Transport of elements in the lithosphere

<p dir="ltr">Understanding how elements move through the lithosphere is essential for addressing a wide range of geological and geochemical questions. These include the application of elements as geochemical tracers, the formation and enrichment of ore deposits, and the mechanisms by which elements are transported.</p><p dir="ltr">This thesis encompasses three projects aimed at investigating: the geochemical behaviour of the critical element germanium (Ge) and its potential as a geochemical tracer; the genesis and timing of ultramafic alkaline massifs of the Aldan Shield, Siberia; and the variable transport of elements in the mantle via aqueous fluids.</p><p dir="ltr">The first project involved high-pressure piston-cylinder experiments conducted at 2.5 GPa across a range of temperatures and compositions to systematically assess the partitioning behaviour of Ge in mantle and crustal minerals and melts. Results revealed that Ge exhibits more complex geochemical behaviour than previously recognized. Contrary to earlier assumptions, Ge is not strongly partitioned into micas and amphiboles, and it displays a variable compatibility in clinopyroxene. Further investigation indicated that these observations can be explained by partial substitution into different crystal lattice sites, as well as by the effects of temperature and melt composition. These findings refine the use of Ge as a tracer for magma processes.</p><p dir="ltr">The second project focused on detailed petrological, geochronological, and geochemical analysis of two platinum-bearing ultramafic alkaline massifs in the Aldan Shield: the previously understudied Bilibin massif and the better-known Inagli massif. New Rb-Sr isotopic dating confirmed a Mesozoic age for the Bilibin massif. Petrological and geochemical evidence supports a model in which these massifs, along with others in the region, formed through fractional crystallization of lamproitic magmas derived from a stagnant subducted slab.</p><p dir="ltr">The final project examined the transport mechanisms of elements in the mantle, focusing on the role of aqueous fluids. Piston-cylinder experiments using mantle assemblages and fluids of variable composition were carried out, and trace elements within the fluids were analysed in situ using a novel cryo-ablation technique. The results show that pyroxenite assemblages facilitate more efficient transport of key elements than peridotite assemblages. Additionally, the findings suggest that fluids at greater mantle depths have a higher capacity to mobilize elements, implying the existence of a depth threshold below which element transport is significantly enhanced, and above which elements may become effectively ‘frozen’ in the sub continental lithospheric mantle (SCLM).</p><p dir="ltr">In summary, this thesis refines the geochemical application of germanium as a tracer, proposes a new genetic model for ultramafic alkaline massifs in the Aldan Shield, and demonstrates the critical role of aqueous fluids in mediating element transport within the lithosphere.</p>