Refinements in BaSO4 to CO2 Preparation and δ18 O Calibration of the Sulfate Reference Materials NBS-127, IAEA SO-5 and IAEA SO-6

Refinements have been made to achieve over 99% yield in the conversion of CO to CO₂ in order to improve the reproducibility and accuracy of δ¹⁸ O measurements in sulfates. BaSO₄ (10-15 mg) was mixed with an identical amount of spectrographic-grade graphite and loaded into a Pt boat. The mixture was gradually heated to 1100 °C to reduce sulfate to CO and CO₂; the former gas was simultaneously converted to CO₂ by a glow discharge between Pt electrodes immersed in a magnetic field (produced by a pair of external neodymium magnets). A small memory effect was noticed during the analysis (less than 0.3‰ per 10‰ difference in δ¹⁸ O between two subsequently analysed samples). The memory effect, however, was suppressed by repetitive preparation of the same specimen. CO₂ produced in this way from sulfate reference samples was analysed on a dual inlet and triple collector mass spectrometer along with CO₂ equilibrated with VSMOW, GISP and SLAP water reference samples. To avoid large departures of measured isotope ratios from ¹⁸O/¹⁶O of the working calibrator we used CO₂ gas prepared from ocean water sulfate for this purpose. The calibrated δ¹⁸ O values (in ‰) obtained in this way for NBS-127, IAEA SO-5 and IAEA SO-6 reference materials were 8.73 ± 0.05, 12.20 ± 0.07 and -10.43 ± 0.12, respectively.     

A Simplified, Accurate and Fast Method for Lithium Isotope Analysis of Rocks and Fluids, and δ7Li Values of Seawater and Rock Reference Materials

We report a new approach to conduct fast and accurate lithium isotope ratio measurements by MC-ICP mass spectrometry after wet chemical sample preparation. In contrast to most previously published methods our MC-ICP-MS set-up did not use a desolvating system to achieve appropriate ion beam intensities and, therefore, was less affected by matrix-induced shifts of the instrumental mass bias. As the total lithium background and build-up in the sample introduction system was low, previous sample residues could be washed out by an extended uptake of the new sample. Elimination of a nitric acid rinse step increased the sample throughput by a factor of two and allowed the instrumental mass bias drift to be tracked more precisely. δ⁷Li values of powdered silicate rock reference materials and seawater obtained in this study revealed good accuracy and an overall analytical uncertainty of typically 0.5‰ (2s). On the basis of a comparison between our lithium isotope data and compiled literature data, we recommend preliminary average δ⁷Li values for seawater (+30.8‰) and several silicate rock reference materials (BHVO-1: +5.0‰; JA-1: +5.6‰; JB-2: +4.8‰). The compilation of published δ⁷Li values for seawater suggests that the observed large lithium isotope differences are due to inter-method and/or interlaboratory bias. Most recently published δ⁷Li values for seawater show little variation and confirm a constant lithium isotope composition (at the sub ‰ level) of seawater in well mixed ocean basins.     

Long-term (105) or short-term (103) δ13C excursion near the Palaeocene-Eocene transition: evidence from the Tethys

Expanded sedimentary records from the Tethys reveal unique faunal and isotopic changes across the Palaeocene-Eocene (P-E) transition. Unlike in the open oceans, the Tethys exhibits a gradual decrease of 1.5% in δ¹³C values prior to the rapid δ¹³C excursion. Associated with the 6¹³C excursion is a decrease in calcite burial, increase in detrital content and appearance of a unique opportunistic planktic foraminifera1 assemblage (e.g. compressed acarininids). The existence of a prelude decrease in δ¹³C values in the Tethys suggests that the P-E δ¹³C excursion may have occurred in two steps and over a few hundred thousand years, rather than as one step over a few thousand years as previously suggested. This slower excursion rate is readily explained by changing organic carbon weathering or burial rates and avoids the need of invoking ad hoc scenarios.     

