V51B-0559
Overview of Uranium Isotopic Reference Materials at IRMM
For many applications in the geological sciences, in particular in geochemistry research, isotope ratio measurements play a significant role. For instance, in geochronology isotope abundances of uranium and its daughter products thorium and lead have been used since more than five decades to determine the age of various samples of geological interest. However, in order to validate mass spectrometric measurement procedures and to calibrate detector systems, suitable isotope reference materials are needed. IRMM is a well recognized provider for nuclear isotope reference materials to the nuclear industry and nuclear safeguards authorities, which can also be used for geological applications. This paper gives an overview of isotope reference materials for uranium prepared and certified at IRMM. These materials are synthetic isotope reference materials prepared based on proven methods of purifying and mixing highly enriched oxides. Firstly, a set of 10 mixtures of 233U, 235U and 238U was made in which the 235U/238U ratios were kept at 1:1 and the 233U/235U ratios varied from 1.0 to 10-6 (IRMM-072). This set is ideal for checking the linearity response of detectors used in isotope mass spectrometry. Recently, after the IRMM-072 series was exhausted, it has been replaced by the IRMM-073 and IRMM-074 series. Secondly the double spike IRMM-3636 with a 233U/236U ratio of 1:1 was prepared which allows internal mass fractionation correction for high precision 235U/238U ratio measurements. The 234U abundance of this double spike material is low enough to allow an accurate and precise correction of 234U/238U ratios, even for measurements of close to equilibrium uranium samples.
V51B-0560
NBL CRM 112-A: A new certified isotopic composition
NBL CRM 112-A Uranium Metal Assay Standard is commonly used as a natural uranium isotopic reference material within the earth science mass spectrometry community. The metal is from the same parent material as NBS SRM 960, the uranyl nitrate solution, CRM 145, and the high-purity uranyl nitrate solution CRM 145-B. Because CRM 112-A has not yet been certified for isotopic composition, it has been assumed that this material has a natural 235U/238U (0.0072527), and the δ234U has been determined by measurement (e.g. -37.1‰; Cheng et al., 2000). These values have been widely used to calibrate the concentration of spikes and standards, and to correct measurements for instrument or mass bias. New, preliminary, isotopic measurements on CRM 145 and CRM 112-A performed at New Brunswick Laboratory suggest that these reference materials have a slightly lower 235U/238U and δ234U than have been commonly used. If this is the case, then data using the accepted values may be slightly biased. The significance of this bias will depend on the uncertainty of the measurement, how the CRM 112-A data is used to correct measurement data, the cited values that were used to correct the data, and the final certified values of the CRM. This fall, New Brunswick Laboratory is certifying the isotopic composition of the CRM 112-A metal using high precision thermal ionization mass spectrometry techniques. Upon completion of certification, the new CRM 112- A standard with certified isotopic ratios will provide the earth science community with a well characterized and traceable reference for calibrating and correcting their mass spectrometry measurement systems.
V51B-0561
The Ins and Outs of U and Th Isotopic Measurements Using a Nu Plasma 1700 MC-ICP- MS
The Nu Plasma 1700 is a relatively new type of large-geometry MC-ICP-MS that offers the potential to improve the accuracy and precision of U and Th isotopic measurements compared to previous instruments (e.g., the Plasma 54-30, or P54-30). The higher resolution of the Nu Plasma 1700 translates to more than a factor of 6 improvement in the abundance sensitivity at high mass (compared to the P54-30), and virtually eliminates the necessity of a tail correction on 230Th during the measurement of 232Th/230Th ratios (the major source of uncertainty using the P54-30). However, the Nu Plasma 1700 uses a discrete dynode secondary electron multiplier for ion-counting, which has a smaller dynamic range than the Daly detector used by the P54- 30. This prevents the placement of a relatively large 235U signal on the ion-counter, and thus, requires alternative methods to correct for the effects of instrumental mass bias. Initial efforts in our laboratory have focused on the development of standard-sample bracketing (SSB) techniques to correct for mass bias, with a careful evaluation of possible instrumental artifacts such as matrix effects, drift in the bias between the Faraday collectors and the ion-counting detector (Faraday/IC bias), and non-linearity of both the ion-counting detector and the retardation lens. Experiments with solution standards suggest that the major source of uncertainty on U and Th isotopic measurements using the Nu Plasma 1700 in SSB mode is drift in the Faraday/IC bias. Nevertheless, SSB analyses for U and Th solution standards are reproducible to 0.3% for 238U/234U and 0.4% for 232Th/230Th (±2σ), which is similar to the results obtained on the Plasma 54-30 (Luo et al., 1997; Pietruszka et al., 2002). So far, accuracy has been evaluated by repeatedly analyzing the UCSC Th isotopic standard against a previously characterized in-house Th isotopic standard in SSB mode. The result agrees within error of previous determinations for UCSC Th using the Plasma 54-30. Our future development work will focus on a more thorough characterization of the accuracy and precision of the SSB method and a detailed exploration of alternative techniques to correct for mass bias.
