Radioactive Isotopes Used For Dating

Parent

Daughter

Half-life of the parent

87Rb (rubidium)

87Sr (strontium)

48.8 billion years

l43Nd (neodymium)

l43Sm (samarium)

106 billion years

238U (uranium)

206Pb (lead)

4.47 billion years

235U (uranium)

207Pb (lead)

0.704 billion years

232 Th (thorium)

208Pb (lead)

14.01 billion years

l87Re (rhenium)

l87Os (osmium)

42.3 billion years

l76Lu (lutetium)

l76Hf (hafnium)

35.4 billion years

40K (potassium)

40Ar (argon)

11.93 billion years

l4C (carbon)

l4N (nitrogen)

5,700 years

l29I (iodine)

129 Xe (xenon)

15.7 million years

26Al (aluminum)

26Mg (magnesium)

0.7 million years

This may reflect transient, hot areas in the very early solar system, or perhaps these are simply the very first materials to condense from the solar nebula. Some chondrules also contain special pieces of material called calcium aluminum inclusions (CAIs). These special inclusions often consist of concentric shells like an onion and contain minerals that form at very high temperatures, such as melilite and spinel. Some also have nuggets of rare metals such as platinum (these nuggets were named fremdlinge by early German petrologists, meaning "little strangers"). Forming a CAI requires temperatures of 3,100°F (1,700°C) or more, with slow cooling. CAIs may be the very earliest of the early material, since they formed before the chondrules themselves and consist of materials that condense at the highest temperatures.

Chondrites contain relatively little oxygen compared to other solar system materials. On Earth and the other terrestrial planets, elements such as calcium, manganese, magnesium, zinc, iron, and chromium usually bind together with oxygen to balance their electric charges and produce a neutral particle. In chondrites, these elements bind with sulfur or even nitrogen because of the lack of oxygen. This lack of oxygen is also consistent with being a very primitive material.

Chondrites show differing amounts of evidence for processing (heating, or exposure to water) after their early formation but while still in space. The material can be heated by the heat of accretion of a parent body and also by radionuclides. Radioactive elements release heat when they decay.This is still an important source of heat in planets today, but it was even more important in the very early solar system because of short-lived radioactive elements that are now gone. In particular, 26Al was a highly radioactive heat-producing isotope of aluminum, but its half-life is only 700,000 years. 26Al has long been gone from the solar system. Its stable daughter element, a particular isotope of magnesium that can be still measured, proves the original presence of 26Al in material billions of years old.The heat of decay of now-extinct radionuclides may have been an important source for heating early solar system materials. CAIs, those very primitive inclusions inside chon-drules, have especially high concentrations of the magnesium daughter product of 26Al. This high concentration is further evidence that CAIs are the most primitive materials and formed earlier than any others.

Chondrite meteorites generally consist of chondrules and the minerals olivine and pyroxene, with a fine-grained matrix filling in the gaps. Some chondrites show evidence of heating in the fuzzy outlines of their mineral grains:The grains were heated to the point that they started to diffuse quickly or almost melt at their edges. This indicates that temperatures reached about 1,800°F (1,000°C). Other chondrites have minerals containing water in veins through the meteorite, as well as minerals such as carbonates, sulfates, and magnetite, which have to form in water.This is fascinating evidence that liquid water existed in large bodies in the asteroid belt even early in solar system evolution.

If chondrites can be shown to be primordial solar system material, then they should have an age of formation that is close to the age of the solar system. Dates of the Allende meteorite show an age of approximately 4.566 billion years before present, now considered to be the age of the solar system, the age at which the planets began to form. These numbers are obtained by measuring the decay rates of radioactive elements.

When rocks form, their crystals contain some amount of radioactive isotopes (see table on page 98). Different crystals have differently sized spaces in their lattices, so some minerals are more likely to incorporate certain elements than are others.The mineral zircon, for example, usually contains a measurable amount of radioactive lead (atomic abbreviation Pb).When the crystal forms, it contains some ratio of parent and daughter atoms. As time passes, the parent atoms continue to decay according to the rate given by their half-life, and the population of daughter atoms in the crystal increases. By measuring the concentrations of parent and daughter atoms, the age of the rock can be determined.

Consider the case of the radioactive decay system 87Rb (rubidium). It decays to 87Sr (strontium) with a half-life of 48.8 billion years. In a given crystal, the amount of 87Sr existing now is equal to the original 87Sr that was incorporated in the crystal when it formed, plus the amount of 87Rb that has decayed since the crystal formed.This can be written mathematically as:

now original x original now'

The amount of rubidium now is related to the original amount by its rate of decay. This can be expressed in a simple relationship that shows that the change in the number of parent atoms n is equal to the original number n times one over the rate of decay, called X (the equations will now be generalized for use with any isotope system):

where dn means the change in n, the original number of atoms, and dt means the change in time. To get the number of atoms present now, the expression needs to be rearranged so that the time terms are on one side and the n terms on the other. Integrated, the final result is:

The number of daughter atoms formed by decay, D, is equal to the number of parent atoms that decayed:

Also, from the previous equation, n = neXt. That can be substituted into the equation for D in order to remove the term n :

Then, finally, if the number of daughter atoms when the system began was D , then the number of daughter atoms now is

This is the equation that allows geologists to determine the age of materials based on radiogenic systems. The material in question is ground, dissolved in acid, and vaporized into an instrument called a mass spectrometer. The mass spectrometer measures the relative abundances of the isotopes in question, and then the time over which they have been decaying can be calculated.

