Carbon in silicate liquids: the systems NaAlSi 3 O 8 -CO 2 , CaAl 2 Si 2 O 8 -CO 2 , and KAlSi 3 O 8 -CO 2

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 CO2 vapor. The depression of the temperature of the solidus of albite by CO2 decreases from ∼30° C at 10 kbar, to ∼10° C at 20 kbar, to about 0 at 25 kbar, suggesting that the solubility of CO2 in NaAlSi3O8 liquid in equilibrium with solid albite decreases with increasing pressure and temperature. In contrast, CO2 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-Si3O8 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 SiO2 and CaO and depleted in Al2O3 relative to CaAl2Si2O8. The tendency toward incongruent melting with increasing pressure in albite and anorthite produces an increase in the activity of SiO2 component in the liquid ( $$a_{SiO_2 }^l $$ ). We predict that this increases the ratio of molecular CO2/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 CO2 in plagioclase liquids may partly explain the decrease in solubility of carbon with increasing pressure in near-solidus NaAlSi3O8 liquids, which is in contrast to that in CaAl2Si2O8 liquid. Also, reaction of CO2 with NaAlSi3O8 liquid to form CO 3 2− that is complexed with Na+ must be accompanied by a change in Al3+ from network-former to network-modifier, as Na+ is no longer abailable to charge-balance Al3+ 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 Ca2+ 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 CO2 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 SiO2-rich liquid up to the singular point at ∼15 kbar. Above this pressure, sanidine melts congruently, resulting in a decrease in the $$a_{SiO_2 }^l $$ 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 $$a_{SiO_2 }^l $$ relative to those of albite and anorthite, and CO2 produces a nearly constant freezing-point depression of about 170° C. Because of the low $$a_{SiO_2 }^l $$ at pressures above the singular point, we infer that most of the carbon dissolves as CO 3 2− , resulting in a low CO2/ 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 $$a_{SiO_2 }^l $$ 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.