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On the basis of the glass and mineral textures and elementary melt chemistry, we know that the zoned plagioclases and other relatively large and well-developed minerals in Austin's dacite must have taken more time to grow than the surrounding glass matrix. By using high-temperature ovens in undergraduate university laboratories or even crystal-growing kits and kitchen chemicals, a normally intelligent person can verify that coarse crystals take more time to grow than finer-grained materials. Farrar, 1996, '40Ar/39Ar Phlogopite and U-Pb Perovskite Dating of Lamprophyre Dykes from the Eastern Lake Superior Region: Evidence for a 1.14 Ga Magmatic Precursor to Midcontinent Rift Volcanism', Can. Mafic microphenocrysts within these glassy particles were probably dominated by the strongly magnetic Fe-Ti oxide minerals. The microscopic examination of the 'heavy-magnetic concentrate' also revealed a trace quantity of iron fragments, obviously the magnetic contaminant unavoidably introduced from the milling of the dacite in the iron mortar. Austin simply assumes that the first explanation is correct and then he proceeds to use the 'presence' of 'excess argon' in his samples to question the reliability of all K-Ar dates on other rocks and minerals. The validity of either hypothesis #2 or #3 would provide additional evidence that Austin's application of the K-Ar method is flawed and that he has failed to prove that the K-Ar method is universally invalid. 9), the K-Ar method cannot be used to date samples that are much younger than 6,000 years old (Dalrymple, 1991, p.
I've attempted to separate very fined-grained minerals from glass in coal ashes by using magnetic separation and hydrofluoric and other acids. Austin even admits that the dacite is 45% phenocrysts and 'lithic (rock) inclusions.'Although Austin claimed that he took precautions to avoid laboratory contamination and that he and his team removed the obvious xenoliths from the dacite sample, Austin's own words refute Swenson's illusions that the dacite mineral/glass 'fractions' were suitably 'pure' enough for testing the validity of the K-Ar method.
This sample also had recognizable hornblende, evidently not completely isolated by magnetic separation.' 'The 'pyroxene concentrate' (DOME-IP) was dominated by orthopyroxene and much less clinopyroxene.
Because it was composed of finer particles (170-270 mesh), it contained far fewer mafic particles with attached glass fragments than DOME-IH.
Zoned crystals also may show Carlsbad twinning, which is typical of feldspars (Perkins and Henke, 2000, Plate 10; Klein and Hurlbut, 1999, p. Well-established laboratory studies (Klein and Hurlbut, 1999, p. 86-89) indicate that calcium-rich plagioclases crystallize in melts at higher temperatures than albite or sodium-rich plagioclases.
Because of their crystalline and chemical differences, the calcium-rich plagioclase cores have somewhat different optical properties than the sodium-rich rims, which produce the noticeable concentric zoning in the grains in Austin's thin section photograph.
Clearly, basic crystal chemistry and physics dictates that zoned and other relatively large phenocrysts grew deep within the Earth and existed before the glass matrix that rapidly formed during the 1986 eruption.