Multi-element isotope dilution (ID) sector field ICP-MS: a novel technique that leads to new perspectives on the trace element systematics of ocean island basalts

Trace elements (elements that constitute less than 0.1 wt.% of the analyzed sample) are important tracers for a great variety of processes in many research areas, such as biochemistry, medicine, semi-conductor and nano-technology, environmental science and geosciences (e.g., Anita et al. [2002]; Becker et al. [2004]; Barbante et al. [2004]; Tibi and Heumann [2003]). Accordingly, much scientific effort and financial resources are raised to develop new high-performance analytical techniques and methods for trace element analysis. In geosciences, the capabilities of trace elements analytics have not been used to its full potential because of the complex matrix of the analyzed samples (rocks) and the time consuming procedure to obtain high-quality trace element data by isotope dilution. Accordingly, the aim of this study is to develop of a new, easy-to-use and fast ID-method for the simultaneous determination of many trace elements in geological materials. In addition, the application of this new technique to the analysis of ocean island basalts (OIB) revealed that evolution of geochemical mantle heterogeneities (HIMU, EM-1, EM-2) is far more complex than perviously thought. In the first part of this thesis, a multi-element technique for the simultaneous determination of 12 trace element concentrations in geological materials by combined isotope dilution (ID) sector field inductively coupled plasma mass spectrometry (SF-ICP-MS) following simple sample digestion is presented. The concentrations of additional 14 other trace elements have been obtained using the ID determined elements as internal standards. This method combines the advantages of ID (high precision and accuracy) with those of SF-ICP-MS (multi-element capability, fast sample processing without element separation) and overcomes the most prevailing drawbacks of ICP-MS (matrix effects and drift in sensitivity). Trace element concentration data for the geological reference material BHVO-1 (n = 5) reproduce to within 1-3% RSD with an accuracy of 1-2% relative to respective literature data for ID values and 2-3% for all other values. To test the overall performance of the method the technique has been applied to the analysis of 17 well-characterized geological reference materials from the United States Geological Survey (USGS), the Geological Survey of Japan (GSJ) and the International Association of Geoanalysts (IAG). The sample set also includes the new USGS reference glasses BCR-2G, BHVO-2G, and BIR-1G, as well as the MPI-DING reference glasses KL2-G and ML3B-G and the National Institute of Standards and Technology (NIST) SRM 612. Most data agree within 3-4% with respective literature data. The concentration data of USGS reference glasses agree in most cases with respective data of the original rock powder within the combined standard uncertainty of the method (2-3%), except the U concentration of BIR-1G, which shows a three times higher concentration compared to BIR-1. In the second part of this thesis, this new method is used to determine the trace element concentrations of basaltic samples form the ocean islands St. Helena, Gough and Tristan da Cunha. The results are used to test the validity of established models concerning the trace element systematics of mantle heterogeneities. Since the early 1990's, recycling of altered oceanic crust together with small amounts of 'pelagic' and 'terrigeneous' sediments has become somewhat of a paradigm for explaining the geochemical and isotopic systematics in global OIB. The vastly increased number of data in the literature, in addition to new high-precision trace element data on samples from St. Helena, Gough, and Tristan da Cunha presented here (altogether more than 300 analyses from basalts from 15 key islands), reveals that the trace element systematics in enriched mantle (EM)-type OIB are far more complex than previously thought. In contrast to EM basalts, HIMU (high μ; μ = 238U/204Pb) basalts have remarkably uniform trace element characteristics (systematic depletion in Cs, Rb, Ba, Th, U, Pb, Sr, and enrichment of Nb, Ta relative to La), which are adequately explained by being derived from sources containing subduction-modified oceanic crust. EM-type basalts have La/Th, Rb/Ba, and Rb/K ratios similar to those in HIMU-type OIB, but at the same time, also share some common characteristics that distinguish them from HIMU basalts (e.g., high Rb/La, Ba/La, Th/U, Rb/Sr, low Nb/La, U/Pb, Th/Pb). EM-type OIB also have far more variable very incompatible elements contents (Cs, Rb, Ba, Th, U, Nb, Ta, La) and are less depleted in Pb and Sr than HIMU-type OIB. In addition, each suite of EM-type basalts carries its own specific trace element signature that must ultimately reflect different source compositions. Consequently, although the compositional similarities between HIMU and EM-type basalts suggest that their sources share a common precursor (subducted oceanic crust), their compositional differences can only be explained if EM sources have a more complex evolution and/or contain an additional component compared to HIMU sources. This additional component in EM basalts is likely to originate from a common, although to some degree compositionally heterogeneous, reservoir. Possible candidates are marine sediments; but they do not, at the same time, provide a plausible explanation of the isotopic bimodality in EM-type basalts (EM-1 and EM-2) because the parent/daughter ratios in marine sediments are unimodally distributed. Similar to the bimodal isotopic compositions in EM basalts, the continental crust is composed of two broadly compositionally different parts: the upper and lower continental crust. Relative to the upper continental crust, the lower continental crust is similarly enriched in very incompatible elements, but has systematically lower Rb/Sr, U/Pb, Th/Pb, and higher Th/U ratios. Thus, over time, the upper and lower continental crust evolve along distinct isotopic evolution paths but retain their complex trace element characteristics, similar to what is observed in EM-type basalts worldwide. It is therefore propose here that recycling of oceanic crust together with variable proportions of lower continental crust (scrapped off from the overlying continental crust during subduction at erosive plate margins) and upper continental crust (either in the form of sediments or eroded continental crust) provides a possible explanation for the trace element and isotope systematics in EM-type ocean island basalts

