Methods of Dating the Age of Meteorites Meteorites are among the oldest objects we know about – formed about 4. But how do scientists know this? This article describes the principles and methods used to make that determination. There are well-known methods of finding the ages of some natural objects. Trees undergo spurts in growth in the spring and summer months while becoming somewhat dormant in the fall and winter months. When a tree is cut down, these periods are exhibited in a cross section of the trunk in the form of rings. Simply counting the number of rings will give one a fairly good idea of the age of the tree. Periods of heavy rain and lots of sunshine will make larger gaps of growth in the rings, while periods of drought might make it difficult to count individual rings. When determining the ages of very old objects, the only suitable clocks we have found involve the measurement of decay products of radioactive isotopes. Isotopes are atoms of the same element with different amounts of neutrons.
This article was originally posted by Dr. Henke to the talk. I have placed it on the web with his permission.
The other common dating techniques are K/Ar and Rb/Sr if you want to google them. T_aquaticus UTC #9. Just one note. Geologists don’t directly measure the age of fossils. Instead, they date the igneous rocks around the fossils, be it in the layers where the fossil is found or the layers above and below the fossil.
Example[ edit ] For example, consider the case of an igneous rock such as a granite that contains several major Sr-bearing minerals including plagioclase feldspar , K-feldspar , hornblende , biotite , and muscovite. Rubidium substitutes for potassium within the lattice of minerals at a rate proportional to its concentration within the melt.
The ideal scenario according to Bowen’s reaction series would see a granite melt begin crystallizing a cumulate assemblage of plagioclase and hornblende i. This then causes orthoclase and biotite, both K rich minerals into which Rb can substitute, to precipitate. The resulting Rb-Sr ratios and Rb and Sr abundances of both the whole rocks and their component minerals will be markedly different. This, thus, allows a different rate of radiogenic Sr to evolve in the separate rocks and their component minerals as time progresses.
Calculating the age[ edit ] The age of a sample is determined by analysing several minerals within the sample. If these form a straight line then the samples are consistent, and the age probably reliable. The slope of the line dictates the age of the sample. Several preconditions must be satisfied before a Rb-Sr date can be considered as representing the time of emplacement or formation of a rock. Rb and Sr are relatively mobile alkaline elements and as such are relatively easily moved around by the hot, often carbonated hydrothermal fluids present during metamorphism or magmatism.
Conversely, these fluids may metasomatically alter a rock, introducing new Rb and Sr into the rock generally during potassic alteration or calcic albitisation alteration.
How Old is the Earth
Rubidium and cesium often occur together in nature. Rubidium, however, is more widely scattered and seldom forms a natural mineral; it is found only as an impurity in other minerals, ranging in content up to 5 percent in such minerals as lepidolite , pollucite, and carnallite. Brine samples have also been analyzed that contain up to 6 parts per million of rubidium.
In the principal commercial process of rubidium production, small amounts of rubidium are obtained from the mixture of alkali metal carbonates remaining after lithium salts are extracted from lepidolite. Primarily a potassium carbonate, this by-product also contains approximately 23 percent rubidium and 3 percent cesium carbonates.
The 87 Sr/ 86 Sr and 87 Rb/ 86 Sr ratios also suggest that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite), and that Ivory Coast tektites, with somewhat higher 87 Sr/ 86 Sr and 87 Rb/ 86 Sr ratios, are a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of.
Although they are thought to form from basinal brines, their exact origins are still unclear, partly because of the scarcity of reliable geochronological data. Rb-Sr dating of sphalerites has recently been shown to be a promising technique for the direct dating of ore minerals in MVT deposits. This paper reports the results of a reconnaissance study of sphalerites, their fluid inclusions, and associated minerals from MVT deposits of North America.
Rb-Sr isotopic analyses of K-feldspar from Immel mine preclude the possibility that the Rb-Sr data reflect feldspar inclusions. Sphalerites from the main ore zone of Daniel’s Harbour mine, New foundland, do not form a linear isochron and open behavior of the Rb-Sr system is suspected. These results are not compatible with mineralization models based on regional fluid migration related to early Tertiary Cordilleran deformation. Sphalerites from northern Arkansas have very low Rb and Sr concentrations less than 0.
The sphalerites are suspected to contain clay inclusions; and it is likely that the Sr isotopic compositions of these sphalerites, which have very low Sr concentrations, were affected by small amounts of inherited inclusions. Except for sphalerite from northern Arkansas, SEM studies and isotope dilution trace element measurements have so far failed to identify any suitable phases other than sphalerite that might be a host for the Sr.
Rb sr dating equation
Rubidium—strontium method The radioactive decay of rubidium 87Rb to strontium 87Sr was the first widely used dating system that utilized the isochron method. Because rubidium is concentrated in crustal rocks, the continents have a much higher abundance of the daughter isotope strontium compared with the stable isotopes. A ratio for average continental crust of about 0.
This difference may appear small, but, considering that modern instruments can make the determination to a few parts in 70, , it is quite significant. Dissolved strontium in the oceans today has a value of 0. Thus, if well-dated, unaltered fossil shells containing strontium from ancient seawater are analyzed, changes in this ratio with time can be observed and applied in reverse to estimate the time when fossils of unknown age were deposited.
Peninsula and also of the post-depositional events, we have applied the Rb–Sr method to date clay minerals from three, ﬁne-grained, pelitic formations.
The isochron age for the Magmont glauconite samples is about 30 percent younger than the age of their Cambrian host rocks. Because the Rb-Sr data from Magmont glauconite samples define an isochron, it appears that the glauconites behaved as a cogenetic suite which was isotopically homogenized m. It is very unlikely that episodic or incomplete loss or exchange of radiogenic strontium, or the addition of rubidium, would result in the formation of an isochron.
