An International Geoscience Journal of the Geological Society of Australia

An International Geoscience Journal of the Geological Society of Australia

Collins January Introduction The creationists are asking for equal time in science classrooms to teach that the Genesis stories are valid scientific interpretations of earth history. Equal time for creationists’ interpretations are not likely to occur in secular universities and schools, but if the creationist are serious about equal time, then they should be open to granting equal time in their private Christian schools for presentations of both sides of a scientific issue a literalist biblical view and the modern science view. The origin, age, and other characteristic features of granite are such issues deserving equal time. If I were given equal time in a science classroom at a private, fundamentalist, Christian college or secondary school, advocating creationists’ views, the following would be the kinds of information that I would provide for a modern scientific interpretation of granite to compare with the corresponding creationists’ biblical interpretation. Modern science interpretation of granite Origin. Geologists recognize that granite has several possible origins, depending upon the processes that operate on the rock systems. Some granites form 1 by magmatic processes, depending upon crystal settling and the order of crystallization of minerals from a magma melted silicate rock , 2 by melting of sedimentary rocks whose chemical composition is the same as that in granite, 3 by partial melting of rocks in which the first minerals to melt have the composition of granite, and, finally, 4 by chemical replacement processes Clark, ; Collins, ; Hunt et al. Discussion of these different origins is not further expanded here because of space limitations and because it is sufficient to say that modern scientific studies show that granite is formed in many different ways, and these ways contrast with the creationists’ model in which granite has a single origin, being created nearly instantly by “fiat” e.

Age Dating The Earth

Nicolas Steno and Age Dating The Earth from the January 10, eNews issue Wednesday, January 11 is Nicolas Steno’s th birthday, a fact that matters to a great many geologists who consider him the father of modern stratigraphy. Even Google has honored his birth through their Jan. Steno first developed the important geological principles that younger geological layers were originally laid on top of older layers, horizontally and in a continuous fashion.

Uplifting or bending of those layers or canyons cutting through the layers demonstrated that some major changes had been made to the face of the earth since those layers settled.

This is a spreadsheet that I use in petrology to walk students through calculating Rb-Sr isochrons and talking about isotope heterogeneity and sampling at various scales. The example exercise is .

Radiometric dating is a means of determining the “age” of a mineral specimen by determining the relative amounts present of certain radioactive elements. By “age” we mean the elapsed time from when the mineral specimen was formed. Radioactive elements “decay” that is, change into other elements by “half lives. The formula for the fraction remaining is one-half raised to the power given by the number of years divided by the half-life in other words raised to a power equal to the number of half-lives.

To determine the fraction still remaining, we must know both the amount now present and also the amount present when the mineral was formed. Contrary to creationist claims, it is possible to make that determination, as the following will explain: By way of background, all atoms of a given element have the same number of protons in the nucleus; however, the number of neutrons in the nucleus can vary.

An atom with the same number of protons in the nucleus but a different number of neutrons is called an isotope. For example, uranium is an isotope of uranium , because it has 3 more neutrons in the nucleus. It has the same number of protons, otherwise it wouldn’t be uranium. The number of protons in the nucleus of an atom is called its atomic number. The sum of protons plus neutrons is the mass number.

Radioactive dating method ‘under fire’

As Creation magazine has been continuously published since , we are publishing some of the articles from the archives for historical interest, such as this. For teaching and sharing purposes, readers are advised to supplement these historic articles with more up-to-date ones available by searching creation. So slick and convincing are the presentations of results, particularly in glossy media and museum propaganda, that no one even bothers to question how these dating methods work, what assumptions are involved, and how reliable they are.

Such questions, however, are highly relevant.

Rb-Sr dating relies on correctly measuring the Rb-Sr ratio of a mineral or whole rock sample, plus deriving an accurate 87Sr/86Sr ratio for the mineral or whole rock sample. Several preconditions must be satisfied before a Rb-Sr date can be considered as representing the time of emplacement or formation of a .

Single grain pyrite Rb—Sr dating of the Linglong gold deposit, eastern China This study employs the single grain pyrite Rb—Sr technique to date mineralization, using the exam The gold deposits in this area are interpreted to have a consistent spatial—temporal relationship with widespread Late Jurassic—Early Cretaceous magmatism in eastern China, likely related to lithospheric thinning around Ma. Four pyrite samples from veins of different mineralization stages give an average isochron age of Analyses of one quartz and two sericite samples yield isochron ages between Ma and Ma.

Hence, the successful attempt of the single grain Rb—Sr technique of pyrite, which occurs as a common mineral phase in orebodies and is genetically related to the mineralization, has great potential for precise geochronology of hydrothermal mineral deposits. Compared to the K—Ar and 40Ar—39Ar results previously reported, these mica standards nevertheless show complex characteristics in the Rb—Sr isotopic system.

Analysis on eight individual grains of GA biotite gives a Rb—Sr isochron age of

The Journal of Geology

The initial ratio has particular importance for studying the chemical evolution of the Earth’s mantle and crust, as we discussed in the section on igneous rocks. K-Ar Dating 40K is the radioactive isotope of K, and makes up 0. Since K is one of the 10 most abundant elements in the Earth’s crust, the decay of 40K is important in dating rocks.

