Australia: The Land Where Time Began

A biography of the Australian continent 

A Geodynamo of Hadean to Palaeoarchean Age recorded by Single Zircon Crystals

According to Tarduno et al. it is important for understanding the evolution of Earth’s core, atmosphere and life on Earth, to know when the Geodynamo began to operate. In this paper Tarduno et al. report full vector palaeointensity measurements of zircons from the Archaean to Hadean which bears magnetic inclusions from the Jack Hills conglomerate, Western Australia, to reconstruct the early history of the Geodynamo. Magnetic fields that vary between 1.0 and 0.12 times the recent field strengths at the equator have been recorded from zircons dating to between 4.2 Ga and 3.3 Ga. A core-mantle heat flow that exceeds the adiabatic value and suggests plate tectonics and/or advective magmatic heat transport is required by a Hadean geomagnetic field. Terrestrial nitrogen isotope evidence supports the existence of a terrestrial magnetic field earlier than the Late Heavy Bombardment, and implies that the evolution of an atmosphere on both Earth and Mars was regulated by dynamo behaviour.

The oldest geomagnetic field values that were previously reported, dating to 3.45 to 3.2 Ga from magnetite bearing single feldspar and quartz phenocyrsts from igneous rocks of the Nondweni and Barberton Greenstone Belts, Kaapvaal Craton, South Africa (1-3), indicate a field that was relatively strong, though the earlier history of the Geodynamo is unknown. It is predicted by some thermal evolution models that a geodynamo didn’t exist prior to about 3.5 Ga (4).

Sources & Further reading

Tarduno, J. A., R. D. Cottrell, W. J. Davis, F. Nimmo and R. K. Bono (2015). "A Hadean to Paleoarchean geodynamo recorded by single zircon crystals." Science 349(6247): 521-524.