Si Isotope Research Articles

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.

Silicon and Oxygen Isotope Evolution of the Inner Solar System

Abstract Enstatite chondrites have been regarded as major building blocks of the Earth and other differentiated inner planetary bodies due to the similarity of Δ17O (deviation of the δ 17O value from the terrestrial silicate fractionation line) and nucleosynthetic isotope anomalies. However, this hypothesis has been rebutted by the fact that the Earth and enstatite chondrites show distinct Si isotopic compositions. It has been debated whether the origin of this Si isotope difference is the result of nebular or planetary processes. Here we show that the δ 30Si (deviation of 30Si/28Si relative to NBS 28 standard) and the Δ17O values of chondrules in unequilibrated enstatite chondrites are between −0.20‰ and −0.54‰ and −0.36‰ and +0.26‰, respectively. Furthermore, the chondrules with higher Δ17O values tend to have lower δ 30Si. The data exhibit values consistent with most of the noncarbonaceous group differentiated planetary bodies. This consistency suggests that the Si and O isotopic compositions of enstatite chondrules record those of the major precursors that formed the differentiated planetary bodies in the inner solar system. Model calculations based on the results reveal that the Si and O isotope variations of the enstatite chondrite chondrules were generated by an interaction between the evaporation-driven SiO-rich gas and partially or fully melted forsterite-rich precursor chondrules. The Mg/Si of the evaporated dust-gas mixtures increased with increasing silicate/metal ratio in the evaporated dust, which may have increased the bulk Mg/Si and δ 30Si value of the inner planetary bodies.

Isotopic analyses of Ordovician–Silurian siliceous skeletons indicate silica‐depleted Paleozoic oceans

The Phanerozoic Eon marked a major transition from marine silica deposition exclusively via abiotic pathways to a system dominated by biogenic silica sedimentation. For decades, prevailing ideas predicted this abiotic-to-biogenic transition were marked by a significant decrease in the concentration of dissolved silica in seawater; however, due to the lower perceived abundance and uptake affinity of sponges and radiolarians relative to diatoms, marine dissolved silica is thought to have remained elevated above modern values until the Cenozoic radiation of diatoms. Studies of modern marine silica biomineralizers demonstrated that the Si isotope ratios (δ30 Si) of sponge spicules and planktonic silica biominerals produced by diatoms or radiolarians can be applied as quantitative proxies for past seawater dissolved silica concentrations due to differences in Si isotope fractionations among these organisms. We undertook 446 ion microprobe analyses of δ30 Si and δ18 O of sponge spicules and radiolarians from Ordovician-Silurian chert deposits of the Mount Hare Formation in Yukon, Canada. These isotopic data showed that sponges living in marine slope and basinal environments displayed small Si isotope fractionations relative to coeval radiolarians. By constructing a mathematical model of the major fluxes and reservoirs in the marine silica cycle and the physiology of silica biomineralization, we found that the concentration of dissolved silica in seawater was less than ~150μM during early Paleozoic time-a value that is significantly lower than previous estimates. We posit that the topology of the early Paleozoic marine silica cycle resembled that of modern oceans much more closely than previously assumed.

Microstructure of bismuth centers in silicon before and after irradiation with 15 MeV protons

A decrease of two-gamma annihilation rate of a positron in a strong spin–orbit field of the annihilation site of bismuth impurity center 209Bi (J = 9/2) in silicon with natural isotope composition was revealed (J is the nuclear spin). This decrease was observed along with increasing occupancy of Bi donor states (binding energy E{Bi} ≈ 69 meV). Atoms of 29Si (J = 1/2) isotope are involved in spin interactions of positron with Bi impurity centers. The growth of occupancy of Bi donor states inhibits two-gamma annihilation rate. The estimated cross-section of positron trapping by the Bi impurity center is σ + ≈ (1.23–1.5) × 10−13 cm2. Together with this surprisingly large value, the integral rate of two-gamma annihilation in a hypothetical polyelectron system of the Bi impurity center is by a factor of just Δ ∼ 2.18 higher compared to the magnitude ≈2.09 × 109 s−1 known for elemental isolated polyelectron, (e−e+e−). Possible formation of the positron-containing exciton-like states, (e+)D 0 X (D = Bi, P) is also discussed. Irradiation of material with 15 MeV protons results in decreasing the factor Δ by ∼11% due to forming the radiation complex in which Bi atom is in an open volume ambient it. Such complex is suggested to have D 3d symmetry and be the deep donor. Low-temperature measurements of both the positron annihilation rate and Hall effect have been applied for studying the isochronal annealing of these point radiation defects which were found to be thermally stable up to ∼370 °C; they can be annealed at ∼430 °C – 470 °C. According to available data of ab initio cluster calculations, the complex of Bi atom with a simulated vacancy has D 3d symmetry with the energy gain ∼0.92 eV, thus indicating qualitative agreement between experimental and theoretical data.

