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History recorded in chemistry
How old is the Earth?
The question of when the Earth was formed and when various events on it occurred has long fascinated humanity. In the past, various estimates of the age of the Earth have been made using the available technology. All estimates of this type changed drastically with the modern application of radioactivity.Return to this point in index.
Elements, isotopes, and radioactivity
Matter is made up of atoms, and atoms are made up of a complex array of subatomic particles. Let\'s consider only three of these particles: protons (positively charged), neutrons (no charge), and electrons (negatively charged). Every element has a fixed number of protons that cannot be changed without creating a different element. If, for example, we add a proton to an atom of sulfur, it becomes heavier and is now an atom of chlorine. If we change the number of neutrons in an atom, however, it has almost no effect on the chemical properties and outward appearance but does have an effect on the atomic mass. It can also have an extreme effect on the atomic stability of the element. If we take an atom of potassium that has 39 neutrons in it and add one more, the atom now becomes unstable and can radioactively decay.
Each combination of an element with a different number of neutrons is called an isotope. Isotopes that are radioactive disintegrate or decay in a predictable way and at a specific rate to make other isotopes. The radioactive isotope is called the parent, and the isotope formed by the decay is called the daughter. A radioactive isotope decays at a constant rate proportional to the number of radioactive atoms remaining. A simple way of describing the speed of decay is to see the time it takes for half of the atoms of a radioactive parent to decay and form the daughter element(s). This is called the half life. Various events (especially melting of the rock) will cause the isotopes in a rock to redistribute. When the rock solidifies it can be thought of as starting a stopwatch. By determining the amount of the parent and daughter isotopes present scientists can determine when the stopwatch started.
Naturally occuring radioactive isotopes (called the parent isotope) disintegrate at specific rates to make other isotopes (called daughter isotopes). The amount of time it takes for half the quantity of the original isotope to decay is constant, no matter how much as present at the beginning. Based on this principle, the age of geologic events can be measured.
As an example, the parent-daughter system used to determine the age of the Earth is the uranium-lead system. The decay of the parent uranium isotopes to daughter lead isotopes in samples of the Earth, Moon, and meteorites indicates that all the planets in our solar system formed 4.5 billion years ago.
While determining the age of the Earth is intriguing, radiometric dating has recently been useful in more practical issues like the following: With what age of granite formation are ore deposits in a particular region associated? How recently has a fault been active, and is it likely to be safe to build near it now? How often does a volcano erupt and how often do landslides recur?
On May 18, 1980 the Cascade volcano, Mt. St. Helens, erupted exposively causing a great deal of destruction and a number of deaths.
In addition to telling us the Earth is 4.5 billion years old, geologic dating can answer important questions such as: When was the last time a fault moved? What areas are a safe place to construct a nuclear reactor? How frequently does a particular volcano erupt? Is a volcano nearing an eruptive part of its cycle?
Because different isotopes of an element have different masses, they can be viewed as an arrangement of masses in a spectrum. An instrument that separates and electronically measures a spectra of atomic masses is called a mass spectrometer. There are many types of mass spectrometers, but the most frequently used in earth-science age determinations are magnetic sector mass spectrometers. These magnetic spectrometers operate on the principle that if you put an electric charge on an object and throw it into a magnetic field, the object s path will form a circle. The radius of the circle will depend on the strength of the magnetic field and the mass of the charged atom divided by its electric charge. Thus, if you have a purified portion of an element from a sample with several isotopes, each can be made, in sequence, to travel the same circular path to the detector by varying the strength of the magnetic field. Magnetic sector mass spectrometers consist of at least three components as illustrated in this figure. (1) A source of sample ions, (2) a magnetic field, and (3) a detector.
The atoms on the filament are ionized and accelerated at a specific velocity through a magnetic field, causing them to take a specific curved path depending on the ion\'s mass. This type of mass spectrometer scheme most commonly used in geologic dating shows how ions with a specific mass are directed into the collector for counting, while others, like a race car taking the curves at the wrong speed, are lost.
A difficult chemical procedure is used to concentrate the element of interest so that isotopes can be measured on a mass spectrometer. In many cases the recovered amount is no larger than a spot on the sample filament and could pass through the eye of a needle. Return to this point in index.
Digesting rocks
But how do you take a rock and purify a portion of it for mass spectrometry, and how do you analyze a rock sample on an instrument that only analyzes liquids? In most cases, before a rock s chemical composition can be determined, it must pass through both a physical and a chemical preparation to free the element(s) of interest from the rock and present them in a dissolved or liquid form.
Initially, fist-size pieces of rock are broken down to pea-size fragments using a crusher with steel jaws. A pulverizer grinds this coarse material into a powder as fine as flour.
Next, the powder is further broken down, or decomposed, by using either an acid treatment or fusion. During this chemical decomposition, the weighed sample of powdered rock releases its elements into solution.
