Iron meteorites: cores, cooling and dynamos

Iron meteorites preserve quantitative records of how asteroid cores crystallised and cooled, and even whether they generated magnetic fields.

Fractional crystallisation

In a magmatic iron group, the parent core is modelled as a metallic liquid crystallising inwards. Each element partitions between solid metal and remaining liquid according to a partition coefficient. Siderophile elements such as iridium, osmium and rhenium are strongly compatible in solid metal, while gold and arsenic are much less compatible.

Sulphur and phosphorus are crucial because they are almost excluded from solid metal and increase in the liquid. As they build up, they change the partition coefficients of other elements, producing curved trends on element plots.

Modern models also account for trapped melt, liquid immiscibility and the possibility that cores crystallised from the outside in, with dendrites growing inwards. Groups such as IIIAB and IVA are well modelled, while others require additional processes.

Measuring cooling rates

As a core cools below roughly 700 to 900 °C, depending on nickel content, kamacite nucleates in taenite and grows by diffusion of nickel. The nickel concentration across a taenite lamella develops an M-shaped profile, with high nickel at the edges and lower in the centre.

By measuring these profiles with an electron microprobe or analytical transmission electron microscope, and modelling the diffusion, scientists calculate the cooling rate through the relevant temperature range. Rates typically range from a few to thousands of degrees per million years.

At lower temperatures, around 300 °C and below, taenite edges decompose into a cloudy zone of tetrataenite islands in a matrix. The size of these islands provides an independent cooling rate estimate. Within a group, a single core should show uniform cooling rates at a given temperature, so variation is diagnostic.

The IVA puzzle

IVA irons, including Gibeon and Muonionalusta, have cooling rates that vary by more than an order of magnitude and correlate inversely with nickel content. Studies published in 2007 argued that this pattern fits a metallic core that cooled without an insulating silicate mantle, with nickel-poor, early-crystallised outer layers cooling fastest.

This implies that the IVA parent was stripped of its mantle, perhaps in a hit-and-run collision with a larger body early in Solar System history. Alternative models involving different geometries have been proposed, but the idea that some cores were exposed early has become influential.

Mesosiderites and some IIE irons similarly suggest complex histories involving impacts between differentiated bodies, highlighting that collisions shaped many asteroids while they were still hot.

Ancient dynamos

The tetrataenite in the cloudy zone has a very high magnetic coercivity and can record the magnetic field present as it formed. Using X-ray photoemission electron microscopy, researchers can map the magnetisation of individual islands.

In 2015, a study of pallasites, including Imilac and Esquel, reported evidence that their parent body generated a magnetic field from a core dynamo, long after the parent body's formation. Similar work on irons and other achondrites suggests that dynamos were common in planetesimals.

A dynamo requires convection in a liquid metal core, so these records place constraints on core size, crystallisation and the thermal evolution of parent bodies. They also indicate when a core finally solidified: dynamo activity driven by compositional convection may have persisted during inward crystallisation and ceased when it was complete.

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