High precision Mg isotope measurements of meteoritic samples by secondary ion mass spectrometry

The possibility of establishing an accurate relative chronology of the early solar system events based on the decay of short-lived Al-26 to Mg-26 (half-life of 0.72 Myr) depends on the level of homogeneity (or heterogeneity) of Al-26 and Mg isotopes. However, this level is difficult. to constrain precisely because of the very high precision needed for the determination of isotopic ratios, typically of +/- 5 ppm. In this study, we report for the first time a detailed analytical protocol developed for high precision in situ Mg isotopic measurements ((25)mg/(24)mg and (26)mg/Mg-24 ratios, as well as Mg-26 excess) by MC-SIMS. As the data reduction process is critical for both accuracy and precision of the final isotopic results, factors such as the Faraday cup (FC) background drift and matrix effects on instrumental fractionation have been investigated. Indeed these instrumental effects impacting the measured Mg-isotope ratios can be as large or larger than the variations we are looking for to constrain the initial distribution of Al-26 and Mg isotopes in the early solar system. Our results show that they definitely are limiting factors regarding the precision of Mg isotopic compositions, and that an under- or over-correction of both FC background instabilities and instrumental isotopic fractionation leads to important bias on delta Mg-25, delta(26)mg and Delta Mg-26 values (for example, olivines not corrected for FC background drifts display Delta Mg-26 values that can differ by as much as 10 ppm from the truly corrected value). The new data reduction process described here can then be applied to meteoritic samples (components of chondritic meteorites for instance) to accurately establish their relative chronology of formation.

Isotopic fractionation in proteins as a measure of hydrogen bond length

If a deuterated molecule containing strong intramolecular hydrogen bonds is placed in a hydrogenated solvent, it may preferentially exchange deuterium for hydrogen. This preference is due to the difference between the vibrational zero-point energy for hydrogen and deuterium. It is found that the associated fractionation factor (I) is correlated with the strength of the intramolecular hydrogen bonds. This correlation has been used to determine the length of the H-bonds (donor-acceptor separation) in a diverse range of enzymes and has been argued to support the existence of short low-barrier H-bonds. Starting with a potential energy surface based on a simple diabatic state model for H-bonds, we calculate (I) as a function of the proton donor-acceptor distance R. For numerical results, we use a parameterization of the model for symmetric 0-H. ``.0 bonds R. H. McKenzie, Chem. Phys. Lett. 535, 196 (2012)]. We consider the relative contributions of the 0-H stretch vibration, O-H bend vibrations (both in plane and out of plane), tunneling splitting effects at finite temperature, and the secondary geometric isotope effect. We compare our total (I) as a function of R with NMR experimental results for enzymes, and in particular with an earlier model parametrization (D(R), used previously to determine bond lengths. (C) 2015 AIP Publishing LLC.

Depletion of the heaviest stable N isotope is associated with NH4 +/NH3 toxicity in NH4 +-fed plants

