in sample pre-treatments does not alter Si isotopic composition or silica concentrations. The errors from neglecting isotope fractionation during secondary mineral precipitation are small on the determined albite dissolution rates when the isotope doping is overwhelming. In conclusion, we now have substantial experimental data show that the 29Si doping is a new method applicable to a wide range of silicate mineral dissolution rate determinations. Particularly, our albite experimental results show promise to use isotope doping method to measure silicate dissolution rates at various saturation states and determine the rate – ΔGr relationships.

Experimental determination of the Si isotope fractionation factor between liquid metal and liquid silicate

The conditions of core formation and the abundances of the light elements in Earthʼs core remain debated. Silicon isotope fractionation provides a tool contributing to this subject. We present experimentally determined Si isotope fractionation factors between liquid metal and liquid silicate at 1450 °C and 1750 °C, which allow calibrating the temperature dependence of Si isotope fractionation. Experiments were performed in a centrifuging piston cylinder at 1 GPa, employing both graphite and MgO capsules. Tin was used to lower the melting temperature of the metal alloys for experiments performed at 1450 °C. Tests reveal that neither Sn nor C significantly affects Si isotope fractionation. An alkaline fusion technique was employed to dissolve silicate as well as metal phases prior to ion exchange chemistry and mass spectrometric analysis. The results show that metal is consistently enriched in light isotopes relative to the silicate, yielding average metal–silicate fractionation factors of −1.48±0.08‰ and −1.11±0.14‰ at 1450 °C and 1750 °C, respectively. The temperature dependence of equilibrium Si isotope fractionation between metal and silicate can thus be described as Δ30SiMetal-Silicate=−4.42(±0.05)×106/T2. The Si isotope equilibrium fractionation is thus about 1.7 times smaller than previously proposed on the basis of experiments. A consequence of this smaller fractionation is that the calculated difference between the Si isotope composition of the bulk Earth and that of the bulk silicate Earth generated by core formation is smaller than previously thought. It is therefore increasingly difficult to match the Si isotope composition of the bulk silicate Earth with that of chondrites for metal–silicate equilibration temperatures above ∼2500 K. This suggests that Si isotopes were more sensitive to the early stages of core formation when low oxygen fugacities allowed significant incorporation of Si into metal.

Biogenic silica recycling in sea ice inferred from Si-isotopes: constraints from Arctic winter first-year sea ice

We report silicon isotopic composition (δ30Si vs. NBS28) in Arctic sea ice, based on sampling of silicic acid from both brine and seawater in a small Greenlandic bay in March 2010. Our measurements show that just before the productive period, δ30Si of sea-ice brine similar to δ30Si of the underlying seawater. Hence, there is no Si isotopic fractionation during sea-ice growth by physical processes such as brine convection. This finding brings credit and support to the conclusions of previous work on the impact of biogenic processes on sea ice δ30Si: any δ30Si change results from a combination of biogenic silica production and dissolution. We use this insight to interpret data from an earlier study of sea-ice δ30Si in Antarctic pack ice that show a large accumulation of biogenic silica. Based on these data, we estimate a significant contribution of biogenic silica dissolution (D) to production (P), with a D:P ratio between 0.4 and 0.9. This finding has significant implications for the understanding and parameterization of the sea ice Si-biogeochemical cycle, i.e. previous studies assumed little or no biogenic silica dissolution in sea ice.

The riverine silicon isotope composition of the Amazon Basin

We present here the first large-scale study of riverine silicon isotope signatures in the Amazon Basin. The Amazon and five of its main tributaries were studied at different seasons of the annual hydrological cycle. The δ30Si signature of the dissolved silicon (DSi) exported to the estuary (weighted for DSi flux) for the period considered is estimated at +0.92‰. A river cross-section shows the homogeneity of the Amazon River regarding DSi concentration and isotope ratio. The biogenic silica (BSi) concentration measured in surface water from all rivers is generally small compared to the DSi reservoir but large variations exist between rivers. Very low isotope signatures were measured in the upper Rio Negro (δ30Si=+0.05±0.06‰), which we explain both by an equilibrium between clay formation and dissolution and by gibbsite formation. The Si isotope fractionation in the Andean tributaries and the Amazon main stem can be explained by clay formation and follow either a Rayleigh or a batch equilibrium fractionation model. Our results also suggest that the formation of 2:1 clays induces a fractionation factor similar to that of kaolinite formation.

