Detailed Protracted Crystallization History of Perovskite in Orapa Kimberlite

Abstract

Detailed petrographic and geochemical studies of perovskites from different lithofacies ofOrapa kimberlite, a typical crater facies kimberlite fromBotswana, have been conducted to investigate the crystallization and paragenesis of this very important groundmass phase. We suggest that there is no universal paragenetic sequence of mineral crystallization in the groundmass of kimberlite as it depends on the magma composition, which is highly variable. Our study reveals that most of the perovskite grains in Orapa grew after the macrocrystal phases such as olivine and Cr-bearing spinel, and simultaneously with ‘‘reaction’’ Fe-rich and groundmass spinel from ulvöspinel-magnetite group, as suggested earlier in the literature. However, certain perovskite grains contain inclusions of phlogopite and apatite, which are generally very late-crystallizing phases in kimberlite. Some perovskite grains are also found to appear as late-crystallizing rims around partially resorbed spinel. These textural features suggest protracted perovskite crystallization over a range of P and T from an evolving kimberlite magma. Previous O isotope data have also been used to suggest that perovskite crystallization succeeded late-stage magmatic degassing. Minor and trace element concentrations of Orapa perovskites also support this longer crystallization history as the post-degassed phase perovskite contain less Nb and Zr, which have preferentially partitioned into rutile, an alteration product of early-crystallizing perovskite. Calculated oxygen fugacities fromOrapa perovskites range from-5.5 NNO to -0.2 NNO, emphasizing perovskite crystallization in an evolving magmatic system. Sudden degassing, mainly CO2 release, prompted a change in the oxidation state of the magma, which was recorded by the late-crystallizing perovskites as this group shows an f(O)2 value much higher (-2.3 NNO to -0.2 NNO) than the rest of the perovskite grains. All different lithofacies contain perovskites of different paragenesis with varying quantities while the pyroclastic kimberlite has the maximum abundance of late-stage post-degassing phase perovskites. C.