Background: In plants, nitrate (NO(3)(-)) nutrition gives rise to a natural N isotopic signature (delta(15)N), which correlates with the delta(15)N of the N source. However, little is known about the relationship between the delta(15)N of the N source and the (14)N/(15)N fractionation in plants under ammonium (NH(4)(+)) nutrition. When NH(4)(+) is the major N source, the two forms, NH(4)(+) and NH(3), are present in the nutrient solution. There is a 1.025 thermodynamic isotope effect between NH(3) (g) and NH(4)(+)(aq) which drives to a different delta(15)N. Nine plant species with different NH(4)(+)-sensitivities were cultured hydroponically with NO(3)(-) or NH(4)(+) as the sole N sources, and plant growth and delta(15)N were determined. Short-term NH(4)(+)/NH(3) uptake experiments at pH 6.0 and 9.0 (which favours NH(3) form) were carried out in order to support and substantiate our hypothesis. N source fractionation throughout the whole plant was interpreted on the basis of the relative transport of NH(4)(+) and NH(3). -- Results: Several NO(3)(-)-fed plants were consistently enriched in (15)N, whereas plants under NH(4)(+) nutrition were depleted of (15)N. It was shown that more sensitive plants to NH(4)(+) toxicity were the most depleted in (15)N. In parallel, N-deficient pea and spinach plants fed with (15)NH(4)(+) showed an increased level of NH(3) uptake at alkaline pH that was related to the (15)N depletion of the plant. Tolerant to NH(4)(+) pea plants or sensitive spinach plants showed similar trend on (15)N depletion while slight differences in the time kinetics were observed during the initial stages. The use of RbNO(3) as control discarded that the differences observed arise from pH detrimental effects. -- Conclusions: This article proposes that the negative values of delta(15)N in NH(4)(+)-fed plants are originated from NH(3) uptake by plants. Moreover, this depletion of the heavier N isotope is proportional to the NH(4)(+)/NH(3) toxicity in plants species. Therefore, we hypothesise that the low affinity transport system for NH(4)(+) may have two components: one that transports N in the molecular form and is associated with fractionation and another that transports N in the ionic form and is not associated with fractionation.

Oxygen, carbon and hydrogen isotope studies of contact metamorphism

The O¹⁸/O¹⁶, C¹³/C¹², and D/H ratios have been determined for rocks and coexisting minerals from several granitic plutons and their contact metamorphic aureoles in northern Nevada, eastern California, central Colorado, and Texas, with emphasis on oxygen isotopes. A consistent order of O¹⁸/O¹⁶, C¹³/C¹², and D/H enrichment in coexisting minerals, and a correlation between isotopic fractionations among coexisting mineral pairs are in general observed, suggesting that mineral assemblages tend to approach isotopic equilibrium during contact metamorphism. In certain cases, a correlation is observed between oxygen isotopic fractionations of a mineral pair and sample distance from intrusive contacts. Isotopic temperatures generally show good agreement with heat flow considerations. Based on the experimentally determined quartz-muscovite O¹⁸/O¹⁶ fractionation calibration curve, temperatures are estimated to be 525 to 625°C at the contacts of the granitic stocks studied.

Small-scale oxygen isotope exchange effects between intrusive and country rock are observed over distances of 0.5 to 3 feet on both sides of the contacts; the isotopic gradients are typically 2 to 3 per mil per foot. The degree of oxygen isotopic exchange is essentially identical for different coexisting minerals. This presumably occurred through a diffusion-controlled recrystallization process. The size of the oxygen isotope equilibrium systems in the small-scale exchanged zones vary from about 1.5 cm to 30 cm. A xenolith and a re-entrant of country rock projecting into on intrusive hove both undergone much more extensive isotopic exchange (to hundreds of feet); they also show abnormally high isotopic temperatures. The marginal portions of most plutons have unusually high O¹⁸/O¹⁶ ratios compared to "normal" igneous rocks, presumably due to large-scale isotopic exchange with meta-sedimentary country rocks when the igneous rocks were essentially in a molten state. The isotopic data suggest that outward horizontal movement of H₂O into the contact metamorphic aureoles is almost negligible, but upward movement of H₂O may be important. Also, direct influx and absorption of water from the country rock may be significant in certain intrusive stocks.

Except in the exchanged zones, the O¹⁸/O¹⁶ ratios of pelitic rocks do not change appreciably during contact metamorphism, even in the cordierite and sillimanite grades; this is in contrast to regional metamorphic rocks which commonly decrease in O¹⁸ with increasing grade. Low O¹⁸/O¹⁶ and C¹³/C¹² ratios of the contact metamorphic marbles generally correlate well with the presence of calc-silicate minerals, indicating that the CO₂ liberated during metamorphic decarbonation reactions is enriched in both O¹⁸ and C¹³ relative to the carbonates.

