 |
| Titel |
Ubiquitous subaerial weathering at regional and temporal scales during emersion of the Fortescue Late Archean igneous province, Western Australia |
| VerfasserIn |
Pascal Philippot, Yoram Teitler, Martine Gerard |
| Konferenz |
EGU General Assembly 2011
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250055887
|
|
|
|
|
|
| Zusammenfassung |
| The 2.77Gy old Mount Roe paleosol at Whim Creek, Fortescue Group, Western Australia, is
a reference weathering profile, used for constraining maximum concentrations of atmospheric
oxygen and greenhouse gases (CO2, CH4) during the Late Archean (Rye et al., 1995;
Sheldon, 2006). Inferences on Early Earth atmospheric composition, however, are based on
the interpretation of reconstructed chemical profiles which are not supported by direct
mineralogical observations. This in turn resulted in a number of controversies that remain to
be solved.
The Whim Creek paleosol is developed at the top of a vesicular, subaerial basaltic flow
exposed along two km-scale outcrops located about 5 km away from each other. It mainly
consists of fresh footwall basalt progressively grading to a 5 meters thick brecciated
chloritic-rich zone showing evidence of corestone weathering, overlain by a 5 to 20 meters
thick sericitic-rich zone. Clastic sediments are often inter-bedded between the top sericite
zone and the overlying basaltic flow. This is consistent with a significant time gap between
deposition of footwall and hanging wall lava flows, a required condition to develop a thick
weathering profile.
Similar weathering profiles have been identified both along the same stratigraphic level
some 100 km away from the Whim Creek locality (Sherlock River), but also in other
formations of the Fortescue Group, such as the 2.74 Gyr Kylena Formation. This indicates
that basalt weathering was a long-lasting process that affected large geographical
areas. Meter scale blue-greenish titanite-rich bodies preserved as hard cores within
sericite zones were discovered both in Whim Creek and Sherlock River outcrops. In
addition, remnants of bedded-parallel diaspore/pyrophyllite deposits containing
carbonaceous material were found in the Whim Creek sericite zone. Field and petrological
investigations indicate that both horizons predate sericitization. Micro-mineralogy
analyses are in progress in order to determine whether these horizons contain early
weathering-related mineral phases such as greenalite or berthierine (Rye et al., 1995;
Sheldon, 2006).
Rye, R., Kuo, P.H., Holland, H.D. (1995). "Atmospheric carbon dioxide concentrations
before 2.2 billion years ago." Nature 78: 603-605.
Sheldon, N. (2006). "Precambrian paleosols and atmospheric CO2 levels." Precambrian
Research 147: 148-155. |
|
|
|
|
|
|