Methods of Dating the Age of Meteorites
The meteorite that exploded over a Russian city in February was around meteor was determined by a technique called radiometric dating, observing The scientists said the meteor was typical in its makeup, being a. Lunar meteorites, or lunaites, are meteorites from the Moon. However, as explained in more detail below, all meteorites contain certain isotopes (nuclides) chemical and mineral composition) that the “YAMM” meteorites. Meteorites date the earth with a ± Ga Pb-Pb isochron by the Pb isotopic composition of a modern oceanic sediment sample.
It was already known that radium was an intermediate product of the decay of uranium. Rutherford joined in, outlining a decay process in which radium emitted five alpha particles through various intermediate products to end up with lead, and speculated that the radium-lead decay chain could be used to date rock samples.
Boltwood did the legwork, and by the end of had provided dates for 26 separate rock samples, ranging from 92 to million years. He did not publish these results, which was fortunate because they were flawed by measurement errors and poor estimates of the half-life of radium. Boltwood refined his work and finally published the results in His studies were flawed by the fact that the decay series of thorium was not understood, which led to incorrect results for samples that contained both uranium and thorium.
However, his calculations were far more accurate than any that had been performed to that time. Refinements in the technique would later give ages for Boltwood's 26 samples of million to 2. Rutherford remained mildly curious about the issue of the age of Earth but did little work on it.
Age of the Earth - Wikipedia
Robert Strutt tinkered with Rutherford's helium method until and then ceased. However, Strutt's student Arthur Holmes became interested in radiometric dating and continued to work on it after everyone else had given up.
Holmes focused on lead dating, because he regarded the helium method as unpromising. He performed measurements on rock samples and concluded in that the oldest a sample from Ceylon was about 1. For example, he assumed that the samples had contained only uranium and no lead when they were formed.
More important research was published in It showed that elements generally exist in multiple variants with different masses, or " isotopes ". In the s, isotopes would be shown to have nuclei with differing numbers of the neutral particles known as " neutrons ". In that same year, other research was published establishing the rules for radioactive decay, allowing more precise identification of decay series.
Many geologists felt these new discoveries made radiometric dating so complicated as to be worthless. His work was generally ignored until the s, though in Joseph Barrella professor of geology at Yale, redrew geological history as it was understood at the time to conform to Holmes's findings in radiometric dating. Barrell's research determined that the layers of strata had not all been laid down at the same rate, and so current rates of geological change could not be used to provide accurate timelines of the history of Earth.
Holmes published The Age of the Earth, an Introduction to Geological Ideas in in which he presented a range of 1. No great push to embrace radiometric dating followed, however, and the die-hards in the geological community stubbornly resisted.
They had never cared for attempts by physicists to intrude in their domain, and had successfully ignored them so far.
Holmes, being one of the few people on Earth who was trained in radiometric dating techniques, was a committee member, and in fact wrote most of the final report. Questions of bias were deflected by the great and exacting detail of the report. It described the methods used, the care with which measurements were made, and their error bars and limitations.
Techniques for radioactive dating have been tested and fine-tuned on an ongoing basis since the s. Forty or so different dating techniques have been utilized to date, working on a wide variety of materials.
Dates for the same sample using these different techniques are in very close agreement on the age of the material. The quoted age of Earth is derived, in part, from the Canyon Diablo meteorite for several important reasons and is built upon a modern understanding of cosmochemistry built up over decades of research.
These cool more slowly and have larger crystals, often forming granite. Both of these tend on the average to have wide biostrategraphic limits, meaning that a large spread of ages will be regarded as non-anomalous. And if we recall that most radiometric dating is done of igneous bodies, one sees that the percentage of anomalies is meaningless.
Thus we really need some evidence that the different methods agree with each other. To make the case even stronger, "Many discrepant results from intrusives are rationalized away immediately by accepting the dates but reinterpreting the biostrategraphic bracket," according to John Woodmorappe. This of course means that the result is no longer anomalous, because the geologic period has been modified to fit the date.
Finally, the fact that the great majority of dates are from one method means that the general but not universal agreement of K-Ar dating with itself is sufficient to explain the small percentange of anomalies if it is small. Back to top Now, the point about agreement is that whatever figure is given about how often ages agree with the expected age, is consistent with the fact that there is no agreement at all between K-Ar and other methods, since so many measurements are done using K-Ar dating.
