Additional experiments have been done with the May 18, 1980, Mount St. Helens dacite and a more mafic, October 1980 sample to resolve questions concerning amphibole stability and dissolved volatiles in the magma chamber prior to the May 18 eruption. The experiments were done at 920°C, at fluid pressures of 220 or 320 MPa, and, in contrast to previous work, at an ƒO2 between the NNO and MnO-Mn3O4 oxygen buffers. Fe-Ti oxides are present in the melt under these conditions, and amphibole is stable when XH2O in the fluid is greater than 0.67. The An content of plagioclase in equilibrium with melt decreases with decreasing XH2O in the fluid and, in the amphibole-bearing experiments, reaches the natural plagioclase rim compositions (An49) at an XH2O of 0.67. Under these H2O-undersaturated conditions the experimentally produced amphibole, low-Ca pyrpxene, and Ca-rich pyroxene are compositionally equivalent to phenocrysts in the May 18 white pumice. The melts (glasses) in amphibole-bearing experiments range from the average plagioclase melt inclusion composition [Rutherford et al., 1985] to slightly less evolved compositions as XH2O approaches 1.0.
Melt inclusions in natural amphiboles were analyzed, and the compositions were plotted on SiO2 variation diagrams along with experimental glass analyses. The amphibole melt inclusions define a liquid line of descent for the magina which extends from relatively primitive compositions (68 wt % SiO2, anhydrous basis) to the more evolved average plagioclase melt inclusion composition (73 wt % SiO2). The volatile content of the amphibole melt inclusions (difference method) reaches 5.0±0.5 wt %, which compares favorably with the volatile content of the amphibole-bearing experimental melts produced at XH2O = 0.67.