V51B-0562
An Inter-Laboratory Assessment of the Th Isotopic Composition of Synthetic and Rock Standards.
We present a concerted international effort to cross-calibrate five synthetic Th isotope standards (UCSC Th ‘A'; OU Th ‘U'; WUN; IRMM35; and IRMM 36), and six rock standards (UCSC TML, Icelandic ATHO; USGS BCR-2; USGS W2; USGS BHVO-2; LV18) using plasma ionization multi-collector inductively coupled mass spectrometry (PIMMS). We then compare our new values with the compilation of the literature mass spectrometric data for these standards and derive recommended ‘consensus' 230Th/232Th values for each. We also present isotope dilution U and Th concentration data for four rock standards (UCSC TML, Icelandic ATHO; USGS BCR-2; USGS W2).
V51B-0563
Rapid U separation and its precise isotopic measurements using ICP-QMS
Here we present a largely simplified analytical separation technique for U from marin carbonates and sediments and U isotopic measurements obtained by inductively coupled plasma-source quadrupole mass spectrometer (ICP-QMS) Xseries II - Thermo Scientific. The separation of U is done from dissolved carbonates and sediments using a single ion exchange column packed with ~500 μg of UTEVA resin from EICHROM industries. The column is pre-cleaned and loaded by several rinses of MilliQ water and 3N HNO3. Then earth alkali, transition metals and lanthanides are eluted quantitatively using 3N HNO3. Pure Th and U solutions are then successively extracted from the column using 3N HCl and 1N HCl at ~100% yield. U solutions at ~25-50 ppb were injected into the ICP-QMS at conventional sample flow rates of approximately 1ml/minute, without particular injection systems such as a desolvator or μ - nebuliser. 30 scans with 180 sweeps and a dwell time of 50 ms per isotope were used to collect 233U, 234U, 235U and 236U on an electron multiplier. Baseline sensitivity was followed on mass 228 with <1cps at ~ 1000cps on mass 234. Then, mass discrimination was corrected using the 233U/236U spike of known isotopic ratio and HU1 reference solutions were used to test the reproducibility and to correct drifts using standard - sample bracketing. Overall ICPMS analyses yield a stunning reproducibility of <0.4 % at 2 σ, which is close to the one obtained by conventional TIMS instruments ~0.2-0.4 %. We have applied this technique to organic rich sediments and marine carbonate samples previously measured by TIMS and found a perfect agreement for both U concentration and its isotopic composition. This rapid and effective chemical purification and isotopic measurement of U allows to process more than 20 samples a day allowing to investigate large numbers of natural samples for weathering, tracer and geochronological studies.
