Multiple-mineral inclusions in diamonds from the Snap Lake/King Lake kimberlite dike,Slave craton,Canada:a trace-element perspective

摘要

Multiple inclusions of minerals in diamonds from the Snap Lake/King Lake kimberlites of the southeastern Slave craton in Canada have been analyzed for trace elements to elucidate the petrogenetic history of these inclusions,and of their host diamonds.As observed worldwide,the harzburgitic-garnet diamond inclusions (DIs) possess sinusoidal REE patterns that indicate an early depletion event,followed by metasomatism by LREE-enriched,HREE-depleted fluids.Furthermore,these fluids appear to contain appreciable concentrations of LILE and HFSE,based on the increasing abundances of these elements in the olivine inclusion that occurs at the outer portion of a diamond compared to that near the core.The compositions of these fluids are probably a mixture of hydrous-silicic melt,carbonatitic melt,and brine,similar to the compositions of micro-inclusions in diamonds reported by Navon et al.(2003).Comparison between the compositions of majoritic and normal harzburgitic garnets shows that the former are more depleted in terms of major/minor elements (higher Cr) but significantly more enriched in the REE (up to approx10 x).This characteristic may indicate the higher susceptibility for metasomatic enrichment of previously more depleted garnets.Garnets of eclogitic paragenesis show strong LREE-depleted patterns,whereas the coexisting omphacite inclusion has relatively flat light-and middle-REE but depleted HREE.Whole-rock reconstruction from coexisting garnet and omphacite inclusions indicates that the protolith of these inclusions was probably the extrusive section of an oceanic crust,subducted beneath the Slave craton.