The Sub-lithospheric Source of North Atlantic Basalts: Evidence for, and Significance of, a Common End-member

Palaeogene basalts from the margins of the North Atlantic oftenshow geochemical variations that are consistent with their parentalmagmas having interacted with the lithosphere en route to theEarth’s surface. These geochemical trends vary dependingon the nature of the local lithospheric contaminants. Usingexamples from the British Tertiary Igneous Province and SE Greenland,we construct coherent contamination trends, which converge ona restricted Pb isotope composition, apparently indicating acommon uncontaminated asthenospheric mantle component. Significantly,this composition is also suitable as one end-member of the Pbisotope arrays recorded in Recent Icelandic basalts. We concludethat this composition has been a persistent component of theIceland plume over 60 my, dominating the mantle contributionto the Palaeocene phase of flood basalt magmatism but constitutingonly one end-member on Iceland. The Pb isotope composition ofthis ‘North Atlantic end-member’ is consistent with,but not necessarily demanding of, a primordial source. Recentevidence suggesting a lower-mantle origin for mantle plumesencourages investigation of whether the geochemical evidencesupports that hypothesis. Helium isotope data from PalaeogeneNorth Atlantic basalts support a lower-mantle contribution.However, mixing models suggest that it is unlikely that thelower-mantle contribution is large enough to dominate the Sr–Nd–Pbisotope compositions and lithophile trace element signaturesof any plume-derived basalts. KEY WORDS: North Atlantic; Iceland; lower mantle; mantle plumes; flood basalts; isotopes     

Host rock dolomitization and secondary porosity development in the Upper Devonian Cairn Formation of the Fairholme carbonate complex (South‐west Alberta,Canadian Rockies): diagenesis and geochemical modelling

The Upper Devonian carbonate reefs in West‐central Alberta are important petroleum reservoirs that are well‐known for their extensive secondary porosity. An outcrop analogue study indicates that an early matrix‐selective dolomitization event occurred which is characterized by a major Late Devonian sea water component with increased salinity because of evaporation. It is interpreted that the matrix (replacive) dolomite formed during the Famennian as the result of a combination of both seepage and latent reflux dolomitization, although an additional type or overprinting of later intermediate burial dolomitization cannot be excluded. Formation of the moulds is attributed mainly to the dissolution of undolomitized fossil cores, most typically stromatoporoids. Geochemical modelling indicates that carboxylic acid fluids have the highest potential for dissolving residual calcite in this case. Geochemical models consistent with this analysis and interpretations can reproduce the secondary porosity and suggest a viable dolomitization process for the localities studied.