Poles apart:A discussion of 'Geochemistry of cumulates from the Bjerkreim-Sokndal Intrusion(S.Norway).Part I:Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent' by J.C.Duchesne and B.Char

摘要

We dispute Duchesne and Charlier's(Duchesne,J.C,Charlier,B.,2005.Geochemistry of cumulates from the Bjerkreim-Sokndal Intrusion(S.Norway).Part I:Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent.Lithos 83,229-254.)postulate that the major-element compositions of cumulates in the Bjerkreim-Sokndal Intrusion vary linearly between plagioclase and mafic "poles" and their inference that this supports an origin by in situ crystallisation.We use a larger set of major-element data for plagioclase-orthopyroxene-ilmenite cumulates to show that some linear trends in Harker diagrams simply reflect varying amounts of hemo-ilmenite relative to plagioclase and orthopyroxene,while others are probably spurious and induced by variations in modal plagioclase,the most abundant cumulus mineral.Ratios of oxides that enter almost exclusively into orthopyroxene and hemo-ilmenite are shown to be highly dispersed,reflecting differential sorting of the mafic minerals.