3.2 billion-year-old surprise: Earth had strong magnetic field

ot be good enough. The technique was developed more than four decades ago, and has changed little. With the old method, an igneous rock about an inch across is heated and cooled in a chamber that is shielded from magnetic interference. The magnetism is essentially drained from the particles in the rock and then its refilled while scientists measure how much the particles can hold.

Tarduno, however, isolates choice, individual crystals from a rock, heats them with a laser, and measures their magnetic intensity with a super-sensitive detector called a SQUIDa Superconducting Quantum Interface Device normally used in computing chip design because its extremely sensitive to the tiniest magnetic fields.

Certain rocks contain tiny crystals like feldspar and quartznano-meter sized magnetic inclusions that lock in a record of the Earths magnetic field as they cool from molten magma to hard rock. Simply finding rocks of this age is difficult enough, but these rocks have also witnessed billions of years of geological activity that could have reheated them and possibly changed their initial magnetic record. To reduce the chance of this contamination, Tarduno picked out the best preserved grains of feldspar and quartz out of 3.2 billion-year-old granite outcroppings in South Africa. Feldspar and quartz are good preservers of the paleomagnetic record because their minute magnetic inclusions essentially take a snapshot of the field as they cool from a molten state. Tarduno wanted to measure the smallest magnetic inclusions because larger magnetic crystals can lose their original magnetic signature at much lower temperatures, meaning they are more likely to suffer magnetic contamination from later warming geological events.

Once he isolated the ideal crystals, Tarduno employed a carbon dioxide laser to heat individual crystals much more quickly than older methods, further lessening the chance of contamination. With the Universitys ultra-sensitive SQUID he could me

Contact: Jonathan Sherwood
University of Rochester

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