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The Formation of Kelyphite and Associated Sub-Kelyphitic and Sculptured Surfaces On Pyrope from Kimberlite

DOI: 10.1016/b978-0-444-42273-6.50031-6
  • Earth Science


Abstract The mineralogy, structure and mode of formation of kelyphite rinds surrounding pyrope garnet in kimberlite concentrates and mantle-derived xenoliths are discussed. Kelyphite rimming garnet in peridotite xenoliths exhibits crudely concentric shells consisting mainly of a radially-arranged fine-grained two pyroxene-spinel assemblage often complicated by the introduction of hydrous phases from the host kimberlite. The proposed conditions for the formation of pyroxene kelyphite in peridotite xenoliths are at temperatures of 650° to 1350° C and pressures of 5kb and 15kb within the upper mantle. By contrast, the coronas around garnet in some eclogite xenoliths are devoid of any structural pattern and are phlogopite-rich, with accessory amphibole, spinel, plagioclase and glass. This type of phlogopite kelyphite is considered to form in eclogite xenoliths by the action of kimberlitic alkalies and volatiles in the region of the lower crust. Kelyphite shells are most commonly developed on mauve garnets, which are probably derived from disaggregated peridotites, and are less commonly found on orange varieties originating from eclogites. Most of the garnets without kelyphite exhibit either sub-kelyphitic or sculptured surfaces. A relationships is noted between the various surface features on sub-kelyphitic garnet surfaces and the structure and mineralogy of kelyphite surrounding garnet. Sculptured surfaces consisting of a combination of hillocks and etch pits are more commonly found on red and orange garnet, and less commonly occur on mauve garnet. Textural evidence shows that sculpturing or high temperature etching of garnet post-dates kelyphite formation, and predates kimberlite groundmass crystallisation, and is most likely to occur at temperatures of 600° to 900° C by resorption processes in the final stages of kimberlite emplacement.

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