Because the total thickness of stratigraphic cover in the southeast Missouri region has never exceeded a kilometer, it is unlikely that the anomalously young glauconite isochron age resulted from heating associated with deep burial. Mississippi Valley-type ore fluids offer a hot, chemically reactive medium which could promote isotopic homogenization in glauconites.
A review of other attempts at dating early Paleozoic glauconites from the midcontinental United States indicates that all Rb-Sr and K-Ar glauconite ages are 10 to greater than 30 percent younger than their known stratigraphic ages. These glauconites are from Paleozoic formations on the stable craton, and again, resetting due to burial metamorphism is unlikely. Because there was no immediately obvious explanation for the discordant young ages, early Paleozoic glauconites were simply considered to be unsuitable geochronometers.
It is suggested that the anomalously young radiometric ages for early Paleozoic glauconites record disturbances in the Rb-Sr and K-Ar isotopic systems, the result of Mississippi Valley-type fluid migration in the midcontinent region. You could not be signed in.
Idealized and simplified diagram of the Grand Canyon There are a number of lava flows on the plateau that the canyon is cut into yellow in Figure 1, above. These lava flows are Cenozoic in age, and some of them spill into the canyon. The walls of the canyon are mostly cut into horizontal rock layers of Paleozoic age green in Figure 1, above. There is an angular unconformity at the bottom of the Paleozoic layers. An angular unconformity is the result of tilting and eroding of the lower layers before the upper ones are deposited.
These tilted and eroded layers are Precambrian in age blue in Figure 1, above.
Applications of Rb/Sr dating on ore deposits using common gangue minerals are tested. The only conditions a deposit must meet are: 1) it should have minerals with a low Rb/Sr ratio, such as calcite and fluorite; 2) it should be enclosed by wall rock with a high Rb/Sr ratio.
Reference to a case where the given method did not work This is perhaps the most common objection of all. Creationists point to instances where a given method produced a result that is clearly wrong, and then argue that therefore all such dates may be ignored. Such an argument fails on two counts: First, an instance where a method fails to work does not imply that it does not ever work. The question is not whether there are “undatable” objects, but rather whether or not all objects cannot be dated by a given method.
The fact that one wristwatch has failed to keep time properly cannot be used as a justification for discarding all watches. How many creationists would see the same time on five different clocks and then feel free to ignore it? Yet, when five radiometric dating methods agree on the age of one of the Earth’s oldest rock formations Dalrymple , p. The claim that the methods produce bad results essentially at random does not explain why these “bad results” are so consistently in line with mainstream science.
Claims that the assumptions of a method may be violated Certain requirements are involved with all radiometric dating methods. These generally include constancy of decay rate and lack of contamination gain or loss of parent or daughter isotope. Creationists often attack these requirements as “unjustified assumptions,” though they are really neither “unjustified” nor “assumptions” in most cases. Rates of radiometric decay the ones relevant to radiometric dating are thought to be based on rather fundamental properties of matter, such as the probability per unit time that a certain particle can “tunnel” out of the nucleus of the atom.
Rb—Sr and U—Pb dating of bentonites H. Baadsgaard and , J. Lerbekmo Canadian Journal of Earth Sciences, , 20 8:
Rb decays with a half-life of billion years to Sr This half-life is so long that the Rb-Sr method is normally only used to date rocks that are older than about million years.
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In most forest ecosystems, sulfate is derived mostly from the atmosphere; weathering of ore minerals and evaporites contribute some sulfur. Sulfur burns with a blue flame with formation of sulfur dioxide, which has a suffocating and irritating odor. Though sometimes found in pure, native form, sulfur on Earth usually occurs as sulfide and sulfate minerals. Being abundant in native form, sulfur was known in ancient times, being mentioned for its uses in ancient India, ancient Greece, China, and Egypt.
Rb-Sr dating of sphalerites has recently been shown to be a promising technique for the direct dating of ore minerals in MVT deposits. This paper reports the results of a reconnaissance study of sphalerites, their fluid inclusions, and associated minerals from MVT deposits of North America.
Slowly and painstakingly, geologists have assembled this record into the generalized geologic time scale shown in Figure 1. This was done by observing the relative age sequence of rock units in a given area and determining, from stratigraphic relations, which rock units are younger, which are older, and what assemblages of fossils are contained in each unit.
Using fossils to correlate from area to area, geologists have been able to work out a relative worldwide order of rock formations and to divide the rock record and geologic time into the eras, periods, and epochs shown in Figure 1. The last modification to the geologic time scale of Figure 1 was in the s, before radiometric dating was fully developed, when the Oligocene Epoch was inserted between the Eocene and the Miocene.
Although early stratigraphers could determine the relative order of rock units and fossils, they could only estimate the lengths of time involved by observing the rates of present geologic processes and comparing the rocks produced by those processes with those preserved in the stratigraphic record. With the development of modern radiometric dating methods in the late s and s, it was possible for the first time not only to measure the lengths of the eras, periods, and epochs but also to check the relative order of these geologic time units.
Radiometric dating verified that the relative time scale determined by stratigraphers and paleontologists Figure 1 is absolutely correct, a result that could only have been obtained if both the relative time scale and radiometric dating methods were correct. Nonetheless, stratigraphy and radiometric dating of Precambrian rocks have clearly demonstrated that the history of the Earth extends billions of years into the past. Radiometric dating has not been applied to just a few selected rocks from the geologic record.
Literally many tens of thousands of radiometric age measurements are documented in the scientific literature. Since beginning operation in the early s, the Geochronology laboratories of the U.