An Investigation of the Oxygen Isotope Geochemistry of the Stillwater Complex

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Abstract
Oxygen isotopic analyses of samples spanning the entire stratigraphic height of the Stillwater Complex show that the intrusion has retained its magmatic isotopic composition. The melt had a calculated δ18O value of 5·9 per mille which agrees well with the reported isotopic compositions of the Kiglapait Intrusion and other mantle-derived basic melts. While there has been alteration, the isotopic data confirm the petrograhpic observation that the alteration took place at low temperature (most likely in the weathering environment). Isotopic thermometry yields magmatic temperatures. The temperatures define two trends: a high temperature trend extending from the base of the Ultramafic Series to the top of Norite-2 in the Banded Series and a lower temperature trend extending from the top of Norite-2 to the top of the intrusion. The high temperature trend is interpreted as indicating an ultramafic magma while the low temperature trend suggests a more gabbroic melt. However, the temperature break does not coincide with any significant petrologic break. Fractional crystallization modelling of the isotopic variation of appropriate compositions shows that most of the isotopic variation within the complex can be accounted for by simple fractional crystallization. δ18O depletions observed in the Ultramafic Series and in the olivine-bearing zones of the Banded Series indicate the presence of magmatically derived late stage hydrous fluids in those zones.

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