What olivine, the neglected mineral, tells us about kimberlite petrogenesis

N. T. Arndt, A. M. Boullier, J. P. Clement, M. Dubois, D. Schissel
2006 eEarth Discussions  
We report here the results of a petrographic and geochemical study of remarkably well-preserved kimberlites from the Kangamiut region in Greenland. The samples contain between 5 and 45% of olivine in the form of rounded "nodules", each 1 to 5 mm in diameter. Most originally were single crystals but many consist of polycrystalline, 5 15 pyroxene and an aluminous phase, which make up about half of most mantle-derived rocks. It appears that these minerals were removed from the material that was to
more » ... aterial that was to become the nodules, perhaps by fluids that immediately preceded the passage of the kimberlites. We speculate that this mantle "defertilization" process was linked to interaction between CO 2 -rich fluid and mantle and that this interaction controlled the 20 geochemical and isotopic composition of kimberlites. 25 gopite and carbonate. Most previous descriptions of the mineralogy of kimberlite have 38 eED 1Abstract 20 similar to that of NCR 27. Deformation features The samples were heated in air at 900 • C for 1 h to "decorate" the internal structures (Fig. 1d) . Anhedral matrix olivine is characterized by undulating extinction and the presence of numerous close-spaced fractures and dislocation walls. The dislocations are 25 39 eED 1EGU which the kimberlite passed on its way towards the surface, most probably the lithospheric mantle. The diamonds may therefore have been present in the "defertilized" material processed from peridotite to dunite. If this is so, the incorporation of diamond into kimberlite will depend on the nature of the reprocessing and the manner in which the reprocessed material is incorporated into the kimberlite. 5 Appendix A Analytical methods Major elements concentrations were determined at the Service d'Analyses of the Centre de Recherche Petrographique et Geochimique (CRPG) in Nancy, France. Details of the analytical procedure and estimated precision and accuracy are found at
doi:10.5194/eed-1-37-2006 fatcat:wp4m2265lvetpmdef6vfqzxm2e