V51B-0564
A Rapid, Easy, and Precise Method for Oxygen and Hydrogen Isotope Analysis of Water of Crystallization in Hydrated Minerals
Oxygen and hydrogen isotope analysis of water of crystallization is not trivial. Commonly used techniques involve off-line extraction of water from hydrated minerals and subsequent isotope analysis. Such methods are time consuming, require relatively large sample sizes and the stepwise procedure has to be carried out with extreme caution to avoid erroneous results. We present a new on-line method for analysis of oxygen and hydrogen isotope compositions of water of crystallization. Our results for gypsum (CaSO4 · 2H2O) show that the precision (1 sigma) of the method on 0.2 mg samples is better than 0.5 per mil for oxygen and 3 per mil for hydrogen isotope measurements.
V51B-0565
In-Situ Silicon Isotope Analysis of Archean Cherts by Laser Ablation MC-ICPMS
We present in-situ stable silicon isotope results for Archean Cherts from the Pilbara region, Western Australia. Analyses were performed using a Geolas Laser Ablation (LA) system equipped with a 193nm Excimer laser and a ThermoFinnigan Neptune MC-ICPMS. The MC-ICPMS was used in medium resolution mode (RP=4000) to resolve molecular isobaric interferences (e.g., 12C16O+, 14N2+, 14N16O+). We used an ablation pit size of 49 by 300μm with a 7Hz repetition rate and 5 J.cm-2. Tuning conditions and cup settings were similar as those described by Van den Boorn et al. (2006; 2007) for solution work. To assess precision and accuracy of the LA technique, chert samples were analyzed that were previously characterized for silicon isotopes by micro-drilling and subsequent liquid chromatographic purification. A chemically homogenous chert sample that is well characterized for silicon isotopes was used as a standard. This in-house standard has a δ30Si of 0.50 ± 20 (2sd, n=4) relative to NIST RM8546 (=NBS28). Our precision with the LA technique of 0.2‰ (2sd, n=11), based on repeated measurements of the standard, is slightly better than the long-term precision of 0.3‰ for solution work (Van den Boorn et al. 2006). Micro-drill and laser data are in excellent agreement (less than 0.4‰), which is well within the variations recorded in individual mm-cm sized chert laminae. By producing 3cm scans across chert bands, inhomogeneities of up to 0.5‰ can be resolved within a single band. Matrix effects might be significant in LA work. For example, borate silicate glass was up to 2.15‰ heavier than values obtained by solution work. This suggests that ablation induces isotopic fractionation and/or that matrix elements cause a shift in mass bias for silicon in the plasma. Because Archean cherts generally contain more than 95% SiO2, offsets due to matrix effects will be small. However, the use of a standard with a composition close to samples is recommended. References: Van den Boorn et al. (2006) J. Anal. At. Spectrom., 21, 734–742. Van den Boorn et al. (2007). Geology, 35: 939-942.
V51B-0566
Natural Rhenium Isotope Variations
Analyses of industrial and natural materials show ‰-scale Re isotope variation. Data are expressed using δ notation where δ187Re = \Large(\normalsize \frac{187Re/185Resmpl}{187Re/185Restd} \Large)\normalsize-1 × 10,000 relative to NIST SRM-989 (187Re/185Re = 1.7025; Gramlich et al., 1973). Re is extracted from solid samples with an aggressive chemical leach and is purified by anion chromatography using AG1x8 100-200 mesh resin. Column-induced fractionation is observed with early eluant δ187Re = showing a positive anomaly of up to 1‰. The pooled isotopic composition of the eluted Re converged with that of the reservoir after elution of ~85% of total Re. Total Re recovered was 100% within uncertainty. Analyses are done by MC-ICP-MS (WHOI Plasma Mass Spectrometry Facility, ThermoFinnigan Neptune) after doping with W (SRM 3163) to allow internal exponential correction of mass bias (186W/183W = 1.98594; Völkening et al., 1991). Analyte concentrations are 20 ng Re g-1 and 75 ng W g-1. Masses 180(W), 