Chemical Separation and Isotopic Variations of Cu and Zn From Five Geological Reference Materials

This paper presents an adapted anion exchange column chemistry protocol which allowed separation of high-purity fractions of Cu and Zn from geological materials. Isobaric and non-spectral interferences were virtually eliminated for consequent multiple-collector ICP-MS analysis of the isotopic composition of these metals. The procedure achieved ∼ 100% recoveries, thus ensuring the absence of column-induced isotopic fractionation. By employing these techniques, we report isotopic analyses for Cu and Zn from five geological reference materials: BCR-027 blende ore (BCR), δ⁶⁵Cu = 0.52 ± 0.15‰ (n = 10) and δ⁶⁶Zn = 0.33 ± 0.07‰ (n = 8); BCR-030 calcined calamine ore (BCR), δ⁶⁶Zn = -0.06 ± 0.09‰ (n = 8); BCR-1 basalt (USGS), δ⁶⁶Zn = 0.29 ± 0.12‰ (n = 8); NOD-P-1 manganese nodule (USGS), δ⁶⁵Cu = 0.46 ± 0.08‰ (n = 10) and δ⁶⁶Zn = 0.78 ± 0.09‰ (n = 9); SU-1 Cu-Co ore (CCRMP), δ⁶⁵Cu = -0.018 ± 0.08‰ (n = 10) and δ⁶⁶Zn = 0.13 ± 0.17‰ (n = 6). All uncertainties are ± 2s; copper isotope ratios are reported relative to NIST SRM-976, and zinc isotope ratios relative to the Lyon-group Johnson Matthey metal (batch 3-0749 L) solution, JMC Zn. These values agree well with the limited data previously published, and with results reported for similar natural sample types. Samples were measured using a GVi IsoProbe MC-ICP-MS, based at the Natural History Museum, London. Long-term measurement reproducibility has been assessed by repeat analyses of both single element and complex matrix samples, and was commonly better than ± 0.07‰ for both δ⁶⁶Zn and δ⁶⁵Cu.     

Three Secondary Reference Materials for Lithium Isotope Measurements: Li7-N, Li6-N and LiCl-N Solutions

The CRPG (Nancy, France) has prepared secondary reference materials for Li isotope measurements by mixing ⁷Li or ⁶Li spikes and either L-SVEC or IRMM-016 certified reference materials to produce solutions having a known Li concentration and isotopic composition. The Li7-N and Li6-N solution samples (1.5 mol l⁻¹ HNO₃) have nominal δ⁷Li isotopic compositions of 30.1‰ and -9.7‰ respectively relative to L-SVEC and concentrations of 100 mg l⁻¹. Repeated measurement of these samples using the QUAD-ICP-MS at the CRPG yielded δ⁷Li of 30.4 ± 1.1‰ (n = 13) and -8.9 ± 0.9‰ (n = 9) at the 2s level of confidence. An additional LiCl-N solution was measured and yielded a delta value of 9.5 ± 0.6‰ (n = 3). Identical results were obtained at the BRGM (Orléans, France) from determinations performed with a Neptune MC-ICP-MS (30.2 ± 0.3‰, n = 89 for the Li7-N, -8.0 ± 0.3‰, n = 38 for the Li6-N and 10.1 ± 0.2‰, n = 46 for LiCl-N at the 2s level of confidence). The deviation of measured composition relative to the nominal value for the Li6-N solution might be explained by either contamination during preparation or an error during sample weighing. These secondary reference materials, previously passed through ion exchange resin or directly analysed, may be used for checking the accuracy of Li isotopic measurements over a range of almost 40‰ and will be available to the scientific community upon request to J. Carignan or N. Vigier, CRPG.     

40Ar-39Ar Spectra of Alunite from Phreatic Craters in the Muine Volcano, Hokkaido, Japan

Abstract. ⁴⁰Ar-³⁹Ar analyses of two alunite samples from phreatic craters in the Pliocene Muine volcano in southwest Hokkaido, Japan, were carried out. The alunite with 17.4 permil δ³⁴S_{V_CDT} value in hydrothermal breccia from the Nagaoyama crater and that with 14.3 permil δ³⁴S_{V_CDT} value in silicified andesite from the Konuma crater give total fusion ages of 1.40 ± 0.04 Ma (la uncertainty) and 1.24 ± 0.08 Ma, respectively. However, the spectra of these samples indicate they have been effected by thermal overprinting and/or the existence of excess argon. These preliminary ⁴⁰Ar-³⁹A analyses suggest that the alunite underwent multiple hydrothermal activity by magmatic gas and vapor subsequent to the main hydrothermal activity.     

