Silicon Isotope Fractionation Research Articles

Silicon and oxygen isotope fractionation in a silicified carbonate rock

Silicon isotope fractionation during silicification is poorly understood and impedes our ability to decipher paleoenvironmental conditions from Si isotopes in ancient cherts. To investigate isotope fractionation during silica-for‑carbonate replacement we analyzed the microscale Si and O isotope composition in different silica phases in a silicified zebra dolostone as well as their bulk δ18O and Δ’17O compositions. The subsequent replacement of carbonate layers is mimicked by decreasing δ18O and δ30Si. The textural relationship and magnitude of Si and O isotope fractionation is best explained by near-quantitative silica precipitation in an open system with finite Si. A Rayleigh model for silicification suggests positive Ɛ30/28Si during silicification, conforming with predictions for isotope distribution at chemical equilibrium from ab-initio models. Application of the modelled Ɛ30Si-T relationship yields silicification temperatures of approx. 50 °C. To reconcile the δ18Ochert composition with these temperatures, the δ18O of the fluid must have been between −2.5 and − 4 ‰, compositions for which the quartz phases fall close to the oxygen equilibrium fractionation line in three-isotope space. Diagenetic silica replacement appears to occur in O and Si isotopic equilibrium allowing reconstructions of temperatures of silicification from Si isotopes and derive the δ18O composition of the fluid – a highly desired value needed for accurate reconstructions of the temperature- and δ18O histories of the oceans.

Silicon isotopes in an Archaean migmatite confirm seawater silicification of TTG sources

Unraveling ancient melting processes is key to understanding how the earliest, tonalite-trondhjemite-granodiorite (TTG)-dominated continental crust formed from partial melting of amphibolite. Application of silicon isotope analysis to ancient crust reveals that Archaean TTGs exhibit consistently high Si isotope values (δ30Si) compared to modern granitoids, attributed to seawater-derived silica introduced by either (a) partial melting of variably silicified basalts or (b) assimilation of authigenic silica-rich marine lithologies in the melt source. However, both mechanisms can introduce highly variable δ30Si, conflicting with the strikingly consistent δ30Si compositions of Archaean TTGs. This study investigates an alternative model, whereby the distinct mineralogy and chemistry of TTG melt sources impart a distinct silicon isotope composition to the melt, compared with “modern” granitoids. We measured δ30Si in component parts (melanosome and leucosome) of an Archaean (2.7 Ga) mafic migmatite and coeval amphibolites and mafic granulites from the Kapuskasing uplift, Canada, to explore how Si isotopes fractionate during incipient TTG melt formation. Our data reveal leucosome (i.e., melt) exhibits consistently high δ30Si values compared to a relatively isotopically lighter melanosome (i.e., residuum). We also derive inter-mineral silicon isotope fractionation factors for mineral separates that agree well with those of ab initio estimates for the same minerals and show that the magnitude of equilibrium fractionation between TTG source rock and melt replicates that in Phanerozoic granitoids. We conclude the effects of magmatic differentiation on δ30Si have remained consistent throughout Earth history, meaning that Archaean TTGs must require a source isotopically heavier than unaltered basalt, as reflected by our amphibolites and mafic migmatite components. The consistently heavy δ30Si of seawater through Earth history, and the high SiO2 content of amphibolites relative to coeval leucosome-free granulites in our study area, imply seawater silicification is the source of the observed high δ30Si. Thus, the consistently heavy Si isotope compositions measured in Archaean melt products define a unique aspect of ancient crust formation: that of the silicification of TTG source rock, implying the intrinsic involvement of a primeval hydrosphere.

