Radiometric dating

• 1. Introduction

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  Radiometric dating is how scientists read time written in stone. Buried inside a grain of zircon or a fragment of ancient meteorite are radioactive atoms that have been ticking away at a fixed, measurable rate since the moment that rock or mineral first formed. By counting what remains of those atoms, and comparing that to the decay products left behind, geologists can calculate how long ago a material formed.

  Ernest Rutherford pioneered this technique in 1906. Just one year later, in 1907, Bertram Boltwood refined it further. The questions their work made answerable are staggering: How old is the Earth? When did a particular layer of rock crystallize? How long ago did an ancient organism die? These are not questions previous generations could answer with confidence. Radiometric dating made it possible to answer them with precision.

  Today, the technique is the principal source of information about the absolute age of rocks, geological features, fossilized life forms, and Earth itself. What drives it is a property of matter that is both simple and profound: some atoms are unstable, and they transform at a rate that nothing in the external world can alter.

• 2. The Clock Inside the Atom

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  Radioactive decay is the ticking mechanism at the heart of every radiometric date. An unstable atom of a given element will, at some unpredictable moment, transform into a different element. The transformation can happen through alpha decay, beta decay, electron capture, or spontaneous fission into two or more fragments.

  No one can predict the precise moment a single atom will decay. But enormous numbers of atoms behave with perfect statistical regularity. That regularity is expressed as the half-life: the time it takes for exactly half of a given quantity of radioactive atoms to transform into their decay products. Some half-lives are brief; tritium's is only about 10 years. Samarium-147, at the other extreme, has a half-life of over 100 billion years.

  Temperature, pressure, chemical environment, and magnetic or electric fields have no effect on the decay rate for virtually all nuclides. The decay constant is a property of the nucleus itself. The only known exceptions are nuclides that decay by electron capture, such as beryllium-7, strontium-85, and zirconium-89, whose rates can be affected by local electron density.

  Because the decay constant is so stable and so well characterized, the ratio of original atoms to decay products in a sample behaves like a clock. That clock started ticking at the moment the material formed and became a closed system, trapping those atoms inside. What scientists measure today is the accumulated record of every tick since then.

• 3. The Closure Temperature Problem

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  A radiometric clock does not start at the moment a mineral forms from a melt. It starts later, at a specific temperature called the closure temperature. Above that threshold, atoms diffuse freely in and out of the mineral's crystal structure. Any daughter isotopes that have accumulated simply escape, effectively resetting the clock to zero.

  As the mineral cools below its closure temperature, its crystal structure tightens enough to trap the daughter nuclides inside. From that point forward, the isotopic ratio records elapsed time. An igneous or metamorphic rock's radiometric age therefore represents when it cooled through closure, not when the original melt existed.

  Closure temperatures vary by mineral and by isotopic system. Mica has a fairly low closure temperature, around 350 degrees Celsius. Hornblende closes at around 500 degrees Celsius. Zircon sits much higher still, making it exceptionally useful for preserving ancient isotopic records. Because different minerals in the same rock close at different temperatures, measuring multiple minerals in a single sample can reconstruct the full cooling and metamorphic history of a rock body over time. The study of this thermal history is called thermochronology or thermochronometry. Closure temperatures are determined experimentally by artificially resetting sample minerals in a high-temperature furnace.

• 4. The Main Dating Methods

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  Uranium-lead dating, often performed on the mineral zircon, is one of the most precise tools geologists possess. Zircon is almost ideally suited to the method: it incorporates uranium atoms into its crystal structure but strongly rejects lead, meaning virtually all the lead measured in an old zircon crystal is the product of decay rather than contamination from the environment. Zircon's high closure temperature and resistance to mechanical and chemical weathering preserve its isotopic record through tremendous geological upheaval. The error margin in uranium-lead dates for very old rocks can be as low as less than two million years across two-and-a-half billion years.

  Potassium-argon dating works through the decay of potassium-40 to argon-40, with a half-life of 1.3 billion years. This makes it useful for dating the oldest rocks on Earth. It is common in micas, feldspars, and hornblendes, though its relatively low closure temperatures in those minerals require care in interpretation.

  Samarium-neodymium dating involves the alpha decay of samarium-147 to neodymium-143, with a half-life of 106 billion years. Accuracy within twenty million years for samples two-and-a-half billion years old has been achieved. Rubidium-strontium dating, based on the beta decay of rubidium-87 to strontium-87 with a half-life of 50 billion years, has been used on lunar samples and on fault systems to decode episodes of movement in the Earth's crust.