Tungsten isotope composition of the Acasta Gneiss Complex

AbstractHigh-precision tungsten (182W/184W) isotope measurements on well-characterised mafic and felsic samples of the ca. 3960 Ma Acasta Gneiss Complex (AGC; Northwest Territories, Canada) show radiogenic ε182W values between +0.06 to +0.15. Two ca. 3600 Ma felsic samples from this terrane have ε182W ∼ 0 and are the oldest samples so far documented to have a W isotopic composition indistinguishable from that of the modern mantle. The ε182W data are correlated with ε142Nd (Roth et al., 2014) and we attribute this variability to incomplete metamorphic homogenisation of the 3960 Ma protolith with more recent material in a 3370 Ma tectono-thermal event. Notably, the value of the positive ε182W anomalies seen in the 3960 Ma AGC samples that are least affected by metamorphic homogenisation is comparable to that observed in other early Archean rocks (Isua Supracrustal Belt, Greenland; Nuvvuagittuq Supracrustal Belt, Canada) and the late Archean Kostomuksha komatiites (Karelia). This demonstrates a globally constant signature. We infer that the presence of a pre-late veneer mantle represents the most straightforward interpretation of a uniform distribution of εW182∼+0.15 value in Archean rocks of different ages. We show that such a notion is compatible with independent constraints from highly siderophile element abundances in mafic and ultra-mafic Archean mantle-derived rocks. The absence of anomalous ε182W and ε142Nd so far measured in samples younger than ca. 2800 Ma suggests progressive convective homogenisation of silicate reservoirs. The downward mixing of an upper mantle rich in late-delivered meteoritic material might account for these combined observations

High-Precision Mass-Dependent Molybdenum Isotope Variations in Magmatic Rocks Determined by Double-Spike MC-ICP-MS

Small mass‐dependent variations of molybdenum isotope ratios in oceanic and island arc rocks are expected as a result of recycling altered oceanic crust and sediments into the mantle at convergent plate margins over geological timescales. However, the determination of molybdenum isotope data precise and accurate enough to identify these subtle isotopic differences remains challenging. Large sample sizes – in excess of 200 mg – need to be chemically processed to isolate enough molybdenum in order to allow sufficiently high‐precision isotope analyses using double‐spike MC‐ICP‐MS techniques. Established methods are either unable to process such large amounts of silicate material or require several distinct chemical processing steps, making the analyses very time‐consuming. Here, we present a new and efficient single‐pass chromatographic exchange technique for the chemical isolation of molybdenum from silicate and metal matrices. To test our new method, we analysed USGS reference materials BHVO‐2 and BIR‐1. Our new data are consistent with those derived from more involved and time‐consuming methods for these two reference materials previously published. We also provide the first molybdenum isotope data for USGS reference materials AGV‐2, the GSJ reference material JB‐2 as well as metal NIST SRM 361.ISSN:1639-4488ISSN:1751-908

Rare earth elements and neodymium and strontium isotopic constraints on provenance switch and post-depositional alteration of fossiliferous Ediacaran and lowermost Cambrian strata from Arctic Norway.

