Cordierite-H2O-CO2-Sodium-Equilibria

Research project P 14030 Cordierite-H2O-CO2-Sodium-Equilibria Peter W. MIRWALD 11.10.1999 Large areas of the continental earth`s crust are build up by metamorphic rocks. For elucidating of the geological processes which led to the development of continental crust it is necessary to study the pressure and temperature conditions during the metamorphic history. Certain chemical cation exchange equilibria between individual mineral phases are highly sensitive to pressure or temperature. They may serve as so-called geobarometers and geothermometers. These chemical equilibria change with pressure and temperature conditions, what can be explored experimentally in the laboratory under defined pressure and temperature conditions. In a similar way one may obtain informations about the content of H20 and other volatiles present during metamorphosis. An example of such a geothermobarometer is the mineral cordierite (Mg,Fe)2 A14Si5Ol8*(nH2O, CO2, Na+), which is involved in exchange equilibria with various coexisting mineral phases, such as garnet, biotite and albite. Cordierite is frequently found in the important and wide-spread group of metapelitic rocks. So, for example, the Na-content of this mineral depends strongly on the temperature of formation as our recent experimental results have shown. Apart from this temperature dependence, the H20- and C02-fluids present during the metamorphic processes have a strong influence on the sodium incorporation, in addition. This project aims at the elucidation of these questions in more detail by further experiments. Moreover, cordierite can be used as pressure indicator if it coexists with garnet or biotite. These mineral equilibria have been already investigated by other researchers, however, recent experimental observations obtained in our previous project make an re-examination of these geobarometers very desirable. The results of the proposed investigations are expected to be of threefold use, the application of cordieirite as an indicator for pressure, temperature and fluid composition. With respect to geological application, these results should be very helpful for a better understanding of the complex conditions encountered in metamorphism of pelitic rocks.

The project P1 4030-GEO had a duration of one year starting out with 1.11.1999. The basic reason for that proposal was a considerably progressed PhD-work of Mag. Erich Knop (working title: "the experimental incorporation of sodium in cordierite and its petrological implications") which should be finished within that year. Two topics should be experimentally adressed: i) further crystal chemical incorporation experiments in cordierite and ii) petrological experiments for the stability of the Mg-Fe-endmembers of cordierite. Ad i): A considerable part of that project could be realized; about 60 high-pressure experiments were conducted. The experiments started out with sodium incorporation experiments in the system NMASH (Na-Mg-Al-Si-O- H2O) and with melting experiments in the binary section albite - Mg-cordierite. These experiments were hampered by a problem of unclear origin: the gold capsules melted far below the expected melting curve. We inferred tiny water leakages out of the capsules into the salt cell what resulted into fatal melting. A complete elucidation of that problem is difficult because a check of the water content after a failed experiment is impossible. To avoid further problems an apparatus for sealing large volumed capsules was developed. This construction bases on the technique of cold deformative wealding. The specific parts have been maschined by Fa. Dummer, specialised in the field tool fabrication. With that appartus it is possible to seal gold capsules of 4 and 6 mmm diameter at a minimal failure rate. In augmentation of his experimental work by field data Mr. Knop took samples in two areas, Sauwald und Ötztal- Migamtite, being characterized by cordierite bearing migmatite series. This should allow a comparison between laboratory results and field reality. The basic results of this experimental study are predominantely of petrological importance: - The incorporation of sodium (in coextistence with albite as buffering phase) is inversely proportional to the temperature in the range between 500-800C up to 8 kbar. - The pressure dependence is very small - An eutectic melting is observed at medium pressures (some 5 kbars) at ca. 680C which is simultaneously correlated with an dramatic increase in the sodium content of cordierite. This does not, however, affect the invers temperature relation! - The experimentally determined sodium-contents correlate well to that found in natural cordierites of metapelitic provenance (<0.1 Na p.f.u.). - By addition of CO2 to the hydric fluid phase a reduction in H2O-activity is produced. As a consequence of this a nonlinear decrease of sodium content in cordierite is observed. At very high CO2 concentration in the fluid phase the sodium contents become very small. - These results suggest two petrological applications: i) in case the fluid phase of metapelites contains little CO2 the sodium content may be used for geothermometric use, ii) in case of a considerable CO2 content, there is a possiblity to estimate the H2O-CO2 composition of the cogenetic fluid phase. Prerequisite for this is an independent temperature estimate from coextisting mineralphases of the paragenesis. This finding was a surprising result in the project, it offers a number of interesting petrological aspects to be followed up in future. The results of the investigations of E. Knop have been presented on a number of meetings (see publications list). Ad ii): These petrological investigations have been performed in the frame of a diploma thesis. The basic results are: - The upper pressure stability of sodium containing Mg- and Fe-cordierite is strictly dependent on the composition of the H2O-CO2-composition of the fluid phase. - The various components have different stabilising effects. While the upper pressure stability in presence of pure H2O-fluidphase is characterised by a positive dP/dT-slope, a CO2-fluid phase leads to a negative slope of the stability boundary! - In case of mixed H2O-CO2 composition of the fluid phase - which is the predominant case in nature - one obtains a slope of dP/dT = +/- 0. That means, the Mg-Fe composition of cordierite may be used as geobarometer. This finding is of of considerable petrological significance. The results have been presented on three national and international meetings. The results are in preparation for final publication. In the frame of the project part i) two students provided help in biblographic work - a large data file on cordierite literature crystal chemistry, structure and petrology) had already been compiled in the previous project - and assisted in laboratrory work (synthesis of starting materials and high-pressure experiments).