And one of the strongest arguments for the validity of radiometric dating is that the methods agree. So when one combines all of the above figures, the statement that there are only 10 percent anomalies or 5 percent or whatever, does not have any meaning any more. This statement is made so often as evidence for the reliability of radiometric dating, that the simple evidence that it has no meaning, is astounding to me.
I don't object to having some hard evidence that there are real agreements between different methods on the geologic column, if someone can provide it.
The precambrian rock is less interesting because it could have a radiometric age older than life, but this is less likely for the rest of the geologic column. It's not surprising that K-Ar dates often agree with the assumed dates of their geological periods, since the dates of the geological periods were largely inferred from K-Ar dating. By the way, Ar-Ar dating and K-Ar dating are essentially the same method, so between the two of them we obtain a large fraction of the dates being used.
Some information from an article by Robert H. History of the Radioisotope based Geologic Time Scale Before the discovery of radioactivity in the late nineteenth century, a geological time scale had been developed on the basis of estimates for the rates of geological processes such as erosion and sedimentation, with the assumption that these rates had always been essentially uniform.
On the basis of being unacceptably old, many geologists of the time rejected these early twentieth century determinations of rock age from the ratio of daughter to radioactive parent large. Byincreased confidence in radioisotope dating techniques and the demands of evolution theory for vast amounts of time led to the establishment of an expanded geological time scale. The construction of this time scale was based on about radioisotope ages that were selected because of their agreement with the presumed fossil and geological sequences found in the rocks.
Igneous rocks are particularly suited to K-Ar dating. The crucial determiners are therefore volcanic extrusive igneous rocks that are interbedded with sediments, and intrusive igneous rocks that penetrate sediments. This verifies what I said about almost all of the dates used to define correct ages for geologic periods being K-Ar dates.
Also, the uncertainty in the branching ratio of potassium decay might mean that there is a fudge factor in K-Ar ages of up to a third, and that the occasional agreements between K-Ar ages and other ages are open to question. So the point is that there is now no reason to believe that radiometric dating is valid on the geologic column.
Back to top Another issue is that sometimes the geologic periods of rocks are revised to agree with the ages computed. This also makes data about percentages of anomalies less meaningful.
It sometimes seems that reasons can always be found for bad dates, especially on the geologic column. If a rock gives a too old date, one says there is excess argon. If it gives a too young date, one says that it was heated recently, or cannot hold its argon. How do we know that maybe all the rocks have excess argon? It looks like geologists are taking the "majority view" of K-Ar dating, but there is no necessary reason why the majority of rocks should give the right date.
The following quote is from the article by Robert H. What is a Radioisotope Age? The relationship of a radioisotope age with real-time must be based on an interpretation. A discussion of rubidium-strontium ages in the Isotope Geoscience Section of the journal, Chemical Geology, specifically states that a radioisotope age determination "does not certainly define a valid age information for a geological system.
Any interpretation will reflect the interpreters presuppositions bias. Back to top Concerning the need for a double blind test, it would seem that there are many places where human judgment could influence the distribution of measured radiometric dates.
It could increase the percentage of anomalies, if they were regarded as more interesting. It could decrease them, if they were regarded as flukes. Human judgment could determine whether points were collinear enough to form an isochron.
It could determine whether a point can justifiably be tossed out and the remaining points used as an isochron. It could determine whether one should accept simple parent-to-daughter K-Ar ratios or whether some treatment needs to be applied first to get better ages. It could influence whether a spectrum is considered as flat, whether a rock is considered to have undergone leaching or heating, whether a rock is porous or not, or whether a sample has been disturbed in some way.
Since one of the main reasons for accepting radiometric dates at least I keep hearing it is that they agree with each other, I think that geologists have an obligation to show that they do agree, specifically on the geologic column. Since we do not know whether or how much human judgment is influencing radiometric dating, a double blind study is most reasonable. And it should not be restricted to just one or two well-behaved places, but should be as comprehensive as possible. Back to top The following information was sent to me by e-mail: Radiometric dating is predicated on the assumption that throughout the earth's history radioactive decay rates of the various elements have remained constant.
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Is this a warranted assumption? Has every radioactive nuclide proceeded on a rigid course of decay at a constant rate?
