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Radioactive dating

Adapted from commons.wvc.edu/rdawes/G101OCL/Basics/geotime.html

In geology, an absolute age is a quantitative measurement of how old something is, or how long ago it occurred, usually expressed in terms of years.

Most absolute age determinations in geology rely on radiometric methods.

Earth is billions of years old. The most useful methods for measuring the ages of geologic materials make use of radioactive parent isotopes and their stable daughter products, as preserved in rocks, minerals, or other geologic materials.

An isotope is a particular type of atom of a chemical element, which differs from other isotopes of that element in the number of neutrons it has in its nucleus.

By definition, all atoms of a given element have the same number of protons.
However, they do not all have the same number of neutrons.
The different numbers of neutrons possible in the atoms of a given element correspond to the different possible isotopes of that element.

For example, all carbon atoms have 6 protons.

Carbon-12 is the isotope of carbon that has 6 neutrons.

Carbon-13 is the isotope of carbon that has 7 neutrons.

Carbon-14 has 8 neutrons in its nucleus, along with its 6 protons, which is not a stable combination.

That is why carbon-14 is a radioactive isotope-it contains a combination of protons and neutrons in its nucleus that is not stable enough to hold together indefinitely. Eventually, it will undergo a spontaneous nuclear reaction and turn into a stable daughter product – a different isotope, which is not radioactive.

Each type of radioactive isotope has a half-life, a length of time that it will take for half of the atoms in a sample of that isotope to decay into the stable daughter product.

Physicists have measured the half-lives of most radioactive isotopes to a high level of precision.

The properties of radioactive isotopes and the way they turn into their stable daughter products are not affected by variations in temperature, pressure, or chemistry. Therefore the half-lives and other properties of isotopes are unaffected by the changing conditions that a rock is subjected to as it moves through the rock cycle. If a granite crystallizes with minerals containing radioactive isotopes, it is as though the rock crystallizes with a built-in batch of stopwatches that begin ticking away as soon as the granite has cooled…..

Parent Isotopes, Daughter Isotopes, and Half-Lives

The dots in the cartoon below represent atoms of a parent isotope, decaying to its stable daughter product, through two half-lives.

At time zero in the diagram, which could represent the crystallization of minerals in a rock, there are 32 red dots.
After one half-life has passed, there are 16 red dots and 16 green dots.
After two half-lives have passed, there are 8 red dots and 24 green dots.

schematic half-lives diagram with three boxes, from left to right:<br /><br /><br /> 0 half-lives, contains 32 red dots; 1 half-life, contains 16 red dots and 16 green dots; 2 half-lives, contains 8 red dots and 24 green dots

For Uranium-238 the half life is 4.5 billion years
{ http://en.wikiversity.org/wiki/Age_of_the_Earth }

The following graph illustrates radioactive decay of a fixed amount of an isotope.

You can see how the proportions of the isotopes from the cartoon above are graphed as percentages at half-lives 0, 1, and 2 below.

The following table lists a selection of isotope pairs that are used in making radiometric age determination.

Carbon-14 has a relatively short half-life, which makes it useful only for young, carbon-rich geologic materials, less than about 70,000 years old.

Igneous rocks and high-grade metamorphic rocks are the most likely to be entirely formed of minerals that crystallized when the rocks formed.  As most fossils are found in clastic sedimentary rocks, which are made of weathered and eroded minerals and bits of rock of various ages, it is unlikely to be able to make an radiometric age determination of a rock in which a fossil is found.

The age of a rock containing fossils can usually be narrowed down by measuring the ages of metamorphic or igneous rocks in stratigraphic relation to it, such as a lava flow on top of a layer of sedimentary rock.

Parent Daughter Half Life (years) Dating Range (years) Minerals/materials
238U
Uranium-238
206Pb
Lead-206
4.5 billion 10 million –
4.6 billion
Minerals include zircon, uraninite. Igneous or metamorphic rocks.
40K
Potassium-40
40Ar
Argon-40
1.3 billion 0.05 million –
4.6 billion
Minerals include muscovite, biotite, K-feldspar. Volcanic rocks.
87Rb
Rubidium-87
87Sr
Strontium-87
47 billion 10 million –
4.6 billion
Minerals include muscovite, biotite, K-feldspar. Igneous or metamorphic rocks.
14C
Carbon-14
14N
Nitrogen-14
5,730 years 100 – 70,000 years Not used for dating rocks, except carbonates from earth’s surface such as recent coral reefs. Used for young organic materials, or surface-water samples:
Wood, charcoal, peat, bone, tissue, carbonate minerals from surficial environments, water containing dissolved carbon.

