Practical Aspects of Mineral Thermobarometry
This section of the site contained examples generated in 1996: it will be updated with revised material, making use of more recent data sets. For the moment, we review the initial steps in choosing mineral data representative of an approach to equilibrium at the metamorphic peak.
Setting up the problem
A summary of what you have to do in order to use THERMOCALC to calculate pressures and temperatures of equilibration for metamorphic assemblages.
- Determine the set of equilibrium phases.
- Select appropriate compositions (means of populations if possible).
- Calculate activities, according to the preferred models.
- Select appropriate end members, and compose input data file.
- Decide on best mode of operation (average-P, average-T, average-PT).
Example: Himalayan metapelite L429
A typical medium-grade P-T determination problem, in which compositional variation in minerals needs to be taken into account, and temperature and water activity turn out not to be separable.
L429 is a garnet-mica schist from the Donara Nappe, within the High Himalayan slab of western Zanskar (see Searle et al, 1992, J Geol Soc London, 149, 753-773). It contains the peak metamorphic assemblage Qtz + Bt + Ms + Pl + Grt + Ky (minerals listed in decreasing order of abundance). This is an eminently suitable assemblage for P-T determination, but first the appropriate analyses have to be chosen, so as to minimise "geological error". The minerals show the following textural and compositional features.
Garnet forms poikiloblasts up to about 6 mm across. It contains abundant inclusions of quartz and clinozoisite. Clinozoisite and certain other minor phases included in the garnet are no longer present in the rock matrix. The garnet is compositionally zoned in a style which strongly suggests prograde growth zoning.
The garnet profile seems unmodified by post-growth diffusional effects at the rim, and the Mn profile decays effectively to zero at the rim in accordance with segregation or reaction-partitioning growth models. We infer that the rim composition is most likely to represent the peak metamorphic conditions and to approach equilibrium with the phases in the rock matrix. For peak thermobarometry it seems sensible to choose the rim analysis with the highest XMg, and the lowest Mn and Ca (left-hand end of the profile).
The interior of the garnet clearly coexisted with a different, clinozoisite-bearing mineral assemblage.
Muscovite forms small flakes and sheaf-like aggregates in the rock matrix, defining a crenulated, spaced foliation.
The most important substitutions in metamorphic muscovites are those towards celadonite (phengites) and towards paragonite (NaK-1). L429 muscovites show substantial variation:
Various cations are plotted against Si content, which monitors the extent of celadonite substitution. There is a strong, almost 1:1 correlation between Mg and Si, but Fe remains uniform. The celadonite content varies between 11 and 24%, and individual grains appear to be compositionally zoned (see the three analyses from grain 1). The Na content shows a negative correlation with Si.
The celadonite content is expected to decrease with increasing T and decreasing P. The Na content is expected to increase with T as the muscovite-paragonite miscibility gap narrows. The trends are consistent with incomplete re-equilibration of muscovite during prograde changes in P and T, and we might expect grain 2, with lowest Si and highest Na, to represent the equilibrium composition at the peak. As it happens, this grain is close to garnet, and thus the most likely composition to represent equilibrium with garnet rims. We could average the two analyses of this grain for use in thermobarometry.
Biotite forms laths in the matrix, broader, coarser, and more poorly oriented than muscovite. The biotite compositions are fairly uniform in XMg and tetrahedral Al content, but there are differences in Ti content, which appear to correlate with octahedral Al as shown below.
Rutile is a ubiquitous accessory in L429, so the Ti content may largely reflect temperature of equilibration. For whatever reason, grain no.5 is clearly the odd one out, and the mean of the remaining analyses should serve as the best estimate of the peak biotite. Biotite no.2, in the high-Ti group is adjacent to garnet. It has a slightly, though perhaps not significantly, lower XMg.
Two groups of plagioclase grains were analysed in the rock. Those adjacent to garnet have distinctly more Ca-rich compositions (An36 as against An30).
If we follow the reasoning that phases close to garnet rims are most likely to represent peak equilibrium with them, we should use the mean of the Ca-rich compositional group for thermobarometry.
Next steps: activity calculations
A number of assumptions have been made about the rock's equilibration history. They may not all be correct. Nevertheless, we now have a "best set" of mineral compositions.
Click here to see a table of these compositions.
This page last modified 12 October 2004