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| Titel |
Crustal growth, thermal evolution of the Earth, and Archaean emerged land surface |
| VerfasserIn |
Nicolas Flament, Nicolas Coltice, Patrice Rey |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250044490
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| Zusammenfassung |
| In the long term, the total amount of emerged land at Earth’s surface and the depth of
mid-oceanic ridges are controlled by the growth of the continental crust and by the secular
cooling of Earth’s mantle that implies changes in the isostatic balance between continents and
oceans. The evolution of the area of emerged land and of oceanic bathymetry are of
fundamental importance to the geochemical coupling of mantle, continental crust, ocean and
atmosphere.
We developed a model to evaluate the area of emerged continental crust as a function of
mantle temperature, continental area and hypsometry. For constant continental hypsometry
and for three different thermal evolution models, we find that a constant continental freeboard
(± 200 m) throughout Earth’s history is possible as long as the potential temperature of the
upper mantle never exceeded its present value by more than 110–210-C. This implies either a
very limited cooling of the planet or, most likely, a change in continental freeboard since the
Archaean. As for the area of emerged land, our calculations suggest that less than
~12% of Earth’s surface were emerged in the Archaean, compared to ~28% at
present.
Of importance to the evolution of the area of emerged land is the shape of the continents.
During the Archaean, a greater radiogenic crustal heat production and a possibly greater
mantle heat flow would have reduced the strength of the continental lithosphere, thus limiting
crustal thickening due to mountain building processes and the maximum elevation in Earth’s
topography (Rey and Coltice, 2008). Taking this effect into account, we show that the
continents were mostly flooded until the end of the Archaean, with 2-3% of Earth’s area
emerged by 2.5 Ga. These results are consistent with the widespread occurrence of
submarine continental flood basalts in the Archaean, and with the appearance and
strengthening of the geochemical fingerprint of felsic sources in the sedimentary record
from 2.5 Ga. In order to investigate the influence of crustal growth models on the
area of emerged land and on the evolution of oceanic 87Sr/86Sr, we developed an
integrated model based on the thermal evolution model of Labrosse and Jaupart (2007).
Modelling results suggest that the area of emerged land does not closely depend on
crustal growth models, and that less than 5% of Earth’s area was emerged in the
Archaean. Furthermore, our models reconcile early crustal growth models with the
evolution of oceanic 87Sr/86Sr as recorded by marine carbonates when a reduced
emerged area and lower continental elevations are accounted for. Thus, a delayed
crustal growth model is not needed to account for the observed trend in oceanic
87Sr/86Sr.
References
Labrosse, S., Jaupart, C., 2007. Thermal evolution of the Earth: Secular changes and
fluctuations of plate characteristics. Earth Planet. Sc. Lett. 260, 465–481.
Rey, P. F., Coltice, N., 2008. Neoarchean strengthening of the lithosphere and the
coupling of the Earth’s geochemical reservoirs. Geology 36, 635–638. |
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