This has been challenged by studies involving Carbon C At the temperature or pressure, collisions with stray cosmic rays or the emanations of other atoms may cause changes other than those of normal disintegration. It seems very possible that spontaneous disintegration of radioactive elements are related to the action of cosmic rays and the rate of disintegration varying from century to century according to the intensity of the rays.
The evidence for a strongly increasing change in the cosmic ray influx is most favorable especially in light of the decay of the earth's magnetic field. Most geochronologists maintain that pleochroic haloes give evidence that decay constants have not changed.
Crystals of biotite, for example, and other minerals in igneous or metamorphic rocks commonly enclose minute specks of minerals containing uranium or thorium. The a- alpha particles emitted at high velocity by the disintegrating nuclides interact, because of their charge, with electrons of surrounding atoms which slow them down until they finally come to rest in the host material at a distance from their source that depends on their initial kinetic energy and the density and composition of the host.
Where they finally stop to produce lattice distortions and defects there generally occurs discoloring or darkening. Each of the 8 a-particles emitted during the disintegration of U to Pb produces a dark ring in biotite. Each ring has its own characteristic radius in a given mineral in this case biotite. This radius measures the kinetic energy, hence the probability of emission of the corresponding a-particle and also the half-life of the parent nuclide according to the Geiger-Nuttall law.
The Geiger-Nuttall law is an empirical relation between half-life of the a-emitter and the range in air of the emitted a-particles. If the radii of these haloes from the same nuclide vary, this would imply that the decay rates have varied and would invalidate these series as being actual clocks. Are the radii in the rocks constant in size or are there variable sizes?
Most of the early studies of pleochroic haloes were made by Joly and Henderson. Joly concluded that the decay rates have varied on the basis of his finding a variation of the radii for rocks of alleged geological ages. This rather damaging result was explained away saying that enough evidence of correct radii for defferent geologic periods and sufficient variation in the same period have been obtained that one is forced to look for a different explanation of such variations as were observed by Joly.
Measurements were later made in an excellent collection of samples with haloes.Radioactive Dating
It was found that the extent of the haloes around the inclusions varies over a wide range, even with the same nuclear material in the same matrix, but all sizes fall into definite groups. The measurements are, in microns, 5,7,10,17,20,23,27, and More recent studies have been made by Robert V.
Gentry also finds a variation in the haloes leading him to conclude that the decay constants have not been constant in time. Gentry points out an argument for an instantaneous creation of the earth. He noted form his studies of haloes: For the Po half-life of 3 minutes only a matter of minutes could elapse between the formation of the Po and subsequent crystallization of the mica; otherwise the Po would have decayed, and no ring would be visible.
The occurrence of these halo types is quite widespread, one or more types having been observed in the micas from Canada Pre-CambrianSweden, and Japan. So, then, careful scientists have measured variations in halo radii and their measurements indicate a variation in decay rates. The radioactive series then would have no value as time clocks. The following quotation also suggests a cause for a change in the decay rate: Jueneman Industrial Research, Sept.
The remnant of that local big bang is a pulsar called Vela-X PSRwhich recent observations have positioned in the southern sky some 1, light years away, and which is considered to have given rise to the huge Gum Nebula Being so close, the anisotropic neutrino flux of the super-explosion must have had the peculiar characteristic of resetting all our atomic clocks.
This is significant because it is known that neutrinos do interact with the nucleii of atoms, and it is also believed that much of the energy of supernovae is carried away by neutrinos.
Back to top Isochrons are an attempt to avoid the need for an absence of daughter element initially in computing radiometric ages. The idea is that one has a parent element, X, a daughter element, Y, and another isotope, Z, of the daughter that is not generated by decay. One would assume that initially, the concentration of Z and Y are proportional, since their chemical properties are very similar. Radioactive decay would generate a concentration of Y proportional to X.
A good general introduction to isochrons from an evolutionary perspective can be found at http: If the concentration of K varies in a rock, that it is unlikely for the concentration of added argon 40 to vary in a way that will yield an isochron.
But if the concentration of K does not vary, then one can still get an isochron if the concentration of the non-radiogenic isotope Ar36 of the daughter product varies. So let's call an isochron a "super-isochron" if the concentration of the parent element varies from one sample to another. Let's call it a "wimpy isochron" otherwise. The question is, what percentage of isochrons are super-isochrons, and how do their dates agree with the conventional dates for their geologic period? I would think that it may be rare to have a super-isochron.
