PGE geochemistry and mineralogy within the Jurassic Dufek-Forrestal layered mafic intrusion,Antarctica

摘要

Selected samples from the Dufek-Forrestal layered mafic intrusion,Antarctica,have been analysed for platinum-group elements and their mineralogical siting to study their low-pressure fractionation behaviour within the ～3.5 km thick exposed profile.Samples analysed comprise variably composed gabbros,anorthosites,pyroxenites and magnetitites from both the Dufek Massif and the Forrestal Range sections.The PGE totals,generally dominated by the Pt-PGE,range from 5 to 792 ppb with the highest contents analysed in Ti-magnetite-rich gabbros and Ti-magnetitites from the basal ～300 m of the Forrestal Range section,which represents the upper ～ 1.7 km of the intrusion.The observed PGE enrichment until Ti-magnetite fractionation indicates sulphur-under-saturation during low-pressure differentiation of a tholeiitic magma originated by high-degree partial melting of a S-poor subcontinental lithospheric mantle source.The final PGE fractionation is caused by a delayed S-saturation ultimately released by Fe-Ti oxide crystallisation at an advanced level of magma differentiation.However,the lack of distinct trace element correlations as well as the platinum-group mineral assemblage identified in these Ti-magnetite-rich rocks indicate that their PGE composition is not only controlled by the base-metal sulphides present.Thus,both high-temperature and subsolidus redistribution processes are responsible for the PGE distribution found within the stratigraphic profile of the Dufek-Forrestal intrusion.