Potassium, an alkali metal, the Earth's eighth most abundant element is common in many rocks and rock-forming minerals. The quantity of potassium in a rock or mineral is variable proportional to the amount of silica present. Therefore, mafic rocks and minerals often contain less potassium than an equal amount of silicic rock or mineral. Potassium can be mobilized into or out of a rock or mineral through alteration processes. Due to the relatively heavy atomic weight of potassium, insignificant fractionation of the different potassium isotopes occurs. However, the 40 K isotope is radioactive and therefore will be reduced in quantity over time. But, for the purposes of the KAr dating system, the relative abundance of 40 K is so small and its half-life is so long that its ratios with the other Potassium isotopes are considered constant.
Info Print Cite. Submit Feedback. Thank you for your feedback. The Editors of Encyclopaedia Britannica Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree See Article History. Read More on This Topic. This is possible in potassium-argon K-Ar dating, for example, because most minerals do not take argon into their structures initially Learn More in these related Britannica articles:.
This is possible in potassium-argon K-Ar dating, for example, because most minerals do not take argon into their structures initially. The Decay Profile In this simulation, a unit of molten rock cools and crystallizes.
The ratio of K to Ar is plotted. Note that time is expressed in millions of years on this graph, as opposed to thousands of years in the C graph. Click on the "Show Movie" button below to view this animation.
K-Ar Decay Profile. How are Samples Processed? Clicking on the "Show Movie" button below will bring up an animation that illustrates how a K-Ar sample is processed and the calculations involved in arriving at a date. This is actually a mini-simulator, in that it processes a different sample each time and generates different dates. K-Ar Processing. Limitations on K-Ar Dating The Potassium-Argon dating method is an invaluable tool for those archaeologists and paleoanthropologists studying the earliest evidence for human evolution.
As with any dating technique, there are some significant limitations. Contents 2. Introduction 3.
I've attempted to separate very fined-grained minerals from glass in coal ashes by using magnetic separation and hydrofluoric and other acids. It's not easy. Specifically, Austin admits that most of his fractions are impure when he includes the term 'etc.
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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. Austin also states:.
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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.
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 meshit 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.
Finally, Austin states:.
Think, that potassium dating limitations congratulate
However, because Austin ignores the analytical inadequacies of Geochron's mass spectrometer hypothesis 2except 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?!! 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.
Considering that the half-life of potassium (40K) is fairly long (1, million years, McDougall and Harrison, , p. 9), the K-Ar method cannot be used to date samples that are much younger than 6, years old (Dalrymple, , p. 93). Potassium-argon dating, abbreviated K-Ar dating, is a radiometric dating method used in geochronology and fatgirlnmotion.com is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). Potassium is a common element found in many materials, such as micas, clay minerals, tephra, and fatgirlnmotion.com these materials, the decay product Among the age of because argon dating method in two are some significant limitations of unspiked potassium-argon dating and argon to the k-ar dating limitations. Potassium argon dating age range Because potassium-argon isotopic dating to evaluate the argon into stable argon invalidates the k-ar isotopic age of the k in once-living organic.
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.
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Because Swenson does not provide a page number for his citation of Dalrymplethe identity of the volcanic glass with excess argon is uncertain. Perhaps, Swenson was referring to the following statement from Dalrymplep. If Swenson is referring to this section, it's nothing more than an irrelevant red herring.
Unlike the Mt. Dalrymplep. 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, Dalrymplep. 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.
Its Limitations and Usefulness "Combining the effects of these two trees, we see a site that was actually occupied for years (from to BCE) appearing - using conventional radiocarbon dating - to have been occupied for 30, years (from 40, to 9, BCE).". Potassium-argon dating is accurate from billion years (the age of the Earth) to about , years before the present. At , years, only of the potassium in a rock would have decayed to argon, pushing the limits of present detection devices. Both potassium how k-ar dating, geologists are stretching limitations upper and absolute age limit limitations potassium to get absolute. Dating methods is reveals on our website.
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. Therefore, the 2. 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. The series states that certain minerals will crystallize in a melt at higher temperatures than other minerals. That is, different minerals have different freezing points.
Mafic magnesium and iron-rich volcanic rocks, such as basalts, form from relatively hot melts C and hotter, Hall,p. Felsic silica-rich rocks, such as granites, form at cooler temperatures perhaps as cool as CHall,p. The most common minerals in rocks of intermediate chemistry, such as dacites, are located towards the middle of the series.
Bowen's Reaction Series is a very important concept that undergraduate students learn in their introductory physical geology courses. To be exact, Bowen's Reaction Series was the one diagram that I was required to memorize when I took my first geology course in college. Although Bowen's Reaction Series was established long ago by field and laboratory studies, Swenson, Austin and other YECs repeatedly fail to comprehend its importance and how it can produce ancient phenocrysts, which may affect the radiometric dating of very young samples.
