Oxygen Isotope Constraints on the Significance of Greenstone Belts for Archean Tectonics
Abstract
The discovery of ^1^8O-enriched zircons from the Hadean and ^1^8O-enriched character of Archean greenstone belts (e.g. the Pilbara block, mean value δ^1^8O= 8.8 ± 1.5), relative to their primary mantle-derived values +5.7, as well as the ^1^8O-enriched character of Archean sandstones (e.g. Pilbara block, mean value δ^1^8O= 9.6 ± 0.5) indicates that ^1^8O enrichment resulting from near surface fluid-rock interaction dominated the early exchange between the hydrosphere and volcanic crust. By mass balance, ^1^8O enrichment of the volcanic crust must result in a complementary ^1^8O depletion of the hydrosphere. During the Hadean, this interaction probably shifted the oceans from an ^1^8O-enriched state (>+6 per mil) downward towards zero per mil. As the surface temperature dropped, this indirect exchange with the mantle reservoir could drive the hydrosphere to a ^1^8O-depleted state relative to present day ocean water. Meteoric waters would reflect the ^1^8O shift of the oceans, also lowering the ^1^8O concentration of minerals produced during chemical weathering. A world dominated by thick submarine basaltic volcanic sections altered by seawater (e.g. Archean greenstone successions) would only drive the δ^1^8O value seawater downward by 3-4 per mil. As a consequence, at steady state, the subsequent greenstones would exhibit more normal mantle-derived values, instead of the observed values near +9 per mil. The actual record, populated with ^1^8O-enriched zircons, greenstones, and sandstones, without a secular shift to ^1^8O-depleted values or normal mantle values, argues strongly that the current plate tectonic regime became dominant very early in Earth history. In the modern Earth, continental weathering and the ^1^8O enrichment of metavolcanic rocks and graywackes do not drive the hydrosphere to strongly negative δ^1^8O values because of the complementary high T exchange (a source of ^1^8O enrichment for the oceans) with the plutonic rocks of oceanic layer three. This ocean-lithosphere exchange occurs in a thin crust (< 6 km thick) plate tectonic regime with a geometry that is robust over an order of magnitude change in spreading rates. The scenarios capable of generating the oxygen isotope data indicate that thick (10-20 km) Archean greenstone successions cannot be representative of typical Archean oceanic crust.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V11D0626G
- Keywords:
-
- 1021 Composition of the oceanic crust;
- 1022 Composition of the hydrosphere;
- 1030 Geochemical cycles (0330);
- 1034 Hydrothermal systems (0450;
- 3017;
- 3616;
- 4832;
- 8135;
- 8424);
- 1041 Stable isotope geochemistry (0454;
- 4870)