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Title: New insights into deep mantle melts: the carbonatite-melilitite connection
Author: McMahon , Sorcha Ciara
ISNI:       0000 0004 5915 3765
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2014
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The study of carbonatites and their genesis is an active area of research, with significant implications for carbon in the mantle and the global carbon cycle. In this thesis, possible theories of carbonatite petrogenesis are investigated using two main approaches. Firstly, natural examples of extrusive calciocarbonatites in the Calatrava Volcanic Province (CVP), Spain, are explored. Secondly, experiments at mantle conditions in piston cylinder apparatus are used to reproduce characteristic textures and compositions observed at the CVP, and examine the melting behaviour of potassic carbonated lherzolite in the mantle. The extrusive pyroclastic carbonatite deposits at the CVP can potentially provideĀ· insights into deep mantle melts. Carbonate-rich volcanism is prevalent, and closely associated with silica-undersaturated magmas such as melilitites, nephelinites and leucitites. Rounded calcium carbonate globules within silicate melt fragments in carbonate tuffs are described and analysed in detail in this study. A variety of methods are used to characterise the textures and geochemistry of the carbonate globules, to investigate whether they represent quenched immiscible carbonatite liquids or solid calcite crystals with a rounded shape. Results from trace element partitioning studies of the natural samples, and experiments based on the Calatrava compositions, suggest that the carbonate globules are rounded carbonate crystals, and do not support an immiscible origin in their formation. In an effort to understand carbonatite melting at a more general scale, experimental results in the simple synthetic system, CaO-MgO-Ah03-Si02-K20-C02, at 30 kbar are presented. A low variance approach is utilised to maximise the proportion of melt to aid in identification and analysis of small-degree partial melts. Potassium-rich carbonatite melt is found to be in equilibrium with phases typical of a carbonated lherzolite assemblage over a large temperature interval. Melt composition changes very little over the temperature range, corresponding to a region of highly potassic (rv20 wt.% K20) carbonatite metasomatism at mantle depths.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available