  For shorter timescales, uranium-thorium dating captures the decay of uranium-234 into thorium-230, which has a half-life of about 80,000 years. Because thorium and protactinium are not water-soluble while uranium is, they precipitate selectively into ocean-floor sediments, making that material a natural archive. The scheme covers a range of several hundred thousand years.

• 5. Carbon-14 and the Recent Past

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  Carbon-14 occupies a peculiar place among radiometric tools because it is continuously replenished in the atmosphere rather than being a fixed inheritance from the formation of the solar system. Neutrons produced by cosmic rays collide with nitrogen in the upper atmosphere, generating carbon-14, which then enters the carbon dioxide cycle and becomes incorporated into all living things.

  While an organism is alive, it keeps exchanging carbon with its environment, maintaining a roughly constant proportion of carbon-14 in its tissues. Death stops that exchange. From then on, the carbon-14 decays with a half-life of 5,730 years, and the proportion that remains is a measure of how long ago the organism died. After about 60,000 years, so little carbon-14 is left that meaningful measurement becomes impossible. At the younger end of the range, the steep drop-off in carbon-14 concentration allows ages to be pinned to within a few decades.

  The method has complications. Volcanic eruptions that release large quantities of carbon dioxide can locally dilute carbon-14 and skew results. Industrial emissions have also depressed the proportion of carbon-14 in the biosphere by a measurable amount. Running in the opposite direction, above-ground nuclear weapon tests conducted into the early 1960s injected extra carbon-14 into the atmosphere, temporarily elevating concentrations. An unusually strong solar wind or an elevated terrestrial magnetic field can also suppress carbon-14 production in the upper atmosphere. Cross-checks against other dating methods confirm that, absent these disturbances, the rate of carbon-14 creation has been roughly constant.

• 6. Reading the Solar System's Birth

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  For rocks that date back to the formation of the solar system itself, the familiar long-lived isotopes become a problem. Their half-lives are so long that the precision of any resulting date is limited. To get a finer time resolution for those earliest moments, scientists turn to isotopes that are now entirely extinct.

  At the beginning of the solar system, short-lived radionuclides were present in the solar nebula: aluminum-26, iron-60, manganese-53, and iodine-129, among others. These were likely produced by a nearby supernova. All have since decayed completely, leaving only their daughter products locked inside the oldest meteorites.

  By measuring those daughter products with a mass spectrometer and plotting the results on isochron diagrams, scientists can determine relative ages of events in the earliest solar system with remarkable precision. The aluminum-26 to magnesium-26 chronometer, for instance, gives an estimate that chondrule formation took only about 1.4 million years.

  The manganese-53 to chromium-53 system has a half-life of 3.80 plus or minus 0.23 million years. Because manganese and chromium are moderately volatile elements, their distribution in the cooling solar nebula is sensitive to temperature changes. Applying this chronometer to meteorites has allowed scientists to establish that the volatile element depletion of the proto-Earth was complete no later than roughly 3 million years after the formation of calcium-aluminium-rich inclusions. Full planetary accretion was completed within roughly 70 million years. The iodine-129 to xenon-129 chronometer, which uses samples of a meteorite called Shallowater as a conversion monitor, provides another independent timeline for comparing when different bodies in the early solar system cooled and sealed their isotopic records.

• 7. Precision and Its Limits

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  Radiometric dating rests on a set of conditions that must hold for any result to be trusted. The parent and daughter isotopes must not have entered or left the material since it formed. Contamination from the surrounding environment is a constant concern. So is diffusion, which can cause daughter isotopes to migrate out of a mineral at high temperatures before the closure temperature is reached.

  To guard against these problems, scientists take measurements on multiple samples from different parts of the same rock body. They also look for agreement across different isotopic systems in the same rock. When several minerals from a single sample, all assumed to have formed at the same time, yield a consistent pattern on an isochron plot, confidence in the result grows substantially.

  The age of the Amitsoq gneisses from western Greenland illustrates this principle. Uranium-lead dating placed their age at 3.60 plus or minus 0.05 billion years ago. Lead-lead dating independently yielded 3.56 plus or minus 0.10 billion years ago. The overlap between those two results, using two independent methods, supports the conclusion.

  Modern mass spectrometers, developed in the 1940s and applied to radiometric dating starting in the 1950s, can perform measurements on samples as small as one nanogram. The instrument ionizes atoms from the sample, sends the beam through a magnetic field, and sorts the ions by mass into detectors called Faraday cups. The tiny electrical currents generated on impact reveal the relative concentrations of different isotopes with great accuracy, making the precision of today's dates something that Ernest Rutherford, working in 1906, could not have imagined.