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

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

Beschreibung

vor 19 Jahren
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.

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