Journal of Conference Abstracts

Constraints on the Nature of Palaeocene Iceland Plume-Related Magmatism: Geochemical Evidence from the Mafic Dykes of the Ammassalik Region, SE Greenland

S. J. Brookes (brookess@geol.port.ac.uk), R. P. Hall (hallp@geol.port.ac.uk) & D. J. Hughes (hughesd@geol.port.ac.uk)

Department of Geology, University of Portsmouth, Portsmouth PO1 3QL.

Mantle melting in the ancestral Iceland plume produced thick seaward-dipping reflector sequences (SDRS), continental flood basalts (CFBs) and abundant mafic intrusives along the North Atlantic margins. This activity appears to have been centred around Kangerlussuaq on the SE Greenland coast where there are thick sequences of basalts and numerous coast parallel dyke swarms. In the Ammassalik region of SE Greenland, about 300 km south of Kangerlussuaq, the only onshore evidence for Palaeocene igneous activity are tholeiitic dolerite dykes, although thick SDRS's were sampled offshore during ODP leg 152. The dykes are normative olivine tholeiites and can be divided into two main groups, High-Ti and Low-Ti, on the basis of differences in their petrography and major trace element geochemistry. The High-Ti group has an average TiO₂ content of c. 3.4 wt% and the Low-Ti group has an average of c. 1.7 wt% TiO₂ for similar levels of MgO. Trace element differences further discriminate both the High-Ti and Low-Ti groups into 4 subgroups.

The range of geochemical signatures observed within continental flood basalts is known to be the result of the variable mixing of melts derived from several sources, including plume, subcontinental lithospheric mantle and crustal components. The compositions of the High-Ti and Low-Ti dykes are compared with possible end-member source contributors, manifest by Icelandic basalts and estimated crustal and mantle lithospheric components, in an attempt to constrain the input from different sources of the Ammassalik dykes. The High-Ti(b) group dykes show strong geochemical affinities with enriched plume-related Icelandic basalts, whereas the Low-Ti(b) dykes are more representative of depleted Iceland plume melts. The compositions of the Low-Ti(a) and High-Ti(a) dykes suggest the modification of these plume-derived magmas by interaction with continental lithosphere.

Quantifying the Effect of a Solidifying Crust on Lava Flow Morphology: Experimental Data

B. C. Bruno^1,2 (bruno@oce.orst.edu) & S. Blake¹ (s.blake@open.ac.uk)

¹ Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK.

² College of Oceanic and Atmospheric Science, Oregon State University, Corvallis, Oregon 97331, USA.

Recent studies by Bruno et al. (1992, 1994) have shown that lava flow margins are fractal, and their shapes can be quantified by a parameter called the fractal dimension. The present work investigates the effect of a solidifying crust on lava flow morphology and explores the use of fractal parameters in quantifying this physical effect. We conducted a series of experiments at The Open University, England, to simulate lava flow emplacement using polyethylene glycol wax (PEG 600). Liquid PEG was pumped into the base of a tank filled with cold water at various wax temperatures and flow rates. Based on earlier work by Fink and Griffiths (1990), we calculate two dimensionless parameters: ½ (a modified Peclet number) and tau (a measure of the time required for a crust to form). In our experiments, ½ was largely controlled by flow rate and tau was largely controlled by wax temperature. As expected, a surface crust formed most readily in experiments characterized by low values of ½ (i.e., low flow rates) and tau (i.e., low wax temperatures).

We videotaped each experiment so as to quantify the temporal evolution of the flow margin shape using the fractal dimension (D). For the final, steady-state shape, we found differing experimental conditions resulted in systematically different D, covering the range 1.05 < D < 1.19, similar to the volcanic range. High flow rates (high ½) and high wax temperatures (high tau) both served to lower D in such a systematic way that D could be effectively used to divide ½ - tau space. This implies that D is an effective measure of the relative importance of crust in controlling flow morphology and that field measurements of D may give insight into the physical conditions under which lavas were emplaced.