Because most rocks are composed of a combination of many types of minerals, each having different chemical and physical properties, digestion is accomplished by using a combination of acids. Most commonly used is a mixture of hydrofluoric, nitric, hydrochloric, and perchloric acids, which will decompose all but the most resistant minerals. The acids are heated with the sample powder in Teflon containers, on a hot plate, or in a specially designed microwave oven.
In the fusion technique, a powdered inorganic reagent (known as a flux) is mixed with the rock powder and heated above the melting point of the flux; the molten flux then attacks the sample and decomposes it into a uniform melt. The melt may then be poured into a mold and cooled for methods that require a uniform solid such as X-ray fluorescence spectrometry (scroll down to picture of arm pouring red hot samples for a discussion of XRF) or dissolved in a diluted acid to create a liquid solution. The higher temperatures (500 to 1,200 C) and caustic nature of the molten chemicals used for fusions increases the efficiency of the decomposition as compared to acid techniques and renders most minerals soluble. Each form of sample decomposition, acid or flux, has its advantages and disadvantages that must be considered. In addition, the importance of safety and simplicity must not be ignored.Return to this point in index.
Disaster from space
One of the mysteries of the history of the earth is the layer of clay that was deposited around the entire globe 65 million years ago. The layer marks the K-T boundary the end of the Cretaceous and beginning of the Tertiary periods. It is best known as the time when not only the dinosaurs but nearly half of all life forms became extinct.
Chemical evidence in this layer of clay preserved from 65 million years ago in Caravaca, Spain, indicates an asteroid or comet struck the Earth at up to 170 times the speed of sound, possibly causing a disaster resulting in the extinction of half of all life forms, including the dinosaurs.
At the beginning of the last decade, Nobel Laureate Luis Alvarez and his team members discovered a 9 ppb abundance of the element iridium while using neutron activation analysis to study 1-cm-thick samples at the K-T boundary layer. The fact that the high level of iridium coincided exactly with the classic end of the Cretaceous mass extinction event led them to propose a theory linking these two observations. They theorized that an asteroid between 6 and 14 km in diameter struck the Earth, and the impact lofted enormous amounts of pulverized target material high into the Earth s atmosphere. They speculated that this dust- size, impact ejecta caused an environmental catastrophe.
Under a microscope, these quartz grains show lines that are characteristic of high shock and are found only with meteorite impacts or atomic explosions. This 1/3 millimeter grain is from the K-T boundary clay at Teapot Dome, Wyoming.
Additional research by other scientists suggests that if the extraterrestrial object was an asteroid, it most likely impacted the Earth at a velocity of 50 times the speed of sound and measured 15 km in diameter. Because asteroids of this size are very few in number in our solar system, the object could also have been a comet, most likely moving even faster, possibly 170 times the speed of sound but measuring only 10 km in diameter.
To test the impact theories, we have applied a new analytical technique called laser ablation, inductively coupled plasma, quadrupole mass spectrometry (LA-ICP-QMS). To allow efficient, rapid, spatial sampling, a laser is used. The technique is highly sensitive for almost all elements.
As depicted below, the energy of the laser is focused onto a spot about 80 micrometers in diameter (slightly more than the diameter of a human hair) to vaporize and sputter material from small zones of the sample. The operating conditions of the laser range from 1 million to 1 trillion watts per square centimeter. This incredibly high energy density is created when the energy is packed into small bursts of 160 microseconds, which are then focused with a lens onto a very small spot.
A laser ablation, induction coupled plasma, quadrupole mass spectrometer vaporizes a small spot on the sample. The vapor is then ionized in the plasma. The four charged rods (the quadrupole) then cause only the appropriate ions to arrive at the detector for counting; all others are lost. By changing the electric charge on the rods, different elements can be determined.
The vapor from the sample is then carried by a stream of argon gas into a 7,000 C argon plasma, where the vapor is ionized. These ions are then drawn into a quadrupole mass spectrometer (QMS). The QMS consists of two sets of electrically charged, machined rods. A radio-frequency signal is applied to both sets of rods. Under specific operating conditions, one unique, mass-to-charge ratio of ions will be directed down the opening between the four rods and exit to the detector. All other ions will be lost.
Based on these 250- micron-wide, black laser trails across the brown layer of clay from the K-T boundary in Caravaca, Spain, the quadrupole mass spectrometer found abnormally high abundances of platinum-group elements (up to 1,000 ppb), most likely coming from an extraterrestrial source.
The LA-ICP-MS is sensitive for all the platinum group elements (PGEs) that would appear from an asteroid impact. The laser, which has fine sampling resolution, was used to sample the 1-cm layer analyzed by Alvarez and coworkers but in bands only 0.25-mm thick. In this way, we were able to sample just the layer of PGE-enriched material and found the concentration in this zone to be nearly 1 ppm, a factor of 100 times higher than that previously reported. This greater concentration of the PGEs gives additional support to the theory that an extraterrestrial body collided with the Earth 65 million years ago. Return to this point in index.
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