Carbon, hydrogen and nitrogen contents of the Earth's mantle are poorly constrained. Mantle xenoliths brought to the surface by alkaline lavas at 6 volcanic centers along the northwestern margin of North America provide a way to estimate the contents of these volatiles in the upper mantle. Carbon (C), hydrogen (H$ sb2$) and nitrogen (N$ sb2$) are contained as several C-H-N phases in these xenoliths and lavas; light isotopic C-H-N, C in carbonates, C and N$ sb2$ in fluid inclusions, and dissolved C, H$ sb2$ and N$ sb2$ in anhydrous and hydrous minerals. The light isotopic C-H-N phase, which represents 30 to 95 % of the volatiles extracted from these xenoliths by combustion, contributes 10 to 250 ppm C and 10 to 100 ppm H$ sb2$, with $ delta sp{13}$C and $ delta$D values ranging from $-$31 to $-$23$ perthous$, and $-$150 to $-$100$ perthous$, respectively. This phase is composed of C-H-N hydrocarbon molecules, which are thought to occur along grain boundaries or fractures. Calcite contributes between 1 to 24 ppm C in the temperature range 600-800$ sp circ$C but reaches up to 240 ppm in host volcanic rocks. / Carbon of mantle origin is present as a supercritical CO$ sb2$-rich fluid contained in inclusions within the minerals of the xenoliths. Maximum pressures of entrapment are estimated from fluid densities in type I "primary" inclusions to range from 11 to 14 kb, in agreement with the spinel lherzolite stability field of these host xenoliths. / The hydrogen content extracted at temperatures $>$600$ sp circ$C from anhydrous spinel lherzolite and harzburgite xenoliths ranges from 3 to 15 ppm H$ sb2$, and from 10 to 25 ppm H$ sb2$ in anhydrous pyroxenites. The H$ sb2$ content extracted at temperatures $>$600$ sp circ$C from xenoliths containing hydrous minerals varies from 18 to 345 ppm H$ sb2$. Nitrogen reaches 8 ppm in the xenolith most enriched in fluid inclusions and 58 ppm in phlogopites from an amphibole clinopyroxenite. / Partial melting calculations indicate that the C, H$ sb2$ and N$ sb2$ contents of primary MORB magmas derived from a mantle source similar to the anhydrous spinel lherzolite of this study would contain 4-145 ppm C, 20-215 ppm H$ sb2$ and 0.2-7.0 ppm N$ sb2$. / The isotopic signature ranges of C, H$ sb2$ and N$ sb2$ in the upper mantle beneath the northwestern margin of North America are estimated to be; $-$10 to $-$4$ perthous$, $-$95 to $-$45$ perthous$, and $-$5 to +1.5$ perthous$ respectively, and to contain between 3 and 7 ppm C and 7 to 30 ppm H$ sb2$ with an average content of 4 ppm C and 8 ppm H$ sb2$. If these latter values are representative of the Earth's mantle as a whole, then the Earth is significantly more degassed than previously estimated. (Abstract shortened by UMI.)
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.74645 |
| Date | January 1991 |
| Creators | Nadeau, Serge |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Geological Sciences.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001237172, proquestno: AAINN67709, Theses scanned by UMI/ProQuest. |
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