International Geologiical Congress - Oslo 2008

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MPM-10 Fluids and melts in the Earth?s mantle: From natural observation to HT-HP experiment

 

The complexities of accessory minerals in the Catoca kimberlite as revealed by a LA-ICP-MS investigation

 

Sandra Robles, Universitat de Barcelona (Spain)
Manuel Watangua, Endiama (Angola)
Leonardo Isidoro, Endiama (Angola)
Antonio Olimpio, Universidade Agostinho Neto (Angola)
Salvador Galí, Universitat de Barcelona (Spain)
Joan Melgarejo, Universitat de Barcelona (Spain)
 

 

The Catoca kimberlite, Lunda Sul province, is the most important primary diamond deposit in Angola. This kimberlite is hosted by Precambrian rocks and covered by Mesozoic sedimentary deposits. The structure of the Catoca kimberlite is well preserved, and a set of core sampling down to 600 m allowed us to identify complete crater and diatreme facies. Diverse xenoliths, comprising lherzolite, eclogite, harzburgite, carbonatite, gneiss, amphibolite, shales and sandstones are distributed through the kimberlite. Accessory minerals and xenocrysts comprise garnet, zircon, Cr-rich diopside, amphibole, phlogopite, chromite and several generations of ilmenite. Secondary minerals include serpentine-group minerals, heazlewoodite, calcite, barite, barytocalcite, witherite and strontianite. Textural evidence indicates different a complex history of growth in the xenocrysts. The aim of this study is to asess the chemical differences among different populations of zircon and ilmenite, based on petrographic study, SEM-BSE images, and electron-microprobe and LA-ICP-MS analyses.
Zircon xenocrysts are partially replaced by fine-grained baddeleyite, and at least two populations exist according to the trace element distribution. All of these crystals are enriched in HREE, but with a noticeable positive Ce anomaly, similar to that reported in zircon in MARID xenoliths and in kimberlite. The crystals are not optically zoned, but there is a slight depletion in REE from the core to the rim. Up to six petrographic types of ilmenite can be distinguished in Catoca. Unaltered megacrystic ilmenite (ilmenite I) is rich in Fe3+, indicating a crystallization under high fO2 conditions; this ilmenite contains Nb, and Cr, Ni and Ta in low contents. Its composition is similar to intercumulus ilmenite crystals that occur in the peridotite xenoliths (ilmenite II).
Ilmenite I is replaced along discontinuities (grain borders, cracks) by magnesian ilmenite (ilmenite III); the elemental distribution of Mg in these grains points to processes of replacement through solid-state diffusion in a typical reducing environment. A late generation of Mn-rich ilmenite (ilmenite IV) is found rimming all the above mentioned generations, and it is strongly enriched in Nb, Ta, Zr, W, Hf, Th and U, and poor in Mg and Fe3+. The composition of this is similar to that of the fine-grained euhedral ilmenite crystals found in the kimberlitic matrix (ilmenite V) and to that of the ilmenite crystals found in the carbonatite xenoliths (ilmenite VI). Crystals of Mn-rich ilmenite are not replaced or zoned.
Both the late generations of ilmenite and the baddeleyite replacing zircon can be produced by interaction of a carbonate-bearing kimberlitic magma enriched in Mn and HFSE. The replacement of Fe3+-rich ilmenite by Mg- and Mn-rich ilmenite implies that the early ilmenite was formed under oxidizing conditions in the mantle, and the last compositions of ilmenite were produced by reaction with the kimberlitic magma.

 

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