182(W), 183(W), 185(Re), 186(W), 187(Re), 190(Os), and 192(Os) are monitored in static mode. Tailing effects of Re into W and vice-versa have been evaluated and are negligible. Some Re-rich samples with unknown but presumably very large 187Os/188Os, such as ReS2 and MoS2, are stripped of Os by sparging (Hassler et al., 2000 ). Comparison of sparged and unsparged solutions splits can show significant differences up to 1.5 per mil, for an Archaean MoS2. Uncertainties for raw 187Re/185Re and 186W/183W ratios typically range from 8 to 30 ppm. Long-term reproducibility of NIST SRM-989 is 0.04 ‰ (2s). Industrial Re, represented by SRM-989, a Re filament from H. R. Cross, and commercially available perrhenic acid (HReO4) exhibit a total isotopic range of ~0.3‰ with δ187Re values of 0, 0.17, and 0.27‰ respectively. To date, variations in δ187Re have been observed in all classes of natural materials analyzed. Samples of Devonian Ohio shale from a Kentucky weathering profile with highly variable Re concentrations (Jaffe et al., 2002) display a range of about 0.6‰ and are themselves 0.5‰ lighter than an unweathered drill core sample taken from the site. Meteorites show variation of a similar range as represented by Allende (-0.69‰) and Canyon Diablo Troillite (-0.21‰). Sulfides analyzed include two MoS2 samples, one Archaean (0.55‰) one Proterozoic (-0.10‰), and a modern ReS2 (- 0.27‰). Finally, two water samples taken from the Berkeley Pit show δ187Re of - 0.13‰ (surface) and -0.27‰ (76 m depth). As sample data were generated and reduced throughout the developmental period and therefore according to some variable parameters (e.g. analyte levels, frequency of standard bracketing, different bracketing standards . . .) it is difficult to properly determine sample reproducibility at this stage. When evaluated under all conditions, 2s sample reproducibility can be as good as ±0.03‰ (3 separately processed sample aliquots, 6 total analyses), and as poor as 0.35‰ (2 separately processed sample aliquots, 5 total analyses). The effect of observed Re isotope variations on Re-Os dating is negligible.
V51B-0567
Fe Isotopic Analysis by a Nu Plasma 1700 MC-ICP-MS With a 58Fe-57Fe Double-spike
High-resolution MC-ICP-MS is well suited for Fe isotope analyses in systems with small natural variations. In addition, the use of a double spike may improve reproducibility through precise correction of potential mass- dependent fractionation that may result from chemical separation of Fe using ion-exchange chromatography and from instrumental artifacts (e.g., mass bias and matrix effects). Therefore, we performed an error analysis to calculate theoretically attainable precision for both iterative and non-iterative approaches to data reduction and different double-spike compositions. We investigated a full range of double spikes with either a 57Fe or 54Fe spike mixed with a 58Fe spike. Each double spike is theoretically mixed over the full range from pure double spike to pure standard and randomly fractionated simulating machine bias. For each mixture, we randomly perturb the input ratios one thousand times, at most by our in-run analytical errors. The standard deviation of a thousand differences with the true standard provides a precision estimate for each mixture. The results of this analysis show that the combination of an iterative procedure with a 58Fe-57Fe double spike has the potential for higher precision (to below 0.01 per mil) than a 58Fe-54Fe double spike. This lower limit is possible with our internal precision of 0.01 per mil (unspiked standard, 2SD) on the 56Fe/54Fe ratio. Such internal precision is not uncommon in other published data, however matrix effects and machine bias often degrade the external precision [e.g. Dideriksen et al., 2006]. Experiments with spiked solution standards and samples are required to determine if use of a 58Fe-57Fe double spike will translate into a significant improvement in the external precision of Fe isotope measurements using the Nu 1700 MC-ICP-MS.