Measurement of SIMS Instrumental Mass Fractionation of Pb Isotopes During Zircon Dating

An igneous zircon reference material (OG1) was characterised for U-Pb isotopes by ID-TIMS, and utilised to evaluate SIMS (SHRIMP) instrumental mass fractionation (IMF) of radiogenic Pb isotopes (²⁰⁷Pb*/²⁰⁶Pb*). The TIMS ²⁰⁷Pb*/²⁰⁶Pb* reference value for OG1 was 0.29907 ± 0.00011 (95% confidence limit), 3465.4 ± 0.6 Ma. The high ²⁰⁷Pb* (∼ 30 μg g⁻¹), negligible common Pb, and isotopic homogeneity permitted precise (± 1–2‰) ²⁰⁷Pb*/²⁰⁶Pb* measurements within the analytical sessions. External reproducibility of mean ²⁰⁷Pb*/²⁰⁶Pb* ratios between sessions was demonstrated for one instrument, yielding a mean IMF of +0.87 ± 0.49‰. The mean ²⁰⁷Pb*/²⁰⁶Pb* ratios between instruments were dispersed beyond uncertainties, with session IMF values from +3.6 ± 1.7‰ to −2.4 ± 1.3‰, and a grand mean IMF value (twenty-six sessions) of +0.70 ± 0.52‰, indicating a tendency towards elevated ²⁰⁷Pb*/²⁰⁶Pb*. The specific causes of variability in IMF are unclear, but generally reflect subtle differences in analytical conditions. The common practice in SIMS of assuming that IMF for Pb⁺ is insignificant could result in systematic age biases and underestimated uncertainties, of critical importance for precise correlation of Precambrian events. Nevertheless, a zircon RM such as OG1 can be readily incorporated into routine dating to improve ²⁰⁷Pb*/²⁰⁶Pb* accuracy and external reproducibility.     

Tying platform drowning to perturbations of the global carbon cycle with a δ13COrg-curve from the Valanginian of DSDP Site 416

The carbonate carbon isotope record of the Phanerozoic is marked by repeated high-amplitude excursions towards more positive values. Although the interpretation of C-isotope events remains controversial, they are regarded as a proxy of the global C-cycle. Using δ¹³C_Org-measurements of samples from DSDP Site 416, it is shown that a classic example of platform drowning coincided with the onset of the mid-Valanginian carbon-cycle excursion. Widespread carbonate platform drowning would have contributed to the observed positive shift in the C-isotope record. As choking of carbonate production was compensated by increased organic production, both processes favoured a shift in marine carbon partitioning from the oxidized to the reduced carbon reservoir. This would have resulted in an increased flux of CO₂ from the atmosphere into the marine and sedimentary carbon reservoir.     

A 3.1 billion year old marble and the 87Sr/86Sr of late-Archean seawater

A marble band in the ≈ 2.75 Gyr old Ramagiri schist belt in the Dharwar craton of south India gave a Pb–Pb age of 3.075 ± 0.095 Gyr. The geochemical data, including high Sr and low Ba and Mn indicate seawater origin for the parent rock, and that there was insignificant geochemical exchange between the marble and the surrounding rocks. The calculated initial Nd isotopic composition and μ₁ indicate an older continental crustal source for the Nd and Pb. The initial ⁸⁷Sr/⁸⁶Sr of the marble is 0.70128, which is higher than the calculated mantle value at ≈ 3 Ga. Although pre-3 Gyr old marine carbonate rocks are thought to be buffered by mantle Sr, the Ramagiri marble contains evolved, crustal Sr. Despite this, the marble has the lowest measured ⁸⁷Sr/⁸⁶Sr among carbonates and represents one of the least radiogenic periods in seawater Sr isotope composition.     

