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Journal of Petrology Advance Access published online on September 20, 2008
Journal of Petrology, doi:10.1093/petrology/egn044
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© The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

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

Shoshana B. Goldstein and Don Francis*

Department of earth and Planetary Sciences, Mcgill University, 3450 University Street, Room 238, Montreal, que., Canada H3A 2A7

Received January 11, 2008; Revised typescript accepted August 21, 2008


  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 in Ti 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 Fe- and 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/cm3) 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.

KEY WORDS: Archaean; ferropicrite; Fe-rich; komatiite; mantle; petrogenesis

*Corresponding author. Telephone: 514-398-4885. Fax: 514-398-4680. E-mail: donf{at}eps.mcgill.ca


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