Pyroxenes in the rapidly-cooled upper portions of thick, layered komatiite lava flows have significantly different compositions and structures from pyroxenes in the cumulate lower portions of the same flows. Rocks in the flow tops contain composite pyroxene megacrysts made up of needles with cores of pigeonite and margins of subcalcic, aluminous augite. The pigeonite is more magnesian [Mg/(Mg + Fe) ≈ 0.84] than any previously reported in terrestrial rocks. In the cumulate zones, the pyroxenes are bronzite and augite.

The pigeonite probably formed under conditions of strong undercooling developed during the solidification of the upper parts of the flows, whereas the cumulus pyroxenes formed under conditions approaching equilibrium in the more slowly-cooled central parts of the flows.

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