Silicon isotope fractionation dynamics during uptake and translocation by various crop species under three soil types

Silicon isotope fractionation dynamics during uptake and translocation by various crop species under three soil types

Silicon isotope fractionation with a low‐silica rice mutant reflects plant uptake strategy in response to different silicon supply levels

AbstractPlants may modulate uptake pathways when the nutrient concentration varies. The identified transporters Lsi1 and Lsi2 belong to low‐affinity systems; however, it is unknown whether a high‐affinity system responsible for silicon (Si) transport exists or how plants modulate Si uptake pathways. We aimed at developing isotope fractionation as a novel tool for nutrient uptake and cycling in plants. We grew the wild‐type rice ‘Oochikara’ and its mutant lsi1 under low (Si‐L: 0.17 mM) and high (Si‐H: 1.70 mM) Si supply levels and evaluated Si isotope fractionation between different Si pools. The isotope fractionations between Oochikara and nutrient solution were –0.15 and –0.63‰ at Si‐L and Si‐H, respectively; between lsi1 and nutrient solution they were –0.18 and –1.33‰ at Si‐L and Si‐H, respectively. The qualitative isotope fractionation caused by Lsi1 were –0.09 and –0.49‰ at Si‐L and Si‐H, respectively. We thus verified the hypothesis that Lsi1 preferred 28Si transport over 30Si and that this preference was independent of Si supply levels. Changes in Δ30Siplant‐solution suggested that a high‐affinity system exists and that plants may regulate Si uptake strategy by changing the participation of low‐affinity and high‐affinity pathways. Another root–shoot translocation that favors 28Si has been found in Si‐H‐lsi1 that previously occurred in banana (Musa spp.), which suggested that Si xylem loading is related to the different Si transport abilities of Lsi1 and Lsi2. Our results demonstrated that Si isotope fractionation can shed light on plants’ response to variable nutrient conditions.

Physical weathering by glaciers enhances silicon mobilisation and isotopic fractionation

Physical weathering by glaciers enhances silicon mobilisation and isotopic fractionation

Competition of equilibrium and kinetic silicon isotope fractionation during silica precipitation from acidic to alkaline pH solutions in geothermal systems

As an important volcanic geothermal region in China, Tengchong belongs to the Mediterranean-Himalayan geothermal belt and is characterized by a wide distribution of geothermal systems. The study area- Rehai- is the sole geothermal area in Tengchong in mainland China that discharges both acidic and alkaline hot springs with pH values ranging from 1.6 to 9.7 at temperatures between 50 and 97 °C. Natural geothermal environments with large variations in pH and temperature are ideal study sites to investigate silicon (Si) isotope fractionation mechanisms which are still insufficiently understood. In this study, we investigated 3 different geothermal systems, which have distinct fluid characteristics: (1) type I fluid from Diretiyan Spring (pH: 1.6–2.5, HSO4 water, saturated with respect to quartz), (2) type II fluid from Zhenzhuquan Spring (pH: 6.4, NaClSO4 water, saturated with respect to quartz), and (3) type III fluid from Dagunguo Spring (pH: 7.8–9.7, NaCl water, oversaturated with respect to amorphous silica: SiO2am). The mechanisms controlling Si isotope fractionation during Si precipitation differ significantly between the three fluid types. In type I fluid, equilibrium fractionation controls Si isotope variability during silica precipitation, causing the highest natural Si isotope equilibrium fractionation (Δ30Si) detected to date, ranging in Δ30Siquartz-fluid from +1.09‰ to +2.78‰. The equilibrium isotope fractionation is essentially caused by balanced precipitation and dissolution rates and high ionic strength of the geothermal fluids. In type II fluid, amorphous silica formed during evaporation and/or cooling of the spring water during capillary rise and a kinetically controlled, uni-directional Si isotope fractionation is observed. In type III fluid, the Δ30SiSiO2am-fluid ranges from −1.57‰ to +0.55‰ at the temperature range of 80–100 °C. The Δ30SiSiO2am-fluid show a shift from kinetic to equilibrium controlled fractionation and demonstrate that isotopic re-equilibration can reset the original kinetic Si isotope values in dependence of the hydrochemical conditions like pH, temperature and ionic strength as well as reaction time. Here we show that isotopic re-equilibration after solid formation can shift the isotopic signatures originally inherited during precipitation, challenging the interpretation of Si isotope values in the ancient rock record.