The D/H ratios of biotites in the contact metamorphic rocks and their associated intrusions show a geographic correlation that is similar to that shown by the D/H ratios of meteoric surface waters, perhaps indicating that meteoric waters were present in the rocks during crystallization of the biotites.

Isotopic fractionation in sputtering

Isotopic fractionation due to sputtering has been investigated via a collector type experiment in which targets of known isotopic composition have been bombarded with several keV Ar⁺ and Xe⁺ ions with fluences down to 3.0x10¹⁴ ions/cm² , believed to be the lowest fluences for which such detailed measurements have ever been made. The isotopes were sputtered onto carbon collectors and analyzed with Secondary Ion Mass Spectroscopy (SIMS.) There is clear indication of preferential effects several times that predicted by the dominant analytical theory. Results also show a fairly strong angular variation in the fractionation. The maximum effect is usually seen in the near normal direction, measured from the target surface, falling continuously, by a few percent in some cases, to a minimum in the oblique direction. Measurements have been made using Mo isotopes: ¹⁰⁰Mo and ⁹²Mo and a liquid metal system of In:Ga eutectic. The light isotope of Mo is found to suffer a 53 ± 5‰ (note: 1.0‰ ≡ 0.1%) enrichment in the sputtered flux in the near normal direction, compared to the steady state near normal sputtered composition, under 5.0 keV Xe⁺ bombardment of 3.0 x 10¹⁴ ions/cm². In the liquid metal study only the angular dependence of the fractionation could be measured due to the lack of a well defined reference and the nature of the liquid surface, which is able to 'repair' itself during the course of a bombardment. The results show that ¹¹³In is preferentially sputtered over ¹¹⁵In in the near normal direction by about 8.7 ± 2.7‰ compared to the oblique direction. ⁶⁹Ga, on the other hand, is sputtered preferentially over ⁷¹Ga in the oblique direction by about 13 ± 4.4‰ with respect to the near normal direction.

Stable carbon isotope variations in surface bloom scum and subsurface seston among shallow eutrophic lakes

Carbon stable isotope analysis of surface bloom scum and subsurface seston samples was conducted in shallow eutrophic lakes in China during warm seasons from 2003 to 2004. delta C-13 values of bloom scum were always higher (averaged 5 parts per thousand) than those of seston in this study, and the possible reasons were attributed to (i) direct use of atmospheric CO2 at the air-water interface, (ii) decrease in C-13 fractionation due to higher carbon fixation, (iii) active CO2 transport, and/or (iv) HCO3 accumulation. Negative correlation between delta C-13(scum) - delta C-13(seston) and pH in the test lakes indicated that phytoplankton at the subsurface water column increased isotopic enrichment under the-carbon limitation along with the increase of pH, which might in turn decreased the differences in 313 C between the subsurface seston and the surface scums. Significant positive correlations of seston 8 13C with total concentrations of nitrogen and phosphorus in water column suggested that the increase in delta C-13 of seston with trophic state was depending on nutrient (N or P, or both) supply. Our study showed that delta C-13 of phytoplankton was indicative of carbon utilization, primary productivity, and nutrient supply among the eutrophic lakes. (C) 2007 Elsevier B.V All rights reserved.

Genesis of Th-230 excess in basalts from mid-ocean ridges and ocean islands: Constraints from the global U-series isotope database and major and rare earth element geochemistry