V51B-0568
Mass Dependent Fractionation of Hg Isotopes in Source Rocks, Mineral Deposits and Spring Waters of the California Coast Ranges, USA
We present here the first study of the isotopic composition of Hg in rocks, ore deposits, and active hydrothermal systems from the California Coast Ranges, one of Earth's largest Hg-depositing systems. The Franciscan Complex and Great Valley Sequence, which form the bedrock in the California Coast Ranges, are intruded and overlain by Tertiary volcanic rocks including the Clear Lake Volcanic Sequence. These rocks contain two types of Hg deposits, hot-spring deposits that form at shallow depths (<300 m) and silica-carbonate deposits that extend to greater depths (200 to 1000 m), as well as active springs and geothermal systems that release Hg to the present surface. The Franciscan Complex and Great Valley Sequence contain clastic sedimentary rocks with higher concentrations of Hg than volcanic rocks of the Clear Lake Volcanic Field. Mean Hg isotope compositions for all three rock units are similar, although the range of values in Franciscan Complex rocks is greater than in either Great Valley or Clear Lake rocks. Hot spring and silica-carbonate Hg deposits have similar average isotopic compositions that are indistinguishable from averages for the three rock units, although δ202Hg values for the Hg deposits have a greater variance than the country rocks. Precipitates from dilute spring and saline thermal waters in the area have similarly large variance and a mean δ202Hg value that is significantly lower than the ore deposits and rocks. These observations indicate there is little or no isotopic fractionation during release of Hg from its source rocks into hydrothermal solutions. Isotopic fractionation does appear to take place during transport and concentration of Hg in deposits, especially in their uppermost parts. Boiling of hydrothermal fluids is likely the most important process causing of the observed Hg isotope fractionation. This should result in the release of Hg with low δ202Hg values into the atmosphere from the top of these hydrothermal systems and a consequent enrichment in heavy Hg isotopes in the upper crust through time.
V51B-0569
An Assessment of the Total Uncertainly in Hf Isotope Analyses by LA-MC-ICPMS
The analysis of Hf isotopes in zircon by LA-MC-ICPMS is a promising new analytical method that is increasingly being used by many laboratories. The total uncertainty of these laser Hf isotopic analyses, however, has several potential components that should be fully accounted for in assigning total analytical uncertainty. Currently, this full respresentation of uncertainty is not widely done by the laser Hf community. The sources of potential uncertainty in a laser Hf isotope analysis include: 1) uncertainty during the Hf analysis (within-run uncertainty); 2) uncertainty in the correction of 176Yb and 176Lu on 176Hf (included in this must be the allowance for the differences in mass bias behavior between Yb/Lu and Hf, which are not equivalent, sensu stricto); 3) matrix effects between zircon grains (this effect has been demonstrated for U-Pb zircon analysis and may be significant for some zircon grains); 4) any instrumental biases in the determination of Hf isotopic composition (most MC- ICPMS analyses of the JMC 475 Hf standard, for example, do not give exactly 176Hf/177Hf=0.282160 and require some normalization to a common value, which is difficult to do with laser Hf analyses). Furthermore, the determination of initial Hf isotope values requires both precise and accurate age determinations and accurate 176Lu/177Hf ratios in addition to the above factors that contribute uncertainty in present-day Hf isotope ratios. The effect of 176Lu/177Hf uncertainty on the initial values is small because of the very low ratio in zircon but the age correction is highly significant because the CHUR reference varies by ~2.3 εHf units/100 my. For zircons with a highly complex growth history (e.g., early Archean and Hadean zircons) linking precise and accurate age determinations to the Hf isotopic analysis is especially challenging and represents a potentially large source of uncertainty for these zircons. Because only in-run uncertainty can be determined during a single analysis of an unknown, most laboratories only report in-run precision of the laser Hf analysis. This clearly underestimates (in some cases drastically) the total uncertainty. One solution to determining overall uncertainty in a laser Hf analysis is to analyze multiple standards with Lu/Hf and Yb/Hf ratios that closely match the unknown zircons being analyzed. The variance of these analyses (estimated by 2σ SD of the population) relative to precisely determined Hf isotopic composition by analysis of purified solutions will provide a fair and reasonable estimate of the overall uncertainty of individual analyses of unknowns.
