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., Anitha 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 HIMUtype 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.