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Phenocrysts and microscopic phenocrysts microphenocrysts are crystals that grow in a melt magma deep within the Earth. In some cases, the entire melt solidifies before reaching the Earth's surface and an intrusive igneous rock develops Hyndman, , p. 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, , p. Although Austin and Swenson will not admit it, some of the grains in Figure 4 may be xenocrysts rather than phenocrysts. In some cases, the magma may not be hot enough to melt or entirely dissolve the xenocrysts and they may survive after the melt cools. For even the best mineralogists and petrologists, xenocrysts may be difficult to distinguish from phenocrysts for example, Hyndman, , p.
As clearly shown in Figure 4 of Austin's essay , many of the mineral grains are zoned. 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. Zoned crystals also may show Carlsbad twinning, which is typical of feldspars Perkins and Henke, , Plate 10; Klein and Hurlbut, , p.
In thin section and under crossed-polarized light, Carlsbad twinning has a 'half and half' appearance, where one half of the grain is darker than the other half Perkins and Henke, , Plate As the sample is rotated on a microscope stage, one twin will darken as the other lightens in crossed-polarized light. A large grain with very noticeable Carlsbad twinning is located at the top of Figure 4.
Well-established laboratory studies Klein and Hurlbut, , p.
That is, as the magma cools, calcium-rich plagioclases crystallize first, which causes the remaining melt to become depleted in calcium and relatively enriched in sodium. Once temperatures further decline, more sodium-rich plagioclase begins to solidify from the melt and may surround the calcium-rich grains.
This process produces zoning, where the older and more calcium-rich plagioclases are located in the core of the grains and the younger and more sodium-rich plagioclases occupy the rims. 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.
Besides plagioclase feldspars, chemicals in cooling magmas deep within the Earth may organize into pyroxenes, amphiboles and a large variety of other minerals. In contrast, any melt that reaches the Earth's surface during an eruption will immediately quench into volcanic glass if it comes into contact with seawater or other surface waters.
The quenching process freezes the atoms in place and prevents them from organizing into crystals. In the presence of air, the lava may cool slowly enough that some VERY small minerals may grow. The highly disorganized volcanic glass matrix in Austin's Figure 4 appears black or 'isotropic' in crossed-polarized light. Unlike most minerals, which lighten and darken in crossed-polarized light as the microscope stage is rotated, volcanic glass always remains consistently dark under crossed-polarized light. Furthermore, unlike disorganized and quickly chilled volcanic glass, well-zoned and developed feldspar crystals, such as those shown in Figure 4, don't form overnight.
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.
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 eruption. Nevertheless, it is clear from Austin's essay that he has failed to incorporate the obviously diverse ages of the phenocrysts and the volcanic glass into his explanation for the origin of the dacite. Similarly, Swenson also fails to comprehend the indisputable history that is associated with the plagioclase zoning and to properly recognize the important age differences between the coarsest phenocrysts and the volcanic glass.
Obviously, if Austin wanted a sample that only represented the material that solidified during the eruption, he would have had to remove ALL of the plagioclase and other phenocrysts from the glass component.
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Even when phenocrysts as in Austin's Figure 4 and xenocrysts can be seen with an optical microscope, they can be extremely difficult, if not impossible, to effectively separate from the glass. I've attempted to separate very fined-grained minerals from glass in coal ashes by using magnetic separation and hydrofluoric and other acids.
Specifically, Austin admits that most of his fractions are impure when he includes the term 'etc. Furthermore, Austin's descriptions in the following statements clearly indicate that he FAILED to adequately separate the phenocrysts and possible xenocrysts from the volcanic glass.
Because Austin did NOT separate the plagioclase from the glass, we would expect this sample to contain a mixture of young glass, plagioclases with relatively old calcium-rich cores and moderately old sodium-rich rims. Because Austin clearly understands the heterogeneous composition of this 'fraction', he should have known that a K-Ar date on this mess would be meaningless. 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.
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.
No attempt was made to separate the hornblende from the Fe-Ti oxides, but further finer milling and use of heavy liquids should be considered.
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At this point Austin admits that the iron mortar probably contaminated his sample. Although the contamination might have seriously affected any iron analyses, K and Ar analyses may not have been affected. The description of another one of Austin's 'fractions' indicates that it is also highly impure: These mafic microphenocrysts and fragments of mafic phenocrysts evidently increased the density of the attached glass particles above the critical density of 2.
