Hydrogen Contents of the Lithospheric Mantle Beneath the Canadian Cordillera and Slave Craton



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Trace levels of hydrogen in mantle minerals can significantly influence the physical properties of the mantle, and identifying the processes that affect hydrogen incorporation in the main minerals of the lithospheric mantle is vital to our understanding of geodynamics. In particular, results from this dissertation are used to investigate the role of hydrogen in the longevity of ancient cratonic lithosphere and how melting, metasomatism and oxygen fugacity conditions affect hydrogen in the lithosphere. This is accomplished by measuring hydrogen contents, as well as major and trace elements contents of peridotite minerals from mantle xenoliths derived from the lithosphere beneath Alligator Lake in the northern Canadian Cordillera and three locales from the Slave craton. The Alligator Lake lithosphere has a heterogeneous composition resulting from modification by fluids derived from a subducting slab, followed by melting and metasomatism by carbonatite-like melts initiated by the opening of a slab window. Not only is subduction lithosphere not more hydrous than lithosphere in other tectonic settings, but it is also highly heterogeneous in hydrogen content locally. Moreover, in Alligator Lake, oxidation is not correlated to subduction metasomatism. The Slave cratonic lithosphere is chemically stratified vertically, and the hydrogen contents of its peridotite minerals replicate this stratification. Xenoliths from the Slave lithosphere provide evidence of localized metasomatism unique to each region (Jericho in the north, Lac de Gras in the center and Gahcho Kue in the southeast) and each chemical stratum at depth. These metasomatic processes resulted in addition and/or removal of hydrogen in the lithosphere, including by silicate melts, hydrous fluids, kimberlite-like melts and carbonatite-like melts. High hydrogen contents in olivine near the base of cratonic lithosphere resulting from metasomatism by H-bearing melts and fluids, as observed beneath the Slave craton, do not necessarily result in cratonic root delamination. Hydrogen addition caused by metasomatism might be localized in some lithospheres, and kimberlites that entrain mantle xenoliths follow conduits of weakness through the lithosphere, preferentially sampling the most metasomatized parts of the lithosphere. Therefore, the highest hydrogen contents recorded in mantle xenoliths might not be representative of the overall lithosphere.



Hydrogen, Water, Mantle, Lithosphere, Craton, Subduction, Mantle wedge, Melting, Metasomatism, Craton stability, Oxygen fugacity, Oxidation


Portions of this document appear in: Kilgore, McKensie L., Anne H. Peslier, Alan D. Brandon, and William M. Lamb. "Water and Oxygen Fugacity in the Lithospheric Mantle Wedge beneath the Northern Canadian Cordillera (Alligator Lake)." Geochemistry, Geophysics, Geosystems 19, no. 10 (2018): 3844-3869.