Substantial Isotopic Fractionation Research Articles

Iron isotope constraints on the genesis of giant Beiya Au-base metal deposits, Yunnan, Southwest China

The Beiya Au-base metal deposit in southwest China is characterised by a huge amount of iron associated with gold mineralization. The formation of the Beiya deposit is generally thought to be related to Tertiary potassic intrusion, as porphyry- and skarn-type mineralization are the two most important types of mineralization in the region. However, the lack of direct evidence to constrain the source of iron and gold as well as other metals makes it difficult to accurately conceptualise the ore genesis model. In this study, we report high-precision (0.03 ‰; 2sd) Fe isotope data on Fe-bearing minerals including magnetite, pyrite and chalcopyrite, and major igneous rocks exposed in the area, including monzogranite porphyry (MGP), mafic microgranular enclave (MME), lamprophyre and basalt. Magnetites have a wide range of δ Fe from 0.15 to 0.64 ‰, reflecting substantial isotopic fractionation at different mineralization stages. MMEs have heaviest Fe isotopic composition (0.35 ‰ to 0.88 ‰) compared to MGPs (0.19 ‰ to 0.48 ‰), lamprophyre (∼0.2 ‰) and basalt (∼0.0 ‰). A negative correlation between δ56Fe and TFe2O3 is found for both MPGs and MMEs, which indicates the leaching of iron by hydrothermal fluids mobilizes light Fe and leaves behind isotopically heavy Fe in the remaining source rock. Fe isotopic compositions of MPG and MME overlap with those of magnetite, both of which are proposed to be the source of Fe in magnetite. The basalt has δ56Fe value of 0 ± 0.03 ‰, similar to those of global basalts, suggesting a negligible contribution to the formation of iron ores. Therefore, we propose that magmas parental to the MGP and MME may be the main source of iron to form the Beiya deposit; post-magmatism hydrothermal activities play a key role in leaching the metals out and transporting them to the mineralization area.

Stable carbon isotopic composition of biomass burning emissions – implications for estimating the contribution of C<sub>3</sub> and C<sub>4</sub> plants

Abstract. Landscape fires are a significant contributor to atmospheric burdens of greenhouse gases and aerosols. Although many studies have looked at biomass burning products and their fate in the atmosphere, estimating and tracing atmospheric pollution from landscape fires based on atmospheric measurements are challenging due to the large variability in fuel composition and burning conditions. Stable carbon isotopes in biomass burning (BB) emissions can be used to trace the contribution of C3 plants (e.g. trees or shrubs) and C4 plants (e.g. savanna grasses) to various combustion products. However, there are still many uncertainties regarding changes in isotopic composition (also known as fractionation) of the emitted carbon compared to the burnt fuel during the pyrolysis and combustion processes. To study BB isotope fractionation, we performed a series of laboratory fire experiments in which we burned pure C3 and C4 plants as well as mixtures of the two. Using isotope ratio mass spectrometry (IRMS), we measured stable carbon isotope signatures in the pre-fire fuels and post-fire residual char, as well as in the CO2, CO, CH4, organic carbon (OC), and elemental carbon (EC) emissions, which together constitute over 98 % of the post-fire carbon. Our laboratory tests indicated substantial isotopic fractionation in combustion products compared to the fuel, which varied between the measured fire products. CO2, EC, and residual char were the most reliable tracers of the fuel 13C signature. CO in particular showed a distinct dependence on burning conditions; flaming emissions were enriched in 13C compared to smouldering combustion emissions. For CH4 and OC, the fractionation was the other way round for C3 emissions (13C-enriched) and C4 emissions (13C-depleted). This indicates that while it is possible to distinguish between fires that were dominated by either C3 or C4 fuels using these tracers, it is more complicated to quantify their relative contribution to a mixed-fuel fire based on the δ13C signature of emissions. Besides laboratory experiments, we sampled gases and carbonaceous aerosols from prescribed fires in the Niassa Special Reserve (NSR) in Mozambique, using an unmanned aerial system (UAS)-mounted sampling set-up. We also provided a range of C3:C4 contributions to the fuel and measured the fuel isotopic signatures. While both OC and EC were useful tracers of the C3-to-C4 fuel ratio in mixed fires in the lab, we found particularly OC to be depleted compared to the calculated fuel signal in the field experiments. This suggests that either our fuel measurements were incomprehensive and underestimated the C3:C4 ratio in the field or other processes caused this depletion. Although additional field measurements are needed, our results indicate that C3-vs.-C4 source ratio estimation is possible with most BB products, albeit with varying uncertainty ranges.