Isotope Systematics of Presolar Silicate Grains: New Insights from Magnesium and Silicon

Abstract We report on Mg and Si isotope data of 86 presolar silicate grains identified through NanoSIMS oxygen ion imaging in thin sections of carbonaceous and ordinary chondrites. The O, Mg, and Si isotope data of 106 presolar silicates (including grains studied previously by our group) suggest division of O isotope Group 1 grains into four subpopulations: (i) “normal,” (ii) 25Mg-rich, (iii) 26Mg-rich, and (iv) 25Mg-poor. Normal Group 1 grains (∼60% of Group 1 grains) formed in the winds of low-mass asymptotic giant branch (AGB) stars, with Mg and Si defining linear arrays with slopes of ∼0.9 and 1.37, respectively, in three-isotope representations, most likely representing Galactic chemical evolution (GCE). The 25Mg-rich grains (∼25%) show enrichments in 25Mg of up to a factor 2.4 relative to solar composition and most likely formed in supernova (SN) ejecta or the winds of intermediate-mass AGB stars. The 26Mg-rich and 25Mg-poor Group 1 grains lie below the Mg GCE line and their isotopic compositions favor origins from supergiants or SNe. The O isotope Group 2 grains show a wide range of Mg-isotopic compositions, similar to Group 1 grains, with likely origins from massive AGB stars, super-AGB stars, supergiants, and SNe. The Mg- and Si-isotopic compositions of Group 4 grains are compatible with previously proposed SN origins. Our results suggest that >30% of presolar silicates formed in the winds of supergiants and in SN ejecta, and that low-mass AGB stars appear to have contributed only some 50% to presolar silicates, less than previously thought.

Imprint of chemical weathering and hydrothermalism on the Ge/Si ratio and Si isotope composition of rivers in a volcanic tropical island, Basse-Terre, Guadeloupe (French West Indies)

A significant portion of the disproportionally high chemical weathering flux in volcanic island arcs may originate from hydrothermal fluid-rock interaction, thereby compromising the accurate estimate of atmospheric CO2 consumption rates. The objective of this study is to evaluate how the riverine Ge/Si ratio and Si isotopes, two well-established tracers of weathering, respond to hydrothermal inputs. The work took place in Basse-Terre, Guadeloupe, a tropical volcanic island with a dense river network, high chemical weathering fluxes and various hydrothermal surface manifestations. We characterized the Ge/Si ratio and δ30Si of 15 thermal springs, nine non-impacted (NI) rivers and 13 hydrothermally-impacted (HI) rivers. The soil solution from a highly weathered soil profile (Ferralsol) and a clayey-rock corresponding to the material exposed in an extinct hydrothermal system were also measured. A new purification method was successfully developed in order to allow the reliable measurement of Si isotopes in SO42−- and Cl−-rich thermal spring and HI river waters by mass spectrometry. Basse-Terre's thermal springs have variable Ge/Si ratios (0.05–21.03 μmol.mol−1) and δ30Si (0.71–1.50‰), but with no apparent relationship to the water compositional type. The Ge/Si ratio (0.15–2.57 μmol.mol−1) and Si isotope composition (0.26–1.21‰) values of the NI rivers reveal differences in the watersheds' weathering degree. Dissolution of Ge- and 28Si-rich secondary minerals explains the high Ge/Si and isotopically light composition of the northern NI rivers draining strongly weathered terranes. The Ge/Si ratio and δ30Si values measured for the NI and HI rivers overlap, implying that they cannot be used to diagnose hydrothermal contributions to river basins unambiguously. However, when combined with the Cl− and SO42− concentrations, the analysis of Ge and Si in the HI rivers suggests that water seeping through an extinct hydrothermal system produces SO4-rich drainages with distinctively lower Ge/Si ratios than those inferred for watersheds receiving thermal spring discharges associated with an active hydrothermal system. Overall, our results provide new constraints for applying and interpreting Ge/Si and Si isotope measurements to study weathering in volcanic environments.

