Siderophile element systematics and Hf-Os isotope signatures of carbonatites: Insights into the origin of Earth’s most unusual magmas

Abstract

Carbonatites are considered to be one of the most unusual magmas on Earth, but their source and genesis remain poorly understood and the subject of much debate. Chemical exchange with the mantle and/or crust, and re-equilibration during post-magmatic processes can lead to open system behaviour of carbonatites on a whole-rock scale, thereby hampering identification and characterisation of the carbonatitic source. To date, only a small number of studies have addressed carbonatite genesis using Lu-Hf or Re-Os isotope systems. In this thesis I present the first combined Lu-Hf, Sm-Nd, Rb-Sr and Re-Os isotope data together with highly siderophile element abundances to address carbonatite genesis, identify the source of carbonatites and to understand secular changes of the mantle they are sourced by throughout time.
Samples from ten different global localities were studied at the whole-rock, carbonate, non-carbonate, and mineral scale, varying in age from present day to ca. 3 Ga and covering four different carbonatite compositional types. Existing methods to successfully analyse the different isotope systems were assessed and improved in order to deal with the unusually Ca-rich matrix of carbonatites. These methods were then applied to three separate studies: 1) a global lithophile element isotope study, 2) a global whole-rock Re-Os isotope and HSE abundances study and 3) a combined lithophile and Re-Os isotope case study of the Fen complex in Norway. In all three projects, carbonatites are shown to record open system behaviour, resulting in implausible age corrected εHf values (<initial of Earth) for most carbonate fractions and many whole-rock samples, Lu-Hf isotopic disequilibrium between carbonate and non-carbonate fractions that yield isochron ages that do not match the published ages, as well as equally inapplicable 187Os/188Os ratios at time of eruption (<0 to 259).
The combined geochemical and petrographic data presented in this thesis demonstrate that: 1) carbonatites have experienced a complex petrological history influenced by post-magmatic processes; 2) results obtained on the whole-rock, carbonate and non-carbonate scale must be considered with great caution; 3) young carbonatites (e.g. Oldoinyo Lengai and Fogo) are less influenced by post-emplacement radiogenic ingrowth and can thus retain source information; and 4) the non-carbonate fractions of carbonatites are less affected by post-emplacement processes and for Hf and Nd isotopes record a close to chondritic uniform mantle source over time. Further understanding on the origin and petrogenetic evolution of carbonatites requires isotope investigation at the scale of individual minerals (e.g. calcite, dolomite, apatite, magnetite, phlogopite, BMS) together with a detailed geochemical and petrographic characterisation.