Mg and Si isotopic fractionation patterns in types B1 and B2 CAIs: Implications for formation under different nebular conditions

AbstractMagnesium and silicon isotopic profiles across melilite grains in two type B1 and two type B2 calcium‐aluminum‐rich inclusions (CAIs) reveal differing but constant enrichments in heavy isotopes everywhere except ≤1000 μm from the CAI margins. There is no close correlation in the B1s or the B2s between isotopic composition and åkermanite content of the melilite, a measure of progressive igneous crystallization, yet such a correlation might be expected in a type B2: without a melilite mantle (as in B1s) to seal the interior off and prevent further evaporation, the melt would have maintained communication with the external gas. These observations indicate a model in which B1s and B2s solidified under differing conditions. The B2s solidified under lower hydrogen pressures ( ≤ 10−4 – 10−5 bars) than did B1s ( &gt; 10−4 bars), so surface volatilization was slower in the B2s and internal chemical and isotopic equilibrium was maintained over the interval of melilite crystallization. The outermost zones of the CAIs (≤1000 μm from the edge) are not consistently enriched in heavy isotopes relative to the interiors, as might be expected from diffusion‐limited surface evaporation of the melt. In all cases, the magnesium in the CAI margins is lighter than in the interiors. In one case, silicon in the margin also is lighter, but locally in some CAIs, it is isotopically heavier near the surface. If melt evaporation played a role in the formation of these outer zones, a later event in many cases caused isotopic re‐equilibration with an external and isotopically near‐normal reservoir.

Silicon isotope variations in the inner solar system: Implications for planetary formation, differentiation and composition

Accurate and precise Si isotope measurements were obtained using magnesium doping and high-resolution plasma source mass spectrometry for samples representative of the Earth, as well as lunar samples, meteorites from Mars (SNC), eucrites, a howardite, carbonaceous chondrites (CC), ordinary chondrites (OC) and enstatite chondrites (EC). Our data confirm that significant Si isotope fractionations exist among the inner solar system planetary bodies. They show that the Earth and the Moon share the same Si isotopic composition, which is heavier than all other measured bodies, in agreement with most of previous studies. At the other end of the spectrum, enstatite chondrites have the lightest Si isotope compositions. In order to precisely estimate the amount of Si that may have entered the Earth’s core, we developed a refined model of Si partitioning based on continuous planetary accretion that takes into account the likely variations in T, P and fO2 during the Earth’s accretion, as well as isotopic constraints involving metal–silicate partitioning derived from both experimental and natural sample data sets.Assuming that the difference between the isotopic signature of the bulk silicate Earth (BSE) and chondrites solely results from Si isotope fractionation during core formation, our model implies that at least ∼12wt% Si has entered the Earth’s core, which is greater than most of the estimates based on physical constraints on core density or geochemical mass balance calculations.This result leads us to propose two hypotheses to explain this apparent contradiction: (1) At least part of the Earth’s building blocks had a Si isotope composition heavier than that observed in chondrites (i.e., δ30Si>−0.39‰). (2) If on the contrary the Earth accreted only from material having chondritic δ30Si, then an additional process besides mantle–core differentiation is required to generate a stronger isotope fractionation and lead to the observed heavy isotope composition of the bulk silicate Earth. It may be the loss of light Si isotopes during partial planetary vaporization in the aftermath of the Moon-forming giant impact. This process, which may have affected metallic cores, required a thorough isotopic re-equilibration between core and silicate to explain the similar heavy isotope composition of the silicate portions of the Earth and the Moon.

