Journal of Conference Abstracts

The Dissolution Mechanisms of Silicate and Oxide Minerals

D. G. Fraser¹ (thermo@vax.ox.ac.uk), A. J. Berry¹, K. Refson¹ & R. Wogelius²

¹ Dept. of Earth Sciences, Oxford, University, Parks Road, Oxford OX1 3PR.

² Dept. of Geology, The University, Oxford Road, Manchester M13 9PL.

An important step in many geochemical reactions involves the detachment of ions from the mineral surface and their diffusion through the near-surface region. The initial detachment step may be rate-determining and involves formation of an ordered surface-solution complex followed by a combination of proton-cation exchange and hydrolysis of the siloxane framework. The details of these processes are being studied by combining:-

1. Careful measurements of dissolution rates from well-orientated single crystal faces;

2. Ab initio quantum mechanical calculations of the energetics of different postulated reactions of such faces with solution;

3. Surface-specific measurements of the structure and coordination environment of ions on the mineral surface in the near surface region of the crystal and in the aqueous solution boundary layer;

4. Measurements of the structure of the surface itself.

We have shown in recent papers that this approach can be successfully applied to understand the dissolution behaviour of simple oxides and we shall describe separately at this meeting the results of in situ synchrotron X-ray studies of silicate surface hydration.

In the present paper we shall show results of studies of MgO surfaces during dissolution at different pH. In agreement with our previous work, MgO dissolves by crystallographically-controlled dissolution from particular planes to form an etched surface with pyramidal etch pits. However, careful measurement of the angles of these inclined surfaces gives a range of values and suggests that hydration of MgO may not proceed by the topotactic growth of Mg(OH)₂ along the MgO (111) planes. Observation of highly localised birefringent platelets on the MgO surface indicates that simple ERDA measurements of H-penetration into mineral surfaces may be in error.

Dating Volcanic Ash by Luminescence

Manfred Frechen¹ (MFrechen@chelt.ac.uk) & Ulrich Schweitzer² (Schwtzru@geocip.geo.uni-koeln.de)

¹ Centre for Environmental Change & Quaternary Research, CGCHE, Francis Close Hall, Swindon Road, Cheltenham, GL50 4AZ, UK.

² Geologisches Institut, Universität zu Köln, Zülpicher Str., 49a, D-50674, Köln, Germany.

Since the early days of thermoluminescence (TL) and Optical Stimulated Luminescence (OSL) dating, various attempts have been made to determine the cooling or eruption age of volcanic material. The dating of lava and pyroclastic deposits has its own set of problems like anomalous fading, low signal background ratio and the disequillibrium in the uranium series. To overcome these difficulties and to prove the capability of thermoluminescence for dating of volcanic material systematic investigations have been carried out to investigate TL and OSL properties of various types of proximal and distal air fall tephras, pyroclastic flow deposits and hyaloclastites. The indirect approach has been successful in dating aeolian sediments above and below the tephra horizons to bracket the eruption age by the deposition age the sediments. These techniques are used as routine dating methods for eolian sediments younger than 100,000 (300,000) years as demonstrated for the Eltville Tephra and Laacher See Pumice in the West and East Eifel Volcanic Field, Germany. The combined OSL and TL age estimates are in agreement with the geological estimates for which independent age control is available. The second and direct approach is more difficult due to the above mentioned problems. Extensive fading experiments have been carried out for pumice of the Thera Pyroclastic Formation on Santorini island, Greece. The results of the Minoan and lower Pumice seem to be consistent with other independent chronological estimates (Schweitzer, unpublished PhD thesis). Dating results of Hekla ash H3 and unexpected old TL ages for hyaloclastites of the Krafla Volcanic System in Northern Iceland suggests that either the geological estimates are not reliable or that this class of volcanics has some unrecognised cause of inaccurate TL ages (Frechen et al., in press). By selecting suitable material which is free of complicating factors, reliable physical ages can be achieved for volcanic material.

The extensive dating study indicates that volcanic glass from both proximal and distal tephra has the potential to yield accurate TL ages and thus eruption ages up to 100,000 (500,000) years depending mainly on the annual dose rate and complicating factors such as poor reproducibility of TL, a low signal background ratio and anomalous fading.

The TL dating of volcanic glass up to 200,000 yr is still in an experimental stage but promising results have been determined for hyaloclastites from the Krafla Volcanic System, and from pumice of the Thera Pyroclastic Formation of the East Eifel Volcanic Field.