Zircon Reveals the Age of Earth’s Magnetic Field
The Earth’s magnetic field is absolutely essential to life on our planet. It protects us from the solar winds, streams of charged particles emitted by the sun which constantly threaten to strip away the atmosphere, and has allowed life to flourish. Until 2010 the best estimate we had for the antiquity of the field put its age at 3.45 billion years but new data has pushed back that age by 500 million years.
The Earth’s magnetic field is produced by the movement of its inner liquid core, which is predominantly made of iron, but in order to operate there also needs to be a regular release of heat. Today this happens through plate tectonics but exactly when this tectonic movements started on Earth is still hotly debated. Understanding when the magnetic field formed would give an important clue into the origins of plate tectonics, as well as revealing when the Earth first became hospitable for life.
The new research, carried out by the University of Rochester, New York, focused on the mineral magnetite. Magnetite is a magnetic form of iron oxide and each grain has its own magnetization which records the strength of the Earth’s magnetic field at the time they formed. This offers an excellent record of any changes in the past magnetic field. The problem is finding undisturbed magnetite samples from 4 billion year old rocks. In this study, the magnetite grains were preserved in zircon crystal from the Jack Hills, Australia. Even though the zircon was the right age, there was still a possibility that the magnetization recorded was not. If a magnetic mineral is heated past a certain temperature (called its Curie Point) it loses its magnetization, which then resets to the magnetic field of the time the mineral is cooled again. Around 2.6 billion years ago the temperature of the rocks in the Jack Hills reached 475oC, which is beyond the Curie Point of magnetite. In order to check this wasn’t the case, the team tested numerous magnetite grains from the zircons. If all had been heated and then reset they should all have had the same magnetization, especially as it is clear the zircons have not been moved relative to each other since they formed. They didn’t. This strongly suggests that the magnetite is recording a real ancient magnetic field.
This new discovery suggests that the Earth’s magnetic field formed very early in the history of our planet. It also suggests that plate tectonics may have started around the same time, supporting some earlier evidence which put tectonics at around 4.4 billion years.
Tarduno, J.A., et al. 2015. A Hadean to Palaeoarchean geodynamo recorded by zircon crystals. Science. 349. pp.521-524. DOI: 10.1126/science.aaa9114
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