Silicon isotope enrichment in diatoms during nutrient-limited blooms in a eutrophied river system

We examined the Si isotope fractionation by following a massive nutrient limited diatom bloom in a eutrophied natural system. The Oder River, which is a eutrophied river draining the western half of Poland and entering the southern Baltic Sea, exhibits diatom blooms that cause extreme Si isotope fractionation. The rapid nutrient depletion and fast BSi increase observed during the spring bloom suggest a closed system Rayleigh behavior for DSi and BSi in the river at certain time scales. A Si isotope fractionation factor (30εDSi–BSi) of −1.6±0.31‰ (2σ) is found based on observations between April and June, 2004. A very high δ30Si value of up to +3.05‰ is measured in BSi derived from diatoms. This is about 2 times higher than previously recorded δ30Si in freshwater diatoms. The Rayleigh model used to predict the δ30Si values of DSi suggests that the initial value before the start of the diatom bloom is close to +2‰, which is relatively higher than the previously reported values in other river water. This indicates that there is a biological control of the Si isotope compositions entering the river, probably caused by Si isotope fractionation during uptake of Si in phytoliths. Clearly, eutrophied rivers with enhanced diatom blooms deliver 30Si-enriched DSi and BSi to the coastal ocean, which can be used to trace the biogeochemistry of DSi/BSi in estuaries.

Si isotope fractionation between Si-poor metal and silicate melt at pressure–temperature conditions relevant to metal segregation in small planetary bodies

Experimental investigations of Si isotope fractionation between Si-bearing metal alloy and silicate phases have to date been limited to high pressure (1–7GPa) and high temperature (1800–2200°C) conditions at highly reducing conditions, to optimize applicability of results to early core formation processes in the Earth. Here, we assess the extent and mechanism of Si isotopic fractionation at conditions relevant to metal segregation in small (km-scale) planetary bodies, using samples obtained from an industrial-scale blast furnace of Tata Steel (IJmuiden, the Netherlands).During the low-pressure, high-temperature process of steelmaking inhomogeneous blast furnace burden consisting of pre- and untreated iron ore, iron silicates and coke is reduced to oxygen fugacities near the C–CO buffer, resulting in the segregation of a metal phase containing only ∼0.3wt% Si. Seven sample sets, each comprising a metal alloy and a silicate slag, were taken during tapping of the blast furnace at tapping temperatures between 1400°C and 1600°C. We find large isotopic mass fractionation between metal and silicate, with Δ30Sisilicate–metal varying between 0.7‰ and 1.6‰, values that are as high as previously obtained in high-pressure, highly reduced experiments. A model for metal–silicate Si isotope fractionation in blast furnaces can explain both the sense and magnitude of fractionation, if the presence of SiO-bearing vapour is explicitly taken into account. Our data indicate that significant Si isotope fractionation can occur between metal and silicate at low-pressure, high-temperature and only mildly reducing conditions for which Si solubility in molten Fe-rich metal is low. This suggests an important role for SiO at low confining pressures. Our data can be applied to models of aubrite meteorite formation through high-temperature differentiation of an enstatite chondrite parent body. Our calculations suggest a far larger degree of rehomogenisation during differentiation than previously thought on the basis of metal–silicate Si isotopic fractionation measured in natural meteorites that re-equilibrated at low temperature.

The silicon isotope composition of the upper continental crust

The upper continental crust (UCC) is the major source of silicon (Si) to the oceans and yet its isotopic composition is not well constrained. In an effort to investigate the degree of heterogeneity and provide a robust estimate for the average Si isotopic composition of the UCC, a representative selection of well-characterised, continentally-derived clastic sediments have been analysed using high-precision MC-ICPMS.Analyses of loess samples define a narrow range of Si isotopic compositions (δ30Si=−0.28‰ to −0.15‰). This is thought to reflect the primary igneous mineralogy and predominance of mechanical weathering in the formation of such samples. The average loess δ30Si is −0.22±0.07‰ (2 s.d.), identical to average granite and felsic igneous compositions. Therefore, minor chemical weathering does not resolvably affect bulk rock δ30Si, and loess is a good proxy for the Si isotopic composition of unweathered, crystalline, continental crust.The Si isotopic compositions of shales display much more variability (δ30Si=−0.82‰ to 0.00‰). Shale Si isotope compositions do not correlate well with canonical proxies for chemical weathering, such as CIA values, but do correlate negatively with insoluble element concentrations and Al/Si ratios. This implies that more intensive or prolonged chemical weathering of a sedimentary source, with attendant desilicification, is required before resolvable negative Si isotopic fractionation occurs. Shale δ30Si values that are more positive than those of felsic igneous rocks most likely indicate the presence of marine-derived silica in such samples.Using the data gathered in this study, combined with already published granite Si isotope analyses, a weighted average composition of δ30Si=−0.25±0.16‰ (2 s.d.) for the UCC has been calculated.

