It was previously assumed that pallasites formed at the boundary between the iron core and the rocky mantle in a planetary body. However a team of geophysicists, led by John Tarduno at the University of Rochester, discovered that they most likely formed when a smaller asteroid crashed into a planetoid about 30 times smaller than Earth. This would have resulted in the materials mixing before solidifying to produce the distinctive meteorites. Tarduno and his team discovered that the tiny metal grains within the olivine were magnetised to a particular direction; previous work had theorised that the iron intruded from the core into the olivine in the mantle.
The scientists used a carbon dioxide laser at the University of Rochester to heat the metal grains past their Curie temperatures (the point at which a metal loses its magnetisation). The metal grains were then cooled in the presence of a magnetic field to re-magnetise them; at the same time a measuring instrument known as SQUID (superconducting quantum interference device) was used to record the data. The team was able to calculate the strength of the past magnetic field and then the rate of cooling. The measurements from the experiment, combined with a computer model, indicated the parent body had a radius of about 200 km, qualifying it as a proto-planet.
For the metal grains within the olivine to be magnetised, the planetary body in which they formed must have had a molten iron core, to create a magnetic field. Temperatures at the core-mantle boundary would have been close to 930°C and too hot for magnetisation to take place. Pallasites therefore would have formed at somewhat shallow depths in the much cooler mantle of the proto-planet. This research also provides further evidence that small celestial bodies can have dynamo activity; a rotating liquid iron core that can create a magnetic field.
The iron-nickel in the pallasites is believed to have originated from the collision with the asteroid, where molten iron from the core of the smaller of the two asteroids was injected into the mantle of the larger body, creating the textures observed in the pallasites.
The image is a piece measuring 210 mm by 190 mm by 5 mm of the Esquel meteorite, which contains the pallasites. The meteorite was discovered in Chubut, Argentina in 1951 as a single mass weighing more than 700 kilograms.
John A. Tarduno, Rory D. Cottrell, Francis Nimmo, Julianna Hopkins, Julia Voronov, Austen Erickson, Eric Blackman, Edward R.D. Scott, Robert Mckinley. Evidence for a Dynamo in the Main Group Pallasite Parent Body. Science, 2012 DOI: 10.1126/science.1223932