A helium, oxygen and rhenium-osmium isotope study of some intraplate magmatism

• Main Author: Day, James Martin Dines
• Format: Thesis
• Language: unknown
• Published: 2004
• Subjects: Iceland; muskox; Ocean Island
• Online Access: http://etheses.dur.ac.uk/3670/; http://etheses.dur.ac.uk/3670/1/3670_1231.pdf

Intraplate magmatism provides important constraints on the evolution, dynamics and composition of the Earth's mantle. Uncertainties remain, however, in understanding the mantle sources related to intraplate magmatism. For example, the influence or existence of mantle stratification, core-mantle interaction and recycling of subducted components are poorly understood. The extent to which geochemical signatures of intraplate magmas are modified during melt transportation, eruption and emplacement also requires better definition. This thesis employs He, O and Re-Os isotope systematics on selected volcanic and intrusive rocks from a number of oceanic and continental intraplate settings to address some of these key issues. New constraints on the effect of syn-eruptive degassing and post-emplacement mobility processes on intraplate magmas are presented. Ocean Island Basalts (OIB) from the Western Canary Islands and Coppermine Continental Flood Basalts (CFB) suggest that Re degassing for both modem and ancient alkaline and tholeiitic sub-aerially empted lavas are similar in extent. Crustal contamination is shown to explain some low δ(^18)O (≤5%(_0) in central Icelandic lavas and low (^3)He/'(^4)He (<8R(_A)) in Western Canary Island and central Iceland lavas. Study of the Mackenzie large igneous province (LIP) also reveals that relatively limited (2-4%) crustal contamination can generate large (^187)Os/(^188)Os isotopic variation (≥80 γOs units) in high Os concentration cumulate rocks. This finding provides direct support for the role of crustal contamination in the formation of stratiform chromitite reefs in the Muskox Intrusion. By inference, contamination by ancient (≥0.5Ga) continental crust might also explain coupled (^186)Os-(^187)Os enrichments in some continental intraplate magmas. He isotope ratios of intraplate magmas have been used to infer mantle stratification with both shallow and deep mande origins proposed for a long-lived, primordial, high (^3)He/(^4)He reservoir. In this work the continental ...