Based on Th-230-U-238 disequilibrium and major element data from mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs), this study calculates mantle melting parameters, and thereby investigates the origin of Th-230 excess. (Th-230/U-238) in global MORBs shows a positive correlation with Fe-8, P (o), Na-8, and F-melt (Fe-8 and Na-8 are FeO and Na2O contents respectively after correction for crustal fractionation relative to MgO = 8 wt%, P (o)=pressure of initial melting and F (melt)=degree of melt), while Th-230 excess in OIBs has no obvious correlation with either initial mantle melting depth or the average degree of mantle melting. Furthermore, compared with the MORBs, higher (Th-230/U-238) in OIBs actually corresponds to a lower melting degree. This suggests that the Th-230 excess in MORBs is controlled by mantle melting conditions, while the Th-230 excess in OIBs is more likely related to the deep garnet control. The vast majority of calculated initial melting pressures of MORBs with excess Th-230 are between 1.0 and 2.5 GPa, which is consistent with the conclusion from experiments in recent years that D (U)> D (Th) for Al-clinopyroxene at pressures of > 1.0 GPa. The initial melting pressure of OIBs is 2.2-3.5 GPa (around the spinel-garnet transition zone), with their low excess Ra-226 compared to MORBs also suggesting a deeper mantle source. Accordingly, excess Th-230 in MORBs and OIBs may be formed respectively in the spinel and garnet stability field. In addition, there is no obvious correlation of K2O/TiO2 with (Th-230/U-238) and initial melting pressure (P (o)) of MORBs, so it is proposed that the melting depth producing excess Th-230 does not tap the spinel-garnet transition zone. OIBs and MORBs in both (Th-230/U-238) vs. K2O/TiO2 and (Th-230/U-238) vs. P (o) plots fall in two distinct areas, indicating that the mineral phases which dominate their excess Th-230 are different. Ce/Yb-Ce curves of fast and slow ridge MORBs are similar, while, in comparison, the Ce/Yb-Ce curve for OIBs shows more influence from garnet. The mechanisms generating excess Th-230 in MORBs and OIBs are significantly different, with formation of excess Th-230 in the garnet zone only being suitable for OIBs.

Diet shift of upland buzzards (Buteo hemilasius): evidence from stable carbon isotope ratios

We measured the stable carbon isotope ratios for muscle of the upland buzzards (Buteo hemilasius), plateau pika (Ochotoma curzoniae), root vole (Microtus oeconomus), plateau zokor (Myospalax fontanierii) and passerine bird species at the Haibei Alpine Meadow Ecosystem Research Station (HAMERS), and provided diet information of upland buzzards with the measurement of stable carbon isotopes in tissues of these consumers. The results showed that δ~(13)C values of small mammals and passerine bird species ranged from -25.57‰ to -25.78‰ (n = 12), and from -24.81‰ to -22.51% (n = 43), respectively, δ~(13)C values of the upland buzzards ranged from -22.60‰ to -23.10‰ when food was not available. The difference in δ~(13)C values (2.88‰±0.31‰) between upland buzzards and small mammals was much larger than the differences reported previously, 1‰-2‰, and showed significant difference, while 1.31‰±0.34‰ between upland buzzard and passerine bird species did not differ from the previously reported trophic fractionation difference of 1‰-2‰. Estimation of trophic position indicated that upland buzzards stand at trophic position 4.23, far from that of small mammals, i.e., upland buzzards scarcely captured small mammals as food at the duration of food shortage. According to isotope mass balance model, small mammals contributed 7.89% to 35.04% of carbon to the food source of the upland buzzards, while passerine bird species contributed 64.96% to 92.11%. Upland buzzards turned to passerine bird species as food during times of shortage of small mammals. δ~(13)C value, a useful indicator of diet, indicates that the upland buzzards feed mainly on passerine bird species rather than small mammals due to "you are what you eat" when small mammal preys are becoming scarce.

Stable hydrogen isotope ratios of lignin methoxyl groups as a palaeoclimate proxy

• Stable isotope ratios of organic compounds are valuable tools for determining the geographical origin, identity, authenticity or history of samples from a vast range of sources such as sediments, plants and animals, including humans. • Hydrogen isotope ratios (d2H values) of methoxyl groups in lignin from wood of trees grown in different geographical areas were measured using compound-specificpyrolysis isotope ratio mass spectrometry analysis. • Lignin methoxyl groups were depleted in 2H relative to both meteoric water andwhole wood. A high correlation (r2=0.91) was observed between the d2 H valuesof the methoxyl groups and meteoric water, with a relatively uniform fractionation of –216±19 recorded with respect to meteoric water over a range of d2H values from –110 in northern Norway to + 20‰ in Yemen. Thus, woods from northernlatitudes can be clearly distinguished from those from tropical regions. By contrast, the d2H values of bulk wood were only relatively poorly correlated (r 2 = 0.47) with those of meteoric water. • Measurement of the d 2H values of lignin methoxyl groups is potentially a powerful tool that could be of use not only in the constraint of the geographical origin of lignified material but also in paleoclimate, food authenticity and forensic investigations.