Terrestrial carbon-isotope records from coastal deposits (Algarve, Portugal): a tool for chemostratigraphic correlation on an intrabasinal and global scale

ABSTRACT The carbon-isotope signature of terrestrial organic matter (OM) offers a valuable tool to develop stratigraphic correlations for near-shore deposits. A mid-Cretaceous coastal succession of the western Algarve Basin, Portugal, displays a marked negative δ¹³C excursion ranging from − 21.2‰ to − 27.8‰ in the Early Aptian followed by two shifts towards higher values (up to − 19.3‰) during the Early and Late Aptian, respectively. The dominance of cuticle and leaf debris in the bulk OM fraction is confirmed by optical studies, Rock-Eval pyrolysis and by comparison with the δ¹³C signature of four different types of fossilized land-plant particles. Correlation of two terrestrial δ¹³C_bulk OM records from different study sites leads to a significant enhancement of the intrabasinal stratigraphic correlation within the Algarve Basin. Three prominent excursions in the Portuguese records can be correlated with existing δ¹³C curves from pelagic and terrestrial environments. The general carbon-isotope pattern is superimposed by small-scale fluctuations which can be explained by compositional variations within the OM.     

Very high 18o enrichment in Archean cherts from South India: implications for Archean ocean temperature

Stable carbon and oxygen isotopic compositions of essentially unmetamorphosed Archean (> 2.6 Gyr old) cherts and carbonates of the Dharwar Sequence of southern India, from the northernmost part of the Dharwar-Shimoga supracrustal belt (Kalche and Nagargali), have been determined. The cherts from the Nagargali area, which preserve oolitic texture and cryptocrystalline silica, show highly enriched δ¹⁸O values ranging from 28 to 31.4%o relative to SMOW. Such values are the highest yet reported from Archean nondetrital sediments, but are similar to those of modern marine cherts. On the assumption of a seawater δ¹⁸O of 0%₀, calculation of temperature based on the maximum δ¹⁸O value of 31.4%₀ yields a value of 40°C. This is significantly less than 70–80°C reported for the Archean oceans based on cherts and chert-phosphate pairs. Diagenetically recrystallized microcrystalline chert-dolomite pairs of Kalche area exhibit a range of oxygen isotopic ratios similar to those reported for Archean cherts and carbonates from other parts of the world. The temperature of diagenesis is estimated to be about 68°C.     

Anthropogenic CO2 in Southern Ocean surface waters: evidence from stable organic carbon isotopes

We present an approach for tracing the fate of anthropogenic CO₂, compiling a large data set of stable organic carbon isotope ratios from surface sediments, plankton, and sinking matter in the Atlantic Ocean. The δ¹³C values of sinking matter are generally lower by 0.5–4.6‰ compared to the surface sediments. This difference increases with increasing latitude, which is explained by a stronger modern increase in surface water [CO₂ (aq)] in the Southern Ocean relative to the Tropical/Subtropical Ocean. Preindustrial dissolved CO₂ concentrations in Atlantic surface waters, estimated from the δ¹³C_org of surface sediments, are compared to recently measured surface water [CO₂ (aq)] values taken from literature. We obtain only a slight increase in [CO₂ (aq)] at lower latitudes but a significant change of about 7 ± 2 μ m in high latitudinal surface waters which we attribute to anthropogenic perturbation. Our results suggest that CO₂ released by human activities has been stored in Southern Ocean surface waters.     

High Precision Determination of 48Ca/42Ca Ratio by TIMS for Ca Isotope Fractionation Studies

For studies of mass-dependent fractionation of calcium isotopes in natural materials, the ⁴⁸Ca/⁴²Ca ratio is a superior choice to the conventionally measured ⁴⁴Ca/⁴⁰Ca ratio for two important reasons. These are (1) mass-dependent fractionation can be determined free from the effects of inherited or ingrown radiogenic ⁴⁰Ca and (2) this ratio increases the spread of measured isotopic masses by 50%, resulting in statistically better resolution of fractionation, assuming similar precision. A third, though strictly technical, advantage is the inherent ability of a mass spectrometer to measure ratios close to unity (⁴⁸Ca/⁴²Ca) more precisely than very small or large ratios (⁴⁴Ca/⁴⁰Ca). However, because of the very low natural abundance of both ⁴⁸Ca and ⁴²Ca, their ratio has been very difficult to measure, the only attempt so far being on a high mass resolution MC-ICP-MS with a precision of 0.33%. We report here determination of the ⁴⁸Ca/⁴²Ca ratio by the more commonly available and user-friendly multi-collector TIMS using a ⁴³Ca-⁴⁶Ca double-spike, with a significantly better precision of 0.18% (2s). The ⁴⁸Ca/⁴⁰Ca or ⁴⁴Ca/⁴⁰Ca ratio can also be measured in the same mass spectrometer run to provide complementary information on any radiogenic component.     

