Re-Os and PGE Systematics of Neoarchean Websterite Xenoliths and Diamondiferous Lamprophyres of the Wawa Area, Superior Province, Canada

The Michipicoten greenstone belt (MGB) is situated in the Wawa-Abitibi subprovince, a largely juvenile crustal addition to the southern Superior Province. The MGB comprises three mafic to felsic volcanic successions. The Mesoarchean basal succession also contains komatiites and granitoids unconformably overlain by 2 Neoarchean units. Numerous 2.68 Gyr old lamprophyre dikes and their diatreme facies equivalents occur in the youngest succession and are notable for their diamonds and ultramafic xenoliths; they are, in fact, the world’s oldest igneous diamond hosts. This occurrence is ideal for understanding deep-seated petrogenetic processes beneath greenstone belts, the possible subduction origin of diamond, and how juvenile oceanic crust became cratonized to form the continents. The lamprophyres are calc-alkaline or shoshonitic with original minerals mostly replaced by greenschist grade metamorphic assemblages of actinolite+chlorite+albite+epidote+titanite+chromite. They contain xenocrysts of chromite and diamond, and abundant crustal and ultramafic xenoliths. The ultramafic xenoliths are compositionally websteritic and recrystallized to assemblages of actinolite +/- talc. Both ultramafic xenoliths and lamprophyres were studied previously for their major and trace elements and Nd and Pb isotopes [1]. Re-Os and the platinum group elements (PGE) were analyzed in these xenoliths and their lamprophyre hosts to examine their depletion/enrichment history. As expected, lamprophyres are mildly enriched in the incompatible PGE (Pt, Pd) and depleted in the compatible PGE (Os, Ir, Ru) whereas websterites are the reverse. All samples have low Re content that leads to low 187Re/188Os (lamprophyres 0.14 - 0.30; websterites 0.03 - 0.06) and hence yields accurate initial 187Os/188Os. The lamprophyres have initial 187Os/188Os ranging from 0.114 to 0.122 which is enriched by 4-10% over Neoarchean chondritic mantle. Similar relative enrichments are seen in modern lamprophyres such as those from the Mexican Volcanic Belt and confirm that the Wawa lamprophyres originated from subduction-fluid enriched mantle. The websterites have Re-Os and PGE systematics more like peridotites than pyroxenites. Their low initial 187Os/188Os (0.106 - 0.114) overlaps the composition of peridotite xenoliths in Kirkland Lake kimberlites and the initial isotopic composition of Abitibi komatiites. Two websterites with lower Os content (1.1 - 1.6 ppb) have PGE patterns and Os isotopic compositions suggesting they could be komatiitic melt infiltrated mantle if not high pressure cumulates. The websterite with the highest Os content (3.7 ppb) has a depleted PGE pattern like on-craton south African xenoliths and a low Os isotopic composition like Mesoarchean mantle. Such mantle could have been a host for Wawa diamonds prior to their entrainment in lamprophyric magma. It suggests that incorporation of Mesoarchean residues may have been essential to form thick subcontinental mantle keels and allow for diamond extraction in the Neoarchean. [1] Wyman et al. (2006), Lithos 89, 300-328.