The origin of ultrapotassic magmas

Alkaline mafic magmas, but more specifically the potassic and ultrapotassic varieties enriched with more potassium (K) than sodium (Na), are essential for our understanding planetary habitability, evolution, and volatile cycles. Despite this importance they have been neglected by the Earth Science community, relative to the more widespread basalts like those that erupt at mid-ocean ridges or ocean islands. The enigmatic nature of these melts has hindered their investigation in part due to analytical challenges such as their propensity to be volatile laden, but also because their genesis is wrapped in paradoxical processed. For example, it is well established that the bulk of the Earth's mantle is made of peridotite which is also known to be largely devoid of K. This alone would more or less eliminate the possibility of mantle--derived melting processes producing magmatic compositions that could crystallise at the surface to form potassic or ultrapotassic rocks. The widespread occurrence across all geodynamic settings of K--enriched rock types like lamproites, lamprophyres, leucitites, as well as basanites or shoshonitic basalts is in direct contradiction to this impossibility. Likewise the existence of this rocks, and the fact they represent a spectrum of K enrichments also suggests that for all of them to be derived from melting in the mantle they must be sampling source assemblages with different mineral components. Within the mantle geoscience community this is known as "Mantle Heterogeneity" and as a concept captures processes that produce magmas of all compositions, not just those that are potassic and ultrapotassic. Heterogeneity on this scale is not a widely accepted concept, and debate exists as to the existence and breadth of any non-peridotite assemblage within the mantle. A furtherance to this debate is the complexity added by isolating the type, style, and drivers of any process that could materialise significant changes to both the physical and chemical profiles of the mantle. These are the central questions that this doctoral thesis aims to approach. Through the use of these unique potassium-rich magmas and rocks that are believed to capture the first stages of mantle melting and sample these heterogeneous regions, this work will evaluate both natural and synthetic samples in the field and laboratory to isolate melting and enrichment processes. In doing so this work will also provide a deeper understanding of potassic rocks and their formation processes.