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Again, the mineral textures, as well as the laws of chemistry and physics, dictate that the calcium-rich plagioclase cores grew at higher temperatures before the sodium-rich rims and that glasses only formed once the melt erupted at the surface.

Because intrusive rocks solidify deep within the Earth away from cool water and air, volcanic glass is absent and the grains may be fairly large (that is, easily reaching lengths of one centimeter or more).

In other cases, such as Austin's dacite, a partially crystallized melt erupts on the Earth's surface and produces a volcanic rock, which may be a mixture of rapidly quenched volcanic glass and coarser phenocrysts (Hyndman, 1985, p. The zoning appears as a series of concentric rings of various shades of gray within the grains (see the two obvious examples in the middle of Figure 4).

Steve Austin and his associates at the Institute for Creation 'Research' (ICR) collected a dacite sample from Mt. Helens, Washington State, USA, which probably erupted in 1986 AD. then ineffectively separated the sample into several mineral and glass 'fractions', submitted the dacite and its 'fractions' for potassium 40-argon 40 (K-Ar) dating, and subsequently used the bogus results to inappropriately attack the K-Ar method.

Considering that the half-life of potassium-40 (40K) is fairly long (1,250 million years, Mc Dougall and Harrison, 1999, p.

Y."; also see discussions by advanced equipment, 'memory effects' can be a problem with very young samples (Dalrymple, 1969, p. That is, very tiny amounts of argon contaminants from previous analyses may remain within the equipment, which precludes accurate dates for very young samples.

For older samples, which contain more 40Ar, the contamination is diluted and has insignificant effects. De Paolo, 1998, 'Intercalibration of Standards, Absolute Ages and Uncertainties in 40Ar/39Ar Dating', Chem.

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.

Obviously, if Austin wanted a sample that only represented the material that solidified during the 1986 eruption, he would have had to remove ALL of the plagioclase and other phenocrysts from the glass component.

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