Hints of an Eoarchean magnetic field from the Isua Supracrustal Belt, Greenland
Abstract
Understanding the earliest history of the geodynamo is important for constraining the early thermal and geodynamic history of our planet and establishing the role of planetary magnetic fields for habitability. However, extracting ancient records of Earth's field is extremely challenging: Archean samples typically experienced metamorphism or aqueous alteration during their long histories causing their paleomagnetic records to be overprinted. As a result, records of Earth's field older than 3.5 Ga are sparse and their fidelity is highly debated.
The northeast region of the Isua supracrustal belt (ISB) hosts unusually well-preserved Eoarchean (up to 3.7 Ga) crustal rocks that experienced lower-greenschist to lower-amphibolite facies metamorphism. We conducted a conglomerate test on a rounded pebble conglomerate in the southern part of this area where peak metamorphic temperatures are independently constrained to ~600°C. We found that the natural remanent magnetization (NRM) directions of the pebbles are nonrandom, indicative of an overprint during metamorphism. Further north, estimates of peak metamorphic temperature (360-400°C) suggest primary NRM may be preserved by magnetite-bearing rocks. We conducted three baked contact tests where mafic dykes intrude banded iron formation (BIF). We found that the BIF contains a magnetite-bearing NRM that persists up to 580°C during thermal demagnetization. Furthermore, we found that two baked contact tests were positive, suggesting the NRM in the BIF preceded dyke emplacement and was acquired during diagenetic magnetite formation. The dyke ages (inferred to be 3.2-3.5 Ga) therefore set a minimum age for the BIF NRM. However, U-Pb ages in the BIF indicate the magnetite crystallized at ~3.7 Ga. Given that our diffusion experiments indicate closure temperature for the U-Pb system of ~615°C, this would suggest that the stable component of the NRM in the BIF formed as early as 3.7 Ga. These results hint that the ISB may contain the oldest record of Earth's field. Because the original orientations of these samples can be constrained, they may provide both paleointensity and paleodirectional data. We are conducting additional baked contact tests, rock magnetic studies and U-Pb chronometry at Isua along with Pb diffusion experiments to further test these conclusions.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMDI14A..02N
- Keywords:
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- 1507 Core processes;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1510 Dynamo: theories and simulations;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 7207 Core;
- SEISMOLOGY