http://congdong.bancongxanh.com/2809.php So now we can find a date for the rock. This one additional piece of information about the initial state of the rock allows us to calculate its age. As with the other methods we've discussed so far, the Rb-Sr method will only work if nothing but the passage of time has affected the distribution of the key isotopes within the rock. And of course this is not necessarily the case.
Hydrothermal or metasomatic events may have added or subtracted rubidium and strontium to or from the rocks since their formation; or a metamorphic event may have redistributed the rubidium or strontium among its constituent minerals , which would also interfere with the method. However, barring an extraordinary coincidence, the result of such events will be that when we draw the isochron diagram, the minerals will no longer lie on a straight line.
A small deviation from a straight line tells us that there is some uncertainty about the date, and this degree of uncertainty can be calculated; and if we get something which is nothing like a straight line, then the method simply doesn't supply us with a date. So just as step heating in Ar-Ar dating protects us from error, so too does the isochron method in Rb-Sr dating: There is, however, one potential source of error which will not show up on the isochron diagram, since it is expected to produce a straight line.
Suppose that the original source of the rock was two different magmas call them X and Y imperfectly mixed together so that some parts of the rock will be all X, some all Y, some part X and part Y in varying proportions.
Then these different parts of the rock, when analyzed for their isotopic composition, will plot in a straight line on the isochron diagram; and the slope of this line, and the point at which it intercepts the vertical axis, will have nothing to do with the age of the rock, and everything to do with the compositions of X and Y. About half the time this will produce a straight line with negative slope: Such a line must necessarily be produced by mixing, since a real isochron will always have positive slope: We can also test for mixing using what is known as a mixing plot: It can happen that if we produce a mixing plot for a perfectly good isochron, it will by some statistical fluke produce a straight line on the mixing plot; we would then be throwing out a perfectly good date.
The mobility of rubidium in deep-level crustal fluids and melts that can infiltrate other rocks during metamorphism as well as in fluids involved in weathering can complicate the results. The radioactive decay of samarium of mass Sm to neodymium of mass Nd has been shown to be capable of providing useful isochron ages for certain geologic materials.
Both parent and daughter belong to the rare-earth element group, which is itself the subject of numerous geologic investigations. All members of this group have similar chemical properties and charge, but differ significantly in size. Because of this, they are selectively removed as different minerals are precipitated from a melt. In the opposite sense, their relative abundance in a melt can indicate the presence of certain residual minerals during partial melting.
Unlike rubidium, which is enriched over strontium in the crust, samarium is relatively enriched with respect to neodymium in the mantle. Consequently, a volcanic rock composed of melted crust would have elevated radiogenic strontium values and depressed radiogenic neodymium values with respect to the mantle. As a parent—daughter pair, samarium and neodymium are unique in that both have very similar chemical properties, and so loss by diffusion may be reduced. Their low concentrations in surface waters indicates that changes during low-temperature alteration and weathering are less likely.
Their presence in certain minerals in water-deposited gold veins, however, does suggest mobility under certain conditions. In addition, their behaviour under high-temperature metamorphic conditions is as yet poorly documented. The exploitation of the samarium—neodymium pair for dating only became possible when several technical difficulties were overcome.
Procedures to separate these very similar elements and methods of measuring neodymium isotope ratios with uncertainties of only a few parts in , had to be developed. In theory, the samarium—neodymium method is identical to the rubidium—strontium approach. Both use the isochron method to display and evaluate data. In the case of samarium—neodymium dating, however, the chemical similarity of parent and daughter adds another complication because fractionation during crystallization is extremely limited.
This makes the isochrons short and adds further to the necessity for high precision. With modern analytical methods, however, uncertainties in measured ages have been reduced to 20 million years for the oldest rocks and meteorites.
Mineral isochrons provide the best results. The equation relating present-day neodymium isotopic abundance as the sum of the initial ratios and radiogenic additions is that of a straight line, as discussed earlier for rubidium—strontium. Other successful examples have been reported where rocks with open rubidium—strontium systems have been shown to have closed samarium—neodymium systems.
In other examples, the ages of rocks with insufficient rubidium for dating have been successfully determined. There is considerable promise for dating garnet , a common metamorphic mineral, because it is known to concentrate the parent isotope.
In general, the use of the samarium—neodymium method as a dating tool is limited by the fact that other methods mainly the uranium—lead approach are more precise and require fewer analyses. In the case of meteorites and lunar rocks where samples are limited and minerals for other dating methods are not available, the samarium—neodymium method can provide the best ages possible. The decay scheme in which rhenium is transformed to osmium shows promise as a means of studying mantle—crust evolution and the evolution of ore deposits. Osmium is strongly concentrated in the mantle and extremely depleted in the crust , so that crustal osmium must have exceedingly high radiogenic-to-stable ratios while the mantle values are low.
In fact, crustal levels are so low that they are extremely difficult to measure with current technology. Most work to date has centred around rhenium- or osmium-enriched minerals. Because rhenium and osmium are both siderophilic having an affinity for iron and chalcophilic having an affinity for sulfur , the greatest potential for this method is in studies concerning the origin and age of sulfide ore deposits. The radioactive decay scheme involving the breakdown of potassium of mass 40 40 K to argon gas of mass 40 40 Ar formed the basis of the first widely used isotopic dating method.
