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Normative minerals were calculated in Table 2, with the assemblage representative of dacite.

Thus, this seven-kilogram sample of dacite is representative of the whole lava dome.

The conventional K-Ar dating method was applied to the 1986 dacite flow from the new lava dome at Mount St. Porphyritic dacite which solidified on the surface of the lava dome in 1986 gives a whole rock K-Ar 'age' of 0.35 ± 0.05 million years (Ma).

Mineral concentrates from the dacite which formed in 1986 give K-Ar 'ages' from 0.34 ± 0.06 Ma (feldspar-glass concentrate) to 2.8 ± 0.6 Ma (pyroxene concentrate). The fundamental dating assumption ('no radiogenic argon was present when the rock formed') is questioned by these data. Helens dacite argue that significant 'excess argon' was present when the lava solidified in 1986.

Phenocrysts of the sample are of the kind and abundance representative of the entire lava dome.

The sample even has several gabbroic inclusions of the composition and size representative of the whole lava dome., the average composition of the 1986 flows on the lava dome.

Because the sample comes from the sloping surface of the dome, it most likely represents the upper surface of a flow lobe.The lava dome formed by a complex series of lava extrusions, supplemented occasionally by internal inflation of the dome by shallow intrusions of dacite magma into its molten core.Extrusions of lava produced short (200-400 m) and thick (20-40 m) flows piled on top of one another.The stability of this third dome, along with decrease in the frequency of earthquakes and phreatic steam eruptions in the ten years after October 1986, indicate that the volcano, again, may be approaching a period of dormancy.the rock averages about 55 per cent fine-grained, grey groundmass and 45 per cent phenocrysts and lithic inclusions (see Figure 4).The groundmass of the rock is composed of microphenocrysts of plagioclase, orthopyroxene, and Fe-Ti oxides within a glass matrix.Phenocrysts of plagioclase (30-35 per cent), orthopyroxene (5 per cent), hornblende (1-2 per cent), Fe-Ti oxides (1 to 2 per cent), and clinopyroxene (less than 0.5 per cent) together comprise almost half of the lava dome.Of the lithic inclusions 85 per cent are medium grained gabbros with an average diameter of 6 cm." The high mafic mineral content of gabbroic inclusions makes a small but significant decrease in the overall Si OPhotomicrograph of Mount St. The most abundant phenocrysts are plagioclase which are embedded in a much finer-grained groundmass containing grass end microphenocrysts. Geologists are in general agreement concerning the crustal source of the dacitic magma beneath Mount St. Experimental data from the assemblage of minerals in the dacite indicate that just prior to the May 18, 1980 eruption the upper part of the magma chamber was at a temperature of 930°C and at a depth of about 7.2 km.Photographed in polarised light with 2 mm width of view (dacite sample ' DOME-1 photo by A. Major-element and trace-element abundances in the 1986 dacite lava flow at Mount St. The analysis was performed on dacite groundmass and phenocrysts without lithic inclusions.One kilogram of dacite groundmass with phenocrysts (without gabbroic inclusions) was removed from the sample for potassium-argon analysis.The technique began by crushing and milling the dacite in an iron mortar.

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