Oxygen, hydrogen, and carbon ratios

Oxygen-18/oxygen-16, deuterium/hydrogen, and carbon-13/carbon-12 ratios in rocks are heavy- to light-isotope ratios of oxygen, hydrogen, and carbon, respectively. These ratios can give clues to the geologic conditions under which rocks were formed.

Stable Isotopes

Rocks are divisible into three major types: igneous, sedimentary, and metamorphic. Igneous rocks are formed from hot, molten rock material called magma. Sedimentary rocks are formed by the compaction and cementation of mineral grains and rock fragments, which collectively are called sediments. Metamorphic rocks are formed by the alteration of rocks caused by increased heat and pressure and interaction with water in pore spaces and fractures. The temperature of formation and the types of water that interacted with the rocks are among the geologic conditions that scientists can determine from stable isotope ratios in rocks.

Rocks are aggregates of minerals. Minerals are natural compounds made of atoms that are arranged in an ordered fashion. The atoms in minerals exist in different isotopes. Some isotopes are unstable, or radioactive; others are stable. Radioactive isotopes, or parent isotopes, change into isotopes of other elements, or daughter isotopes. This change, which is called radioactive decay, occurs at a constant rate and can be determined via experimental work. The amounts of parent and daughter isotopes in rocks and the decay constant of the isotope are used by scientists to determine rocks' ages. Unlike radioactive isotopes, stable isotopes do not decay into isotopes of other elements. Thus, stable isotopes are not used for age-dating purposes. Rather, the ratios of stable isotopes—particularly of low atomic number elements, such as hydrogen, carbon, and oxygen—are used to determine the geologic conditions under which rocks are formed.

Ionic and Covalent Bonds

The three elements oxygen, hydrogen, and carbon are key components of the minerals which make up rocks. Oxygen, the most common element in the crust, the topmost layer of the earth, is found in all kinds of rocks. Carbon is abundant in coal. Carbon and oxygen are important components of a fairly common group of sedimentary rocks called limestones and their metamorphosed product, marble. Hydrogen and oxygen are constituents of water and can be used to characterize the types of watery solution that interact with rocks.

Oxygen, hydrogen, and carbon form bonds that range from ionic to covalent. Ionic bonds are formed by the electrostatic attraction of adjacent atoms; covalent bonds are formed by the sharing of electrons and are stronger bonds. Scientists have found that bonding characteristics affect the behavior of isotopes in natural conditions; elements which form only one type of bond in all conditions are not useful for isotopic work.

Isotopic Separation

Oxygen, hydrogen, and carbon all have low atomic weights. The relative mass difference between the heavy and light isotopes is large for such elements, unlike the elements of high atomic weight. For example, deuterium, the heavy stable isotope of hydrogen, is heavier than the light isotope of hydrogen by about 100 percent. In contrast, the stable strontium isotopes differ from each other by only 1.2 percent. Some conditions and processes favor the incorporation of heavy isotopes into a material; others favor light isotopes. Thus, depending on the geologic conditions, there will be a difference between the heavy- to light-isotope ratio in one mineral and the ratio of the same isotopes in another mineral or in the source material. This syndrome is called isotopic separation or fractionation. Isotopic separation is significant and detectable only for elements with low atomic numbers, such as oxygen, carbon, and hydrogen. The conditions and processes that cause separation include temperature, evaporation and diffusion, and oxidation reduction reactions.

A mineral is a compound formed by the chemical bonding of atoms. The internal energy of a mineral is controlled by factors including the vibration of atoms. The atoms of light isotopes vibrate with higher frequencies than do atoms of heavy isotopes. Since the bond strength between atoms depends on the vibration frequencies of atoms, light isotopes are weakly bonded. Such bonds can be broken comparatively easily upon dissolution or by bacterial action. Significantly, with increased temperature, the vibrational frequencies of all isotopes of the same element become nearly equal. Therefore, all other factors being equal, a mineral which is formed at low temperatures will contain a higher heavy- to low-isotope ratio than a similar mineral which is formed at high temperatures. A change in temperature during mineral formation can cause isotopic separation. Consequently, scientists can use isotopic separation to determine temperatures of mineral formation; in other words, isotope ratios can be used as geothermometers. Commonly, oxygen-isotope ratios in two different oxygen-bearing minerals which were formed at the same time and from the same source are used to determine the temperature at which a rock formed.