Interpreting lacustrine bulk sediment δ15N values using metagenomics in a tropical hypersaline lake system

Nitrogen (N) is often a limiting nutrient in lacustrine systems, and bulk organic matter stable isotope ratios of N (δ15N) are widely used in lake sediment studies to interpret N source inputs and lake trophic status. Although records of lacustrine sedimentary δ15N can provide critical information relating to past environmental change, often productivity interpretations from δ15N and lacustrine fossil records yield conflicting interpretations. Furthermore, components of the internal N cycle have substantial isotopic fractionation factors, and likely wield an enormous influence on bulk lacustrine sedimentary δ15N values. Yet apart from cyanobacteria N-fixation, few studies link specific microbial, N-related activity to δ15N variability in lake sediment records. Here, we assess the relationship between lacustrine sedimentary δ15N and microbiome profiles analyzed from extracted sediment DNA using metagenomics. In a ~ 1600-year-long sediment record from a hypersaline lake located on Kiritimati, Republic of Kiribati (1.9° N, 157.4° W), both δ15N and the taxonomy annotations from five unique metagenomes vary with depth. Despite the relatively high abundance of Cyanobacteria, Bacteroidetes, and N-fixation genes in the uppermost sediment, we find the highest δ15N values of the sediment record there. These high values are likely due to denitrification, supported by a relatively high abundance of denitrification genes and taxa responsible for denitrification, such as those found in family Chromatiaceae within the Gamma-proteobacteria. In the deep sediment, N-related biochemical processes are likely suppressed considering the low energy, low nutrient subsurface environment. Low δ15N values observed in deeper sediments co-occur with genes for assimilatory nitrate reduction and ammonification. Thus, metagenomics provides greater clarity with respect to the specific, microbial processes that alter primary δ15N signatures in the subsurface sediment.

Selenium isotopes trace anoxic and ferruginous seawater conditions in the Early Cambrian

Selenium (Se) isotopes can yield substantial isotopic fractionation (up to 20‰) confirmed by experiments and field investigations, depending on various biotic or abiotic redox transformations. Therefore, it is expected that redox changes in the ancient oceans would induce significant isotopic fractionation, and the Se isotopic signatures recorded in old sedimentary rocks might provide new insight into how the redox state of the ancient ocean has evolved. However, previous studies have shown that Se is slightly enriched in the lighter isotope relative to the bulk earth values in most deposited conditions (oxic, anoxic, and even sulfidic). Here, our results reveal that ferruginous conditions can result in excessive accumulation of Se in sediments with an elevated Se/S ratio and significant isotope fractionation (about 6‰), which leads us to propose that Se isotopes are an appropriate geochemical proxy to trace unique oceanic conditions over times. Accordingly, Se isotopic variations measured in three Early Cambrian formations in southern China suggest that anoxic waters with ferruginous conditions must have been present in early Cambrian ocean along the eastern margin of the Yangtze platform, and oceanic circulation was stepwise reorganized. This may have triggered biological diversification from the Ediacaran to the Early Cambrian.

Comment on “The isotopic composition of cadmium in the water column of the South China Sea”

Yang et al. (2012) found that marine biogenic particles and corresponding surface seawater from the South China Sea were characterized by essentially identical Cd isotope compositions, and inferred that biological Cd isotope fractionation is insignificant at this location. Based on results obtained for a box model that represents Cd cycling in the surface layer, and using published data to constrain the Cd source and sink fluxes, we show that this conclusion is likely to be incorrect. The modeling results indicate that the heavy Cd isotope signature of ε114/110Cd≈+9 observed by Yang et al. (2012) for the surface South China Sea is most likely a consequence of substantial isotope fractionation (of about 4–6 ε) during uptake of dissolved seawater Cd by phytoplankton.

