Liquidus phase relations have been determined in the system Qz-Ab-Or at 1 kbar with 1 and 4.5 wt% B2O3 in the melt under both H2O-saturated conditions and H2O-undersat-urated conditions (4.5 wt% B2O3 in the melt), using a H2O-CO2 fluid mixture. Under H2O-saturated conditions, the addition of B2O3 leads to a reduction of the liquidus temperatures for compositions in the feldspar field; compositions in the quartz field are not significantly affected. The minimum liquidus temperature is progressively reduced, and its composition moves toward the Ab comer: Qz38Ab33Or29, 720 °C (B-free, Tuttle and Bowen, 1958), Qz36Ab37Or27, 680 °C (1 wt% B2O3), Qz31Ab46Or23, 640 °C (4.5 wt% B2O3). H2O solubility in the melt increases with increasing B concentration. Isobaric reduction of the H2O content in the melt by about 50% at constant B2O3 increases liquidus temperatures for all of the compositions studied and shifts the minimum liquidus composition toward the Qz-Or side at constant Qz contents: Qz31Ab34Or35, 750 °C (4.5 wt% B2O3)

The contraction of the liquidus field for Ab suggests that the addition of B partially disrupts the aluminosilicate network. B-bearing structural units are formed involving alkalis (mainly Na) that were previously used for charge-balancing IVAl. A fraction of the B atoms may exist in tetrahedral coordination. The increase in the H2O solubility is attributed to the hydrolysis of the borate units. An isobaric increase of the H2O content of the melt has similar effects on phase relations, indicating that H2O is preferentially associated with the Ab-forming components in the melt and that the incorporation of H2O in aluminosilicate melts may involve expulsion of Al from the network. These results allow an estimation to be made of the individual effects of pressure and of the H2O content of the melt on the phase relations in the Qz-Ab-Or system. The B concentration of natural magmas (1 wt% B2O3 maximum) is too low to influence significantly the major-element composition of residual melts. However, the minor- and trace-element chemistry of these melts may be profoundly affected.

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