『Abstract
Re and Os isotopes in shales are increasingly used for precise
and accurate geochronology and for tracking rates of continental
weathering in the geologic past. Moreover, trace element compositions
of organic-rich shales reveal paleoredox conditions and properties
of shale organic matter (OM) useful for lithostratigraphic correlations.
Chemical weathering alters the original shale composition and
hence may compromise geologic interpretations based on geochemical
data. Here we compare Re-Os isotopes, trace elements and OM properties
in correlative shales from outcrops and nearby drillcore to quantify
the effects of weathering. We define chemical parameters that
distinguish weathered from pristine shale in samples that are
macroscopically identical.
Black shale from the Ravnefjeld Formation in east Greenland yields
distributed Re-Os isotope systematics for outcrop samples, and
highly precise Re-Os isochron ages for drillcore. Geochemical
data show that Re, Os and common Os are hosted in the organic
matter rather than sulfides. Re and Os both show mobility during
weathering. Combined use of Rock-Eval pyrolysis indices and sulfur
contents documents oxidation of both OM and pyrite, thereby providing
a valuable chemical criterion to assess invisible shale weathering.
Further studies on chemically diverse shales will test the widespread
application of these criteria as weathering indicators.
Systematic differences in major and trace element content and
kerogen quality between weathered and fresh shale samples are
used to characterize and quantify weathering effects. The character
of the OM in shales is most telling. Ravnefjel data are compared
with data from time correlative shales from the mid-Norwegian
shelf [Georgiev et al., 2011. Hot acidic Late Permian seas stifle
life in record time. Earth Planet. Sci. Lett., 310, 389-400] to
present a strategy for identification of Re-Os isochroneity in
advance of Re-Os analytical work.
Our recent compilation and comparison of Upper Permian shales
from widely separated sequences on Pangea's margins reveal exceptionally
high Re/Os ratios on a global scale. We suggest Upper Permian
shales may have significantly influenced the Cenozoic (e.g. Eocene)
187Os/188Os ratio of seawater on their exposure
and weathering.
Keywords: Shale weathering; Re-Os; Rock-Eval; Trace elements;
Late Permian; East Greenland』
1. Introduction
2. Shale chemistry: from deposition to weathering
3. Geological setting
4. Materials and methods
4.1. samples
4.2. Methods
4.2.1. Re-Os analyses
4.2.2. Rock-Eval analyses
4.2.3. Major and trace element analyses
5. Results
5.1. Re-Os
5.2. Rock-Eval pyrolysis
5.3. Major and trace elements
6. Discussion
6.1. Weathered versus unweathered shales: the Re-Os isotope
criterion
6.2. Effects of weathering on shale geochemistry
6.2.1. Correlation of trace elements with TOC and sulfur
6.2.2. Major and trace element criteria for weathered shales
6.2.3. Organic food for thought
6.2.4. Which shale samples to date
6.3. Mobility of Re and Os during weathering
6.4. Weathering of high-Re/Os shales: implications for seawater
Os isotopic composition
7. Conclusions
Acknowledgments
Appendix A. Supplementary data
References