Equilibrium isotopic fractionation of copper during oxidation/reduction, aqueous complexation and ore-forming processes: Predictions from hybrid density functional theory

Copper exists as two isotopes: 65Cu (∼30.85%) and 63Cu (∼69.15%). The isotopic composition of copper in secondary minerals, surface waters and oxic groundwaters is 1–12‰ heavier than that of copper in primary sulfides. Changes in oxidation state and complexation should yield substantial isotopic fractionation between copper species but it is unclear to what extent the observed Cu isotopic variations reflect equilibrium fractionation. Here, I calculate the reduced partition function ratios for chalcopyrite (CuFeS2), cuprite (Cu2O), tenorite (CuO) and aqueous Cu+, Cu+2 complexes using periodic and molecular hybrid density functional theory to predict the equilibrium isotopic fractionation of Cu resulting from oxidation of Cu+ to Cu+2 and by complexation of dissolved Cu. Among the various copper(II) complexes in aqueous environments, there is a significant (1.3‰) range in the reduced partition function ratios. Oxidation and congruent dissolution of chalcopyrite (CuFeS2) to dissolved Cu+2 (as Cu(H2O)5+2) yields 65–63δ(Cu+2–CuFeS2)=3.1‰ at 25°C; however, chalcopyrite oxidation/dissolution is incongruent so that the observed isotopic fractionation will be less. Secondary precipitation of cuprite (Cu2O) would yield further enrichment of dissolved 65Cu since 65–63δ(Cu+2–Cu2O) is 1.2‰ at 25°C. However, precipitation of tenorite (CuO) will favor the heavy isotope by +1.0‰ making dissolved Cu isotopically lighter. These are upper-limit estimates for equilibrium fractionation. Therefore, the extremely large (9‰) fractionations between dissolved Cu+2 (or Cu+2 minerals) and primary Cu+ sulfides observed in supergene environments must reflect Rayleigh (open-system) or kinetic fractionation. Finally the previously proposed (Asael et al., 2009) use of δ65Cu in chalcopyrite to estimate the oxidation state of fluids that transported Cu in stratiform sediment-hosted copper deposits is refined.

Nickel Isotopic Composition and Nickel/Iron Ratio in the Solar Wind: Results from SOHO/CELIAS/MTOF

Using the Mass Time-of-Flight Spectrometer (MTOF)—part of the Charge, Elements, Isotope Analysis System (CELIAS)—onboard the Solar Heliospheric Observatory (SOHO) spacecraft, we derive the nickel isotopic composition for the isotopes with mass 58, 60 and 62 in the solar wind. In addition we measure the elemental abundance ratio of nickel to iron. We use data accumulated during ten years of SOHO operation to get sufficiently high counting statistics and compare periods of different solar wind velocities. We compare our values with the meteoritic ratios, which are believed to be a reliable reference for the solar system and also for the solar outer convective zone, since neither element is volatile and no isotopic fractionation is expected in meteorites. Meteoritic isotopic abundances agree with the terrestrial values and can thus be considered to be a reliable reference for the solar isotopic composition. The measurements show that the solar wind elemental Ni/Fe-ratio and the isotopic composition of solar wind nickel are consistent with the meteoritic values. This supports the concept that low-FIP elements are fed without relative fractionation into the solar wind. Our result also confirms the absence of substantial isotopic fractionation processes for medium and heavy ions acting in the solar wind.