V51B-0570
An Improved Method for TIMS High Precision Nd Isotopic Analysis of Very Small Aliquots (1- 10ng) With Example Application in Garnet Sm/Nd Geochronology
Technological and scientific developments have demonstrated both the attainability and the utility of very high precision (i.e. 5-20ppm 2 σ) Nd isotopic measurements with TIMS. However such high precision has been limited to relatively large aliquots of Nd, on the order of several hundred nanograms. Several potential applications of precise Nd isotopic measurements, including garnet Sm/Nd geochronology, do not always permit such large samples, instead yielding only a few nanograms of Nd. We have explored and tested an improved method for Nd isotopic analysis of such small (1-10ng) aliquots of Nd using the NdO+ method with a Triton TIMS at Boston University. Analyzing Nd isotopes as the oxide is a well known technique, frequently involving an oxygen bleed valve. Instead, we forego the bleed valve and load samples with a TaO slurry which provides the oxygen source. Using an in-house Nd isotopic standard solution, 4ng loads easily yield stable 2.0-2.5 volt beams resulting in internal precisions of 10ppm 2 σ RSE. Within barrel external precision of 4ng loads of the Nd standard is 13ppm 2 σ RSD (n=20). Long term (6 months, six analysts) external precision of 4ng loads of the standard is currently 23ppm 2 σ RSD (n=55) suggesting that further improvements are possible. As a further test of this method, we dissolved a natural rock sample (a metapelite), separated the Nd using TRU- spec and MLA column chemistry, and loaded nineteen 4ng loads in one barrel. Within barrel external precision was 21ppm 2 σ RSD (n=18). This precision represents a significant advance over previous NdO+ analyses of small samples using an oxygen bleed valve. The TaO loading method for small Nd aliquots is useful in Sm/Nd garnet geochronology as exemplified by two case studies. Garnets from eclogite facies gneisses from Norway ran very well with 2.4-18ng loads and yielded age precision as good as 0.8 million years 2 σ. Conversely, garnets from blueschist facies rocks from Sifnos, Greece, ran poorly with similarly sized 1-17ng loads and consequently yielded generally poorer age precision. Differences between the two garnet sample suites must relate to the garnets themselves (notably including much lower Nd concentration in Sifnos garnets), not the identical column chemistry nor the TaO loading method. Additional procedures may be required to cleanly separate Nd from samples where Nd concentrations are very low (≪1ppm). As always, clean separation and column chemistry represents an unavoidable limiting factor in achieving precise isotopic measurements.
V51B-0571
Isotopic Measurement of Lead in Nanogram Quantities on Multi-Collector Inductively Coupled Plasma Mass Spectrometry
Lead isotopes have been used as geochemical tracers in Earth Sciences, such as geochemistry, paleoclimatology and chronology, due to the diverse ratios and variable elemental abundance. Determination of Pb isotope ratios, with 2-sigma external precisions of 200 ppm for 207Pb/206Pb and 208Pb/206Pb and 800 ppm for for 206Pb/204Pb, can be performed with a Faraday-cup protocol in static mode on a multi-collector ICP-MS (MC-ICP-MS), Thermo Electron Neptune. The sample size is as low as 3 ng of Pb consumed per measurement. Lead blanks, from acid, labware, and airborne particulate, was effectively reduced to less than 10 pg, which causes an isotopic ratio bias of 30-50 ppm at most. Isobaric interference of 204Hg on 204Pb was corrected by monitoring the ion beam intensity of 202Hg. Mass dependant instrumental fractionation was normalized to 205Tl/203Tl value. A desolvation nebulization system, Cetac Aridus, and an X-skimmer cone were used to enhance signal intensity. With a sample uptake rate of 50 μ L/min, Pb concentration of 5-10 ng/ml offers an ion beam intensity of larger than 1 volt for 208Pb. The measured isotope ratios with 2-sigma external uncertainty of an international standard of NIST- Pb 981 are: 206Pb/204Pb= 16.9419 ± 0.012, 207Pb/206Pb= 0.91475 ± 0.0002 and 208Pb/206Pb= 2.1674 ± 0.00035. The key merit of this technique is to provide a possibility of analyzing Pb isotopic composition in trace-quantity of 1-10 ng, mainly for sample with limited Pb content.
