Did the Earth have a magnetic field before 3.5 billion years ago? Previous paleomagnetic studies of the world\u2019s oldest mineral grains – the Jack Hills zircons, which have maximum ages of 4.4 billion years – claimed that tiny inclusions of magnetite within those grains had taken a snapshot of a strong geomagnetic field at the time they formed.\u00a0<\/p>\n\n\n\n
Now, however, a new exhaustive study<\/a> shows that we still don\u2019t know, because the detected magnetisation came much later. The study shows that the carriers of the putative super-ancient magnetisation are not primary inclusions (crystallised from the melt first before the zircon grew around them), but magnetite formed by alteration later on. How much later? We don’t know. But it could have been any point between 4 billion years ago and today. <\/p>\n\n\n\n And thus, the state of the magnetic field in the Hadean and Eoarchean goes back to a big question mark. This is disappointing, but not totally unexpected. The fact that most magnetic minerals contain iron, and iron is redox sensitive, is a real bane for studying ancient magnetisations, because there is always the very real prospect that your rock is one age and the magnetisation you are oh-so-carefully measuring is another, younger, age. If you don’t realise this, then you are putting a continent or crustal block in the wrong place, or mischaracterising the magnetic field for the period you’re interested in. I have a certain amount of experience in this particular area<\/a>.<\/p>\n\n\n\n This is a really nice example of how there is a distinction between ‘good’ data and meaningful<\/em> data. Sometimes, you can have a really nice, precise measurement that nonetheless leads you completely wrong, because you lack the information to put it in the proper context.<\/p>\n","protected":false},"excerpt":{"rendered":" Did the Earth have a magnetic field before 3.5 billion years ago? Previous paleomagnetic studies of the world\u2019s oldest mineral grains – the Jack Hills zircons, which have maximum ages of 4.4 billion years – claimed that tiny inclusions of … Continue reading