Stable nitrogen isotope analysis of chlorophylls by gas chromatography–isotope ratio mass spectrometry

Compound-specific H, C and N isotope analyses by gas chromatography–isotope ratio mass spectrometry (GC/IRMS) have widely been used for various studies, because it allows a rapid and precise isotope analysis of specific molecules of a small amount (nanomolar amount of the element) in complex mixture. However, its application for tetrapyrrole pigments (i.e. chlorophylls) is limited due to their high molecular weight and conjugated structures. In this study, we developed a method to determine nitrogen isotopic composition (dN) of chlorophylls, by the use of chemical degradation treatment prior to the isotope analysis. Chlorophylls were isolated by high-performance liquid chromatography. The isolated chlorophylls were transformed to pyropheophorbide derivatives by HCl treatment, and subsequently degraded to pyrrole units (maleimides) by chromic acid oxidation (Fig. 1). These chemical degradations have no substantial isotopic fractionation for chlorophyll nitrogen, and thus dN values of the pyrrole units can be determined precisely by GC/IRMS. Analytical error (1r, standard deviation) of the isotope measurement was always better than ±0.5&, with the minimum sample amount of 15 ngN.

Ammonium transport and reaction in contaminated groundwater: Application of isotope tracers and isotope fractionation studies

Ammonium (NH4+) is a major constituent of many contaminated groundwaters, but its movement through aquifers is complex and poorly documented. In this study, processes affecting NH4+ movement in a treated wastewater plume were studied by a combination of techniques including large‐scale monitoring of NH4+ distribution; isotopic analyses of coexisting aqueous NH4+, NO3−, N2, and sorbed NH4+; and in situ natural gradient 15NH4+ tracer tests with numerical simulations of 15NH4+, 15NO3−, and 15N2 breakthrough data. Combined results indicate that the main mass of NH4+ was moving downgradient at a rate about 0.25 times the groundwater velocity. Retardation factors and groundwater ages indicate that much of the NH4+ in the plume was recharged early in the history of the wastewater disposal. NO3− and excess N2 gas, which were related to each other by denitrification near the plume source, were moving downgradient more rapidly and were largely unrelated to coexisting NH4+. The δ15N data indicate areas of the plume affected by nitrification (substantial isotope fractionation) and sorption (no isotope fractionation). There was no conclusive evidence for NH4+‐consuming reactions (nitrification or anammox) in the anoxic core of the plume. Nitrification occurred along the upper boundary of the plume but was limited by a low rate of transverse dispersive mixing of wastewater NH4+ and O2 from overlying uncontaminated groundwater. Without induced vertical mixing or displacement of plume water with oxic groundwater from upgradient sources, the main mass of NH4+ could reach a discharge area without substantial reaction long after the more mobile wastewater constituents are gone. Multiple approaches including in situ isotopic tracers and fractionation studies provided critical information about processes affecting NH4+ movement and N speciation.

Carbon and hydrogen isotopic composition of sterols in natural marine brown and red macroalgae and associated shellfish

Carbon (δ 13C) and hydrogen isotopic compositions (δD) of sterols in natural marine brown ( Sargassum filicinum and Undaria pinnatifida), red macroalgae ( Binghamia californica and Gelidium japonica), and in shellfish ( Binghamia californica, Haliotis discus and Omphalius pfeifferi) feeding on the brown algae have been investigated as a first attempt to understand the isotopic compositions of sterols in natural algae and heterotrophs. Brown algae have 24-methylcholesta-5,24(28)-dien-3β-ol with δ 13C value of −19.1 ± 0.1‰ and δD value of −298 ± 4‰, and 24-ethylcholesta-5,24(28) E-dien-3β-ol with δ 13C value of −20.3 ± 0.2‰ and δD value of −301 ± 1‰. Red algae have cholest-5-en-3β-ol with δ 13C value of −20.9 ± 3.0‰ and δD value of −289 ± 0‰. The δ 13C and δD values of these sterols in algae are consistent with those of algal sterols found in marine sediments, suggesting that the isotopic compositions of algal sterols are well preserved in sediments with little change due to heterotrophic effects. The three shellfish have a wide diversity of sterol types but most sterols, except cholest-5-en-3β-ol, have quite similar isotope compositions (δ 13C value of −19.5 ± 0.4‰ and δD value of −296 ± 5‰) to the dietary algal sterols. On the other hand, cholest-5-en-3β-ol is slightly depleted in 13C by up to 1.7‰ and significantly enriched in D by up to 49‰ relative to the other shellfish sterols. This may suggest that shellfish sterols are derived from dietary algal sterols with no substantial isotopic fractionation during either assimilation or modification, but cholest-5-en-3β-ol is derived from an admixture of processes including de novo biosynthesis in shellfish.