Water-Rock Interactions

Applications of oxygen isotope work have provided scientists with additional insights into the formation of rocks. It is now known that groundwater interacts with magmas. Part of this water is incorporated into the magma, and part of it circulates through rocks, changing their nature (metamorphosing them) in the process. Furthermore, it is now possible to determine whether a magma has been contaminated by the incorporation and subsequent melting of roof rocks.

Evaporation and diffusion also cause isotopic separation. Water is a compound of oxygen and hydrogen. During evaporation, the light isotopes of oxygen (oxygen-16) and of hydrogen break through the water surface and escape to the atmosphere in the form of water vapor, while the heavy isotopes, oxygen-18 and deuterium, concentrate in reservoirs. Similarly, molecules with lighter isotopes move across a boundary (diffuse) faster than the same molecules with heavier isotopes. Evaporation and diffusion lead to different waters' having different isotopic ratios. Scientists have determined oxygen-18/oxygen-16 and deuterium/hydrogen ratios in many kinds of water: lakes, rivers, rain, snow, oceans, and water that comes from molten rocks. When these waters enter into interconnected pore spaces and fractures of rocks and circulate through the rocks at depth, they become hot solutions. These solutions interact with the rocks, leading to the diffusion (movement) of atoms from rocks to the solutions and from the solutions to the rocks. Such water-rock interaction changes the nature of both the rocks and the solutions. From stable isotope ratios, scientists can determine the nature of the rock, the nature of solution, and the type of water-rock interaction.

Oxidation and Reduction

Oxidation reactions are another cause of isotopic separation. Atoms are said to be in an oxidized state if they have a lower number of electrons and in a reduced state if they have a higher number of electrons when compared with other atoms of the same element. For example, the element carbon can occur in the form of C⁺⁴, C, and C⁻⁴. C⁺⁴ is a highly oxidized state of carbon, a positively charged ion formed by the loss of four electrons. Such ions combine with negative ions of other elements, such as oxygen ions, to form compounds such as carbon dioxide. In a highly reduced state, carbon atoms gain four electrons, forming the negatively charged carbon ion, C⁻⁴. These ions combine with positively charged ions to form compounds such as methane.

Oxidized carbon is enriched in the heavy carbon isotope carbon-13, and reduced carbon is enriched in carbon-12. Thus, if the same source material were to permit the formation of two compounds, one with oxidized carbon and the other with reduced carbon, the one with the oxidized carbon will be enriched in carbon-13. The application of this simple principle is quite involved, however, because carbon can cycle through living organisms and other environments.

Green leaves of plants photosynthesize carbon dioxide, reducing the carbon and making it part of organic compounds. This reduction is done in stages, which are different in different plants. Thus, some plant types can be distinguished by their carbon-13 values. Generally, land plants have lower values of carbon-13 than marine plants; however, marine algae have values within the range of land plants. Evaporation leads to the enrichment of seawater in carbon-13. Condensation in clouds leads to the enrichment of carbon-13 in raindrops as compared with water vapor. Consequently, repeated evaporation and rain cause seawater to be richer in carbon-13; the atmosphere is lower. Carbon dioxide in soils and groundwater has an even lower carbon-13 value, because the carbon there has cycled through decaying plants.

Diagenesis

It appears that freshwater limestone should have lower carbon-13 values than marine limestone. Also, marine limestone should have variable carbon-13 values, depending on the amount of carbon inherited from algae (lower values indicating higher algal content). Most limestones, however, undergo a change called diagenesis, which involves the reconstitution of carbonate minerals under conditions different from the original ones. That makes it difficult to determine precisely the original conditions under which the rock formed or the subsequent conditions that resulted in the diagenetic change. Commonly, low values of carbon-13 are obtained from the diagenesis of marine limestones.