But this scheme is not used because 40Ca can be present as both radiogenic and non-radiogenic Ca. Note that this is not always true.

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.

Isotopes used for isochron dating How is the half life of an element determined? For something that takes 60 billion years to partially decay, how is an exact measure of the decay rate determined in a few hours? Half-life assessments don’t necessarily take only “a few hours. The statistical uncertainty in an assessment of decay rate is a function of the number of decays counted. Even in a small sample of a long-lived isotope, there will be a constant stream of decays.

If the sample’s size can be measured accurately, and the number of decays can be counted accurately, then the half-life can be computed accurately. That’s the basis for the “direct counting experiments” from which half-lives are calculated. The line is telling us that no matter what size sample we take we always have the same ratio of parent to daughter.

Rubidium-Strontium Isochrons

To conduct radioisotope dating, scientists evaluate the concentration of isotopes in a material. The number of protons in an atom determines which element it is, while the number of neutrons determines which isotope it is. For example, strontium has 38 protons and 48 neutrons, whereas strontium has 38 protons and 49 neutrons. Radioactive elements, such as rubidium but not strontium or strontium , decay over time. By evaluating the concentrations of all of these isotopes in a rock sample, scientists can determine what its original make-up of strontium and rubidium were.

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.

Zircons found from grains in Western Australia are considered slightly older – 4. Geologists depend on radiometric age dating to give dates to different strata and the rocks and bones and artifacts found in those layers. Most will say that the dating methods have been confirmed over and over again, but there are questions about whether the dating methods used are reliable and whether they give scientists true dates – or whether they give dates that fit those secular geologists’ preconceived ideas.

During its history, the earth has suffered constant change. Erosion, volcanic eruptions, earthquakes, catastrophes change the surface of the earth, melting rocks, grinding them up and spitting them back out in different forms than they had when they started. Those who trust in radiometric dating methods point to G. Brent Dalrymple’s book The Age of the Earth, in which he lists the ages of various chondrite samples age-dated through several methods.

Samarium-neodymium Sm-Nd dating of chondrites gives a date of 4. Dalrymple lists other sets of samples and shows that the samples consistently date between 4. This looks like convincing evidence that the earth is truly about 4. Dalrymple’s results appear to be fairly consistent in giving the earth an age of well over 4 billion years. As we noted last week, however, the use of these dating methods depends on a number of assumptions.

Radiometric Dating and the Geological Time Scale

Rubidium silver iodide RbAg4I5 has the highest room temperature conductivity of any known ionic crystal , a property exploited in thin film batteries and other applications. Rubidium forms salts with halides, producing rubidium fluoride , rubidium chloride , rubidium bromide , and rubidium iodide. Isotopes of rubidium Although rubidium is monoisotopic , rubidium in the Earth’s crust is composed of two isotopes: It readily substitutes for potassium in minerals , and is therefore fairly widespread.

Rb has been used extensively in dating rocks ; 87Rb beta decays to stable 87Sr.

Dating sphalerite by the Rb-Sr method method using 3 mL of AGSOW-X8 resin. Total procedure blanks for Rb and Sr were 20 pg and 50 pg, respectively, less than 1% of most of the samples.

We can calculate the half-lives of all of these elements. All the intermediate isotopes between U and Pb are highly unstable, with short half-lives. That means they don’t stay around very long, so we can take it as given that these isotopes don’t appear on Earth today except as the result of uranium decay. We can find out the normal distribution of lead isotopes by looking at a lead ore that doesn’t contain any uranium, but that formed under the same conditions and from the same source as our uranium-bearing sample.

Then any excess of Pb must be the result of the decay of U When we know how much excess Pb there is, and we know the current quantity of U , we can calculate how long the U in our sample has been decaying, and therefore how long ago the rock formed. Th and U also give rise to radioactive series — different series from that of U , containing different isotopes and ending in different isotopes of lead.

Chemists can apply similar techniques to all three, resulting in three different dates for the same rock sample. Uranium and thorium have similar chemical behavior, so all three of these isotopes frequently occur in the same ores. If all three dates agree within the margin of error, the date can be accepted as confirmed beyond a reasonable doubt.

Since all three of these isotopes have substantially different half-lives, for all three to agree indicates the technique being used is sound. But even so, radioactive-series dating could be open to question. It’s always possible that migration of isotopes or chemical changes in the rock could yield incorrect results.

Rubidium–strontium dating

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 primary reason for accepting the isotopic age-dating methods is because, in many places, world-wide, where several different kinds of isotopic age-dating methods have been applied to the same rock, all age determinations were found to be the same.

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.

Rb-Sr can then be used on the altered mineralogy to date the time of this alteration, but not the date at which the rock formed. Thus, assigning age significance to a result requires studying the metasomatic and thermal history of the rock, any metamorphic events, and any evidence of fluid movement. A Rb-Sr date which is at variance with other geochronometers may not be useless, it may be providing data on an event which is not representing the age of formation of the rock.

Sampling error is taken into account in all radiometric dating techniques. I have an excellent science background, although I am no Dr. My “theory” is that God used evolution. It certainly isn’t an original theory, as it was articulated long ago by the Jesuit Teilhard de Chardin, who denominational concerns aside, was an internationally respected thinker.

Script of R to the Rb/Sr system



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