Carbon Isotopic Fractionation of CFCs during Abiotic and Biotic Degradation

Carbon stable isotope ((13)C) fractionation in chlorofluorocarbon (CFC) compounds arising from abiotic (chemical) degradation using zero-valent iron (ZVI) and biotic (landfill gas attenuation) processes is investigated. Batch tests (at 25 °C) for CFC-113 and CFC-11 using ZVI show quantitative degradation of CFC-113 to HCFC-123a and CFC-1113 following pseudo-first-order kinetics corresponding to a half-life (t(1/2)) of 20.5 h, and a ZVI surface-area normalized rate constant (k(SA)) of -(9.8 ± 0.5) × 10(-5) L m(-2) h(-1). CFC-11 degraded to trace HCFC-21 and HCFC-31 following pseudo-first-order kinetics corresponding to t(1/2) = 17.3 h and k(SA) = -(1.2 ± 0.5) × 10(-4) L m(-2) h(-1). Significant kinetic isotope effects of e(‰) = -5.0 ± 0.3 (CFC-113) and -17.8 ± 4.8 (CFC-11) were observed. Compound-specific carbon isotope analyses also have been used here to characterize source signatures of CFC gases (HCFC-22, CFC-12, HFC-134a, HCFC-142b, CFC-114, CFC-11, CFC-113) for urban (UAA), rural/remote (RAA), and landfill (LAA) ambient air samples, as well as in situ surface flux chamber (FLUX; NO FLUX) and landfill gas (LFG) samples at the Dargan Road site, Northern Ireland. The latter values reflect biotic degradation and isotopic fractionation in LFG production, and local atmospheric impact of landfill emissions through the cover. Isotopic fractionations of ?(13)C ~ -13‰ (HCFC-22), ?(13)C ~ -35‰ (CFC-12) and ?(13)C ~ -15‰ (CFC-11) were observed for LFG in comparison to characteristic solvent source signatures, with the magnitude of the isotopic effect for CFC-11 apparently similar to the kinetic isotope effect for (abiotic) ZVI degradation.

Stable isotope composition of smectite in suevites at the Ries crater, Germany: Implications for hydrous alteration of impactites

The 24-km diameter Ries crater, Germany, exhibits well-preserved crater filling and surficial melt-rich breccia deposits that are believed to have been altered by post-impact hydrothermal fluids. The alteration mineralogy of the crater filling breccias is characterized by clay (smectite, chlorite) and a zeolite assemblage, and secondary clay phases (smectite, minor halloysite) in surficial melt-bearing breccia deposits. Using stable isotope analysis of secondary smectitic clay fractions, evidence of significant hydrous alteration of impactites at large water/rock ratios was found. The estimated fluid temperatures, using data derived by delta(18)O and delta D fractionation, suggest smectite precipitation in surficial breccias in equilibrium with meteoric fluids at temperatures 16 +/- 5 degrees C in agreement with the long-term variation of modern precipitation in the area. The stable isotope composition of smectite in crater-fill breccia, however, suggests a trend of monotonously increasing temperatures from 43 to 112 degrees C. with increasing depth through the breccia sequence. This demonstrates a different origin of alteration and temperature distribution for the surficial and crater filling melt-bearing impact breccias in the Ries crater. Our results suggest that the inverted structure of hydrothermal systems observed in some terrestrial impact craters, including the Ries crater, could indicate the initial configuration of a thermal anomaly in the crater filling sequence, but which is replaced with a normal hydrothermal convection in crater proper, during the course of post-impact cooling. (C) 2010 Elsevier B.V. All rights reserved.