A biomimetic peptide has no effect on the isotopic fractionation during in vitro silica precipitation

The stable isotopic composition of diatom silica is used as a proxy for nutrient utilisation in natural waters. This approach provides essential insight into the current and historic links between biological production, carbon cycling and climate. However, estimates of isotopic fractionation during diatom silica production from both laboratory and field studies are variable, and the biochemical pathways responsible remain unknown. Here, we investigate silicon isotopic fractionation through a series of chemical precipitation experiments that are analogous to the first stages of intracellular silica formation within the diatom silicon deposition vesicle. The novelty of our experiment is the inclusion of the R5 peptide, which is closely related to a natural biomolecule known to play a role in diatom silicification. Our results suggest that the presence of R5 induces a systematic but non-significant difference in fractionation behaviour. It thus appears that silicon isotopic fractionation in vitro is largely driven by an early kinetic fractionation during rapid precipitation that correlates with the initial amount of dissolved silica in the system. Our findings raise the question of how environmental changes might impact silicon isotopic fractionation in diatoms, and whether frustule archives record information in addition to silica consumption in surface water.

First-principles calculation of iron and silicon isotope fractionation between Fe-bearing minerals at magmatic temperatures: The importance of second atomic neighbors

In order to elucidate the processes involved in iron and silicon isotopes partitioning during magmatic differentiation, it is essential to know the precise value of equilibrium fractionation factors between the main minerals present in the evolving silicic melts. In this study, we performed first-principles calculations based on the density functional theory to determine the equilibrium iron and silicon isotopes fractionation factors between eleven relevant silicate or oxide minerals in the context of magmatic differentiation, namely: aegirine, hedenbergite, augite, diopside, enstatite, fayalite, hortonolite, Fe-rich and Fe-free forsterites, magnetite and ulvospinel. Results show that Fe2+-bearing silicate minerals display significant differences in iron isotope fractionation factors that cannot be neglected, even at high temperature (1000 °C). Various physical and chemical parameters control the iron isotopic fractionation of silicate minerals. However, the main parameter, after temperature and the iron oxidation state, is the nature and number of iron second neighbors (i.e. the local chemical composition around Fe atoms). This conclusion is also valid for silicon isotopes. In the investigated nesosilicates and inosilicates, silicon isotope reduced partition function ratios (also called β-factors) show no correlation with the average Si-O bond length, which remains almost constant, but Si β-factors are correlated with the local chemical composition of the minerals. Fractional crystallization is one of the mechanisms, which could explain the evolution of iron isotopic compositions during magmatic differentiation. Using the present theoretical set of equilibrium fractionation factors allows us to assess the impact of inter-mineral isotopic fractionations, and shows that pyroxene appears to be the main mineral phase driving the isotopic evolution to a heavier signature in the most evolved lavas.

Silicon uptake and isotope fractionation dynamics by crop species

Abstract. That silicon is an important element in global biogeochemical cycles is widely recognised. Recently, its relevance for global crop production has gained increasing attention in light of possible deficits in plant-available Si in soil. Silicon is beneficial for plant growth and is taken up in considerable amounts by crops like rice or wheat. However, plants differ in the way they take up silicic acid from soil solution, with some species rejecting silicic acid while others actively incorporate it. Yet because the processes governing Si uptake and regulation are not fully understood, these classifications are subject to intense debate. To gain a new perspective on the processes involved, we investigated the dependence of silicon stable isotope fractionation on silicon uptake strategy, transpiration, water use, and Si transfer efficiency. Crop plants with rejective (tomato, Solanum lycopersicum, and mustard, Sinapis alba) and active (spring wheat, Triticum aestivum) Si uptake were hydroponically grown for 6 weeks. Using inductively coupled plasma mass spectrometry, the silicon concentration and isotopic composition of the nutrient solution, the roots, and the shoots were determined. We found that measured Si uptake does not correlate with the amount of transpired water and is thus distinct from Si incorporation expected for unspecific passive uptake. We interpret this lack of correlation to indicate a highly selective Si uptake mechanism. All three species preferentially incorporated light 28Si, with a fractionation factor 1000×ln (α) of −0.33 ‰ (tomato), −0.55 ‰ (mustard), and −0.43 ‰ (wheat) between growth medium and bulk plant. Thus, even though the rates of active and passive Si root uptake differ, the physico-chemical processes governing Si uptake and stable isotope fractionation do not. We suggest that isotope fractionation during root uptake is governed by a diffusion process. In contrast, the transport of silicic acid from the roots to the shoots depends on the amount of silicon previously precipitated in the roots and the presence of active transporters in the root endodermis, facilitating Si transport into the shoots. Plants with significant biogenic silica precipitation in roots (mustard and wheat) preferentially transport silicon depleted in 28Si into their shoots. If biogenic silica is not precipitated in the roots, Si transport is dominated by a diffusion process, and hence light silicon 28Si is preferentially transported into the tomato shoots. This stable Si isotope fingerprinting of the processes that transfer biogenic silica between the roots and shoots has the potential to track Si availability and recycling in soils and to provide a monitor for efficient use of plant-available Si in agricultural production.