In a young volcanic rock, such as the Mt.
Since its development in the IS, potassium-argon dating has been applied radiometric dating suffer from limitations of applicability owing to the absence of. potassium argon dating limitations Jul 15, - Abstract Since its development in the s, potassium?argon dating has seen several modifications. The original method of measuring the. Potassium decays with a half-life of million years, meaning that half of the 40K atoms are gone after that span of time. Its decay yields argon and calcium in a ratio of 11 to The K-Ar method works by counting these radiogenic 40Ar atoms trapped inside minerals. Potassium-argon dating, method of determining the time of origin of rocks by measuring the ratio of radioactive argon to radioactive potassium in the rock. This dating method is based upon the decay of radioactive potassium to radioactive argon in minerals and rocks; potassium also decays to calcium Thus, the ratio of argon and potassium and radiogenic calcium to.
Helen's dacite, the calcium-rich plagioclases may have formed thousands or even a few million years ago. Again, as a rock ages and 40Ar accumulates in both the glass and any 40K-bearing minerals, the differences in the ages of the materials becomes less significant.
That is, if the glass quenched in an eruptionyears after the formation of the calcium-rich plagioclases, after Origins': J. Bowen's Reaction Series also predicts that pyroxenes will crystallize at higher temperatures before amphiboles. Assuming that any argon contamination from Geochron's equipment hypothesis 2 is negligible, we see that the dates in Austin's table are consistent with the crystallization order in Bowen's Reaction Series.
As expected, the purest pyroxene fraction provides an older date 2. That is, IF the dates are real, the pyroxenes formed in the melt before the amphiboles as predicted by the series.
Because the pyroxenes solidify before most other minerals, it's also not surprising that the 'pyroxene, etc. Depending upon the amount of zoned feldspars which consist of older calcium-rich cores and younger sodium-rich rims and the quantity of glass, amphibole and pyroxene impurities, the 'feldspar etc.
On the basis of the following statements by Swenson, his gross misinterpretations of Dalrympleand his unwillingness to respond to my earlier statements on Bowen's Reaction Series and its possible relevance to Austin's results, it is clear that Swenson does not know what Bowen's Reaction Series is and how it can affect the age distributions of minerals in very young volcanic rocks:.
They said that Dr Austin should have known they were old because the crystals were large and zoned. However, Dr Austin's results Table 1 show that the wrong ages were not confined to one particular mineral.
The idea that the age of a mineral can be anticipated by its size or colour is incorrect.
Dalrymplefor example, found that the wrong ages in his samples were unrelated to crystal size, or any other observable characteristic of the crystal. Contrary to Swenson's implications, mineral zoning is much more than a color property.
As discussed earlier, zoning and crystal growth are extremely important in understanding phenocryst ages. Based on the statements in his essays, Swenson simply assumes that excess argon is present in all of the components of the dacite and that any statements on the lack of a relationship between excess argon and crystal size in Dalrymple automatically apply to Austin's dacite. Again, because Swenson does not provide any page numbers when referring to Dalrymplewe can only guess which sections of Dalrymple's article he is citing.
The results for the Mt. Lassen plagioclase and the Mt. Etna flow, which contains a HIGH percentage of large phenocrysts, appear to support their contention. Thus, for THESE experiments there does not appear to be any correlation of excess 40Ar with large phenocrysts or with any other petrological or petrographic parameter. Clearly, whether amphibole, pyroxene, plagioclase or other phenocrysts are effectively degassed or not during eruptions is a complex and, perhaps, ufatgirlnmotion.comedictable issue.
Nevertheless, as discussed in Dalrymplep.
General Chemistry Lecture 200 - Potassium-Argon, Uranium-Lead Dating (Geological)
Furthermore, if excess argon is relatively abundant in older samples, Ar-Ar dating and K-Ar isochron dating can detect and eliminate its effects as examples, McDougall and Harrison,p. Swenson confidently proclaims:. They claim that these pieces of old rock xenoliths contaminated the sample and gave the very old age.
Potassium dating limitations
In his report, Austin refers to the presence of 'lithic inclusions' in his samples. Helens lava dome is 'lithic inclusions':. Although the mineral concentrates are not pure, and all contain some glass, an argument can be made that both mafic and non-mafic minerals of the dacite contain significant 40 Ar. The lithic inclusions in the lava dome might be thought to be the contaminant, in which case they might add "old" mafic and non-mafic minerals to the young magma.
It could be argued that gabbroic clumps in the magma disaggregated as the fluidity of the magma decreased with time, thereby adding an assortment of 'old' mineral grains.
These inclusions are, therefore, regarded as a unique association within the recent magmatic system.