Petrogenesis of Kaapvaal lamproites (aka orangeites) constrained by the composition of olivine and similarities with kimberlites and other diamondiferous lamproites

Abstract

Kaapvaal lamproites, also known as orangeites, are H2O-rich, diamondiferous, highly micaceous, ultrapotassic rocks. Olivines in kimberlites have been shown to be extremely useful in tracking melt evolution, highlighting the importance of the chemical effects of SCLM assimilation on asthenosphere-derived melts. Kaapvaal lamproites are derived from melting metasomatised SCLM and may be expected to form an endmember to the asthenosphere melt-SCLM trend defined by kimberlites. In this contribution, we use olivine composition in Kaapvaal lamproites to further understand melt evolution in the SCLM and assess the similarity between Kaapvaal lamproite, other diamondiferous lamproites, and kimberlite petrogenesis in cratonic regions.We present olivine composition for representative on- and off-craton Kaapvaal lamproites from Finsch and Melton Wold, respectively. Olivines from these Kaapvaal lamproites are characterized by distinct core and rim zones, regardless of the size of individual grains. Polycrystalline grains are abundant at Finsch but relatively rare at Melton Wold. The olivine cores from both occurrences are predominantly Mg-rich (Fo>89) whereas Fe-rich cores (Fo<89) are rare. Mg-rich cores are interpreted to be derived from the disaggregation of mantle peridotites, including sheared peridotites, whereas Fe-rich cores are derived from olivines of the Cr-poor megacryst suite. The average Fo and NiO concentrations of the Melton Wold cores are lower than Finsch cores, likely related to less refractory off-craton mantle. The olivine rims at Finsch and Melton Wold are characterized by reverse zoning with ranges of Fo89-92 and Fo90-91, respectively. The rims are interpreted to represent crystallisation related to a complex interplay between increasing oxidation, assimilation of orthopyroxene, and increasing alkali content of the melt during evolution.The average core and rim compositions of Finsch and Melton Wold, in conjunction with data from diamondiferous lamproites of other cratonic regions, define a broad positive correlation. Kaapvaal lamproites have Mg-rich core and rim compositions, similar to that of Lac de Gras kimberlites, and interpreted to reflect sampling and equilibration of low volume Kaapvaal lamproite melt with refractory mantle. In contrast to Lac de Gras kimberlites, Kaapvaal lamproites have high abundances of groundmass phlogopite that reflect metasomatic material in the SCLM source, likely present as veins within refractory peridotite. This suggests that increasing proportions of melt-metasomatised SCLM interactions are not always linked with increasing Fe content of melts.We show that the petrogenesis of Kaapvaal lamproites is similar to that of kimberlites and lamproites from other cratonic regions, however, the high abundance of phlogopite and Fo-rich olivine rims suggest a distinct metasomatic lithology in the source and that olivine composition; i.e., a proxy for melt composition, may be strongly controlled by melt volume during melt-SCLM interactions.