Carmody, L; (2012) Geochemical characteristics of carbonatite-related volcanism and sub-volcanic metasomatism at Oldoinyo Lengai, Tanzania. Doctoral thesis , UCL (University College London).

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Abstract

The eruption of xenolithic material during large explosive eruptions, at any volcano, supply vital samples of the sub-surface lithologies upon which it is built, which in turn provides an indication of the evolution of the volcanic complex, in particular the volcanic conduit, magma storage zones and crustal / mantle lithologies. This is particularly important at alkaline-carbonatite complexes which are known to have “exotic” chemistries and also cause extensive zones of alteration through fenitisation processes. As the only active carbonatite volcano on Earth and also the unusual nature of Oldoinyo Lengai, Tanzania, it is an excellent study site to better understand the generation and chemical influence of carbonate-rich melts and fluids from source to surface. This study has attempted to better constrain the sub-volcanic environment, the source of the carbon within the material and the processes which lead to the formation of such unique rocks. Using geochemistry, isotopic studies and fluid inclusions, this thesis highlights the importance of fluid circulation within the volcanic system, both at the surface but also within the sub-volcanic mantle, leading to metasomatised material rich in carbon and alkali elements from which natrocarbonatite and potentially kimberlitic material could be derived. Almost all of the geochemical evidence and composition of fluid inclusions trapped within fenitised aureoles indicates a mantle derivation of carbonatitic material with isotopic signatures typical of the pre-defined “mantle-box”. The nature of the fluids is also investigated using trace element modelling and argued to be both carbonatitic and siliceous in origin, which have been circulating within the mantle beneath the Gregory Rift since before the establishment of Oldoinyo Lengai. These themes of research are discussed in terms of the genesis of natrocarbonatite, focussing upon the notion that it may be an evolutionary feature of Oldoinyo Lengai rather than a constant eruptive product.

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