Silicon Isotope Analyses of Soil and Plant Reference Materials: An Inter‐Comparison of Seven Laboratories

The use of silicon (Si) isotopes has led to major advances in our understanding of Si cycling in modern and past environments. This inter‐laboratory comparison exercise provides the community with the first set of soil and plant reference materials with an analytically challenging matrix containing organic material that is known to induce isotopic bias, for use as secondary reference materials in Si isotope measurement. Seven laboratories analysed four soil reference materials (GBW‐07401, GBW‐07404, GBW‐07407, TILL‐1) and one plant reference material (ERM‐CD281). Participating laboratories employed a range of chemical preparation methods and analytical setups but all analyses were performed by MC‐ICP‐MS. Irrespective of the chemical preparation method or analytical conditions, the results show excellent agreement among laboratories within 2s for at least three replicates. Data were combined together to calculate δ29Si and δ30Si mean values (relative to NBS 28) and their expanded uncertainties (U, coverage factor k = 2). The δ30Si values are as follow: GBW‐07401: −0.27 ± 0.06‰, GBW‐07404: −0.76 ± 0.12‰, GBW‐07407: −1.82 ± 0.17‰, TILL‐1: −0.16 ± 0.06‰ and ERM‐CD281: −0.28 ± 0.11‰. Also, a compilation of published data provides an up‐to‐date mean δ30Si for BHVO‐2 of −0.28 ± 0.08‰.

Silicon uptake and translocation in low‐silica rice mutants investigated by isotope fractionation

AbstractSilicon (Si) alleviates biotic and abiotic stress in plants and many studies have been conducted in Si uptake by the roots. However, there is limited information about Si translocation during grain filling and the Si transport pathways regulating plant Si uptake. In the present study, we performed a 2‐yr field trial on the wild‐type rice (Oryza sativa L.) varieties Oochikara and Taichung‐65 and their corresponding mutants lsi1 and lsi2. We used multi‐collector, inductively coupled plasma mass spectrometry to evaluate Si isotope fractionation by the Lsi1 and Lsi2 transporters. The qualitative isotope fractionation caused by Lsi1 and Lsi2 (δ30SiLsi1 and δ30SiLsi2) were −2.95 and −2.61‰, respectively, in 2016 and −1.35 and −1.67‰, respectively, in 2017, indicating that transporter Lsi1 and Lsi2 preferentially absorbed 28Si rather than 30Si. Moreover, there was net 28Si enrichment in the mutants, indicating that except for Lsi1 and Lsi2, other Si transport pathways belong to ion channels and electrogenic pumps occur in rice. The unexpected observed decrease in 30Si from the husk to the grain suggests xylem‐to‐phloem Si transfer during grain filling. Hence, Si isotope fractionation is a useful tool to investigate plant Si uptake and translocation.

Testing hypotheses of albite dissolution mechanisms at near-equilibrium using Si isotope tracers

Here, we demonstrate the potential advantages of using isotope tracers to test hypotheses of reaction mechanisms near-equilibrium. Using non-traditional stable Si isotopes as tracers, we measured albite unidirectional dissolution rates (r+) across a range of Gibbs free energy of reaction (ΔrG) close to equilibrium (−26 to −2 kJ/mol). Thirteen batch experiment series were conducted at 50 °C and pH ∼ 8 ± 0.25. Different distances from equilibrium were achieved by a stepwise increase of concentrations of Si (0–600 μM), Al (0–10 μM), and Na (0–1000 μM). The temperature, pH, sample preparation, and reaction duration were kept identical to isolate the ΔrG effect. Secondary phase precipitation, which is difficult to avoid in near-equilibrium, near-neutral pH experiments renders the rate measurement method based on changes in Si and Al concentration unworkable, but it should not impact the Si isotope ratios-based rates.The resulting r+ values were nearly constant in the experimental ΔrG range, signaling no major ΔrG-related switch of reaction mechanisms. Our results suggest that the switch from etch pit opening at far-from-equilibrium to step retreat at near-equilibrium does not operate under circum-neutral pH in low-temperature systems; this mechanism switch was proposed based on experimental data in alkaline solutions at hydrothermal temperatures. The nearly constant r+ values at pH 5–8 also suggest that an H2O-catalyzed reaction mechanism dominant at circumneutral pH, in addition to the H+- and OH–-catalyzed reaction mechanisms dominant at acidic and alkaline pH, respectively.The experimental results have implications for geochemical modeling of low-temperature geological and environmental processes. The results suggest that a term of H2O-catalyzed reaction mechanism should be included in rate laws and that the parallel rate law with a mechanism-switch is not applicable in the pH range of 5–8.