Analytical Techniques

Scientists use many analytical techniques, including mass spectrometers, mass spectrographs, and ion microprobes, to determine isotopic ratios. All these techniques utilize the fact that different isotopes separate from each other and arrive at a detector at different speeds when they travel through a magnetic field from an ionization chamber, in which a sample containing the elements is bombarded by electrons or by (in the case of the microprobe) a beam of negatively charged oxygen atoms.

The separated isotopes are detected electronically in mass spectrometers. In mass spectrographs, they are detected by nonelectronic methods, such as photographic devices. For these methods of analysis, the elements or molecules of interest are chemically separated and introduced into an ionization chamber in a gaseous form, or they are deposited as solids on filaments that are then vaporized in the ionization chamber. In the ion microprobe method, chemical separation of the sample is not necessary, and the original sample does not have to be destroyed. A small sample of a rock is polished and then coated with gold or carbon. A beam of negatively charged oxygen is focused on the sample, over an area of less than 0.01 millimeter. That causes ionization of the sample; the ions are accelerated through a magnetic field to a detector, and the isotopes are measured by a mass spectrometer or spectrograph.

Mass Spectrometers and Spectrographs

There are many different spectrometers and spectrographs, but the principles involved can be understood by considering one type of spectrometer. In this device, an appropriate voltage applied across a filament, possibly of tungsten, produces a stream of electrons. These electrons bombard a sample and cause the removal of electrons from the atoms of the sample. The resulting positive ions of different isotopes are accelerated by a high-voltage field and are collimated into a beam. Since the kinetic energies of isotopes of the same element are identical, the lighter isotopes travel faster than the heavier isotopes. The ion beam passes through a magnetic field, which is constructed in such a way that different isotopes are separated from each other as they exit the field and enter a collector cup. The accelerating voltage and the magnetic field can be adjusted so that an ion beam of one isotope can be focused through a collector slit to enter a detector cup. The focused ions are neutralized by electrons which flow through a resistor, and the voltage difference across the resistor can be measured with a voltmeter. The ensuing electrical signals can be digitized or, more commonly, displayed on a strip-chart record. A series of peaks and valleys—each peak representing an isotope, with the peak height being proportional to the abundance of the isotope—can be recorded by adjusting the accelerating voltage or the magnetic field, which would vary the ions being focused through the collector slits. In this way, the various isotopes and their relative abundances can be determined. In turn, the ratios of heavy to light isotopes can be calculated.

Modern commercial mass spectrometers, equipped with multiple collectors for the simultaneous detection of different isotopes and with digital computers, have improved both the speed of acquisition and the reliability of isotopic data.

Applications of Stable Isotopic Geochemistry

Many kinds of chemical analysis can be used to determine the concentration of ions in groundwater. If different ions originate from different source areas, then these areas can be distinguished by such analysis. When the source areas produce the same ions, the isotopic ratios of the ions may be different because of different environmental conditions. In such instances, stable isotope geochemistry can be used to specify the source area, such as a groundwater-polluting factory. Stable isotope geochemistry, then, is useful in groundwater pollution studies. One of the uses of mass spectrometers is to identify elements by their isotopes, and by using a procedure called isotopic dilution, scientists can identify elements whose amounts in a sample are extremely small. The isotope dilution method provides the added advantage of identifying the groundwater pollutants.

Scientists have used stable isotopic geochemistry to study the diet of prehistoric humans. The remains of wood in ancient campfires have been analyzed for their stable isotopic ratios. Such ratios can help the researcher to distinguish between land and marine plants and between groups of land plants. The ion microprobe, with its ability to analyze particles smaller than 0.01 millimeter, has a potential application in forensic science, where trace amounts of hair and clothes are used to identify criminals. The isotopic dilution method is another procedure that can be applied to such an effort.

