The main intentions of this research are to identify and characterise enriched components in the sub-Icelandic mantle and to determine their origins and relationships with other mantle components. Helium, strontium, neodymium and lead isotope data, in conjunction with major and trace element data, allow the characterisation of enriched mantle sources in the flank-zones. Postglacial olivine- and pyroxene-bearing basalt samples have been collected from each of three flank-zones. Pyroxene <sup>3</sup>He/<sup>4</sup>He may have been affected by pre- or post-eruptive addition of radiogenic <sup>4</sup>He, but olivine <sup>3</sup>He/<sup>4</sup>He represent the time-integrated <sup>3</sup>He/(U+Th) of the mantle source. Published oxygen isotope data indicate that the flank-zone basalts have not been affected by crustal contamination. Coherent trends in He-Sr-Nd-Pb isotope co-variation diagrams allow the identification of three distinct enriched components in the North Atlantic mantle. Each enriched component is characterised by lower <sup>3</sup>He/<sup>4</sup>He (< 6 R<sub>a</sub>)<sub> </sub>and <sup>143</sup>Nd<sup>/ 144</sup>Nd<sup> </sup>and more radiogenic Pb and Sr isotope ratios than the depleted mantle components sampled in the rift-zones and along the MAR. One enriched component, EI<sub>l</sub>, is most prevalent in the mantle beneath the SFZ. It is distinguished from the other enriched components by its extreme Pb isotope composition (<sup>206</sup>Pb/<sup>204</sup>Pb<sup> </sup>> 19.4). It is probably derived from recycled oceanic crust (a young-HIMU-like component). The second enriched component, EI<sub>2, </sub>is only sampled at Öræfajökull (EFZ), and is distinguished by its extreme <sup>87</sup>Sr/<sup>86</sup>Sr (> 0.7037) and positive D7/4. Its isotopic composition most likely has its origins in recycled pelagic or terrigenous sediments. The third enriched component, E13, dominates the Jan Mayen (North Atlantic) and Snæfell (EFZ) mantle sources. It has similar Sr, Nd and He isotopic compositions to EI<sub>l</sub>, but is characterised by less-radiogenic Pb isotope ratios ( <sup>206</sup>Pb/<sup>204</sup>Pb<sup> ~ </sup>18.6). It is also most likely derived from young recycled oceanic crust. Enriched mantle beneath Snæfellsnes appears to be derived from mixing of EI<sub>l</sub> and EI<sub>3</sub>. Two depleted components (DI<sub>1</sub> and DI<sub>2</sub>) are required to explain negative trends in He-Sr, He-Nd or He-Pb co-variation diagrams. They have similar Sr, N d and Pb isotopic compositions to each other, but can be distinguished on the basis of <sup>3</sup>He/<sup>4</sup>He. DI<sub>1</sub> is characterised by <sup>3</sup>He/<sup>4</sup>He ~45 R<sub>a </sub>and represents the depleted Iceland plume component. DI<sub>2</sub> is characterised by lower <sup>3</sup>He/<sup>4</sup>He (7-9 <i>R</i><sub>a</sub>)<sub> </sub>and represents the depleted component in the N-MORB mantle. Only a very small contribution of DI<sub>1</sub> can change the <sup>3</sup>He/<sup>4</sup>He of a source without affecting its Sr; Nd or Pb isotopic composition. A mixing model is presented which enables plume-derived enriched components to be distinguished from those present in the shallow asthenospheric mantle. Only EI<sub>l</sub> appears to be intrinsic to the plume, whereas EI<sub>2</sub> and EI<sub>3 </sub>are upper-mantle heterogeneities.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:663806 |
| Date | January 2005 |
| Creators | Williams, Alice Josiane |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/14667 |
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