Human influence on the continental Si budget during the last 4300 years: δ30Sidiatom in varved lake sediments (Tiefer See, NE Germany)

The continental silicon (Si) cycle, including terrestrial and freshwater ecosystems (lakes, rivers, estuaries), acts as a filter and modulates the amount of Si transported to the oceans. In order to link the variation in the terrestrial Si cycle to aquatic ecosystems, knowledge on changes in vegetation cover, soil disturbance and the impact of human activity are required. This study on varved lake sediments from Tiefer See near Klocksin (TSK) in northeastern Germany investigates Si isotope variations in diatom frustules (δ30Sidiatom) over the last ∼4300 years. δ30Sidiatom values vary between 0.37 and 1.63‰. The isotopic signal measured in centric (mostly planktonic) and pennate (mostly benthic) diatoms shows the same trend through most of the record. A decrease in δ30Sidiatom coinciding with early deforestation between 3900 and 750 a BP in the catchment area, points to an enhanced export of isotopically light dissolved silica (DSi) from adjacent soils to the lake. The burial flux of biogenic silica (BSi) observed in the lake sediments increases with cultivation due to enhanced nutrient supply (N, P and Si) from the watershed and nutrient redistribution caused by wind-driven increased water circulation. When the cultivation intensifies, we observe a shift to higher δ30Sidiatom values that we interpret to reflect a diminished Si soil pool and the preferential removal of the lighter 28Si by crop harvesting. Human activity influences the DSi supply from the catchment and appears to be the primary driver controlling the Si budget in TSK. Our data shows how land use triggers variations in continental Si cycling on centennial timescales and provides important information on the underlying processes.

Study of the Nuclear Structure of some Neutron Rich Si Isotopes Using Shell Model with Skyrme Interaction

Abstract
 The nuclear structure of 28-40Si isotopes toward neutron dripline has been investigated in framework of shell model with Skyrme-Hrtree-Fock method using certain Skyrme parameterizations. Moreover, investigations of static properties such as nuclear densities for proton, neutron, mass, and, charge densities with their corresponding rms radii, neutron skin thicknesses, binding energies, separation energies, shell gap, and pairing gap have been performed using the most recent Skyrme parameterization. The calculated results have been compared with available experimental data to identify which of these parameterizations introduced equivalent results with the experimental data. For all dynamic properties, sdpf shell model space has been used to generate one body transition density matrix element with SDPFK two body effective interaction. The calculations also reproduced the low and higher-laying 2+ energy level scheme, and reduced transition probability B(E2) for even Si-isotopes.

Isotope analysis of highly enriched “silicon-28” by high-resolution inductively coupled plasma mass spectrometry using an internal standard

In order to study the isotopic effects in semiconductor materials, single crystals of high chemical and isotopic purity are required. The reliability of the obtained data on the magnitude and the direction of isotopic shifts depends on the accuracy of determining the concentration of all stable isotopes. In the isotopic analysis of enriched “silicon-28” with a high degree of enrichment (> 99.99%), it is necessary to determine the impurities of 29Si and 30Si isotopes at the level of 10-3 ¸ 10-5 at. %. At this concentration level, these isotopes can be considered as impurities. It is difficult to achieve high measurement accuracy with simultaneous registration of the main and “impurity” isotopes in such a wide range of concentrations. The registration of analytical signals of silicon isotopes must be carried out in the solutions with different matrix concentrations. The use of the solutions with the high concentration of the matrix element requires the introduction of corrections for matrix noise and the drift of the instrument sensitivity during the measurement. It is possible to reduce the influence of the irreversible non-spectral interference and sensitivity drift by using the method of internal standardization. The inconsistency of the literature data on the selection criteria for the internal standard required studying the behavior of the signals of the “candidates for the internal standard” for the ELEMENT 2 single-collector high-resolution inductively coupled plasma mass spectrometer on the matrix element concentration and the nature of the solvent, as well as on the solution nebulizing time. Accounting for the irreversible non-spectral matrix noise and instrumental drift in isotopic analysis of enriched “silicon-28” and initial 28SiF4 by inductively coupled plasma mass spectrometry had allowed us to reduce by 3-5 times the random component and by more than an order of magnitude the systematic component of the measurement error in comparison with the external standard method. This made it possible to carry out, with sufficient accuracy, the operational control of the isotopic composition of enriched “silicon-28”, both in the form of silicon tetrafluoride and polycrystalline silicon obtained from it, using a single serial device in the range of isotopic concentrations 0.0001–99.999%.

