Magmatic iron
An iron meteorite from a group whose chemistry records fractional crystallisation of a molten metal core.
A magmatic iron belongs to a chemical group whose members are thought to have crystallised from a single, once-molten metallic core. As such a core cooled, solid metal grew from the liquid and trace elements partitioned between solid and melt. Elements that prefer the solid, such as iridium, were concentrated early, while those that prefer the liquid, such as gold and arsenic, rose in later crystals. Plotting iridium against nickel or gold for a group therefore produces long, tight fractional-crystallisation trends.
Groups usually counted as magmatic include IC, IIAB, IIC, IID, IIF, IIIAB, IIIE, IIIF, IVA and IVB. Each is interpreted as sampling a separate parent body, which means iron meteorites alone represent dozens of melted asteroids. Cooling rates from metallographic profiles add estimates of core size and depth. Typical examples are the IIIAB irons Henbury and Cape York and the IIAB iron Sikhote-Alin.
The contrast is with non-magmatic groups, chiefly the IAB complex and IIE, which show scattered element trends and often contain silicate inclusions. Assignment requires neutron activation or mass spectrometry for nickel, gallium, germanium and iridium.
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