Extreme silicon isotope fractionation due to Si organic complexation: Implications for silica biomineralization

A combination of theoretical predictions and isotopic equilibration experiments using the three-isotope method have been performed to assess Si isotope fractionation among minerals, and aqueous species in the presence of dissolved catechol. Aqueous Si in abiotic ambient temperature aqueous solutions is dominated by the IV-coordinated H4SiO40 species, but the presence of aqueous catechol provokes the formation of a VI-fold Si-catechol complex. Results show an equilibrium Si fractionation factor of ∼19‰ between the VI-fold coordinated Si-catechol complex and the IV-fold coordinated aqueous silicic acid, an amplitude never previously observed for silicon. The fractionation between V-fold Si-organo complexes (with diolate, glyconate or methyllactate groups) and silicic acid has also been estimated through theoretical predictions to be about −10‰. These extreme fractionations can be used to improve our ability to interpret the Si isotope compositions of natural solids, and in particular those associated with marine silica biomineralization processes (e.g. sponge spicules).

Silicon isotopic fractionation during metamorphic fluid activities: constraints from eclogites and ultrahigh-pressure veins in the Dabie orogen, China

To understand Si isotope fractionation during metamorphic fluid activities in the subduction zone, this study presents the Si isotopic compositions of two high- to ultrahigh-pressure (HP–UHP) eclogite–vein systems from the Ganghe and Hualiangting areas in the Dabie orogen, eastern China. The results show that Si isotopes are significantly fractionated between the veins and their host eclogites in both areas. The δ30Si values of Ganghe eclogites range from −0.50‰ to −0.39‰, higher than that of the studied omphacite−epidote vein (−0.63 ± 0.07‰). The Hualiangting eclogites have δ30Si values of −0.36‰ to −0.29‰, whereas the δ30Si values of the Hualiangting multi-stage veins show greater variation, from −0.45‰ to +0.05‰, revealing significant Si isotope fractionation during metamorphic fluid evolution and vein formation.For the Hualiangting and Ganghe samples, the enrichment of heavy Si isotopes in minerals follows the order of δ30Siphengite (−0.01 to +0.13‰) ≈ δ30Siquartz (−0.14 to +0.10‰) > δ30Siomphacite (−0.63 to −0.33‰) ≈ δ30Siepidote (−0.60 to −0.30‰) ≈ δ30Sikyanite (−0.42 to −0.28‰) > δ30Sigarnet (−0.92 to −0.44‰). The Si isotope fractionation between metamorphic minerals was generally consistent with the equilibrium Si isotope fractionation factors calculated by the first-principles methods. These results suggest that the vein minerals are likely in Si isotopic equilibrium with each other.The δ30Si of the Hualiangting veins are linearly correlated to SiO2 contents with a steeper slope than that of the magma differentiation trend. By also considering the mineralogy of the veins, we conclude that this linear relationship reflects the sequential variation of mineral composition on the Si isotope signature of the veins. The SiO2 content and δ30Si of veins increased with continuing crystallization from the fluid, and the δ30Si of the fluid also increased. The results suggest that Si isotope data can be used to constrain element recycling and metamorphic fluids activities in the subduction zones.