Since radiogenic argon was first detected in by the American geophysicist Lyman T. Nier , the method has evolved into one of the most versatile and widely employed methods available. In fact, potassium decays to both argon and calcium , but, because argon is absent in most minerals while calcium is present, the argon produced is easier to detect and measure.
Argon dating involves a different technology from all the other methods so far described, because argon exists as a gas at room temperature. Thus, it can be purified as it passes down a vacuum line by freezing out or reacting out certain contaminants. It is then introduced into a mass spectrometer through a series of manual or computer-controlled valves. Technical advances, including the introduction of the argon—argon method and laser heating, that have improved the versatility of the method are described below. In conventional potassium—argon dating , a potassium-bearing sample is split into two fractions: After purification has been completed, a spike enriched in argon is mixed in and the atomic abundance of the daughter product argon is measured relative to the argon added.
The amount of the argon present is then determined relative to argon to provide an estimate of the background atmospheric correction.
The equation relating present-day neodymium isotopic abundance as the sum of the initial ratios and radiogenic additions is that of a straight line, as discussed earlier for rubidium—strontium. Say, then, that their initial amounts are represented by quantities of A and cA respectively. In theory, the samarium—neodymium method is identical to the rubidium—strontium approach. Both parent and daughter belong to the rare-earth element group, which is itself the subject of numerous geologic investigations. The age of a sample is determined by analysing several minerals within the sample. When we produced the formula for K-Ar dating , it was reasonable enough to think that there was little to no argon present in the original state of the rock, because argon is an inert gas, does not take part in chemical processes, and so in particular does not take part in mineral formation.
In this case, relatively large samples, which may include significant amounts of alteration, are analyzed. Since potassium is usually added by alteration, the daughter—parent ratio and the age might be too low. A method designed to avoid such complexities was introduced by American geochronologist Craig M. Merrihue and English geochronologist Grenville Turner in In this technique, known as the argon—argon method, both parent and daughter can be determined in the mass spectrometer as some of the potassium atoms in the sample are first converted to argon in a nuclear reactor.
In this way, the problem of measuring the potassium in inhomogeneous samples is eliminated and smaller amounts of material can be analyzed. An additional advantage then becomes possible. However, if strontium 87 was present in the mineral when it was first formed from molten magma, that amount will be shown by an intercept of the isochron lines on the y-axis, as shown in Fig Thus it is possible to correct for strontium initially present. The age of the sample can be obtained by choosing the origin at the y intercept. Note that the amounts of rubidium 87 and strontium 87 are given as ratios to an inert isotope, strontium However, in calculating the ratio of Rb87 to Sr87, we can use a simple analytical geometry solution to the plotted data.
Again referring to Fig. Since the half-life of Rb87 is When properly carried out, radioactive dating test procedures have shown consistent and close agreement among the various methods. If the same result is obtained sample after sample, using different test procedures based on different decay sequences, and carried out by different laboratories, that is a pretty good indication that the age determinations are accurate. Of course, test procedures, like anything else, can be screwed up.
Mistakes can be made at the time a procedure is first being developed. Creationists seize upon any isolated reports of improperly run tests and try to categorize them as representing general shortcomings of the test procedure. This like saying if my watch isn't running, then all watches are useless for keeping time. Creationists also attack radioactive dating with the argument that half-lives were different in the past than they are at present.
There is no more reason to believe that than to believe that at some time in the past iron did not rust and wood did not burn. Furthermore, astronomical data show that radioactive half-lives in elements in stars billions of light years away is the same as presently measured. On pages and of The Genesis Flood, creationist authors Whitcomb and Morris present an argument to try to convince the reader that ages of mineral specimens determined by radioactivity measurements are much greater than the "true" i.
The mathematical procedures employed are totally inconsistent with reality. Henry Morris has a PhD in Hydraulic Engineering, so it would seem that he would know better than to author such nonsense. Apparently, he did know better, because he qualifies the exposition in a footnote stating:. This discussion is not meant to be an exact exposition of radiogenic age computation; the relation is mathematically more complicated than the direct proportion assumed for the illustration.
Nevertheless, the principles described are substantially applicable to the actual relationship. Morris states that the production rate of an element formed by radioactive decay is constant with time.
This is not true, although for a short period of time compared to the length of the half life the change in production rate may be very small. Radioactive elements decay by half-lives.
At the end of the first half life, only half of the radioactive element remains, and therefore the production rate of the element formed by radioactive decay will be only half of what it was at the beginning. The authors state on p.
If these elements existed also as the result of direct creation, it is reasonable to assume that they existed in these same proportions. Say, then, that their initial amounts are represented by quantities of A and cA respectively. Morris makes a number of unsupported assumptions: This is not correct; radioactive elements decay by half lives, as explained in the first paragraphs of this post. There is absolutely no evidence to support this assumption, and a great deal of evidence that electromagnetic radiation does not affect the rate of decay of terrestrial radioactive elements.
He sums it up with the equations: