The Petrogenesis and Mantle Source of Archaean Ferropicrites from the Western Superior Province, Ontario, Canada

Abstract

Archaean ferropicrites have been under-appreciated in the past because they have been frequently misidentified as enriched komatiites or Al-depleted komatiites. To investigate the nature of Archaean ferropicrite magmatism, we sampled ferropicrites from the Steep Rock, Lumby Lake, Grassy Portage Bay, and Dayohessarah Lake greenstone belts in the Western Superior Province, Ontario, Canada. Ferropicrite samples that are thought to approximate liquid compositions have 18 wt % Fe2O3 at 19 wt % MgO, and frequently contain less than 5 wt % Al2O3.They are enriched inTi and high field strength elements relative to komatiites, and have fractionated trace element profiles (La/Yb 11). These distinctive geochemical characteristics require that ferropicrites and komatiites have different mantle sources, with that of the ferropicrites being Feand incompatible element-enriched compared with that of komatiites. A consideration of recent 5 GPa melting experiments on pyrolite and Fe-rich Martian mantle compositions indicates that Archaean ferropicrites could be generated by melting of an olivine-dominated mantle source with a Mg-number of 85 at 5 GPa. The high densities calculated for the ferropicrite magmas (e.g. 3 33 g/cm) suggest that more Fe-rich magmas would have difficulty rising to the Earth’s surface and would tend to stagnate or sink within the mantle.