If one is dealing with minerals that exclude parent or daughter, then one cannot get an isochron at all. If one is dealing with minerals that do not exclude parent and daughter elements, then most likely the parent element will be evenly distributed everywhere, and one will have a wimpy isochron that cannot detect added daughter product, and thus may give unreliable ages.
Whole rock isochrons may also tend to be wimpy, for the same reason. Even super isochrons can yield ages that are too old, due to mixings, however. False K-Ar isochrons can be produced if a lava flow starts out with a lot of excess Ar40 which becomes well mixed, along with potassium. Then while cooling or afterwards, a mixture of Ar36 and Ar40 can enter the rock, more in some places than others. Other isotopes of argon would work as well.
I believe that this will produce a good K-Ar isochron, but the age calculated will be meaningless. There is another way that false isochrons can be produced. For a wimpy isochron, say a K-Ar isochron, we can assume that initially there is a uniform concentration of K everywhere, and concentrations of Ar40 and Ar36 that form an isochron. Then a lot of Ar40 enters, uniformly, through cracks in the rock or heating.
This will retain the isochron property, but will make the isochron look too old. My reasoning was that if the lava is thoroughly mixed, then the concentration of parent material should be fairly constant.
If the concentration of parent substance is not constant, it could indicate that the lava is not thoroughly mixed. Or it could have other explanations. If the lava is not thoroughly mixed, it is possible to obtain an isochron from the mixing of two different sources, in which case the radiometric age is inherited from the sources, and does not necessarily yield the age of the flow.
Someone pointed out to me that many Rb-Sr isochrons are super isochrons. I find this information very interesting, and thank him for it.
I'd be curious to know which strata they occur in, as my main interest is the geologic column of Cambrian and above. My impression is that these are not on this part of the geologic column.
And how well do the dates correlate with others for the same formation? There are also mixing scenarios that can produce even super isochrons having invalid ages. And geologists admit in any event that isochrons can sometimes give false ages. Here is a mixing scenario for false isochrons.
Specifically, they are called mare basalts because they formed when magmas from inside the Moon erupted petrology into the basins formed by the impacts of small asteroids early in lunar history to form the maria. NWA is a fragmental breccia 2. Note that in this and other brecciated lunar meteorites, the clasts are not particularly colorful.
Some lunar meteorites from hot deserts are more colorful than lunar meteorites from Antarctica because the hot-desert meteorites have suffered a greater degree of chemical alteration from interaction with liquids since landing on Earth. Many lunar meteorites from Oman e. Breccias Breccias are rocks made up of bits and pieces of other rocks clasts in a matrix of finer-grained rock fragments, glass, or crystallized melt.
Monomict breccia is a term applied to a breccia that is made up entirely one kind of rock. Monomict breccias are rare on the Moon because meteoroid impacts tend to mix different kinds of rocks. Dimict breccias or dilithologic breccias are made up of only two lithologies. The term is usually applied to a common type of rock collected on the Apollo 16 mission that consists of anorthosite light color and mafic dark, iron rich crystallized impact melt in a mutually intrusive textural relationship.
SaUhowever, could be regarded as a dilithologic breccia. Types of polymict breccias are glassy melt breccias, impact-melt breccias, granulitic breccias, regolith breccias, and fragmental breccias. Each of these breccia types has a different texture because the set of conditions that formed them differed. An impact-melt breccia can be regarded as in igneous rock because it formed from the cooling of a melt.
Regolith and fragmental breccias are the closest lunar equivalents to terrestrial sedimentary rocks. Granulitic breccias are metamorphic rocks in that they were some other type of breccia that was metamorphosed recrystallized by the heat of an impact. Most brecciated lunar meteorites are regolith breccias. Some kinds of terrestrial rocks strongly resemble lunar regolith breccias e.
Igneous anorthosites are rare in the lunar highlands, but some were found on the Apollo missions. Impacts of asteroidal meteorites on the Moon both break rocks of the lunar crust apart and glue them back together. An impact can melt rock, forming impact melt. The melt usually collects rock fragments called clasts as it is forced away from the point of impact within a crater.
