Stable and metastable pyroxene crystallization in layered komatiite lava flows

Pyroxenes in the rapidly-cooled upper portions of thich layered komatiite lava flows have significantly different compositions and structures from pyroxencs in the cumuliate lower portions of the same flows. Rocks in the flow tops contain composite pyroxene mcgacrysts made up of needles with cores of pigeonite and margins of subcalcic, aluminous augite. The pigeonite is more magnesian [Mgl(Mg + Fe) = 0.841 than any previously reported in terrestrial rocks. In the cumulate zon€s, the pyroxenes are bronzite and augite. The pigeonite probably formed under conditions of strong undercooling developed during the solidifcation of the upper parts of the flows, whereas the cumulus pyrox€nes formed under conditions approaching equilibrium in the more slowly-cooled ccntral parts of the flows.