Orthopyroxenes water content

Dobson et al. (1995) measured water contents in melt inclusions in pyroxenes in boninites from the Bonin Islands. These melt inclusions contained 2.8-3.2 wt.% H2O, whereas quenched glass from pillow lava rims had 2.2-2.4wt.% H2O, which Dobson et al. interpreted as a result of degassing between the time of entrapment of the melt inclusions and the eruption on the seafloor. They also measured the water content of the orthopyroxene, which contained 80-120 ppm H2O. 

The amount of water that can dissolve in orthopyroxene depends on pressure, temperamre, and composition. In a study at 1,100 °C, Rauch and Keppler (2002) found that water content in pure enstatite increased from 55 ppm H2O at 0.2 GPa to 867 ppm at 7.5 GPa, then decreased slightly to 714 ppm at 10 GPa. They also looked at the effect of changing pyroxene composition the most impressive change occurred with increasing aluminum content of the pyroxene. The addition of 1 wt.% AI2O3 increased water solubility from 199 ppm to 1,102 ppm H2O at 1.5 GPa and... 

In the case of a high iron content in the carbonate, olivine is formed instead of orthopyroxene. A 10% increase in magnesium content leads to a 5-8° shift of the P-T curve into the higher temperature region. The shift is relatively small, but it plays a definite role in the metamorphic redistribution of iron among the minerals, particularly in reactions in which iron oxides are formed in the absence of water ... 

The association pyroxene + olivine + cummingtonite + quartz, often encountered in highly metamorphosed BIF, is also very important for judging the thermodynamic parameters of mineralization. This association fixes the temperature at 700-720°C, which depends little on pressure and iron content of the silicates, while pressure can be estimated fairly precisely from the iron content of orthopyroxene. A decrease in partial pressure of water in conjunction with mechanical equilibrium P =Ff) due to dilution of the fluid by other volatiles, for instance carbon dioxide, can lead to some shift of the P-T curve into the lower-temperature region. However, the amount of such a shift, especially at high pressures (8-10 kbar) cannot be significant because Fe-Mg carbonates which are stable in carbon dioxide fluids already appear at 610-650°C (Fig. 93b), and their paragenesis with anhydrous silicates is not typical of iron formations. 

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