The H13CN/HC15N Abundance Ratio in Dense Cores: Possible Source‐to‐Source Variation of Isotope Abundances?

We have observed the H13CN (J = 1-0) and HC15N (J = 1-0) lines simultaneously toward three dense cores in the Taurus molecular cloud complex and have found a significant source-to-source variation of the column density ratio, N(H13CN)/N(HC15N). The lowest ratio of 2.6-3.1 is observed toward L1521E, while the highest ratio of 8-11 is observed toward L1498. We have also observed the 12C18O (J = 1-0, 2-1) and 13C18O (J = 1-0) lines, and the 12C/13C ratio has been derived to be 58.8 ± 3.7 and ≥117 toward L1521E and L1498, respectively. These results imply that a substantial isotope fractionation effect may exist in the formation processes of these molecules, or elemental abundances are not uniform among the observed sources. If the source-to-source variation of the (14N/15N)/(12C/13C) ratio in HCN and the 12C/13C ratio in CO originates from the elemental abundance variation, the 14N/15N ratio is calculated to be 151 ± 16 and ≥813 for L1521E and L1498, respectively. These ratios significantly deviate from the values obtained toward Galactic disk sources.

Intracrystalline fractionation of oxygen isotopes between hydroxyl and non-hydroxyl sites in kaolinite measured by thermal dehydroxylation and partial fluorination

Thermal dehydroxylation and partial fluorination techniques were used to measure intracrystalline fractionation of oxygen isotopes between hydroxyl and non-hydroxyl sites in kaolinite. Several aliquots of a well characterized, fine-grained (<1 μm) kaolinite from Macon, GA, were dehydroxylated in vacuo using a variety of heating procedures, heating rates, and target temperatures. Measured δ18O values of both the liberated water and the dehydroxylated residue are consistent over a wide range of temperatures (550–850°C) when dehydroxylation is performed in a single-step fashion at a rapid heating rate (>50°C/min.). Similar dehydroxylation experiments indicate that brucite dehydroxylation occurs without any significant isotopic fractionation of the oxygen isotopes. By extrapolation we postulate that no significant fractionation occurs during single-step thermal dehydroxylation of fine-grained kaolinite, provided that dehydroxylation is performed under well controlled conditions. In contrast, gibbsite dehydroxylation is accompanied by substantial isotopic fractionation. This is probably the result of the complex, multi-pathway dehydroxylation reaction of this mineral. Similarly, thermal dehydroxylation of coarsegrained (>1 μm) kaolinites and dickites of weathering and hydrothermal origin yield results that are dependent on the temperature of dehydroxylation. We suggest that this effect may be caused by isotopic exchange during diffusion of water molecules through coarse particles.Partial fluorination of fine-grained kaolinite in the presence of excess F2 at low temperatures (<185°C) yields unreproducible δ18O values. When performed at high temperatures (220–240°C) in the presence of insufficient F2, δ18O values are systematically lower than the bulk δ18O value and increase linearly with the percent stoichiometric yield. They are consistent with a greater rate of reaction of hydroxyl oxygen than of non-hydroxyl oxygen, but examination of the isotopic data as well as XRD and IR analyses of the residues after partial fluorination indicates that the separation between the two types of oxygen is not complete. The results, therefore, do not yield a reliable δ18O value of the hydroxyl oxygen.The results of this study suggest that the thermal dehydroxylation technique may be appropriate for analysis of OH groups in fine-grained kaolinite. The partial fluorination approach appears less suitable. Intracrystalline fractionation of oxygen isotopes between hydroxyl and non-hydroxyl sites in kaolinite may be larger than previously reported in experimental studies. Based on the results obtained for the finer-than 1 μm Macon kaolinite a value of 1.0272 is proposed for the intracrystalline fractionation factor, αnon-OH/OH, of kaolinite at 20 ± 10°C.