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.

Hyperfine spectroscopy in a quantum-limited spectrometer.

We report measurements of electron-spin-echo envelope modulation (ESEEM) performed at millikelvin temperatures in a custom-built high-sensitivity spectrometer based on superconducting micro-resonators. The high quality factor and small mode volume (down to 0.2 pL) of the resonator allow us to probe a small number of spins, down to . We measure two-pulse ESEEM on two systems: erbium ions coupled to nuclei in a natural-abundance crystal and bismuth donors coupled to residual nuclei in a silicon substrate that was isotopically enriched in the isotope. We also measure three- and five-pulse ESEEM for the bismuth donors in silicon. Quantitative agreement is obtained for both the hyperfine coupling strength of proximal nuclei and the nuclear-spin concentration.

The Effects of Glacial Cover on Riverine Silicon Isotope Compositions in Chilean Patagonia

Proglacial rivers have been shown to have distinctive silicon (Si) isotope compositions, providing new insights into the mechanisms controlling Si cycling in the subglacial environment and suggesting terrestrial Si isotope exports may have varied between glacial and interglacial periods. However, Si isotope data are currently limited to a small number of glacial systems in the northern hemisphere, and it is unclear how compositions might vary across a spectrum of glacial influence. Using Chilean Patagonia as a unique natural laboratory, we present Si isotope compositions of 0.45 μm filtered (fSi), 0.02 μm filtered (DSi), and reactive amorphous (ASi) fractions from 40 river catchments with variable glacial cover and explore the key controls on Si cycling. The 0.45 μm filtered glacier-fed river samples displayed isotopically light compositions and a positive linear correlation with upstream glacial cover. This relationship was controlled by the inclusion of an isotopically light colloidal-nanoparticulate (0.02–0.45 μm) silicate phase that was only present in glacier-fed rivers and dominated Si budgets in these catchments. This phase was predominately composed of feldspars and its lability in seawater is uncertain, representing a significant unknown in resolving glacial Si isotope exports from this region. When the colloidal-nanoparticulates were removed from solution by ultra-filtration, the resultant DSi isotope compositions of glacier-fed catchments were not isotopically distinct from some non-glacial rivers and exhibited no clear relationship with glacial cover. The colloidal-nanoparticulate concentration of other weathering-sensitive elements (Li, Mg, Ba, Sr) also showed a linear relationship with glacial cover, suggesting that their isotopic compositions could be affected in a similar manner. These findings highlight the benefit of size-fractionated sampling and the need for more research to understand the lability of colloidal-nanoparticulate species, especially in glacier-fed rivers. Finally, we explore the controls on river ASi isotope compositions and show how including these reactive particulate phases is critical to quantifying terrestrial Si isotope budgets, both in Patagonia and other global regions.

New perspectives on lateritic weathering process over karst area – Geochemistry and Si-Li isotopic evidence

Silicon and lithium isotopes are increasingly used to trace current and past weathering processes. Herein, four soil profiles (SL1, SL2, XNC, and DP) in the karst critical zone in southwest China were studied. The bedrock of the SL1, SL2, and XNC profiles are limestone, while the bedrock of the DP profile is shale. By analyzing their major elements, trace elements, and mineral compositions as well as using Si and Li isotope methods, we were able to identify the material source of the laterite in southwest China and study its element behavior and isotope fractionation characteristics during the weathering process. The results show that the δ30Si of the profiles overlying limestone (SL1, SL2, and XNC) range from −0.5‰ to 0.7‰, and those of the δ7Li range from 0.28‰ to 3.54‰, while the profile overlying shale (DP) has a large range of δ30Si variation (from −1.2‰ to 1.1‰). The results reveal a clear inheritance from the underlying bedrock, indicating that the profiles reflect in-situ weathering. Although no obvious relationship exists among the chemical index of alteration (CIA), the δ30Si, and the δ7Li, a significant correlation between δ30Si and δ7Li indicates that the isotope fractionation of the studied profiles is mainly affected by the dissolution and formation of minerals during the weathering process, and the biological effect is relatively small. Further, a diagram of Li/Al versus δ7Li shows a decrease in both Li/Al and δ7Li during weathering.

