Kiglapait geochemistry VI: Oxygen isotopes

Abstract

The Kiglapait layered intrusion is the first major intrusion found to have all whole rock and calculated liquid δ 18 O 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 δ 18 O 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 δ 18 O values lie practically superimposed on the whole rock trend. The whole-rock data and the modelled δ 18 O of the magma and cumulates rigorously demonstrate that the effect of incoming cumulus phases such as magnetite and augite on the δ 18 O 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 δ 18 O of the whole rocks constant to within ±0.1 %.. The minor change in δ 18 O 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 δ 18 O values of the country rocks bracket the estimated δ 18 O 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 δ 18 O≅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.