When the melt cools, it forms an impact-melt breccia - clasts suspended in a matrix of solidified glass or crystalline impact melt. The lunar surface is covered with fine-grained material called soil or regolith. The shock wave associated with an impact can lithify the regolith - it can turn the fine, powdery material into a coherent rock called a regolith breccia. At depth, coarser fragments can be lithified to form a fragmental breccia. Breccia is a textural term that applies to rocks of both the maria and highlands.
Most lunar meteorites are feldspathic regolith breccias, that is, rocks consisting of lithified soil from the lunar highlands. Most highlands rocks are breccias because the highlands crust is very old and the impact rate was greater in early lunar history than during the time since the magmas forming mare basalts erupted.
The lunar crust is formed mainly from a light-colored, aluminum-rich mineral known as anorthite, a plagioclase feldspar. Early in lunar history the crust was impacted by small asteroids to form large craters called basins. Dark, iron-rich magmas generated from melting inside the Moon erupted into the basins. To ancient astronomers the resulting dark, circular features resembled seas. Rocks from the maria contain some feldspar but consist mostly of pyroxene, olivine, and ilmenite, which are minerals that are rich in iron and poor in aluminum.
The concentration of iron or aluminum serves as a useful chemical classification system in lunar rocks. Lunar meteorites that are mare basalts e. In contrast, meteorites from the feldspathic highlands are rich in aluminum and poor in iron.
Glass spherules and basalt fragments from the maria have been found as clasts in most of the highlands meteorites and some e. Such meteorites plot on the high-iron end of the range of highlands feldspathic lunar meteorites. Some intermediate lunar meteorites e. All regolith samples from the Apollo 15 and 17 missions are mixed in this way. Such meteorites have intermediate concentrations of iron and aluminum. We might expect, as more lunar meteorites are found, that the gaps in the aluminum-iron plot above will be filled in.
It may seem, considering that kg of well-documented rock and soil samples were obtained from nine locations by the Apollo and Luna missions, that a few small rocks from unknown points on the lunar surface cannot be very important.
For several reasons, however, the lunar meteorites have provided new and useful information. This existence of this hot spot, sometimes known as the Procellarum KREEP Terrane or PKT, indicates that the mare-highlands distinction of the ancient astronomers is not adequate in a geochemical sense.
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Many rocks collected on the Apollo missions that likely originated from the PKT especially those from Apollos 12, 14, and 15 are neither mare basalts nor feldspathic breccias. K potassiumTh thoriumand U uranium. Almost certainly, it derives from the PKT. This distribution is reasonable in that we believe that the lunar meteorites are rocks from randomly distributed locations on the lunar surface, and most locations on the lunar surface are not high in radioactivity.
A Lot of Lunar Meteorites? This impact event delivered 25— km3 of lunar material to the Earth, i. These massive deposits may be found in proper stratigraphic layers similar to the Ordovician meteorites . The center of the map shows the nearside and the left and right edges show the far side of the Moon. The locations of the six Apollo A and three Russian Luna L landing sites are indicated all on the nearside.
The bottom part of the diagram shows the concentrations of Th in lunar meteorite source craters. Most lunar meteorites have low Th concentrations but a few have high concentrations see last column of the List. The figure shows that 1 the Apollo missions all landed in or near a region of the Moon with anomalously high radioactivity the anomaly, which we call the PKT Procellarum KREEP Terrane was not known at the time of Apollo site selection and 2 most of the lunar meteorites must come from areas of the Moon that are distant from the PKT because most have low Th.
Thus, one of the values of the lunar meteorites is that they are samples from places on the Moon that are more typical of the lunar surface low radioactivity than the Apollo samples. Also, most of the lunar meteorites are breccias composed of fine material from near the surface of the Moon. This fine-grained material has been mixed by many impacts. As a consequence, the composition and mineralogy of a brecciated lunar meteorite is likely to be more representative of the region from which it came than any single unbrecciated igneous rock from the same region.
Map of the surface concentration of iron expressed as FeO on the lunar nearside left and far side rightbased on spectral reflectance measurements taken by the Clementine mission in High-FeO areas occur where volcanic lavas mare basalts filled giant impact craters. Low-FeO areas correspond to the feldspathic highlands. Image courtesy of Jeff Gillis. Because the distributions have the same shape and because the peak occurs at the same concentration, we can reasonably infer that the lunar meteorites are random samples from the surface of the Moon.
The lunar meteorite data are updated end of from Korotev et al