The values of D and n are measured for a number of minerals in the same rock, or a number of rocks from the same outcrop, and the data are plotted on a graph of D versus n (often D and n are measured as ratios of some stable isotope, simply because it is easier for the mass spectrometer to measure ratios accurately than it is to measure absolute abundances). The slope of the line the data forms is ekt — 1. This relation can be solved for t, the time since the rocks formed.This technique also neatly gets around the problem of knowing D , the initial concentration of daughter isotopes: D ends up being the y-intercept of the graph.

Radiodating, as the technique is sometimes called, is tremendously powerful in determining how fast and when processes happened on the Earth and in the early solar system. Samples of many geological materials have been dated: the lunar crustal rocks and basalts returned by the Apollo and Luna missions, many kinds of meteorites—including those from Mars—and tens of thousands of samples from all over the Earth.While the surface of the Moon has been shown to be between 3.5 and 4.6 billion years old, the Earth's surface is largely younger than 250 Ma (million years old).The oldest rock found on Earth is the Acasta gneiss, from northwestern Canada, which is 3.96 billion years old.

If the oldest rock on Earth is 3.96 billion years old, does that mean that the Earth is 3.96 billion years old? No, because scientists believe older rocks have simply been destroyed by the processes of erosion and plate tectonics and also have reason to believe that the Earth and Moon formed at nearly the same time. The ages of the chondritic meteorites, 4.56 billion years old, appears to be the age of the solar system. How is it known that this is when the solar system formed and not some later formation event?

The answer is found by using another set of extinct nuclides (nuclide is a synonym for isotope). An important example is 129I (an isotope of iodine), which decays into 129Xe (xenon) with a half-life of only 16 million years. All the 129I that the solar system would ever have was formed when the original solar nebula was formed, just before the planets began to form. If a rock found today contains excess 129Xe, above the solar system average, then it must have formed very early in solar system time, when 129I was still live. The meteorites that date to 4.56 Ga (billion years) have excess 129Xe, so 4.56 Ga is the age of the beginning of the solar system.

The Allende chondrite and others have ages of 4.566 Ga (billion years). The scientific community is convinced through many examples of radiodating and thorough proofs that the technique works and that 4.56 Ga is the age of the solar system. Though the solar system began forming then, the process of forming the planets took some time. Studies of other isotopic systems can put constraints on how long it took the terrestrial planets to form their cores and begin to look the way they do today. The Earth and the Moon both show an age of about 4.515 Ga. Mars seems to have formed at about 4.536 Ga. Some meteorite parent bodies also formed a little later than the very beginning of the solar system, and even very early parent bodies sometimes hold records of later processing.

Uranium radioisotope dating indicates that most chondrite meteorites cooled within the first 60 million years of the solar system. At some point after cooling the parent bodies, impacts broke the larger parent bodies into the smaller pieces that now fall to Earth. About 20 percent of chondrites also contain or are completely made of breccia, indicating strong impact forces broke the material presumably while breaking the parent body. The breccia indicates that pressure of impact may have reached 75 GPa, the equivalent of 750,000 times atmospheric pressure on Earth.

The times of impact can be dated with some accuracy using the radioisotope 40K (potassium), which decays into 40Ar (argon). Argon is highly volatile and doesn't fit well into rock-forming minerals. As the 40K decays, 40Ar builds up in the rock. When the body is broken by a catastrophic impact, it is also heated by the shock of impact. This heat is thought to be sufficient to release all the 40Ar that has built up in the rock. After impact and breakup, the 40Ar begins to build up again because 40K continues to decay. The concentration of 40Ar can therefore be used to calculate the time between present and the time of shock heating. For chondrites, the catastrophic impacts that broke up the parent bodies seem to have occurred between 100 million and 4.1 billion years ago, almost the entire range of the age of the solar system.

A few chondrite meteorite falls have been photographed. With enough photographs, their original orbits can be calculated. All the orbits for chondrites now known have their aphelions (the point farthest from the Sun) between Mars and Jupiter, so at the moment, scientists believe that all the chondrites may come from the asteroid belt.

The chondrites are divided into classes, which are sets of groups that have similar compositions or textures and may have formed in the same area of the solar system, and then into groups, each of which may be from a single parent body. The principal classes are the ordinary chondrites, the carbonaceous chondrites, and the enstatite chon-drites.There are other minor classes, mostly represented by only one meteorite fall. Groups are designated by letters, such as C, L, I, and H, which are described within the classes below. In the 1960s, the scientists Randy Van Schmus of Kansas University and John Wood of the Smithsonian Astrophysical Observatory added numbers to designate the degree of alteration of the chondrite after its initial formation. Numbers 1 and 2 have been altered by water, 3 is the least altered, and numbers 4 and 5 indicate heat alteration. The number 2 indicates heating to about 70°F (20°C), number 3 to 120°F (50°C), and 4 and 5 to much higher temperatures.

Though the numbers were first developed to indicate alteration by heat and water, they also appear to correlate with numbers of chon-drules: 1 indicates no chondrules, 2, sparse chondrules, 3 and 4, abundant and distinct chondrules, and 5 and 6, increasingly indistinct chondrules.Types 3 and 4 are thought to be the most primitive material known in the solar system.

Ordinary chondrites (groups H, L, LL) seem to have originated in parent bodies 50—60 miles (85—100 km) in radius. All chondrites contain evidence of very early impacts, showing they had already formed by the time these very early impacts occurred. The group names H, L, and LL refer to how much oxygen is bound into the crystals in the meteorite (the meteorite's oxidation state): H chondrites

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