While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sediments , from which their ratios are measured. This is meaningless - paleontologists do not use carbon dating to assess dinosaur fossils; dinosaurs became extinct 66 million years ago, more than a thousand times farther back than carbon dating can be used. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature. The uranium content of the material can then be calculated from the number of tracks and the neutron flux. Carbon is a radioactive isotope of carbon, with a half-life of 5, years,   which is very short compared with the above isotopes and decays into nitrogen. Potassium has a half-life of 1. In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter. For instance, carbon has a half-life of 5, years. This is well-established for most isotopic systems. After an organism has been dead for 60, years, so little carbon is left that accurate dating cannot be established. However it is less useful for dating metal or other inorganic objects. This makes carbon an ideal dating method to date the age of bones or the remains of an organism. This transformation may be accomplished in a number of different ways, including alpha decay emission of alpha particles and beta decay electron emission, positron emission, or electron capture. Plants take up atmospheric 14C for as long as they live, through the process of photosynthesis.
The proportion of carbon left when the remains of the organism are examined provides an indication of the time elapsed since its death. Rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample. This is meaningless - paleontologists do not use carbon dating to assess dinosaur fossils; dinosaurs became extinct 66 million years ago, more than a thousand times farther back than carbon dating can be used. Carbon has different isotopes , which are usually not radioactive. Less astute creationists often conflate carbon dating with other forms of radiometric dating, attempting to "disprove" the true age of dinosaur fossils by "refuting" carbon dating. The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation. Carbon has a half-life of about years, so researchers use the process to date biological samples up to about 60, years in the past. In uranium—lead dating , the concordia diagram is used which also decreases the problem of nuclide loss. The decay of uranium and thorium, among other isotopes, produces radiation which can create 14C from 12C. Carbon, though, is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere and thus remains at a near-constant level on Earth. This scheme is used to date old igneous and metamorphic rocks , and has also been used to date lunar samples. Older materials can be dated using zircon , apatite , titanite , epidote and garnet which have a variable amount of uranium content. This leaves out important information which would tell you how precise is the dating result. However, under about 20, years the results can be compared with dendrochronology , based on tree rings. This normally involves isotope-ratio mass spectrometry. Plants take up atmospheric 14C for as long as they live, through the process of photosynthesis. Radioactive decay[ edit ] Example of a radioactive decay chain from lead Pb to lead Pb. Animals take up atmospheric 14C indirectly, by eating plants or by eating other animals that eat plants. Ironically, given how supposedly useless carbon dating is claimed to be, Creation Ministries International rests part of their " Evidences" on carbon dating being a useful method for within several thousand years. Instead, they are a consequence of background radiation on certain minerals. Helens, creationists attempted discredit the discipline through dishonest practices. Other elements used for dating, such as rubidium, occur in some minerals but not others, restricting usefulness. A related method is ionium—thorium dating , which measures the ratio of ionium thorium to thorium in ocean sediment. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron. Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years e. It is a type of radiometric dating.
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