Technical note: The silicon isotopic composition of choanoflagellates: implications for a mechanistic understanding of isotopic fractionation during biosilicification

Abstract. The marine silicon cycle is intrinsically linked with carbon cycling in the oceans via biological production of silica by a wide range of organisms. The stable silicon isotopic composition (denoted by δ30Si) of siliceous microfossils extracted from sediment cores can be used as an archive of past oceanic silicon cycling. However, the silicon isotopic composition of biogenic silica has only been measured in diatoms, sponges and radiolarians, and isotopic fractionation relative to seawater is entirely unknown for many other silicifiers. Furthermore, the biochemical pathways and mechanisms that determine isotopic fractionation during biosilicification remain poorly understood. Here, we present the first measurements of the silicon isotopic fractionation during biosilicification by loricate choanoflagellates, a group of protists closely related to animals. We cultured two species of choanoflagellates, Diaphanoeca grandis and Stephanoeca diplocostata, which showed consistently greater isotopic fractionation (approximately −5 ‰ to −7 ‰) than cultured diatoms (−0.5 ‰ to −2.1 ‰). Instead, choanoflagellate silicon isotopic fractionation appears to be more similar to sponges grown under similar dissolved silica concentrations. Our results highlight that there is a taxonomic component to silicon isotope fractionation during biosilicification, possibly via a shared or related biochemical transport pathway. These findings have implications for the use of biogenic silica δ30Si produced by different silicifiers as proxies for past oceanic change.

First-principles calculations of equilibrium silicon isotope fractionation in metamorphic silicate minerals

In this study, silicon isotope fractionation factors for metamorphic silicate including muscovite, epidote, and kyanite are calculated using first-principles methods based on density-functional theory. The reduced partition function ratio (103lnβ) of 30Si/28Si decreases in the order of muscovite > epidote ≈ kyanite. Combining with previous calculations, our results show that the equilibrium silicon isotope fractionation factors are correlated with average Si–O bond length. These results predict that measurable Si isotope fractionation can occur during metamorphic fluid activities. At 1000 K, the calculated 103lnα values for quartz–epidote, quartz–kyanite, and quartz–muscovite are 0.51‰, 0.49‰, and 0.08‰, respectively. The calculation results are generally consistent with the observed Si isotopic data in two metamorphic eclogite-vein systems, showing that our work can provide useful information in investigating the Si isotope fractionation in metamorphic fluid evolution.

Theoretical analyses of chemical and magnesium and silica isotope fractionation during vaporization of ca-al-inclusion of chondrite

Experimental study of changes in the composition of Ca-Al-inclusions of chondrites during evaporation indicates a close relationship of chemical and isotopic fractionation of this substance. The theoretical description of the coupling is carried out using the equations of the evaporation rate of the melt component (the Hertz-Knudsen equation) and isotope Rayleigh fractionation. The form of Rayleigh equation taken in foreign literature, derived from the Hertz-Knudsen equation, faces difficulties in interpreting experimental data. The discrepancy between the experimental and model data is explained by the fact that the «ideal» isotope fractionation factor used in the Rayleigh equation does not take into account its dependence on the temperature and composition of the evaporating melt. The article presents an alternative expression of the Rayleigh equation and a new expression of the rate of evaporation of Hertz-Knudsen, taking into account the activity of the melt component. The activity of the component is determined by the acidity-basicity index of the melt Ca-Al-inclusion, which, in turn, affects the evaporative fractionation of magnesium and silicon isotopes.

