Did the Earth have a magnetic field before 3.5 billion years ago? Previous paleomagnetic studies of the world’s oldest mineral grains claimed. New exhaustive study shows we still don’t know. [1/n] https://advances.sciencemag.org/content/6/15/eaav9634

This study shows that the magnetite within the zircon grains that carries the putative super-ancient magnetisation are not primary inclusions (crystallised from the melt first before the zircon grew around them), but formed by alteration later on. [2/n]

How much later? We don't know. But it could have been any point between 4 billion years ago and today. The state of the magnetic field in the Hadean and Eoarchean goes back to a big question mark. Very disappointing, but not totally unexpected... [3/n]

Fact that most magnetic minerals contain iron, and iron is redox sensitive, is a real bane for studying ancient magnetisation. There is always the very real prospect that your rock is one age & the magnetisation you are oh-so-carefully measuring is another (younger) one. [4/n]

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, e.g., http://all-geo.org/highlyallochthonous/2008/02/greigite-mineral-of-evil/ [5/n]

This is a really nice example of how there is a distinction between 'good' data and *meaningful* 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 /fin.

PS Adding a tip of the hat to @NanoPaleoMag for bringing this to my attention - and being involved in such a cool and careful study.