V51B-0572
Laser Ablation MC-ICP-MS measurements of Pb isotope composition: Controls on Precision and Accuracy and Comparison between Faraday and Faraday-Ion Counter detection systems
In this contribution I discuss controls on precision and accuracy and compare in-situ laser ablation multicollector ICP-MS (LA-MC-ICP-MS) measurements of Pb isotope composition using parallel Faraday cup and ion counter detector array to those made using solely Faraday cups. In this configuration ion counting is used for the low abundance isotope 204Pb as well as 200Hg and 202Hg isotopes required for correction of isobaric interference of 204Hg. Mass bias is effectively controlled by external normalization to 208Pb/206Pb ratios measured in standard glasses. Provided that He sweep gas flow rates are kept constant and that adequate time is allowed for stabilization after changes in He flow, mass bias can be controlled to within ± 0.05% 2s. Accurate measurement of the differences in gain between ion counters and Faraday cups, required for determination of 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb ratios, can also be made by analysis of the same standard glasses to within ± 0.3 % 2s. The precision of measurements of Pb isotope ratios is strongly dependent on ion beam intensities. Internal precision of < 0.02 % (2SE) on 208Pb/206Pb and 207Pb/206Pb ratios requires Pb ion beam intensity of > ~30-50 mV. At total beam intensities <100 mV measurement ratio to 204Pb also benefit significantly from the use of ion counting to measure 204Pb intensity, with an overall improvement of 1\-2 orders of magnitude in internal standard error. However, the additional uncertainty associated with calibration of the Faraday cup \– ion counter relative gain suggest that use of Faraday cup to measure 204Pb is only appropriate at signal intensities > ~200 mV, equivalent to ~100,000 counts per second (cps) 204Pb. Correction for 204Hg interference does not substantially contribute to uncertainty as long as 204Hg comprises < ~20% of the total 204 ion beam.
V51B-0573
Chemical, Isotopic, and Ar-Geochronologic Evidence for Three-Phase Volcanic History in the Northwest Basin & Range and High Lava Plains
The transition between active extension in the northern Basin and Range and the dextral accommodation in the High Lava Plain (HLP) of southern Oregon represents a long-lived lithospheric boundary located along the 87Sr/86Sr 0.704 isopleth. New element geochemistry, 40Ar/39Ar geochronology, and Sr and Nd isotopic data from 37 bimodal Eocene to late-Miocene age rocks from southern Oregon and northwest Nevada were analyzed to investigate spatial and geochemical patterns of arc, hot-spot, and extension-related volcanism as modified by this boundary. Samples were collected throughout northwest Nevada and southeastern Oregon, and represent three tectonomagmatic episodes of volcanism: 1) 38.1-20.3 Ma, low- volume, episodic, calc-alkalic magmatism (47-75.5 wt% SiO2) interpreted to have an arc signature, 2) 17.2-15.0 Ma, tholeiitic to weakly alkalic flood basalt followed by widespread peralkaline rhyolite (70.2-76.5 wt% SiO2) attributed to impingement of the Yellowstone hot-spot, and 3) 11.8-5.5 Ma, bimodal assemblage of migrating silicic magmatism (67.2-77.7 wt% SiO2) and widespread high-alumina olivine tholeiites (48.1-52.6 wt% SiO2) in the HLP accompanied by lithospheric extension. The isotopic compositions of all analyzed Eocene to late- Miocene silicic ignimbrites and lavas (87Sr/86Sri 0.7036-0.7075; ε Nd0 -0.5 to +4.9) are more heterogeneous and differ from coeval basalts. The 38.1-20.3 Ma mafic rocks show isotopic variability which appears to be spatially correlated with the abrupt crustal transition (87Sr/86Sr 0.704 isopleth) that broadly follows the boundary of accreted Mesozoic terrains. Eocene to Early Miocene mafic samples east of the 87Sr/86Sr 0.704 line in the Basin and Range have more evolved 87Sr/86Sri and ε Nd0 values than samples to the northwest where the isotopic compositions are closer to MORB. High K2O/TiO2 and low Zr/Ba differentiate 38.1-20.3 Ma mafic magmas that have an arc-signature, whereas the mid-Miocene Steens and Columbia River Basalts, and younger high-alumina olivine tholeiites, have compositions closer to MORB or OIB's.
