Intellectual Merit. This project focuses on the evolution of magmas emplaced in mafic sills associated with the Proterozoic Franklin Igneous event as exposed on Victoria Island, Canada. Through cooperation with the Canadian Geological Survey, the PI has been involved in field mapping of these sills and has access to three complete drill cores through one of the sills. Detailed studies of this sill are proposed to decipher the differentiation mechanisms that have operated during and after emplacement of the intrusion. The sill is relatively thin (26 to 44 m thick), laterally extensive (>36 kms of strike length), and remarkably differentiated for such a thin magma body (Mg#s vary systematically from 35 to 82). Upper and lower chilled margins with ~10% olivine phenocrysts give way abruptly to olivine-free diabasic rocks in the interior, suggesting that any suspended phenocrysts settled out rapidly, or were segregated in some way from the sill margins. Indeed an olivine-rich zone 5 to 8 m thick occurs near the base of the sills. Near-constant olivine composition throughout isconsistent with injection of an olivine-porphrytic magma, followed by en-masse settling of olivine. A thin (1 to 2 m thick) layer above the olivine zone contains abundant, euhedral, sector-zoned augite. Diabase, which makes up the upper 80% of the sill, becomes progressively Mg-poor and Fe-rich upward from the base and downward from the roof to an Fe-Ti-excluded element rich "sandwich" zone near the center of the sill. The variation trend in the diabase cannot be explained by the removal of olivine and augite phenocrysts from the initial magma, and must have resulted from in-situ differentiation of the diabase. A Mg# ?kick? in the lower diabase can be traced horizontally for over 10 kms, and appears to represent a magma recharge event. Although the sill is Proterozoic, geochemical modeling of preliminary data, suggests that neither major nor trace elements have been significantly altered. The summed bulk composition of the sill very closely matches a chilled margin composition. Petrography and chemical analyses of the constituent minerals is proposed to interpret rock textures with regards to possible post-emplacement compaction and melt migration in the crystal mush. CSD grain size variations will be examined to determine the nucleation and growth history of the major minerals present. Goals of the proposed study are to evaluate: [1] the origin of, and compaction and re-equilibration within, the olivine-rich basal zone, [2] mineral textural and compositional variations above and below the recharge horizon, and [3] the origin of in-situ differentiation trends recorded in the diabase at the floor and roof of the sill, and the role of compaction in these two sequences.
Broader Impacts. This project is being carried out in collaboration with a five-year regional mapping program of the Geological Survey of Canada. The Victoria Island sills are similar in composition and tectonic setting to sills in Siberia that host Noril'sk-type Ni-Cu-PGE mineralization; a better understanding of their emplacement and crystallization history may lead to improved exploration strategies for similar deposits. Although these thin sills represent one extreme end-member of the differentiated magma chamber suite, they may provide critical insights into initial crystallization processes in larger magma chambers where the record of the early crystallization history has been overwritten by later re-equilibration during slow cooling. This project will also provide excellent educational opportunities for the student participants.
