Journal of Conference Abstracts

Evidence for Pyroclastic Activity from the Faroes-Greenland Region During the Paleocene

Julie C. Hardiman (jcha@wpo.nerc.ac.uk)

British Geological Survey, Nicker Hill, Keyworth, Notts. NG12 5GG.

Sequences of ash layers found in Upper Paleocene sediments in North Sea cores give evidence for 2 phases of pyroclastic activity associated with rifting within the Faroes-Greenland region. Whole-rock analysis, concentrating on the trace elements that remain apparently immobile, and the feldspar assemblage and mineral suite are used to identify the source of the eruptions and correlate the sequences. These suggest that all ashes are alkaline including alkali-basalt, hawaiite, mugearite, trachy-andesite and phonolites. Multi-element signatures of the majority of the alkali-basalt layers are comparable to basaltic lavas exposed onland in east and west Greenland and the Faroes Lower and Middle Basalt Series. Basalt discrimination diagrams suggest generation from either a within-plate or a plume-influenced MORB environment. All ashes are REE-enriched and the LREE enrichment and the proportion of evolved ashes decreases stratigraphically upwards through the sequences which is thought to be consistent with an increase in an asthenospheric relative to lithopheric component in the magmas as rifting progressed towards the opening of the North Atlantic ocean around the Paleocene-Eocene boundary.

With a source deduced to lie greater than 800 km from the North Sea region, the core sequences only contain the eruptions with this minimum downwind eruptive dispersal distance. The eruptive spacing of these large eruptions, inferred from average sedimentation rates within each sequence, is 3-10 ka. The largest ash layer thicknesses of 5 cm at this distance also imply an erupted volume of the order of 10-100 km³ magma which is consistent with generation from a divergent setting and suggests that pyroclastics were produced from the Faroes-Greenland eruption sites in equal volumes to the lavas exposed onland. The erupting centres are presumed to be fissure-fed central volcanoes.

Local Fluctuations in Feldspar Frameworks

Stuart A. Hayward¹ (sah21@esc.cam.ac.uk), Ekhard K. H. Salje^1,2 (es10002@esc.am.ac.uk) & Jutta Chrosch^1,2 (jc10023@esc.cam.ac.uk)

¹ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK.

² IRC in Superconductivity, Madingley Road, Cambridge CB3 0HE, UK.

At any set of thermodynamic conditions, a mineral will have some well defined equilibrium crystal structure. However, this structure can be locally disturbed by crystal defects, such as domain walls or solute atoms. This distorted structure will only affect a finite volume within the crystal, but the need to retain continuity within the crystal means that the relaxation cannot be totally sharp.

This means, for example, that the boundary between two twin domains will include a transition zone from one domain's crystal structure to that of the other domain. Thick twin domain walls can be studied quantitatively, by measuring the intensity of diffuse diffraction between pairs of twin-related Bragg peaks. In alkali feldspar (Or₃₀) at room temperature, these walls are approximately 25Å thick.

Similarly, a single solute atom in a mineral will only affect a small region within a crystal. As a result, chemical mixing will only occur in a substitutional solid solution once there is significant overlap between the strain fields around individual solute atoms. This causes the "plateau effect", where the properties of a phase transition are independent of composition. In alkalli feldspar, this plateau extends from albite to 2% Or, which corresponds to a strain field radius of 10Å.

These phenomena can be modelled using Ginzburg-Landau theory, which predicts that the range of these strain fields will increase as the temperature is raised to T_c. This has been confirmed by measuring the thickness of twin walls as a function of temperature.