The origin of rare alkali metals in geothermal fluids of southern Tibet, China: A silicon isotope perspective

Geothermal waters from the Semi, Dagejia and Kawu hot springs in the Shiquanhe-Yarlung Zangbo geothermal field of southern Tibet (China) are highly enriched in rare alkali metals (RAM). However, the enrichment mechanism is still hotly debated. Here, we report the first silicon isotope data of these geothermal waters to unravel the origin of the extreme RAM enrichments. Sinter precipitation in the spring vents and water-rock interaction in the deep reservoir controlled both the silicon budget and silicon isotope fractionation. The rates of water-rock interaction and sinter precipitation in three spring sites decrease in the sequences Semi > Kawu > Dagejia, and Dagejia > Kawu > Semi respectively. Silicon isotope fractionation during sinter precipitation (i.e. Δ30Siprecipitate-solution < −0.1‰) is less than that due to water-rock interaction (i.e. Δ30Sisolution-rocks at least as high as −0.47‰), which makes it possible to use the δ30Si signatures of springs to evaluate the intensity of water-rock interaction. Based on the available evidence, a conceptual model of RAM enrichment is proposed: (i) persistent magmatic activity in southern Tibet provided the initial enrichment of the RAM in host rocks and a heat sources for the deep reservoirs of geothermal systems; (ii) the high Cl− content and long residence time (thousands of years) promote the leaching of RAM from the silicate host rocks.

Putting the silicon cycle in a bag: Field and mesocosm observations of silicon isotope fractionation in subtropical waters east of New Zealand

A mesocosm experiment was used to investigate the fractionation of silicon (Si) isotopes in subtropical surface waters east of New Zealand. Surface waters in this region were characterised by relatively low concentrations of silicic acid (Si(OH)4) (~2 μmol L−1) and higher nitrate (~5 μmol L−1) and dissolved iron (Fe) concentrations (~0.4 nmol L−1) prior to development of the annual springtime phytoplankton bloom. To simulate initiation of the bloom, a large (~700 L) mesocosm experiment was undertaken whereby surface seawater containing the natural plankton community was incubated for a 168-h period. During the mesocosm experiment the concentrations of Si(OH)4, nitrate, phosphate and dissolved iron all decreased while the concentration of biogenic silica (BSi) increased 12-fold. Coupled with the increase in BSi was a change in the Si-isotope composition of BSi (δ30SiBSi) which increased from 1.49‰ to 2.64‰ after 168 h. Complementary observations to those made for the mesocosm experiment were made for corresponding surface waters. For these waters, we observed a small decline in the concentrations of nitrate, phosphate and dissolved Fe, but little change in the concentrations of Si(OH)4 and BSi. In contrast to the mesocosm experiment, surface water δ30SiBSi values became lighter during bloom initiation, suggestive of Si(OH)4 being replenished into surface waters. These differences in the drawdown and utilisation of nutrients and dissolved Fe between the mesocosm and surface waters during bloom initiation likely result from favourable Fe and light supply conditions within the mesocosm. In contrast, water column stability (i.e. vertical mixing), and the supply of dissolved Fe are likely to influence bloom initiation and its longevity. The fractionation of Si-isotopes in the mesocosm experiment followed closed-system Rayleigh fractionation kinetics, and an enrichment factor (ε) of −1.13‰ was calculated for the exponential phase of growth for the diatom community, which was marked by the presence of the diatoms Asterionelopsis glacilis and Mellosira moniliformis. The isotope enrichment factor agreed well with previous observations of Si isotope fraction in diatoms from field communities, and appeared to be independent of variations in the ambient Si(OH)4 concentration, and phytoplankton species composition.

Silicon Isotope Geochemistry: Fractionation Linked to Silicon Complexations and Its Geological Applications.

