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Journal of Petrology Volume 41 Number 7 Pages 919-932 2000
© Oxford University Press 2000
The Sub-lithospheric Source of North Atlantic Basalts: Evidence for, and Significance of, a Common End-member
SCOTTISH UNIVERSITIES ENVIRONMENTAL RESEARCH CENTRE, SURRC, EAST KILBRIDE G75 0QF, UK
Palaeogene basalts from the margins of the North Atlantic often show geochemical variations that are consistent with their parental magmas having interacted with the lithosphere en route to the Earth’s surface. These geochemical trends vary depending on the nature of the local lithospheric contaminants. Using examples from the British Tertiary Igneous Province and SE Greenland, we construct coherent contamination trends, which converge on a restricted Pb isotope composition, apparently indicating a common uncontaminated asthenospheric mantle component. Significantly, this composition is also suitable as one end-member of the Pb isotope arrays recorded in Recent Icelandic basalts. We conclude that this composition has been a persistent component of the Iceland plume over 60 my, dominating the mantle contribution to the Palaeocene phase of flood basalt magmatism but constituting only one end-member on Iceland. The Pb isotope composition of this ‘North Atlantic end-member’ is consistent with, but not necessarily demanding of, a primordial source. Recent evidence suggesting a lower-mantle origin for mantle plumes encourages investigation of whether the geochemical evidence supports that hypothesis. Helium isotope data from Palaeogene North Atlantic basalts support a lower-mantle contribution. However, mixing models suggest that it is unlikely that the lower-mantle contribution is large enough to dominate the Sr–Nd–Pb isotope compositions and lithophile trace element signatures of any plume-derived basalts.
KEY WORDS: North Atlantic; Iceland; lower mantle; mantle plumes; flood basalts; isotopes
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