Paleomagnetism of 3.5-4.0 Ga zircons from the Barberton Greenstone Belt, South Africa

Abstract:

The existence of a core dynamo during the first billion years of Earth history holds broad implications for the thermal state of the Earth’s interior and affects the composition of the early atmosphere. The scarcity of well-preserved rock units older than 3.5 billion years (Ga) has motivated the paleomagnetic analysis of detrital zircons with crystallization ages up to 4.4 Ga. Studies of zircons from Jack Hills, Australia, however, have so far found strong evidence for secondary ferromagnetic inclusions, calling into question the ability of these zircons to retain primary paleomagnetic information.

We conducted paleomagnetic analyses on a set of 19 detrital zircons from the Barberton Greenstone Belt (BGB) of South Africa with crystallization ages 3.5-4.0 Ga. These zircons have undergone lower grade metamorphism compared to all other localities known to produce >3.5 Ga detrital zircons. Further, the pervasive silicification of zircon-bearing host rock shortly after deposition at 3.3 Ga suggests that these samples were better protected from metamorphic fluids similar to those that overprinted Jack Hills zircon inclusions.

Using a combination of quantum diamond microscope (QDM) and SQUID microscope mapping, we find that BGB zircons have magnetic moments at least one order of magnitude weaker than those reported from Jack Hills zircons with 15 out of 19 samples carrying no detectable natural remanent magnetization (NRM). Isothermal remanent magnetization (IRM) acquisition confirmed the lack of ferromagnetic content. As a result, these zircons do not record information about ≥3.5 Ga magnetic fields. This finding corroborates observations from Jack Hills and North China Archean detrital zircon populations that primary ferromagnetic inclusions are readily eliminated from zircons during sedimentary transport and/or moderate metamorphism, likely facilitated by radiation damage-induced permeability. Paleomagnetic determination of geodynamo activity prior to 3.5 Ga may require investigation of other detrital grains with lower radiation damage potential or whole-rock samples that have escaped high grade metamorphism.