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1. Microprobe analyses

2. Mineral groups

3. Solid solutions

4. Thermobarometers

5. Uncertainties

6. P-T calculations

7. Phase diagrams



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Bulk compositions

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Practical Aspects of Mineral Thermobarometry

Estimating rock bulk compositions for pseudosections

From XRF data

One method is to make use of whole-rock geochemical analysis. If your model system is KFMASH and you will have muscovite, quartz and water in excess, you will have to provide a composition in terms of the four components Al2O3, MgO, FeO and K2O. However, you will not be able simply to take the values straight from the bulk analysis. They must be converted to molar proportions and corrected for the A, M, F and K contents of phases outside the model system. The most important corrections will probably be:

Following the above procedure I estimated the model AMFK ratios of two Bushveld aureole rocks, based on XRF analyses of M33 (Timeball Hill Formation, Annesley), and M37 (Silverton Formation, Burgersfort):

       Al2O3   MgO   FeO   K2O
M33       55    10    25    10
M37       42    22    28     8

Warning: Rocks are seldom truly homogeneous. Your probe section, which contains the equilibria you want to model, may not be representative of the different and larger volume crushed for XRF analysis.

From modal data and microprobe analyses

The second method is to estimate the model bulk composition of a rock from the relative proportions of the minerals which fall in the model system, and so define the composition to be modelled. For example, your rock consists of garnet, staurolite, biotite and muscovite with quartz (and perhaps other phases outside KFMASH).

This approach is commonly more accurate than using the XRF analysis. With M33 above, which contains biotite, staurolite and andalusite, I found that the XRF-derived bulk XMg of 0.286 was more Mg-rich than the biotite (XMg = 0.272), so something was not quite right there, and a better estimate could be obtained from the mineral compositions and mode.

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This page last modified 12 October 2004