The anatomy of the ancestral Iceland plume : a chemical and isotopic study of the tertiary basalts and picrites from Baffin Island

• Main Author: Lass-Evans, Solveigh
• Published: University of Edinburgh 2004
• Subjects: 551.9
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.653690

This thesis presents a detailed chemical and isotopic study of picritic and basaltic lava flows and dykes from Baffin Island, the most westerly and least studied part of the North Atlantic Igneous Province (NAIP). Most of the Baffin Island olivine phenocrysts are not in equilibrium with their host-rock compositions (MgO = 8-28 wt. %), and lava flows with > 14 wt.% MgO contain accumulated olivines. The wide range of calculated liquid compositions (9-20 wt. % MgO) indicates a complex magmatic system, in which several batches of magmas have been mixed and of which only a small proportion has reached the surface. The Baffin Island rocks were generated over a wide range of depths with estimated liquidus temperatures of 1211-1430°C and segregation pressures of 1.7-3.1 Gpa. trace element and isotope ratios show that two distinct mantle sources were available: (1) a relatively depleted source with (La/Sm)_n <1, Nb/Zr<0.04, negative ΔNb, ⁸⁷Sr/⁸⁶sr: 0.7030-0.7034 and ¹⁴³Nd/¹⁴⁴Nd ~0.5130m, and (2) a relatively enriched source with (La/Sm)_n >1, positive ΔNb, ⁸⁷Sr/⁸⁶Sr: 0.7039-0.7042 and ¹⁴³Nd/¹⁴⁴Nd: 0.51280-0.51284. with the exception of Pb-isotope ratios, which show evidence for some interaction with Precambrian basement rocks, isotope and trace element ratios are not significantly affected by crustal material. Therefore, the high-⁸⁷Sr/⁸⁶Sr, low-⁴¹Nd/¹⁴⁴Nd samples represent the most enriched endmember of the ancestral Iceland plume which was present at ca. 61 Ma but not evident today. The highest ³He/⁴He (up to 50 <i>R</i>_a) yet recorded from any terrestrial rocks are found in the low-⁸⁷Sr/⁸⁶Sr, high-⁴³Nd/¹⁴⁴Nd rocks. Mixing, at the 670-km thermal boundary, of depleted mantle material with primordial helium form the lower mantle may account for the association of high ³He/⁴He with depleted Sr- and Nd-isotope ratios.