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| Titel |
PGE-Re concentrations in carbonaceous siltstones from the Barberton Drilling Project: Sources and processes |
| VerfasserIn |
Philipp Rammensee, Sonja Aulbach |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250091941
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| Publikation (Nr.) |
 EGU/EGU2014-6260.pdf |
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| Zusammenfassung |
| The emergence, diversification and disappearance of Earth’s life forms are closely tied to the
redox state of the oceans, and the sources and sinks of metabolically cycled metals. It is
generally accepted that the early terrestrial atmosphere contained extremely low levels of free
oxygen [1]. While a significant change to atmospheric oxygen levels has been constrained to
ca. 2.45 Ga ago, the details of the complex prior redox evolution of the oceans and
atmosphere, and their influence on continental weathering, are still blurry [1]. Among the
trace metals that have been applied to this problem, Re and the platinum-group
elements (PGE) have variable redox chemistry that has been successfully exploited to
identify detrital vs. hydrogenous sources and the presence of oxic vs. suboxic or
euxinic conditions both in young and ancient sediments, including predominantly
outcrop samples from the Barberton Greenstone Belt (BGB) [2,3]. As 187Re decays to
187Os, the Re-Os isotope system can additionally be used to date the deposition
of carbonaceous shales through construction of isochrons and obtain the initial
Os isotope composition, which is a tracer for continental input of radiogenic Os
[4].
The sampling approach here was to choose 8+ samples from narrow intervals (-¤1 m, to
avoid initial Os isotope heterogeneity) from the Barberton Drilling Project (two depths in
core BARB5/Fig Tree Group and one interval in core BARB3/Buck Reef Chert. We are
currently finalising institution of the sample preparation and analytical techniques,
involving (1) high-pressure asher digestion and (2) low-temperature leaching of
presumably hydrogenous, acid-soluble components of spiked samples, followed by
solvent extraction of Os and cation exchange column chromatography to isolate
PGE-Re from the residue, further purification with BPHA and measurement of
Ru-Pd-Ir-Pt by ICPMS and of Re-Os by MC-ICPMS. Preliminary tests with the SDO-1
standard have revealed that concentrations of Ir and Pt in low-T dissolutions are
indistinguishable from those in high-T dissolutions within the uncertainty, implying that
these elements are contained fully in the hydrogenous component. In contrast, Re
concentrations in the low-T digest are significantly lower (by 8%), indicating some
control by the detrital component in a Phanerozoic shale. Prior work [3] has shown
that PGE-Re abundance patterns of BGB sediments resemble those of komatiites,
indicating an ultramafic, detrital source. Given the age of the BGB (3.5 – 3.2 Ga) and
the absence of oxidative weathering, we expect to see low concentrations and no
resolvable difference between the two digestion approaches that would be attributable to
the variable redox behavior of Re and PGE. We aim to present results showing
whether any short-scale changes occur in the sources and processes of PGE-Re
incorporation into the sediment, and to eventually obtain robust Re-Os isotope
constraints.
[1] Canfield (2005) Annu. Rev. Earth Planet. Sci. 33:1–36; [2] Lee et al. (2003) Geochim.
Cosmochim. Acta 67: 655–670; [3] Siebert et al. (2005) Geochim. Cosmochim. Acta 69:
1787–1801; [4] Kendall et al. (2009) Geol. Soc. London Spec. Publ. 326: 85-107 |
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