Insights into kimberlite petrogenesis and mantle metasomatism from a review of the compositional zoning of olivine in kimberlites worldwide
Abstract Olivine is the dominant component in kimberlites (~40–60 vol%), where it occurs as individual grains of variable size (>1 cm to 2700 major and minor element analyses of olivine from 17 kimberlite localities from southern Africa, Canada, Greenland and Russia. These data show that the large majority of olivine grains in coherent kimberlites are compositionally zoned regardless of size and shape. The zonation typically includes a core of variable composition (e.g., Mg# = 100 × Mg/(Mg + Fe) = 78–95) that is overgrown by a rim characterised by relatively restricted Mg# (typically ≤1 “unit”; predominantly 88–92), decreasing Ni and Cr, and increasing Mn, Ca and Ti contents. One or more internal zones of variable composition occur between core and rim of some grains. The internal zones can be euhedral, diffuse or partially resorbed (i.e. embayments). Low-Ni, high Mg-Ca rinds (Mg# up to 96–98) commonly fringe olivine rims in fresh (i.e. minimally serpentinised) kimberlites. A comparison between the compositions of olivine cores and olivine from mantle xenoliths (including megacrysts) entrained by kimberlites, demonstrates that olivine cores are xenocrysts derived from disaggregation of mantle wall-rocks. This interpretation is consistent with the inclusion of mantle phases (i.e., orthopyroxene, clinopyroxene, garnet and Cr-spinel) in olivine cores, and evidence of resorption (i.e. embayments) and abrasion (e.g., rounded shapes) of these cores. A variable proportion of olivine cores is sourced from the products of kimberlite metasomatism at mantle depths (e.g., sheared peridotites, megacrysts, ‘defertilised dunites’), which implies variable extent of kimberlite activity in the mantle before kimberlite emplacement at surface. Olivine rims host inclusions of groundmass minerals (e.g., spinel, Mg-ilmenite, rutile), which requires a magmatic origin for the rims. With few exceptions (i.e., Benfontein; Udachnaya-East), olivine rims in each kimberlite locality, cluster (e.g., Kimberley) and, potentially field (e.g., Lac de Gras), form a single compositional trend. This suggests that kimberlites within the same cluster derive from similar parental melts and therefore sources, which is consistent with available radiogenic isotope results, and undergo similar crystallisation processes. Indistinguishable compositions of olivine rims in kimberlites from Lac de Gras that were emplaced as hypabyssal root-zones, dykes and volcaniclastic units, indicate that olivine crystallised during ascent, i.e. before different emplacement processes modified magma compositions. The implication is that the composition of (near-primitive) melt parental to olivine has minimal influence on kimberlite emplacement mechanism. Variations on the compositions of olivine rims in kimberlites from different areas suggest contribution from a range of local processes, such as variable source composition, olivine and spinel fractionation, assimilation of mantle material, CO2 loss, melt oxidation, changing pressure and temperature conditions of crystallisation. Based on their compositional and textural features, three types of internal zones can be distinguished: 1) euhedral early liquidus olivine with higher Mg# and Ni than rims and hosting inclusions of magmatic chromite; 2) diffusional zones with compositions intermediate between those of cores and rims; 3) zones exhibiting resorption features that may be products of earlier kimberlite metasomatism at mantle depths. Olivine grains represent unique capsules that provide a potentially complete record of the evolution of kimberlite systems. Olivine cores store information on the mantle column entrained by kimberlites, including clues to early kimberlite metasomatism. Internal zones can show the effects of mantle metasomatism and/or record early kimberlite crystallisation at mantle depths. Rims (and rinds) testify to the complex interplay of different processes during ascent and emplacement of kimberlite magmas.