Crystal-chemistry and isotope geochemistry of alteration associated with the uranium Nopal I deposit, Chihuahua, Mexico

Thermal dehydroxylation and partial fluorination techniques were used to measure intracrystalline fractionation of oxygen isotopes between hydroxyl and non-hydroxyl sites in kaolinite. Several aliquots of a well characterized, fine-grained (<1 μm) kaolinite from Macon, GA, were dehydroxylated in vacuo using a variety of heating procedures, heating rates, and target temperatures. Measured δ18O values of both the liberated water and the dehydroxylated residue are consistent over a wide range of temperatures (550–850°C) when dehydroxylation is performed in a single-step fashion at a rapid heating rate (>50°C/min.). Similar dehydroxylation experiments indicate that brucite dehydroxylation occurs without any significant isotopic fractionation of the oxygen isotopes. By extrapolation we postulate that no significant fractionation occurs during single-step thermal dehydroxylation of fine-grained kaolinite, provided that dehydroxylation is performed under well controlled conditions. In contrast, gibbsite dehydroxylation is accompanied by substantial isotopic fractionation. This is probably the result of the complex, multi-pathway dehydroxylation reaction of this mineral. Similarly, thermal dehydroxylation of coarsegrained (>1 μm) kaolinites and dickites of weathering and hydrothermal origin yield results that are dependent on the temperature of dehydroxylation. We suggest that this effect may be caused by isotopic exchange during diffusion of water molecules through coarse particles.Partial fluorination of fine-grained kaolinite in the presence of excess F2 at low temperatures (<185°C) yields unreproducible δ18O values. When performed at high temperatures (220–240°C) in the presence of insufficient F2, δ18O values are systematically lower than the bulk δ18O value and increase linearly with the percent stoichiometric yield. They are consistent with a greater rate of reaction of hydroxyl oxygen than of non-hydroxyl oxygen, but examination of the isotopic data as well as XRD and IR analyses of the residues after partial fluorination indicates that the separation between the two types of oxygen is not complete. The results, therefore, do not yield a reliable δ18O value of the hydroxyl oxygen.The results of this study suggest that the thermal dehydroxylation technique may be appropriate for analysis of OH groups in fine-grained kaolinite. The partial fluorination approach appears less suitable. Intracrystalline fractionation of oxygen isotopes between hydroxyl and non-hydroxyl sites in kaolinite may be larger than previously reported in experimental studies. Based on the results obtained for the finer-than 1 μm Macon kaolinite a value of 1.0272 is proposed for the intracrystalline fractionation factor, αnon-OH/OH, of kaolinite at 20 ± 10°C.

On acceleration and motion of ions in corona and solar wind

Assuming a stationary, radial, spherically symmetric solar wind and a radial magnetic field direction in the vicinity of the sun, an equation of motion for ions heavier than protons in the solar wind is derived. The general properties of this equation are discussed and the results of numerical integrations are given. These results are based on the assumption of maxwellian velocity distribution functions for electrons, protons and ions, but the effects of first order deviations from such distributions are also presented and discussed. It is shown that dynamical friction, i.e. momentum transfer from protons to heavier ions accounts for the observed fact that heavier ions - if accelerated at all - normally reach the same velocity as the protons in the solar wind. Because of the non-linear relation between dynamical friction and proton-ion velocity difference a minimum proton flux is required to carry a certain ion species in the solar wind. Formulae comparing the minimum fluxes for different ions are given. It is shown that elements up to and beyond iron will be carried along in the solar wind as long as helium is carried along. Substantial isotopic fractionation is possible, in particular in the case of helium. The effects of ion motion and escape on abundances in the corona and in the outer convective zone of the sun are discussed.