The fundamental advances in silicon isotope geochemistry have been systematically demonstrated in this work. Firstly, the continuous modifications in analytical approaches and the silicon isotope variations in major reservoirs and geological processes have been briefly introduced. Secondly, the silicon isotope fractionation linked to silicon complexation/coordination and thermodynamic conditions have been extensively stressed, including silicate minerals with variable structures and chemical compositions, silica precipitation and diagenesis, chemical weathering of crustal surface silicate rocks, biological uptake, global oceanic Si cycle, etc. Finally, the relevant geological implications for meteorites and planetary core formation, ore deposits formation, hydrothermal fluids activities, and silicon cycling in hydrosphere have been summarized. Compared to the thermodynamic isotope fractionation of silicon associated with high-temperature processes, that in low-temperature geological processes is much more significant (e.g., chemical weathering, biogenic/non-biogenic precipitation, biological uptake, adsorption, etc.). The equilibrium silicon isotope fractionation during the mantle-core differentiation resulted in the observed heavy isotope composition of the bulk silicate Earth (BSE). The equilibrium fractionation of silicon isotopes among silicate minerals are sensitive to the Si–O bond length, Si coordination numbers (CN), the polymerization degrees of silicate unites, and the electronegativity of cations in minerals. The preferential enrichment of different speciation of dissoluble Si (DSi) (e.g., silicic acid H4SiO40 (H4) and H3SiO4− (H3)) in silica precipitation and diagenesis, and chemical weathering, lead to predominately positive Si isotope signatures in continental surface waters, in which the dynamic fractionation of silicon isotope could be well described by the Rayleigh fractionation model. The role of complexation in biological fractionations of silicon isotopes is more complicated, likely involving several enzymatic processes and active transport proteins. The integrated understanding greatly strengthens the potential of δ30Si proxy for reconstructing the paleo terrestrial and oceanic environments, and exploring the meteorites and planetary core formation, as well as constraining ore deposits and hydrothermal fluid activity.

Constraining silicon isotope exchange kinetics and fractionation between aqueous and amorphous Si at room temperature

Abstract Silicon (Si) isotopes are useful tracers for the modern and ancient Si cycle, but their interpretation is limited by inadequate understanding of Si isotope exchange kinetics and fractionation factors at low temperature. This study investigated Si isotope exchange and fractionation between aqueous and amorphous Si at circumneutral pH and room temperature through a series of 29Si-spiked isotope-exchange experiments. Four different amorphous Si solids with varied surface areas were reacted with aqueous Si solutions of high ionic strength similar to seawater, or low ionic strength typical of freshwater, under conditions close to chemical equilibrium with respect to amorphous Si solubility. In contrast to the common perception of negligible Si isotope exchange at low temperature, ∼50–85% isotope exchange was achieved between aqueous and amorphous Si within ∼60 days. Larger solid surface areas and higher aqueous ionic strength generally promoted Si isotope exchange. Drying/aging of Si gel, however, impedes Si isotope exchange between amorphous and aqueous Si relative to freshly prepared Si gels. Excluding the experiments that used the aged Si gel, temporal trajectories of Si isotope evolution of the two phases from all other experiments showed significant curvature in three-isotope space (29Si/28Si and 30Si/28Si). These results can be best explained by a model that comprises two Si isotope exchange processes with different exchange rates and fractionation factors during the interactions between aqueous and amorphous Si towards isotope equilibrium. The faster exchange is associated with surface sites, and slower exchange occurs between exterior and interior Si atoms of the solid. Exchange with surface sites tends to partition heavy Si isotopes in the aqueous phase relative to the solid surface, whereas exchange between surface and interior sites in the solid tends to enrich heavy Si isotopes in the interior. Two experiments that achieved >80% isotope exchange provided the best estimates of equilibrium Si isotope fractionation factors between bulk amorphous Si solid and aqueous monomeric silicic acid H4SiO4 (Δ30Siamorphous–aqueous) at 23 °C: +0.52‰ (±0.15‰, 1sd) at seawater ionic strength, and −0.98‰ (±0.12‰) at freshwater ionic strength. The observed “salt effect” on Si isotope exchange kinetics and fractionation factor is interpreted to reflect an influence of cations on Si speciation of solid surfaces. This work highlights the value of three-isotope method in studying both reaction kinetics and isotope fractionation mechanisms. The observed Si isotope exchange between amorphous and aqueous Si at low temperature implies that Si isotope re-equilibration, a previously neglected process, may be important in controlling Si isotope compositions of natural samples.