V51B-0574
Micro-Raman mass spectrometry for measuring carbon isotopic composition of carbon dioxide fluid inclusions in mantle-originated minerals
Carbon isotopic compositions (13C/12C) of CO2 on the earth vary by their origin. Therefore, carbon isotopic compositions serve as a fingerprint that is useful for clarification of the origin and global circulation of CO2. We investigated the applicability of micro-Raman spectroscopy for determining carbon isotopic compositions (13C/12C) of minute CO2 fluid inclusions in minerals. This method is nondestructive and has sufficiently high spatial resolution (1 μm) to measure each fluid inclusion independently. Isotopically substituted 13 CO2 gas was prepared by heating 13 C-contended amorphous carbon with CuO. Carbon dioxide samples with various 13C/12C ratios were prepared by mixing 13 CO2 with CO2 from a tank (15 MPa, 3 m3, δ13CPDB = -31.52 ± 0.14‰). The carbon isotopic compositions of the obtained CO2 fluids were determined using a mass spectrometer (delta S; Finnigan MAT GmbH). Each CO2 fluid was introduced into a high-pressure cell (PC-400MS; Syn Co., Ltd.); high-density CO2 fluids were prepared in the cell, and Raman spectra was measured. Raman spectra of CO2 have 12 CO2-origin peaks at about 1285 cm-1 and 1388 cm-1 (ν[12]- and ν[12]+) and a 13 CO2-origin peak at about 1370 cm-1 (ν[13]+). The relationship between carbon isotopic compositions and peak intensity ratios of ν[12]+ and ν[13]+ was calibrated (Arakawa et al. (2007) Appl. Spectrosc. 61, 701-705). Considering several factors affecting the peak intensity ratio, the error in obtained carbon isotopic composition was 2% (20‰). The reproducibility of intensity ratio at the same experimental environment was 0.5% (5‰). Within these error values, we can distinguish biogenic CO2 from abiogenic CO2. Micro-Raman spectroscopy also provides density information of CO2 fluid in mantle derived minerals, and the depth the rock came from in the mantle can be calculated using the equation of state of CO2. Several applications on simultaneous determinations of density and carbon isotopic composition for CO2 inclusions in mantle xenoliths will be shown.
V51B-0575
In-situ Sulphur isotope measurements of Archean microbes by NanoSIMS
Sulphur isotope signals preserved within ancient sedimentary sulphides can be used to trace the activity of sulphur-cycling microbes and help to enhance our understanding of the sulphur cycle on the early Earth. Sulphides formed through microbial sulphate reduction have characteristically light, and often highly variable δ34S signatures, compared to their source sulphate. Early Archean sedimentary sulphides are, however, rare and show restricted ranges of δ34S compared to younger examples, meaning that the timing of emergence of sulphate-reducing microbes, and the nature of the Archean sulphur-cycle remains controversial. The lack of clear biogeochemical evidence for such a cycle in sedimentary rocks older than ~2700 Ma is puzzling because sulphur is an essential bio-limiting nutrient and sulphur microbes are widely regarded as branching near the base of the tree of life. We here report the first in situ NanoSIMS δ34S data from synsedimentary, micron-sized pyrite grains, sampled from within the ~3430 Ma Strelley Pool Formation of Western Australia. These show maximum fractionations, relative to their source sulphate, of -54 ‰, with a spread of ~75 ‰. This spread is three times greater than fractionations previously reported for the early Archean, but directly comparable with modern marine settings exhibiting a biologically-coupled redox sulphur cycle. This data combined with associated carbonaceous matter and redox sensitive detrital minerals indicates a biologically-coupled redox sulphur cycle in anoxic beach sediments as early as ~3430 Ma.