Virtual labs & apps

radioactive-dating-game-screenshot

Understand how decay and half life work to enable radiometric dating. Play a game that tests your ability to match the percentage of the dating element that remains to the age of the object.

http://phet.colorado.edu/en/simulation/radioactive-dating-game

Related articles

Carbon dating

New York State High School Regents Exam Prep: Earth Science

http://regentsprep.org/Regents/earthsci/radioactivedecay.htm

Hot topics: Reliable method or Circular reasoning?

Radiometric Dating and the Geological Time Scale: Circular Reasoning or Reliable Tools?
by Andrew MacRae

Radioactive dating of rocks and fossils is often misunderstood, even by some scientists. It is not a circular process, and leads to ever-more-reliable data, supported from a number of different avenues.  This excerpt offers some insight into the issue, follow the link for the entire article:

– The unfortunate part of the natural process of refinement of time scales is the appearance of circularity if people do not look at the source of the data carefully enough. Most commonly, this is characterized by oversimplified statements like: “The fossils date the rock, and the rock dates the fossils.”

Even some geologists have stated this misconception (in slightly different words) in seemingly authoritative works (e.g., Rastall, 1956), so it is persistent, even if it is categorically wrong …

When a geologist collects a rock sample for radiometric age dating, or collects a fossil, there are independent constraints on the relative and numerical age of the resulting data. Stratigraphic position is an obvious one, but there are many others.

* There is no way for a geologist to choose what numerical value a radiometric date will yield, or what position a fossil will be found at in a stratigraphic section.

* Every piece of data collected like this is an independent check of what has been previously studied.

* The data are determined by the rocks, not by preconceived notions about what will be found.

* Every time a rock is picked up it is a test of the predictions made by the current understanding of the geological time scale.

The time scale is refined to reflect the relatively few and progressively smaller inconsistencies that are found. This is not circularity, it is the normal scientific process of refining one’s understanding with new data. It happens in all sciences.

If an inconsistent data point is found, geologists ask the question: “Is this date wrong, or is it saying the current geological time scale is wrong?” In general, the former is more likely, because there is such a vast amount of data behind the current understanding of the time scale, and because every rock is not expected to preserve an isotopic system for millions of years.

However, this statistical likelihood is not assumed, it is tested, usually by using other methods (e.g., other radiometric dating methods or other types of fossils), by re-examining the inconsistent data in more detail, recollecting better quality samples, or running them in the lab again. Geologists search for an explanation of the inconsistency, and will not arbitrarily decide that, “because it conflicts, the data must be wrong.”

If it is a small but significant inconsistency, it could indicate that the geological time scale requires a small revision. This happens regularly. The continued revision of the time scale as a result of new data demonstrates that geologists are willing to question it and change it. The geological time scale is far from dogma.

….Skeptics of conventional geology might think scientists would expect, or at least prefer, every date to be perfectly consistent with the current geological time scale, but realistically, this is not how science works. The age of a particular sample, and a particular geological time scale, only represents the current understanding, and science is a process of refinement of that understanding.

In support of this pattern, there is an unmistakable trend of smaller and smaller revisions of the time scale as the dataset gets larger and more precise (Harland et al. 1982, p.4-5). If something were seriously wrong with the current geologic time scale, one would expect inconsistencies to grow in number and severity, but they do not.

http://www.talkorigins.org/faqs/dating.html

Learning Standards

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012), from the National Research Council of the National Academies.

By the end of grade 12. Radioactive decay lifetimes and isotopic content in rocks provide a way of dating rock formations and thereby fixing the scale of geological time.

College Board Standards for College Success: Science

ES.3 Earth’s History: Relative and Absolute dating. Students understand that various dating methods — relative and absolute — have been used to determine the age of Earth.

Suggested Connections. Between Earth Science and Other Disciplines: Evidence of Common Ancestry and Divergence (LS.1.1); Living Systems and the Physical Environment (LS.3.1); Nuclear Chemistry (C.1.6); Nuclear Interactions and the Conservation of Mass–Energy (P.2.3)

Benchmarks: American Association for the Advancement of Science.

Knowledge of radioactivity helps them understand how rocks can be dated, which helps them appreciate the scale of geologic time… Scientific evidence indicates that some rock layers are several billion years old. 4C/H6** (BSL)

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