Tracing Polymerization in Calcium Silicate Hydrates Using Si Isotopic Fractionation

Silicate-chains polymerization is a crucial process in calcium silicate hydrate minerals, with large relevance for improving the durability and reducing the environmental impact of cement-based materials. To better understand the evolutionary mechanisms underlying the polymerization of silicate-chains in layered calcium silicate hydrates, we herein propose to trace the evolution of the polymerization degree by using silicon isotopes. The method requires tabulating the isotopic fractionation of several basic chemico–physical mechanisms that we obtained by performing atomistic simulations. The calculations reveal that the highly polymerized structures have longer Si–O bonds and that the Ca2+ cations play a dual role in the stretching and bending mode properties of silicates, such as isotopic fractionation is able to discern not only between the polymerization order of calcium silicate hydrate minerals, but even between cement gels suffering calcium leaching. Silicon isotopic fractionation can, therefore, be used to quantify the different evolutions of calcium silicon hydrate phases in a sample of man-made gel cement in order to improve its sustainability along lifetime stages in the quest for green cement.

Processes controlling silicon isotopic fractionation in a forested tropical watershed: Mule Hole Critical Zone Observatory (Southern India)

Assessing the dynamics of the silica cycle in the critical zone remains challenging, particularly within the soil, where multiple processes are involved. To improve our understanding of this cycle in the Tropics, and more specifically the role played by vegetation, we combined elemental Si mass balance with the δ30Si signatures of the compartments involved in the water-plant-rock interactions of a tropical forested watershed, Mule Hole (Southern India). To accomplish this, we analysed (1) the δ30Si values of present-day litter phytoliths from tree leaves and grass, as well as soil amorphous silica (ASi); (2) the Si isotope fractionation induced by phytolith dissolution; (3) the silicon mass balance inferred from isotopes at the soil–plant scale; and (4) the consistency between water sources and the δ30Si signatures in the ephemeral stream.The δ30Si values of present-day litter phytoliths and soil ASi vary within a narrow range of 1.10–1.40‰ for all samples, but two deep vertisol samples which likely trapped phytoliths from different vegetation growing under more humid conditions, as indicated by pollen analysis. A homogeneous signature of litter is a minimum condition for using δ30Si as a proxy for the litter/phytolith source of Si. However, litter-ash dissolution experiments demonstrate that the incipient dissolution of phytoliths fractionates Si isotopes, with the preferential dissolution of 28Si over 30Si yielding δ30Si values as low as −1.41‰. Values close to the whole-sample signatures, i.e., above 1‰, were recovered in the solution after a few hours of water-ash interaction. At the soil–plant scale, the average δ30Si value of soil-infiltrating solutions is slightly lighter than the average phytolith signature, which suggests phytoliths as the source of soil dissolved Si. The isotopic budget of dissolved Si within the soil layer, which was obtained based on previous elemental fluxes, is imbalanced. Equilibrating the isotopic budget would imply that up to 4100 mol ha−1 yr−1 of silica is taken up by vegetation, which is almost twice as large as that initially estimated from the elemental budget. The additional Si flux taken up, and likely stored in woody stems, was estimated assuming that Si isotopes followed a steady-state model for the whole Si plant uptake and then followed a Rayleigh model once in the plants. The δ30Si value of the additional Si flux taken up should be close to 0‰, i.e., enriched in light Si isotopes compared to the litter. If steady-state conditions apply, the source could correspond to soil ASi dissolution or deep (saprolite) root uptake. At the outlet of the watershed, the stream exhibits low δ30Si values (0.28–0.71‰) during peak flows and high δ30Si values (1.29–1.61‰) during the recessions at the end of the rainy season. Heavy δ30Si signatures are consistent with the expected domination of seepage at the end of floods. The light δ30Si values during peak flow are slightly lower than the overland flow signature and reflect either a sampling bias of overland flow or a minor but significant contribution of another Si source within the stream, possibly the partial dissolution of phytoliths from the suspended load, with slight isotopic fractionation. This study confirms that vegetation controls the silicon cycle in this dry tropical forest. It also shows that silicon isotopes yield a better grasp of the mass balance and sources and potential mechanisms involved than the consideration of only silicon concentrations. However, this proxy still relies on working hypotheses, notably steady-state and/or Rayleigh fractionation models, which need to be confirmed in further studies.