Geobarometry in eclogites
A geobarometer is a chemical equilibrium written among minerals which, because of a large volume difference between reactants and products, is sensitive to changes in pressure. Good geobarometers have low slopes (dP/dT) on a P-T diagram, and can be calibrated by precise phase equilibrium experiments or reliable thermochemical data. The defining assemblage should occur in common rock types, over a wide range of pressure and temperature. Many successful barometers depend on the large molar volume contrast between feldspar (anorthite or albite) and end members of denser phases such as garnet or pyroxene:
grossular (in garnet) + 2 kyanite + quartz = 3 anorthite (in plagioclase)
jadeite (in clinopyroxene) + quartz = albite (in plagioclase)
However, feldspars do not occur in true eclogites, and equilibria such as these can give only a lower limit to the actual pressures experienced during eclogite-facies metamorphism. Many of the published pressure estimates for eclogites are minima of this kind, and it would clearly be valuable to search for more relevant geobarometers.
Eclogite-facies rocks commonly contain other minerals in addition to the essential metabasic eclogite association garnet + omphacitic clinopyroxene. White micas (paragonite, phengitic muscovite) are of particular interest. The potential of paragonite-bearing assemblages for barometry was explored by Koons (1986). Of even more widespread applicability are equilibria involving phengite, whose composition can vary, in response to pressure, along the Tschermak exchange vector Al2Mg-1Si-1 between muscovite and celadonite (see Massonne and Schreyer 1987, 1989).
Another problem with high-pressure metamorphic rocks is that it can be difficult to demonstrate chemical equilibrium among minerals, because of compositional zoning in the minerals, or partial overprinting by retrograde mineral assemblages. In such cases the most useful equilibria involve a small number of phases, which have a good probability of being found in mutual contact or with unambiguous paragenetic relationships.
The equilibrium which most nearly answers the criteria for reliable barometry is
pyrope + 2grossular + 3celadonite = 6diopside + 3muscovite
which can also be written
pyrope + 2grossular = 6diopside + 3Al2Mg-1Si-1
It involves only three phases, garnet, clinopyroxene and phengite. It is independent of water activity and of silica saturation. No Fe end members are involved, so that the uncertainty related to formula recalculation for Fe3+ in clinopyroxene or phengite is minimal.
A preliminary calibration was offered by Waters and Martin (1993). For the current recommended calibration, with discussion of activity models and uncertainties, follow the link!
References
- Koons PO (1986) Relative geobarometry from high-pressure rocks of quartzofeldspathic composition from the Sesia Zone, Western Alps, Italy. Contributions to Mineralogy and Petrology 93: 322-334.
- Massonne HJ, Schreyer W (1987) Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite, and quartz. Contributions to Mineralogy and Petrology 96: 212-224.
- Massonne HJ, Schreyer W (1989) Stability field of the high-pressure assemblage talc + phengite and two new phengite barometers. European Journal of Mineralogy 1: 391-410.
- Waters DJ, Martin HN (1993) Terra Abstracts 5 (1): 410-411.
Created 16 September 1996