Fractional crystallisation
Fractional crystallisation is the step-by-step change in a magma's chemistry as early crystals form and are removed.
Fractional crystallisation happens when crystals grow in a melt and are separated from it, by sinking, floating or being left behind as the liquid moves. Each mineral takes certain elements with it, so the remaining melt gradually changes composition.
In silicate magmas, early olivine and pyroxene remove magnesium, leaving the melt richer in iron, calcium and aluminium. This explains the range from diogenites to eucrites on Vesta and from olivine-rich to basaltic rocks on Mars and the Moon.
The same principle works in metal. As an asteroid's molten core froze, solid metal crystallised first, taking up elements such as iridium, while others such as gold and germanium stayed in the liquid. Within a single magmatic iron group, such as IIIAB, iridium falls by a factor of about a thousand as nickel rises, a classic fractional crystallisation trend. Plotting such trends lets scientists check whether a set of iron meteorites came from the same core.
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