This sample also had recognizable hornblende, evidently not completely isolated by magnetic separation. Because it was composed of finer particles mesh , it contained far fewer mafic particles with attached glass fragments than DOME-IH. This preparation is the purest mineral concentrate. Therefore, instead of dating the ages of the pyroxenes, he probably dated a mixture of mostly pyroxenes along with other minerals and volcanic glass. Again, a K-Ar date on such an impure 'fraction' would be meaningless and a waste of time and money. That is, Austin is not dating the volcanic glass or the pyroxenes in the dacite, but artificial mixtures, which result from incomplete separations.
However, because Austin ignores the analytical inadequacies of Geochron's mass spectrometer hypothesis 2 , except for possibly the pyroxenes, there is no evidence that excess argon is present in any of the other mineral or glass components in this sample. Because Austin admits that his separations were impure, how can he, Swenson and other YECs justify their claims that these dacite samples were a fair test of the validity of the K-Ar method? Why did Austin waste precious time and money analyzing samples that were known to contain mineral and glass impurities?
As a geologist, Austin should have known that minerals, especially zoned minerals, take more time to crystallize than quenched disorder glass. How could he expect the relatively large and sometimes zoned minerals to be as young as the glass?!!
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The following additional comments by Swenson demonstrate that he does not understand the mineralogy and chemistry of the dacite: However, Dalrymple  found that even volcanic glass can give wrong ages and rationalized that it can be contaminated by argon from older rock material. I should state that Swenson did not have the courtesy to name this critic it's me or cite even one of my sources that criticize Austin's efforts.
In any debate, the debaters should provide the references or Internet links for their opponents so that the readers can evaluate both sides and really understand what's going on. Clearly, Swenson simply assumes that the volcanic glass contains 'excess argon. In his essay, Austin even admits that the glass still needs to be separated and analyzed for argon. Furthermore, many studies for example, the Haulalai basalt; Funkhouser and Naughton, demonstrate that Swenson and other YECs cannot automatically assume that modern volcanic glass contains excess argon.
Although hypothesis 1 is plausible, until the argon isotope concentrations of the PURE glass are accurately measured for Austin's dacite if this is even possible we cannot properly evaluate this hypothesis. Because Swenson does not provide a page number for his citation of Dalrymple , the identity of the volcanic glass with excess argon is uncertain. Perhaps, Swenson was referring to the following statement from Dalrymple , p.
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If Swenson is referring to this section, it's nothing more than an irrelevant red herring. Although high-pressure ocean water may prevent argon gas from escaping from the rims of a lava flow on the ocean floor, the centers of modern submarine flows typically provide K-Ar dates of 'zero years' Young, , p. Because the centers of the flows cool more slowly, any excess 40Ar and other gases can disperse out of the remaining melt before solidification. While YECs explain geology by invoking talking snakes, magical fruit, and a mythical 'Flood', Dalrymple discusses legitimate chemistry and fluid physics, which is hardly relying on flimsy 'rationalizations' or implausible excuses.
Furthermore, contrary to Swenson's claims, nothing in Dalrymple excuses Austin's sloppy approach to K-Ar dating. In particular, YECs have no justification for automatically assuming that the dacite glass contains excess argon. Even if the dacite glass does contain excess argon, Dalrymple , p. That is, as the volcanics age, the excess argon would be diluted into insignificance by the developing radiogenic 40Ar. Furthermore, if abundant excess argon is present in older rocks, Ar-Ar dating and K-Ar isochron dating can detect and eliminate its effects as examples, McDougall and Harrison, , p.
Austin clearly believes that the ancient dates for his samples entirely resulted from excess argon hypothesis 1: Orthopyroxene retains the most argon, followed by hornblende, and finally, plagioclase. It's certainly plausible that some excess argon could accumulate in small fractures or defects within the crystalline structures of pyroxenes, amphiboles, feldspars and other minerals Dickin, , p. While Austin claims that orthopyroxenes should retain the most argon followed by hornblende an amphibole and finally plagioclase, he provides no references to support this claim.
In reality, the crystalline structures of amphiboles, unlike feldspars and pyroxenes, contain open channels, which can hold argon gas and other fluids Klein and Hurlbut, , p. I'm skeptical that the defects and fractures in the orthopyroxenes and feldspars of Austin's dacites could hold more excess argon per mineral volume than the relatively large open structures within the hornblendes Dickin, , p. Therefore, IF hypothesis 1 was the only factor influencing the dates of Austin's samples, I would expect the hornblende-rich 'fraction' to provide an older date than the pyroxene- and feldspar-rich 'fractions.
From the above discussions, we already know that hypothesis 2 is a likely explanation for Austin's old dates. To evaluate hypothesis 3, we should look at the crystallization order of the phenocrysts as suggested by Bowen's Reaction Series.
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