Compositional Variations in Primitive Kimberlite Melts and Entrained Mantle Cargo from a Global Survey of Trace Element Compositions in Kimberlite Olivine

Abstract

Olivines are the dominant phase of kimberlites and the majority of grains display distinct compositional zoning with xenocrystic cores surrounded by magmatic rims. Previous work has documented large variations in both core and rim major and minor element compositions in kimberlites globally, which has been related to variable entrainment and assimilation of sub-continental lithospheric mantle (SCLM) material. However, there is limited knowledge of trace element variations in olivine from kimberlites and it is unclear whether mantle assimilation has any effect on the trace element composition of kimberlite melts. To fill this gap, we present a global survey of olivine trace element compositions, along with previously reported major and minor element compositions, for samples representing the full spectrum of olivine compositional variations in kimberlites, including samples from Lac de Gras, Canada (Grizzly, Koala), Kimberley, South Africa (Bultfontein, De Beers, Kimberley Big Hole, Wesselton), Kaalvallei, South Africa (New Robinson, Samada), and Alto Paranaiba, Brazil (Limpeza-18, Tres Ranchos-04). Trace element concentrations of olivine cores can discriminate between those derived from the disaggregation of peridotitic material and those related to the megacryst suite. The megacrystic olivine cores exhibit a negative correlation between Al and Mn, which is absent in peridotite-derived cores, and are characterised by high concentrations of temperature dependent elements (e.g., Al, Na, V) as well as Zn, Ti, and Mn. Following pre-screening of cores for megacrystic and spinel peridotite derived grains, we applied the Al-in-olivine thermometer to assess the P-T equilibration conditions of cores in equilibrium with garnet and estimate the sampling depth of kimberlite magmas in the lithospheric mantle. Our results are consistent with predominant entrainment of deep lithosphere xenocrysts in highly diamondiferous compared to diamond-poor kimberlites. Temperature dependent elements display a gradational increase with depth due to higher T with Ca, Cu and to a lesser extent Zn and Ti being higher and Mg# lower toward the base of the SCLM, which is consistent with melt modification of the lower lithosphere. The Zn, Ti, Co, Mn, Li, Al, Cr, Na, and V concentrations of magmatic olivine rims display systematic variations that have a negative correlation with Mg# (whereas Cr is positively correlated). Lac de Gras olivine feature Mg-rich rims (Mg# >90) and low concentrations of these trace elements whereas the Fe-rich olivine rims (Mg# ~85) of the Kaalvallei kimberlites have higher concentrations of these elements, with the Kimberley and Alto Paranaiba kimberlites being intermediate. Direct correlations between average Ti, Zn, Co and Li compositions of olivine cores and rims suggests that the olivine rim (i.e., proxy for primitive melt) variations are related to variable assimilation of metasomatised SCLM and can be effectively used to track the composition of the lithospheric column that is traversed by kimberlite magmas. These observations further imply an intimate link between early proto-kimberlite melt, leading to formation of megacrystic olivine at the base of the SCLM, and the composition of kimberlite melts which entrain and assimilate these products. We conclude that lithospheric mantle assimilation has a major and previously overlooked influence on the trace element composition of kimberlite magmas.

Diamond preservation in the lithospheric mantle recorded by olivine in kimberlites

“…This correlation, combined with extensive petrographic and experimental evidence of assimilation of entrained lithospheric mantle material by kimberlites 29,[33][34][35] , suggests that the composition of kimberlite melts at surface is directly related to the composition of lithospheric mantle wall rocks, which interact with kimberlite melts en route to surface 21 . In addition, more assimilation of metasomatised material yields more Fe and Ti-rich (and Mg-poor) magmatic olivine and kimberlite groundmass 22,36 . Therefore, olivine geochemistry provides a direct link between kimberlite melt composition and traversed lithospheric mantle wall rocks, including the extent of kimberlite-related metasomatism.…”

“…We specifically test the hypothesis, based on previous observations of diamond resorption features 17,18 and recent experimental work 19,20 , that interaction of diamonds with carbonaterich kimberlitic melts at lithospheric mantle depths can reduce the diamond tenor of the lithospheric mantle, potentially rendering kimberlites uneconomic. Olivine in kimberlites has been shown to record the extent of kimberlite-related metasomatism in the lithospheric mantle [21][22][23] and provides a practical means to test this premise by comparing olivine compositions in various kimberlites with their published diamond grades.…”