Dan's Internet Plaice

NEW! You can have a look at this page for a summary of my thesis, chapter by chapter, to get a more detailed flavour of what was covered. The thesis takes a slightly different slant to the paper whose abstract appears below (which I am not going to print as it has not been published yet!), but the bulk of the key results can be accessed.

I have now finished my PhD in Geology looking at rocks from the aureole of the Bushveld Layered Mafic Series to work out metamorphic reaction kinetics. The methods are simple and involve Crystal Size Distribution (CSD) analysis. Tracings are made either from projections of thin sections or the rock faces themselves of the porphyroblasts. The snap below was taken at an andalusite quarry of andalusite (chiastolite) porphyroblasts which are nice and easy to get accurate tracings of. Chiastolites can be about half an inch wide and 4 inches wide - the piece of paper in the drawing is A5 sized.
andalusites at a quarry
Image analysis is then carried out which allows a CSD to be generated by means of a histogram. The characteristics of the histogram can be summarised by a single number which is then compared with forward models of crystal nucleation and growth at the various rates of heating experienced at different parts of the aureole. Small crystals are produced when heating is rapid - it is best viewed from the diagram below.
texure diagram
One related paper has already been published, dealing mainly with calculating overstepping using petrological observations:

Waters, D. J. & Lovegrove, D. P., 2002. Assessing the extent of disequilibrium and overstepping of prograde metamorphic reactions in metapelites from the Bushveld Complex aureole, South Africa. Journal of Metamorphic Geology, 20, 135-149.

And another one dealing with the results is in the pipeline. Here is the title and abstract:

SYSTEMATIC VARIATIONS IN TEXTURAL PARAMETERS OF ANDALUSITE PORPHYROBLAST BEARING METAPELITES FROM THE BUSHVELD AUREOLE, SOUTH AFRICA

D. P. Lovegrove and D. J. Waters


Chiastolite bearing metapelites from the eastern lobe of the Bushveld Aureole, Republic of South Africa, preserve important information about the interaction between the rates of nucleation and growth in response to variable rates of heating. Variations in texture are recorded by means of a parameter, b, which is the slope of a log-linear crystal size distribution within the larger size classes. Porphyroblastic metapelites of the outer aureole exhibit low b parameters of around 5 to 10cm-1 for chiastolite populations, whereas in the more hornfelsic lithologies of the inner aureole this figure rises to over 50cm-1. Such variations are attributable directly to relative rates of heating, which in the inner aureole are much faster and manifest themselves on the resultant texture by causing copious nucleation relative to growth and hence a population of much smaller chiastolite crystals.
When these results are compared with those from synthetically generated populations from which rates of heating and growth are supplied, it becomes possible to evaluate the critical overstepping required for the nucleation of chiastolite. Assumption of experimentally determined growth rates suggests that the magnitude of this overstepping is between 5 and 10ºC.

There's more information about the theory behind this study accessible from this link by Dave Waters whose personal homepage which has a few other bits about this study and much, much more about other things can be reached here.

NEW! Alternatively, you can have a look at this page for a summary of my thesis, chapter by chapter, to get a more detailed flavour of what was covered. The thesis takes a slightly different slant to the paper whose abstract appears above (which I am not going to print as it has not been published yet!), but the bulk of the key results can be accessed.

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Dan Lovegrove
dan@doctor-lovegrove.com

Last updated 27th March 2004.