The effect of water activity on the eutectic point in natural granitic system
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Date
2012-09
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Mineralogical Society of Poland
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Abstract
The water activity except for pressure is known to be the most important parameter
affecting the eutectic point in granitic systems. Water saturated magmas have lower solidus
temperatures than water undersaturated magmas (Johannes, Holtz 1996). Water activity
influences also Ab-Or relation in granitic systems (Holtz et al. 1992). Mineral phases
present in water saturated conditions differ from those present in water poor conditions
(Dall’Agnol et al. 1999).
This effect of water activity on the granitic system has so far been analyzed on synthetic
materials. Other parameters which would affect the eutectic point might exist in a natural
granitic system. Therefore, two natural samples of Wangrah granites from Australia were
taken to analyze their crystallization path. They were chosen so they had very similar
composition in terms of Qz-Ab-Or, but different in terms of Ca and Fe contents. The
homogenous glass powder was used as a starting material. The water was added to reach
water saturated conditions. The water and silver oxalate were added to reach water
undersaturated conditions. Crystallization experiments with both samples were performed
in cold seal pressure vessel (CSPV) in the temperature range of 680-720ºC for water
saturated conditions and 765-815ºC for water undersaturated conditions. The pressure of 2
kbar and oxygen buffer NNO were established for 15 days. The phases were identified and
glass composition was determined by an electron microprobe. The water content of glass
was measured with the use of near infrared spectroscopy.
The results were plotted on the Qz-Ab-Or diagram with application of Blundy and
Cashman corrections (Blundy, Cashman 2001) for the An content. Two very similar
compositions in terms of Qz-Ab-Or show a different position of their cotectic lines for
water saturated conditions. One possible explanation of this fact is that the projection of
Blundy and Cashman does not account accurately for the effect of Ca. The alternative
explanation is that there must be other parameters controlling the position of the cotectic
line, one of them may be Fe. In any case, one cannot simply project natural granite or
rhyolites on the Qz-Ab-Or diagram to extract quantitative information on pressure or water
activity of granites or rhyolites. A new set of experiments should be performed to clarify
this problem.
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Ciążela, J., Bolte, T., Holtz, F., Muszyński, A., and Machowiak, K. (2012). Effect of water activity on the eutectic point in the natural granitic system. 2nd International Conference on “Contemporary problems of geochemistry”, Kielce, September 24-26, 2012, Mineralogia special papers 39, 28-29.
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Mineralogia - Special Papers;39