Laser Ablation and Solution ICP-MS Determination of Rare Earth Elements in USGS BIR-1G, BHVO-2G and BCR-2G Glass Reference Materials

Data are reported for rare earth elements (REE) in three geological glass reference materials (BIR-1G, BHVO-2G and BCR-2G) using a UV (266 nm) laser ablation ICP-MS system and the classical (HF-HClO₄) acid decomposition method, followed by conventional nebulisation ICP-MS. External calibration of laser ablation analyses was performed using NIST SRM reference materials with internal standardisation using ²⁹Si and ⁴⁴Ca. Replicate analyses of reference basaltic glasses yielded an analytical precision of 1-5% (RSD) for all the elements by solution ICP-MS and 1-8% (RSD) by laser ablation ICP-MS. The relative differences between the REE concentrations measured by solution and laser ablation ICP-MS compared with the reference values were generally less than 11 % for most elements. The largest deviations occurred for La determined by solution ICP-MS in BIR-1G. The results of both solution and laser ablation ICP-MS agreed well, generally better than 7%, with the exception of La, Pr and Sm in BIR-1G. The measured REE laser ablation data for BIR-1G, BHVO-2G and BCR-2G agreed with the previously published data on these basaltic reference glasses, within a range of 0-10% for most elements. No significant influences were observed for the predicted spectral interferences on some REE isotopes in the analysis of basaltic glasses.     

Stable isotopic and fluid inclusion evidence for meteoric fluid penetration into an active mountain belt; Alpine Schist, New Zealand

Calcite and quartz veins have formed, and are forming, in steeply dipping fissures in the actively rising Alpine Schist metamorphic belt of New Zealand. The fluids that deposited these minerals were mostly under hydrostatic pressure almost down to the brittle-ductile transition, which has been raised to 5-6 km depth by rapid uplift. Some fluids were trapped under lithostatic pressures. Fluids in the fissure veins were immiscible H₂O + NaCl-CO₂ mixtures at 200-350 C. Bulk fluid composition is 15-20 mol% CO₂ and <4.3 total mol CH₄+ N₂+ Ar/100mol H₂O. Water hydrogen isotopic ratio δD_H2O in the fissure veins spans -29 to -68‰, δ¹⁸O_H2O -0.7 to 8.5‰, and bulk carbon isotopic ratio δ¹³C ranges from -3.7 to -11.7‰. The oxygen and hydrogen isotopic data suggest that the water has a predominantly meteoric source, and has undergone an oxygen isotope shift as a result of interaction with the host metamorphic rock. Similar fluids were present during cooling and uplift. Dissolved carbon is not wholly derived from residual metamorphic fluids; part may be generated by oxidation of graphite.     

Nitrogen isotopes in peridotitic diamonds from Fuxian, China: the mantle signature

Nitrogen isotopes in peridotitic diamonds from Fuxian, China, suggest that the upper mantle δ¹⁵N-value has been globally homogeneous since at least the Proterozoic (−5 to −8‰/ATM), with similar values for subcontinental and MORB mantle.
  In addition, Fuxian diamonds retain the memory of a primary nitrogen lower in δ¹⁵N (down to −25‰). For the first time δ¹⁵N- values in diamonds match those of enstatite chondrites, supporting a formation of the Earth from such meteorites and the idea of a heterogeneous accretion of the Earth's volatiles.
  Nitrogen concentrations in diamonds are believed to depend strongly on the rate of the diamond growth and not to be an indicator of C/N ratios of the fluids from which they grew.     

