The Geochemical Complexity of Kimberlite Rocks and their Olivine Populations: a Reply to the Comment on Cordier et al. (2015) by Andrea Giuliani & Stephen F. Foley
Résumé
We thank sincerely Giuliani & Foley for their discussion of the textures and compositions of olivine in kimberlites. We do not agree with many of their interpretations and therefore welcome the chance to respond, particularly because it provides an opportunity to explain once again the characteristics of olivine in kimberlites that we think are most important—features that have commonly been misinterpreted or overlooked in many other papers. In publications such as those by Kamenetsky et al. (2008), Brett et al. (2009) or Pilbeam et al. (2013), the emphasis has been on mineral compositions and internal zonation whereas in our paper (Cordier et al., 2015) and in an earlier study (Arndt et al., 2010), we emphasized the internal structures and fabrics of olivine grains. In the earlier paper, we described structures produced by solid-state deformation and showed that these could only have formed in the lithosphere. In our 2015 paper, we focused on grain boundary migration zones. These features, which typically develop during fluid-assisted recrystallization, may be unfamiliar to igneous petrologists but are well known to metamorphic and structural geologists. Like the internal deformation features, they form slowly or under high-stress conditions and could only have been produced within the lithosphere. We emphasize that these deformation or recrystallization features formed within rock of dunitic composition: the orthopyroxene, clinopyroxene and garnet–spinel that make up a large proportion of most mantle xenoliths were absent when these textures originated. In our opinion, these observations indicate clearly that orthopyroxene was eliminated before entrainment of xenoliths into the ascending kimberlite magma. If this interpretation is correct (and we have not heard or read any arguments to negate it), it calls into question the commonly accepted model that proposes loss of orthopyroxene within ascending kimberlite magma.