To further our knowledge of the effects of volatile components on phase relationships in aluminosilicate systems, we determined the vapor saturated solidi of albite, anorthite, and sanidine in the presence of CO
2 vapor. The depression of the temperature of the solidus of albite by CO
2 decreases from 30° C at 10 kbar, to 10° C at 20 kbar, to about 0 at 25 kbar, suggesting that the solubility of CO
2 in NaAlSi
3O
8 liquid in equilibrium with solid albite decreases with increasing pressure and temperature. In contrast, CO
2 lowers the temperature of the solidus of anorthite by 30° C at 14 kbar, and by 70dg C at 25 kbar. This contrasting behavior of albite and anorthite is also reflected in the behavior of melting in the absence of volatile components. Whereas albite melts congruently to a liquid of NaAl-Si
3O
8 composition to pressures of 35 kbar, anorthite melts congruently to only about 10 kbar and, at higher pressures, incongruently to corundum plus a liquid that is enriched in SiO
2 and CaO and depleted in Al
2O
3 relative to CaAl
2Si
2O
8.The tendency toward incongruent melting with increasing pressure in albite and anorthite produces an increase in the activity of SiO
2 component in the liquid (
). We predict that this increases the ratio of molecular CO
2/CO
3
2–
in these liquids, but the experimental results from other workers are mutually contradictory. Because of the positive dP/dT of the albite solidus and the negative dP/dT of the anorthite solidus, we propose that a negative temperature derivative of the solubility of molecular CO
2 in plagioclase liquids may partly explain the decrease in solubility of carbon with increasing pressure in near-solidus NaAlSi
3O
8 liquids, which is in contrast to that in CaAl
2Si
2O
8 liquid. Also, reaction of CO
2 with NaAlSi
3O
8 liquid to form CO
3
2–
that is complexed with Na
+ must be accompanied by a change in Al
3+ from network-former to network-modifier, as Na
+ is no longer abailable to charge-balance Al
3+ in a network-forming role. However, when anorthite melts incongruently to corundum plus a CaO-rich liquid, the complexing of CO
3
2–
with the excess Ca
2+ in the liquid does not require a change in the structural role of aluminum, and it may be more energetically favorable.The depression of the temperature of the solidus of sanidine resulting from the addition of CO
2 increases from 50° C at 5 kbar to 170° C at 15 kbar. In marked contrast to the plagioclase feldspars, sanidine melts incongruently to leucite plus a SiO
2-rich liquid up to the singular point at 15 kbar. Above this pressure, sanidine melts congruently, resulting in a decrease in the
with increasing pressure in the interval up to 15 kbar. Above this pressure, the congruent melting of sanidine results in a lower and nearly constant
relative to those of albite and anorthite, and CO
2 produces a nearly constant freezing-point depression of about 170° C. Because of the low
at pressures above the singular point, we infer that most of the carbon dissolves as CO
3
2–
, resulting in a low CO
2/ CO
3
2–
, but a high total carbon content.The principles derived from the studies of phase equilibria in these chemically simple systems provide some information on the structural and thermal properties of magmas. We propose that the
is an important parameter in controlling the speciation of carbon in these feldspathic liquids, but it certainly is not the only factor, and it may be relatively less significant in more complex compositions. In addition, our phase-equilibria approach does not provide direct thermal and structural information as do calorimetry and spectroscopy, but the latter have been used primarily on glasses (quenched liquids) and cannot be used in situ to derive direct information on liquids at elevated pressures, as can our method. Hopefully, the results of all of these approaches can be integrated to yield useful results.Institute of Geophysics and Planetary Physics, Contribution No. 2744
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