The chlorine isotope composition of Earth's mantle

Chlorine stable isotope compositions ( delta Cl-37) of 22 mid- ocean ridge basalts ( MORBs) correlate with Cl content. The high-delta Cl-37, Cl- rich basalts are highly contaminated by Cl- rich materials ( seawater, brines, or altered rocks). The low-delta(37) Cl, Cl- poor basalts approach the composition of uncontaminated, mantle- derived magmas. Thus, most or all oceanic lavas are contaminated to some extent during their emplacement. MORB- source mantle has delta(37) Cl <= -1.6 per mil (%), which is significantly lower than that of surface reservoirs (similar to 0 parts per thousand not equal). This isotopic difference between the surface and deep Earth results from net Cl isotopic fractionation ( associated with removal of Cl from the mantle and its return by subduction over Earth history) and/ or the addition ( to external reservoirs) of a late volatile supply that is Cl-37- enriched.

Microbial isotopic fractionation of perchlorate chlorine

Perchlorate contamination can be microbially respired to innocuous chloride and thus can be treated effectively. However, monitoring a bioremediative strategy is often difficult due to the complexities of environmental samples. Here we demonstrate that microbial respiration of perchlorate results in a significant fractionation (similar to - 15parts per thousand) of the chlorine stable isotope composition of perchlorate. This can be used to quantify the extent of biotic degradation and to separate biotic from abiotic attenuation of this contaminant.

Aqueous geochemistry and oxygen isotope compositions of acid mine drainage from the Río Tinto, SW Spain, highlight inconsistencies in current models

The Rio Tinto river in SW Spain is a classic example of acid mine drainage and the focus of an increasing amount of research including environmental geochemistry, extremophile microbiology and Mars-analogue studies. Its 5000-year mining legacy has resulted in a wide range of point inputs including spoil heaps and tunnels draining underground workings. The variety of inputs and importance of the river as a research site make it an ideal location for investigating sulphide oxidation mechanisms at the field scale. Mass balance calculations showed that pyrite oxidation accounts for over 93% of the dissolved sulphate derived from sulphide oxidation in the Rio Tinto point inputs. Oxygen isotopes in water and sulphate were analysed from a variety of drainage sources and displayed delta O-18((SO4-H2O)) values from 3.9 to 13.6 parts per thousand, indicating that different oxidation pathways occurred at different sites within the catchment. The most commonly used approach to interpreting field oxygen isotope data applies water and oxygen fractionation factors derived from laboratory experiments. We demonstrate that this approach cannot explain high delta O-18((SO4-H2O)) values in a manner that is consistent with recent models of pyrite and sulphoxyanion oxidation. In the Rio Tinto, high delta O-18((SO4-H2O)) values (11.2-13.6 parts per thousand) occur in concentrated (Fe = 172-829 mM), low pH (0.88-1.4), ferrous iron (68-91% of total Fe) waters and are most simply explained by a mechanism involving a dissolved sulphite intermediate, sulphite-water oxygen equilibrium exchange and finally sulphite oxidation to sulphate with O-2. In contrast, drainage from large waste blocks of acid volcanic tuff with pyritiferous veins also had low pH (1.7). but had a low delta O-18((SO4-H2O)) value of 4.0 parts per thousand and high concentrations of ferric iron (Fe(III) = 185 mM, total Fe = 186 mM), suggesting a pathway where ferric iron is the primary oxidant, water is the primary source of oxygen in the sulphate and where sulphate is released directly from the pyrite surface. However, problems remain with the sulphite-water oxygen exchange model and recommendations are therefore made for future experiments to refine our understanding of oxygen isotopes in pyrite oxidation. (C) 2009 Elsevier B.V. All rights reserved.