Stable-isotope (H, O, and Si) evidence for seasonal variations in hydrology and Si cycling from modern waters in the Nile Basin: implications for interpreting the Quaternary record

Seasonal variations in hydrology and Si cycling in the Nile Basin were investigated using stable-isotope (H, O, and Si) compositions and dissolved Si (DSi) concentrations of surface waters, as a basis for interpreting lacustrine diatom sequences. δ18O ranged from −4.7 to +8.0‰ in the wet season and +0.6 to +8.8‰ in the dry season (through 2009–2011). Higher δ18O values during the dry season reflected increased evapotranspiration and open water evaporation under conditions of lower humidity. Progressive downstream enrichment in the heavy isotope 18O also occurred in response to cumulative evaporative losses from open water bodies and swamps. δ30Si values of DSi ranged from +0.48 to +3.45‰ during the wet season and +1.54 to +4.66‰ during the dry season, increasing the previously reported global upper limit for δ30Si values in natural waters by 1‰. Si-isotope fractionation was most intense during the dry season when demand for DSi by aquatic ecosystems exceeded supply. Progressive downstream enrichment in the heavy isotope 30Si, coupled with decreasing DSi concentrations, represented cumulative Si uptake by diatoms, macrophytes and other Si-accumulating aquatic organisms. The pronounced seasonal variations in DSi concentrations and Si-isotope compositions in the River Nile suggest that its DSi flux to the ocean may have varied significantly on a glacial/interglacial time scale, with important consequences for the marine Si budget and consequently the global C cycle. Anthropogenic impacts were evident in both the water- and Si-isotope datasets, especially during the dry season and along the Main Nile, where water management is most intensive.

Species-dependent silicon isotope fractionation by marine diatoms

Fractionation of silicon (Si) isotopes was measured in seven species (nine strains) of polar and sub-polar marine diatoms grown in semi-continuous unialgal cultures under optimal irradiance and temperature for each diatom strain. Results from this work provide the first evidence that Si isotope fractionation by diatoms is species-dependent. The greatest difference in the Si isotope fractionation factor (ε) was observed between two Southern Ocean diatoms, Fragilariopsis kerguelensis (−0.54‰, average for two strains) and Chaetoceros brevis (−2.09‰). The ε for the other species, both polar and sub-polar, ranged from −0.72‰ to −1.21‰. The two remaining polar diatoms had ε values of −0.74±0.05‰ for Thalassiosira antarctica, and −1.21±0.04‰ for Thalassiosira nordenskioeldii, while the sub-polar species had ε values of −0.72±0.04‰ for Thalassiosira weissflogii, −0.88±0.06‰ for Thalassiosira pseudonana (CCCM58), −0.97±0.14‰ for Thalassiosira pseudonana (CCMP1014), and −1.15±0.03‰ for Porosira glacialis. The range in ε for the diatoms evaluated in this study may be large enough to significantly impact the Si isotope composition measured in diatom opal (δ30Si-bSiO2) from marine sediments and its subsequent interpretation. To test the influence of diatom taxonomic composition on δ30Si-bSiO2, we developed a model that considered the relative abundance of diatom species and the ε values (from this study) for each species present within the sediment core (i.e. weighted-average ε). The model was applied to records from a Southern Ocean sediment core (TN057-13) where both diatom abundance and δ30Si-bSiO2 data were available. The analysis indicated that 67% of the variation in δ30Si-bSiO2 could be explained by species-dependent Si isotope fractionation. We suggest that future work should assess phytoplankton taxonomic composition when using δ30Si-bSiO2 as a proxy for Si utilization.