The thermodynamic relations embodied in the Quasicrystalline Model of Burnham and Nekvasil (1986), as recently extended by the author, have been used to quantitatively assess the feldspar-quartz liquidus relations in two I-type (Jindabyne and Moruya) and two S-type (Bullenbalong and Dalgety) suites of Australian granites, using analytical data provided by B. W. Chappell and co-workers. Among the more notable results obtained from these calculations at a constant pressure of 5.0 kbar and X w m = 0.30 (≈2·8wt% H 2 O), for purposes of comparison, are that: (1) felsic melts of remarkably uniform, but distinctive composition can be extracted from each suite, leaving solid residues in amounts up to 65 mol%; (2) all melts from both S-type suites have two feldspars plus quartz on their liquidii, whereas both I-type suites have only plagioclase plus quartz on their liquidii; (3) the total solid residue ranges from 27–63% in the Jindabyne suite, from 15–62% in the Moruya suite, from 30–65% in the Bullenbalong suite, and from 27–65% in the Dalgety suite; (4) liquidus temperatures of the S-type Bullenbalong and Dalgety melts are similar (856° and 860°C), reflecting similar feldspar compositions of An 53 , Or 75 and An 60 , Or 77 , respectively; (5) liquidus temperatures of the I-type Jindabyne and Moruya melts, however, are distinctly different (950° and 894°C), reflecting correspondingly different plagioclase compositions of An 80 , and An 52 ; (6) the calculated liquidus plagioclase composition throughout a given suite is very uniform (±1%) and amounts to as much as 46% of the total rock; and (7) these calculated liquidus and residual plagioclase compositions are also the same, within the uncertainty of measurement, as those of the plagioclase crystal-cores determined optically by A. J. R. White. The only plausible explanation for this remarkable consanguinity in plagioclase liquidus, residue, and crystal-core compositions, hence liquidus temperatures, is that the bulk of the residue is restite, in accordance with the model of White and Chappell (1977). This explanation is corroborated by the very systematic variations in the amounts of individual restite minerals with respect to total restite contents. Accordingly, those members of each suite that contain more than 60% total restite probably closely represent the bulk composition of the source rock, which is dioritic or andesitic for the Jindabyne suite, tonalitic or dacitic for the Moruya suite, pelitic metagreywacke for the Bullenbalong suite, and feldspathic metagreywacke for the Dalgety suite. As a corollary, those members with less than 60% restite must have undergone melt–restite segregation (unmixing), probably during ascent and emplacement.

Abstract Published experimental, thermodynamic, and other geochemical data on H 2 O-bearing silicic melts are used to obtain relationships which show that: (1) the rate of exsolution of H 2 O (vesiculation) from silicic magmas that contain more than a few tenths of 1 wt percent H 2 O is sufficiently rapid to contribute to the explosivity of pyroclastic eruptions; (2) the exsplution of only a few tenths of 1 percent H 2 O from a typical rhyolitic magma by the second boiling reaction—H 2 O-saturated melt crystals + H 2 O vapor releases sufficient mechanical energy (ΡΔV work of expansion)—to cause tensional fracture failure of wall rocks at pressures corresponding to ocean depths of at least 10 km; (3) the ΡΔV energy released by the exsolution of additional H 2 O, as a result of decompression following Wall-rock failure, is fully adequate to produce pyroclastic eruptions, even at these great ocean depths; (4) the crystallinity and vesicularity of the juvenile pyroclasts of the tuff units that host and underlie the Kuroko ores in the Hokuroku district of Japan are consistent with their having been erupted onto the sea floor at an ocean depth of 3.5 ± 0.5 km, from a magma chamber situated 1.1 ± 0.3 km beneath the sea floor; and (5) the submarine caldera model of Ohmoto (1978) for the formation of volcanogenic massive sulfide deposits appears, therefore, to be viable, at least for the deposits in the Hokuroku district. Application of these same relationships to the 1980 eruption of Mount St. Helens suggests that the March through May 18, 1980, eruptive sequence, including the intrusion of magma into the northern bulge and the landslide-triggering earthquake, was initiated by the second boiling reaction at a snbstantial depth beneath the summit. Furthermore, the devastating blast conld have been, but probably was not, cansed entirely by the virtually instantaneons exsolution of H 2 O from a higher level magma upon sudden decompression that accompanied the landslide.

Mineralization in regions of plate convergence is dominantly of the hydrothermal type, either directly associated with high-level (subvolcanic) intrusive bodies or with manifestations of explosive volcanism. In both associations, a strong case can be made for a close connection between mineralization and the processes that operate during generation, emplacement, and solidification of hydrous felsic magmas, whether such magmas are generated from appropriate source rocks in a subduction zone or in the lower continental crust. The thesis espoused here is that magmas of appropriate compositions, including abnormally high metal, S, and H 2 O contents, are normal products of partially melting nonporous mafic amphibolites of typical oceanic tholeiite composition in the upper parts of a subducting plate. Furthermore, the depth at which these magmas are generated is fully consistent with the depth to the top of the seismic zone under many volcanic arcs. There is no need, therefore, to call upon abnormally metalliferous source rocks or other exotic genetic schemes to account for the association of mineralization with plate convergence. Hydrothermal mineralization is linked to convergence, as to other tectonic regimes, through hydrous magmas.

The thermodynamic functions for water that are tabulated here consist of: (1) specific volume, (2) Gibbs free energy, (3) entropy, (4) enthalpy, (5) fugacity and (6) fugacity coefficient. They cover the temperature range 20° to 1,000°C in 20° intervals, and the pressure range 100 to 10,000 bars, in 100-bar intervals. In addition, separate tables are presented for the Gibbs free energy at 0.01 and 1.0 bars, as well as for the coefficients in the three empirical equations of state upon which the tabulated values above 1,000 bars are based. The tabulations up to 800°C and 1,000 bars were obtained, either directly or by computation, from the Steam Tables 1964 (Bain, 1964); all of those above 1,000 bars are based on the specific volume measurements of Burnham, Holloway and Davis (1969). The uncertainty in the tabulated values of specific volume and enthalpy below 1,000 bars is ± 0.1 percent; above 1,000 bars the uncertainty is ± 0.3 percent in specific volume, Gibbs free energy and entropy, ± 0.6 percent in enthalpy and ± 1.3 percent in fugacity and fugacity coefficient.