Other
Scientific paper
May 1963
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1963gecoa..27..431m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 27, Issue 5, pp.431-457
Other
8
Scientific paper
Mixtures of dolomite, quartz and various amounts of water have been subjected to experimental metamorphism in the temperature range from 250° to 550°C under constant total gas-pressure of 2000 bars. The reaction, 3 CaMg ( CO 3 ) 2 + 4 SiO 2 + H 2 O Mg 3 [( OH ) 2 Si 4 O 10 ] + 3 CaCO 3 + 3 CO 2 , leads to the formation of talc, calcite and CO 2 . Only if the developed CO 2 is steadily allowed to leave the system the temperature of reaction is 300°C at 2000 bars H 2 O-pressure. However, caloric data and phase rule considerations suggest that this reaction is endothermic and bivariant in the P-T-region where supercritical gas mixtures of CO 2 and H 2 O are present. This has been ascertained by experiments carried out in the temperature range from 440° to 510°C. For this temperature range approximate values for the equilibrium constant could be worked out after fugacities of CO 2 -H 2 O-gas mixtures have been taken into consideration. The bivariancy of the reaction implies that the equilibrium for the formation of talc + calcite + CO 2 from dolomite + quartz + H 2 O has not a fixed temperature even if the total gas-pressure is fixed. At 2000 bars total gas-pressure talc and calcite are formed at a temperature somewhere between 300° and 530°C; the temperature being the higher the greater the partial-pressure of CO 2 relative to the partial-pressure of H 2 O. Thus, in an extreme case, dolomite + quartz + H 2 O can be preserved and does not react until 530°C; but this has not boon the case in rock metamorphism. From the experiments it seems probable that talc could have been formed from dolomite 4-quartz already during high-grade diagenesis, 300°C, if CO 2 could have left the system completely. In rock metamorphism the reaction has taken place (a) either at the temperature of the beginning of the greonschist facis (about 400 ±20°C at 2000 bars) or (b) at higher temperatures of the quartz-albite-epidote-biotite subfacies of the greenschist facies. This last occurrence proves that during rock metamorphism a rather high fugacity of CO 2 relative to the fugacity of H 2 O can have been present in the pores of the rocks (weight-ratio CO 2 :H 2 O has been about 2:1). Talc + calcite + quartz react to form diopsido + CO 2 + H 2 O. This also is a bivariant reaction which is at present investigated.
Metz Paul W.
Winkler Helmut G. F.
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