Determination of the Absolute Hydrogen Isotopic Ratio of V-SMOW and SLAP

By mixing ¹H₂O and ²H₂O, both with accurately known purity, samples were prepared with ²H/¹H ratios close to those of the international isotopic water standards: V-SMOW and SLAP. A mass spectrometrical comparison of these calibration samples with the actual water standards revealed:
²H/¹H of V-SMOW = (155.95 ± 0.08) × 10^−6
2H/¹H of SLAP = (89.12 ± 0.07) × 10⁻⁶
δ²H_V-SMOW(SLAP) =−428.5 ± 0.5 %     

Boron isotope composition of melt inclusions from porphyry systems of the Central Andes: a reconnaissance study

Quartz-hosted melt inclusions from latite dykes of the Eocene El Salvador copper porphyry system in northern Chile display wide ranges in both boron concentration (15–155 p.p.m. B) and isotope composition (δ¹¹B −7 to +12‰; n  = 10), likely reflecting slab-derived fluid input from seawater-altered oceanic crust. In contrast, the major Miocene tin-silver and tin porphyry systems in the Bolivian back-arc region (Cerro Rico de Potosi, Chorolque, Llallagua) have distinctly different melt inclusion compositions with δ¹¹B of −11.4 ± 2.7‰ ( n  = 10), and magmatic boron enrichment up to several hundred p.p.m. B. The `seawater' signature in the El Salvador melt inclusions explains the oxidized mineral assemblage of the copper porphyry system, as opposed to the more reduced nature of the Bolivian tin porphyry systems, which reflect intracrustal melting of pelitic rocks.     

Chronology and organic chemistry of the Black Creek Swamp Megafauna site (Late Pleistocene), Kangaroo Island, Australia

The timing of the extinction of the Australian Megafauna and whether it was simultaneous and widespread has been a much researched topic in Quaternary geoscience. The Black Creek Swamp Megafauna site on Kangaroo Island was thought to be a refugium for Megafauna; however, recent and multidiscipline age determinations have established that the fossils are considerably older than the well-quoted extinction age of 45 kyr. Further radiocarbon age determinations, δ¹³C isotope analysis and ¹³C-NMR spectroscopy of the fossil containing organic matter demonstrates that it is highly soluble and accumulated as recently as 31–18 cal. kyr BP. These radiocarbon ages are much younger than the 100–50 kyr age bracket ascertained for the fossil material itself, implying separate episodes of death, deposition and burial. The soluble nature of the organic matter and increasing radiocarbon ages with depth suggests lateral accumulation, probably transported by subsurface waters from elevated areas proximal to the low-lying swamp. Such depositional conditions and ¹⁴C age range implies that the site may have experienced an unusually wet Last Glacial Maximum, due maybe to its proximity to the continental shelf and thus to maritime conditions. C3 vegetation dominates the Black Creek Swamp and its organic matter (δ¹³C; −30‰ and −23‰); however, variations in δ¹³C may indicate climatic shifts. ¹³C-enrichment and an abundance of salt-tolerant gastropods within the site's final phase of sediment accumulation (<6 cal. kyr BP) suggest that conditions during this most recent period were not as wet as those of the Last Glacial Maximum.     

Neodymium and Strontium Isotope Data for USGS Reference Materials BCR-1, BCR-2, BHVO-1, BHVO-2, AGV-1, AGV-2, GSP-1, GSP-2 and Eight MPI-DING Reference Glasses

We have measured ⁸⁷Sr/⁸⁶Sr and ¹⁴³ Nd/¹⁴⁴ Nd isotope ratios in different batches and aliquots of the new US Geological Survey (USGS) reference materials (RMs) BCR-2, BHVO-2, AGV-2 and GSP-2 and the original USGS RMs BCR-1, BHVO-1, AGV-1 and GSP-1 by thermal ionisation mass spectrometry. In addition, we also analysed the eight Max-Planck-Institut-Dingwell (MPI-DING) reference glasses. Nearly all isotope ratios obtained in the different aliquots and batches agree within uncertainty limits indicating excellent homogeneity of the USGS powders and the MPI-DING glasses. With the exception of GSP-2, the new USGS RMs are also indistinguishable from the ratios found in the original USGS RMs (⁸⁷Sr/⁸⁶Sr: 0.704960, 0.704958 (BCR-1, -2), 0.703436, 0.703435 (BHVO-1, -2), 0.703931, 0.703931 (AGV-1, -2); ¹⁴³ Nd/¹⁴⁴ Nd: 0.512629, 0.512633 (BCR-1, -2), 0.512957, 0.512957 (BHVO-1, -2); 0.512758, 0.512755 (AGV-1, -2)). This means that for normalisation purposes in Sr and Nd isotope geochemistry BCR-2, BHVO-2 and AGV-2 can well replace BCR-1, BHVO-1 and AGV-1 respectively.