Kiglapait geochemistry VI: Oxygen isotopes |
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| Authors: | Ruth I Kalamarides |
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| Institution: | Department of Geology, Northern Illinois University, DeKalb, Illinois 60115 USA |
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| Abstract: | The Kiglapait layered intrusion is the first major intrusion found to have all whole rock and calculated liquid δ18O values close to a normal uncontaminated gabbroic value of 6.0. The intrusion experienced no detectable oxygen isotope exchange with its surrounding rocks and cooling of the magma was conductive. The δ18O values of average whole rocks vary smoothly from 6.0 at the base of the Lower Zone to 6.3 at the top of the Upper Zone. The calculated liquid δ18O values lie practically superimposed on the whole rock trend. The whole-rock data and the modelled δ18O of the magma and cumulates rigorously demonstrate that the effect of incoming cumulus phases such as magnetite and augite on the δ18O of the liquid and rocks during fractional crystallization is negligible. The cancelling effects of complementary modal variations among the mafic mineral phases and feldspar, keep the δ18O of the whole rocks constant to within ±0.1 %.. The minor change in δ18O that does occur with fractionation is consistent with the enrichment of residual liquids in feldspar component and the increasing fractionation factor δ Liquid-Fsp with falling temperature.The δ18O values of the country rocks bracket the estimated δ18O of the Kiglapait magma. Modelling with oxygen isotopes indicates that contamination of the intrusion, indicated by published radiogenic Sr and Nd isotopic data, was minor. The most probable contaminant had δ18O?7.7 and the contamination most likely occurred at >99% solidified. Subsolidus oxygen isotope exchange with an external source appears to have been very minor. |
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