The Digermulen Peninsula in northeastern Finnmark, Arctic Norway, comprises one of the most complete Ediacaran–Cambrian transitions worldwide with a nearly continuous record of micro- and macrofossils from the interval of the diversification of complex life. Here, we report on the provenance and post-depositional alteration of argillaceous mudstones from the Digermulen Peninsula using rare earth elements and Sm–Nd and Rb–Sr isotopic systematics to provide an environmental context and better understand this important transition in Earth’s history. The studied sections comprise a mid-Ediacaran glacial–interglacial cycle, including the Nyborg Formation (ca. 590 Ma) and Mortensnes Formation (related to the ca. 580 Ma-old Gaskiers glaciation), and the Stahpogieddi ´ Formation (ca. 560–537 Ma), which yields Ediacara-type fossils in the Indreelva Member and contains the Ediacaran–Cambrian boundary interval in the Manndrapselva Member and basal part of the informal Lower Breidvika member (ca. 537–530 Ma). Three sample groups, (1) Nyborg and Mortensnes formations, (2) the lowermost five samples from the Indreelva Member and (3) the remaining samples from the Indreelva as well as from the Manndrapselva and Lower Breidvika members, can be distinguished, belonging to distinct depositional units. All samples have negative εNd(T) values (− 6.00 to − 21.04) indicating a dominant input of terrigenous detritus with an old continental crust affinity. Significant shifts in Sm–Nd isotope values are related to changes in the sediment source, i.e. Svecofennian province vs Karelian province vs Svecofennian province plus in addition likely some juvenile (late Neoproterozoic volcanic) material, and probably reflect palaeotectonic reorganisation along the Iapetus-facing margin of Baltica. The combined Rb–Sr isotopic data of all samples yield an errorchron age of about 430 Ma reflecting the resetting of the Rb–Sr whole-rock isotope systems of the mudstones during the Scandian tectono-metamorphic event in the Gaissa Nappe Complex of Finnmark. Preservation of palaeopascichnids coincides with the sedimentation regimes of sample groups 2 and 3 while other Ediacara-type fossils, e.g. Aspidella-type and frondose forms, are limited to the sample group 3. Our results are similar to those of earlier studies from the East European Platform in suggesting oxic seafloor conditions during the late Ediacaran

MPI-DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented. Copyright 2006 by the American Geophysical Union

Insights into the Mechanism of Ligand Binding to Octopine Dehydrogenase from Pecten maximus by NMR and Crystallography

Octopine dehydrogenase (OcDH) from the adductor muscle of the great scallop, Pecten maximus, catalyzes the NADH dependent, reductive condensation of L-arginine and pyruvate to octopine, NAD+, and water during escape swimming and/or subsequent recovery. The structure of OcDH was recently solved and a reaction mechanism was proposed which implied an ordered binding of NADH, L-arginine and finally pyruvate. Here, the order of substrate binding as well as the underlying conformational changes were investigated by NMR confirming the model derived from the crystal structures. Furthermore, the crystal structure of the OcDH/NADH/agmatine complex was determined which suggests a key role of the side chain of L-arginine in protein cataylsis. Thus, the order of substrate binding to OcDH as well as the molecular signals involved in octopine formation can now be described in molecular detail

Magnesium isotope evidence that accretional vapour loss shapes planetary compositions

It has long been recognized that Earth and other differentiated planetary bodies are chemically fractionated compared to primitive, chondritic meteorites and, by inference, the primordial disk from which they formed. However, it is not known whether the notable volatile depletions of planetary bodies are a consequence of accretion1 or inherited from prior nebular fractionation2. The isotopic compositions of the main constituents of planetary bodies can contribute to this debate3, 4, 5, 6. Here we develop an analytical approach that corrects a major cause of measurement inaccuracy inherent in conventional methods, and show that all differentiated bodies have isotopically heavier magnesium compositions than chondritic meteorites. We argue that possible magnesium isotope fractionation during condensation of the solar nebula, core formation and silicate differentiation cannot explain these observations. However, isotopic fractionation between liquid and vapour, followed by vapour escape during accretionary growth of planetesimals, generates appropriate residual compositions. Our modelling implies that the isotopic compositions of magnesium, silicon and iron, and the relative abundances of the major elements of Earth and other planetary bodies, are a natural consequence of substantial (about 40 per cent by mass) vapour loss from growing planetesimals by this mechanism

Determination of Ce isotopes by TIMS and MC-ICPMS and initiation of a new, homogeneous Ce isotopic reference material

Radiogenic 138Ce/136Ce ratios in geological and cosmological materials are a valuable tracer and dating tool in geo- and cosmochemistry. However, the variation in global 138Ce/136Ce ratios is small (e.g., 0.03% in ocean island basalts). In addition, the isobaric interference of 138Ba and the influence of the dominant 140Ce ion beam (88.5%) on 138Ce (ca. 0.251%) during mass spectrometric analysis present an analytical challenge. Hitherto employed methods generally dismiss the influence of the 140Ce low-mass peak-tail on the accuracy of the determined 138Ce/136Ce ratios. Therefore, the reported reproducibility ranges only from 0.05 to 0.004% (2RSD). In this study, TIMS and MC-ICPMS are used to determine 138Ce/136Ce ratios in reference materials. The results show that only the measurement of Ce as an oxide species by TIMS, combined with the accurate monitoring and correction of the background between the acquired ion intensities, can yield accurate and reproducible 138Ce/136C ratios. This method achieves a more than two-fold improvement in reproducibility (0.002%; 2RSD) compared to reported methods. The identification and quantification of individual sources of error resulted in a combined standard uncertainty of 0.002% (2RSE) for the method. A comparison of published data for the CeO2 reference material JMC 304 reveals its isotopic heterogeneity. Therefore, a new reference material has been prepared from ultra-pure Ce metal from Ames Laboratory. It is now available for distribution. An initial characterization of the new reference material yielded a 138Ce/136C ratio of 1.33738 ± 0.000005 (2σmean; N = 35) as a working value