Cluster radioactivity in superheavy nuclei 299-306122

Cluster radioactivity is an intermediate between alpha decay and spontaneous fission. It is also an exotic decay obtained in superheavy nuclei. When a cluster decay is detected in superheavy nuclei, the daughter nuclei is having near or equal to doubly magic nuclei. We have investigated cluster decay of isotopes of He, Li, Be, Ne, N, Mg, Si, P, S, Cl, Ar and Ca in the superhaevy nuclei region 299-306122. We have also compared the logarithmic half-lives of cluster decay with that of other models such as Univ [1], NRDX [2], UDL [3] and Horoi [4]. From this study it is concluded that cluster decay of 4He, 22Ne, 26Mg, 28Si 30Si, 34S, 40Ca and 46Ca are having shorter logarithmic half-lives compared to exotic cluster decay modes.

Serpentine alteration as source of high dissolved silicon and elevated \u03b430Si values to the marine Si cycle

Serpentine alteration is recognized as an important process for element cycling, however, related silicon fluxes are unknown. Pore fluids from serpentinite seamounts sampled in the Mariana forearc region during IODP Expedition 366 were investigated for their Si, B, and Sr isotope signatures (δ30Si, δ11B, and 87Sr/86Sr, respectively) to study serpentinization in the mantle wedge and shallow serpentine alteration to authigenic clays by seawater. While serpentinization in the mantle wedge caused no significant Si isotope fractionation, implying closed system conditions, serpentine alteration by seawater led to the formation of authigenic phyllosilicates, causing the highest natural fluid δ30Si values measured to date (up to +5.2 ± 0.2‰). Here we show that seafloor alteration of serpentinites is a source of Si to the ocean with extremely high fluid δ30Si values, which can explain anomalies in the marine Si budget like in the Cascadia Basin and which has to be considered in future investigations of the global marine Si cycle.

Behavior of Lithium Donors in Bulk Single-Crystal Isotopically Pure 28Si1 –x72Gex Alloys

The behavior of lithium donors in bulk single-crystal isotopically pure Si1 – xGex alloys (x = 0.0039–0.05) enriched in spinless 28Si and 72Ge isotopes (99.998 and 99.984%, respectively) is investigated by electron spin resonance. The fine structure of the spectrum of electrons localized at Li donors is studied at temperatures of T = 3.5–30 K in the expectation that, much like with pure silicon, enriching Si1 – xGex alloys in spinless isotopes (28SiisoGe, where iso = 70, 72, 74, 76) makes it possible to attain a higher resolution in the spectra of electron spin resonance. The results demonstrate that, despite the irregular arrangement of Ge atoms in the lattice of the 28Si1 – x72Gex alloy, the presence of local distortions in their vicinity, and the broadening of the electron-spin-resonance lines of donor electrons due to random strains, the lines of Li donors observed in the spectra of isotopically pure 28Si1 – x72Gex single crystals with x = 0.39, 1.2, and 2.9 at. % are narrower than the corresponding lines in the spectra of similar crystals with the natural composition of silicon and germanium isotopes. Similarly to the case of silicon crystals, this allowed us to study for the first time the angular dependences of the line positions in the electron-spin-resonance spectra of lithium in Si1 – xGex for different values of x.

Behavior of Phosphorus Donors in Bulk Single-Crystal Monoisotopic 28Si1 – x72Gex Alloys

The behavior of phosphorus donors in bulk single-crystal monoisotopic Si1 – xGex alloys (x = 0.0039–0.05) enriched by zero-spin isotopes 28Si (99.998%) and 72Ge (99.984%) by the electron-spin-resonance method is investigated. The hyperfine structure of the donor-electron spectrum giving information on the density of the donor wave function in the ground state on the 31P nucleus (I = 1/2) as well as the temperature dependences of the spin-relaxation rate (T = 3.5–30 K), which make it possible to analyze the longitudinal component T1 relaxation mechanism and magnitude of the valley–orbit splitting of the donor state, are investigated. Interest in these investigations is also caused by the fact that the Si1 – xGex alloy enriched by zero-spin isotopes (28SiisoGe, iso = 70, 72, 74, 76) is a poorly known material when compared with 28Si. The irregular arrangement of germanium atoms in the lattice of the SiGe solid solution and local distortions formed by it can level isotopic effects under isotopic enrichment. However, despite broadening of the lines of the electron-spin resonance of donor electrons due to random deformations formed by dissolved germanium atoms in silicon, narrower lines of the spectra of the electron-spin resonance are observed in isotopically pure Si1 – xGex single crystals at x = 0.39, 1.2, 2.9 at % when compared with similar crystals with the natural composition of silicon and germanium isotopes.