Towards a new model for kimberlite petrogenesis: Evidence from unaltered kimberlites and mantle minerals

Chemistry of micas from kimberlites and xenoliths—I. Micaceous kimberlites

Mineralogy of primary carbonate-bearing hypabyssal kimberlite, Lac de Gras, Slave Province, Northwest Territories, Canada

We apply an oxygen barometer based on the Fe content of CaTiO3 perovskite to estimate the oxygen fugacity (fO2) during the crystallization and emplacement of kimberlites in different eruptive phases of a single pipe, or between different pipes, clusters or provinces. Mineral chemical data for perovskite were compiled from the literature and obtained in our detailed study of perovskites from 11 kimberlites at Somerset Island and Lac de Gras, Canada. Perovskite compositions in kimberlites record a range in fO2 of many orders of magnitude from NNO 5 to NNOþ 6 [where log fO2 is given relative to the nickel^nickel oxide (NNO) buffer]. The range of fO2 recorded by different parageneses of perovskite within a single pipe can vary up to three orders of magnitude with trends toward both oxidation and reduction during crystallization. Kimberlites record some of the greatest ranges, and the highest known fO2 conditions for any terrestrial magma. This is attributed to the presence of deep and oxidized source regions and the variable interplay of ferric^ferrous vs carbon^fluid equilibria during ascent of kimberlite magmas. Three kimberlite pipes from the Lac de Gras field show that higher fO2 values correlate with higher proportions of more resorbed diamonds, suggesting that this variable has a measurable effect on the physical properties of diamonds in a pipe.

Ferric Iron in CaTiO3 Perovskite as an Oxygen Barometer for Kimberlite Magmas II: Applications

Garnetiferous Ultramafic Inclusions in Minette from the Navajo Volcanic Field

One of the three facies of the newly discovered kimberlite on Somerset Island consists of phenocrysts or xenocrysts of olivine, garnet, phlogopite, enstatite-bronzite, magnetite, magnesian ilmenite, red chrome spinel and chromite, set in a fine-grained groundmass consisting of black chrome spinel, magnetite-spinel-ulvospinel solid solution, perovskite, rutile, pyrite, heazlewoodite, chalcopyrite, dolomite and calcite. Chemical analyses, as well as detailed descriptions of the texture and occurrence of all the major minerals, are given. The texture of the whole rock indicates that an immiscible carbonate liquid was evolved at one stage in the crystallization of the silicate liquid, and that a third sulphide liquid may also have existed. During the period when the two or three liquids existed, some extremely complex epitaxial zonations were formed around oxide minerals which were crystallizing from the silicate liquid. These zonations show that there was a late-stage concentration of TiO 2 in the silicate liquid. The zonations have also been used to reconstruct the history of crystallization of the kimberlite.

D.B. Clarke

Papers

Mineralogy and petrology of the kimberlite from comerset island, N.W.T., Canada