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

Abstract

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 lithospheric mantle, especially Archaean peridotite and eclogite, are shown to possess low (^3)He/(^4)He (<4R(_A)). This observation, combined with low (^3)He/(^4)He (6±1R(_A)) of continental intraplate alkaline volcanics (CIAV), derived from the lithosphere-asthenosphere boundary, provides evidence that a long-lived primordial He component does not exist in the shallow upper mantle. Low δ(^18)O (< 0.9%(_0)) and high (^187)Os/(^188)Os (γOs = +8 to +37) for high Os concentration (> 50 ppt) Western Canary OIB indicate that recycled oceanic crust forms part of the mantle source for these basalts, a theme also common to low δ(^18)O measured in central Icelandic glasses with (^3)He/(^4)He ratios similar to those of mid ocean ridge basalts. Decoupling of lithophile (Sr, Nd, Pb) and siderophile (Os) isotope tracers from He in both settings provides evidence for (^3)He-recharge with (^3)He/(^4)He up to 11.8R(_A) for the Western Canary Islands and 34.3R(_A) for central Icelandic lavas. There is evidence in Western Canary Island lavas, and in OIB globally, that a common or 'universal' mantle component is present with a near-chondritic (^187)Os/(^188)Os (≥0.127), mantle-like δ(^18)O (~5.2%(_0)) and elevated (^206)Pb/(^204)Pb (≥19.1) with respect to the depleted mantle; the so-called FOZO (Focus Zone), PHEM (Primary He mantle), or C (Common) components. Coppermine Re-Os isochron ages (1284 ± 13 Ma) are within error of the accepted 1270 ± 4 Ma U-Pb age for the Muskox Intrusion. γOs initials derived from both the Coppermine CFB (+2.0) and Muskox Intmsion (+1.3) lie on an enstatite or ordinary chondritic Os isotopic evolution curve for the Earth's mantle indicating derivation from a source similar to the universal component recognised in modem-day OIB. These results can be interpreted in the context of a veined or banded peridotite-pyroxenite mantle. Small amounts of partial melting of a heterogeneous mantle source leads to great isotopic variability whilst larger degrees of partial melting will lead to more homogeneous mantle-derived compositions. Pyroxenite sources might also explain the correlated isotopic and elemental compositions, including the possibility of (^186)Os-(^187)Os enrichment in OIB through high Re/Os and Pt/Os ratios. Ultimately, a universal mantle component appears to be present in all intraplate magmatism. This universal component is not always related to high (^3)He/(^4)He and appears to provide an endmember component to all intraplate mantle melting signatures from the Archaean to the present-day.