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    Age and Petrogenesis of Kimberlites and Related Rocks from Finland
    Dalton, Hayden Boyd ( 2022)
    Kimberlites are rare, small volume ultramafic igneous rocks found on every continent on Earth, with eruption ages spanning almost 3 billion years. These rocks are derived from the deepest magmas that reach Earth’s surface (>150-200 km) and provide unique insights into the nature of the convecting mantle. In addition, their cargo of entrained mantle xenocrysts (including diamonds) and xenoliths permit examination of the subcontinental lithospheric mantle (SCLM), while ‘deep’ diamonds give insights into the composition of the underlying asthenospheric mantle. Despite their significant scientific and economic importance, and decades of research, particularly in regions of southern Africa, North America and Siberia, questions remain as to the petrogenesis of kimberlites. Contention persists around the depth of origin of kimberlites, melting trigger(s) including tectonic settings, and the composition and evolution of kimberlite melts during their ascent. To provide new insights on these issues, this study presents a comprehensive petrographic, geochemical and geochronological investigation on samples from three occurrences of kimberlite and related magmatism in Finland, comprising the Lentiira-Kuhmo cluster of olivine lamproites, Kuusamo cluster of kimberlites and ultramafic lamprophyres (UMLs) and the Kaavi-Kuopio kimberlites. Finland represents an optimal location for testing various petrogenetic models, particularly regarding the links to geodynamic processes as the tectonic evolution of the Baltic Shield and its role in supercontinent cycles are well constrained. This work presents the first petrological account of the Kuusamo kimberlites, revealing that they represent highly differentiated magmas with scarce olivine macrocrysts and other mantle-derived xenocrysts. These characteristics contrast with the neighbouring Kaavi-Kuopio kimberlites, which are inferred to have crystallised from less differentiated magmas that were modified by mantle assimilation, as evidenced by correlations between the Mg# of xenocrystic (mantle-derived) olivine cores and the composition of magmatic olivine rims, spinel, and groundmass modal mineralogy. New radiometric ages show that at least ~100 Myr separates the emplacement of the Kuusamo kimberlites (~735-750 Ma) from those at Kaavi-Kuopio (~625-585 Ma). These new age data also indicate temporal overlap between ultramafic lamprophyre magmatism at Kuusamo and the eruption of olivine lamproites at Lentiira-Kuhmo (~1180-1220 Ma), some 100 km to the northeast. As part of this geochronological investigation, the robustness of Rb-Sr phlogopite, U/Pb perovskite and 40Ar/39Ar phlogopite dating methods were evaluated by applying multiple geochronometers to individual intrusions. It is evident that each radiometric system can yield both precise and accurate emplacement ages, with important caveats regarding best practice and interpretation. Radiogenic isotope data (Sr-Nd-Hf) indicates that the olivine lamproites and UMLs were contemporaneous, but have distinct source compositions. The highly unradiogenic Nd-Hf isotope compositions of the former are consistent with derivation from the metasomatised SCLM whereas the UML compositions suggest they were sourced from predominantly asthenospheric melts that were modified by (up to 15%) incorporation of enriched SCLM components. The Mesoproterozoic timing of their emplacement suggests that eruption of the olivine lamproites and UMLs was facilitated by the extensional regime associated with the separation of Baltica from Laurentia. The Kuusamo and Kaavi-Kuopio kimberlites were also emplaced at a time of supercontinent disruption. The Kuusamo eruptions occurred as the break-up of Rodinia was initiated, while the Kaavi-Kuopio rocks were emplaced as Rodinia break-up was completed, contemporaneous with the formation of the Central Iapetus large igneous province. In keeping with their petrographic disparities, the Sr-Nd-Hf isotopic composition of these kimberlites indicates that they were sourced from distinct source regions in the convective mantle. The homogenous composition of the Kuusamo rocks overlaps the prominent PREMA-like signature of kimberlites globally, whereas the Kaavi-Kuopio samples exhibit an extreme range in Hf isotope compositions with a temporal trend from PREMA-like towards lower epsilon Hf(i) values in younger kimberlites. Isotopic modelling suggests that this temporal enrichment of the kimberlite source region was due to increasing entrainment (of up to 10%) of subducted material. These findings are consistent with mounting evidence for subducted material being an important source ‘pollutant’ for kimberlites globally and a petrogenetic link with supercontinent cycles and/or the large mantle plumes that initiate supercontinent disintegration.