Trace-element partitioning in perovskite: Implications for the geochemistry of kimberlites and other mantle-derived undersaturated rocks

Perovskite is a common groundmass phase in many mantle-derived undersaturated rocks (e.g., kimberlites), serving as an important repository for rare-earth elements (REE), high-field-strength elements (especially, Ti, Nb and Ta), Th and U. Perovskite-groundmass partition coefficients, calculated for samples of hypabyssal kimberlite from the Chicken Park (Colorado), Iron Mountain (Wyoming) and Udachnaya East (Russia) are largely consistent with those calculated for perovskite from katungite (potassic olivine melilitite) lava from Bunyaruguru, Uganda. The major and trace elements, analyzed by electron-microprobe and laser-probe techniques, can be grouped into strongly incompatible (D X ≪ 0.1: K, Rb, Zn, Ba, Al), moderately incompatible (D X = 0.1-0.5: Mn, Ga, Sc, V, Fe), compatible (D X = 1.0-5.0: Ca, Sr, Hf, heavy REE), and strongly compatible (D X > 5.0: Y, light- to mid-range REE, Th, U, Ti, Nb, Ta). Sodium gives a wide range of partitioning values in the kimberlitic perovskite, all of which are higher than the Na value for the katungite (D Na = 0.19). This discrepancy is interpreted to indicate loss of Na from the kimberlitic magma during its emplacement, resulting in low Na levels in the groundmass and overestimated partition coefficients. The calculated Pb values corrected for U{single bond}Th decay are also many times higher in the kimberlitic perovskite relative to the katungite (D Pb = 0.3); the latter value is considered a better estimate because it was not affected by U{single bond}Th decay and is in better accord with the partitioning data for other divalent cations. Notably, perovskite has a higher affinity for light REE (LREE), Ho, Ta, Hf and Th relative to heavy REE (HREE), Y, Nb, Zr and U, respectively. Decoupling is particularly strong in the LREE-HREE, Th-U and Nb-Ta pairs, and will produce a noticeable effect on the composition of magma precipitating perovskite even at small degrees of fractionation. Such effects are not observed in kimberlites, even though fractionation of other minerals (notably, olivine) probably does play an important role in the evolution of kimberlites, as demonstrated in the present work on the basis of published data. Perovskite in kimberlites commonly develops a zoning pattern involving a rim-ward decrease in Th/U ratio, Na, REE, Th and Ta contents, and an increase in Nb/Ta value, which can be explained by gradual depletion of the host magma in these elements due to their early sequestration in the core of perovskite crystals and, possibly, Na loss (see above). There are also cases (e.g., Lac de Gras kimberlites) where perovskite develops a discontinuous Nb{single bond}Fe{single bond}Zr-rich rim. These evolved compositions plot away from the normal zoning trend described above owing to their low Na content for the given level of REE enrichment, indicating that a significant proportion of the latter elements is incorporated in this perovskite as REE(Fe,Al)O3. Petrographic observations, combined with the published data and lack of any spectroscopic evidence for the presence of water in this perovskite, indicate that this type of zoning formed due to the loss of a volatile phase from the kimberlitic magma, accompanied by perovskite fragmentation and followed by reaction of perovskite fragments with an evolved trace-element-rich melt.