Principal Terms

carbonate: a mineral containing the carbonate ion, which is composed of one carbon atom and three oxygen atoms

geothermometers: minerals whose components can be used to determine temperatures of mineral formation

ion: an atom that has either lost or gained electrons

isotopes: atoms with an identical number of protons but a different number of neutrons in their nuclei

isotopic fractionation: the enrichment of one isotope relative to another in a chemical or physical process; also known as isotopic separation

limestone: a sedimentary rock composed predominantly of calcite

mineral: a natural substance with a definite chemical composition and an ordered internal arrangement of atoms

radioactive isotope: an isotope of an element that naturally decays into another isotope

stable isotope: an isotope of an element that does not change into another isotope

Bibliography

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Berner, Elizabeth K., and Robert A. Berner. Global Environment: Water, Air, and Geochemical Cycles. Upper Saddle River, N.J.: Prentice Hall, 1996. This book discusses the processes that sustain life and effect change on the earth. Topics include the hydrologic cycle; the atmosphere; atmospheric carbon dioxide and the greenhouse effect; acid rain; the carbon, sulfur, nitrogen, and phosphorus cycles; weathering; lakes; rivers; and the oceans. It is understandable to the college-level reader.

Cazes, Jack. Ewing's Analytical Instrumentation Handbook. 3rd ed. New York: Marcel Dekker, 2005. Updated from Ewing's original publication to include more current methods, techniques and analyses.

Ewing, G. W., ed. Chemical Instrumentation. Easton, Pa.: Chemical Education Publishing, 1971. An excellent publication on instrumentation written for a college-level audience.

Faure, Gunter. Isotopes: Principles and Applications. 3rd ed. New York: John Wiley & Sons, 2004. Originally titled Principles of Isotope Geology. An excellent book that integrates theory with practice and is easy to read.

Greenwood, Norman Neill, and A. Earnshaw. Chemistry of Elements. 2d ed. Oxford: Butterworth-Heinemann, 1997. An excellent resource for a complete description of the elements and their properties. The book is filled with charts and diagrams to illustrate chemical processes and concepts. Bibliography and index.

Hoefs, J. Stable Isotope Geochemistry. 6th ed. New York: Springer-Verlag, 2009. This useful source provides ample field examples.

Krauskopf, Konrad B. Introduction to Geochemistry. 3rd ed. New York: McGraw-Hill, 2003. A section in this book provides a distillation of the nature and uses of stable isotope work.

Krebs, Robert E. The History and Use of Our Earth's Chemical Elements: A Reference Guide. 2d ed. Westport, Conn.: Greenwood Press, 2006. This book defines geochemistry and examines its principles and applications. A good resource for the layperson interested in the field of geochemistry and in the earth's elements. Accessible to high school readers. Illustrations, charts, and bibliography.

Richardson, S. M., H. Y. McSween, Jr., and Maria Uhle. Geochemistry Pathways and Processes. 2d ed. Englewood Cliffs, N.J.: Prentice-Hall, 2003. A chapter in this book offers an accurate summary of stable isotope geochemistry.

Santamaria-Fernandez, Rebeca. “Precise and Traceable Carbon Isotope Ratio Measurements by Multicollector ICP-Ms: What Next??” Analytical & Bioanalytical Chemistry 397 (2010): 973-978. This article reviews a new method of mass spectrometry used to measure carbon ratios. Provides a good description of mass spectrometry methodology for the intermediate or beginner MS analyzer.

Szczepanek, MalGorzata, et al. “Hydrogen, Carbon, and Oxygen Isotopes in Pine and Oak Tree Rings from Southern Poland as Climatic Indicators in Years 1900-2003.” Geochronometria: Journal on Methods & Applications of Absolute Chronology 25 (2006): 67-76. This article discusses the use of carbon, hydrogen, and oxygen ratios in isotope-dating methods. Written technically for a reader with a scientific background, but still accessible to the advanced layperson.

Valley, J. W., H. P. Taylor, Jr., and J. R. O'Neil, eds. Reviews in Mineralogy. Vol. 1b, Stable Isotopes in Higher Temperature Geologic Processes. Washington, D.C.: Mineralogical Society of America, 1986. A collection of work from authorities on various aspects of the topic, with a primary emphasis on igneous and metamorphic rocks and ore deposits.