Water-rock interactions

Water-rock interactions occur as fluids circulate through rocks of the earth's crust. Isotopes of common elements are exchanged between a fluid and its host rock. As a result of these reactions, a rock may preserve a record of the fluids that have passed through its pore spaces. Studies of water-rock interactions provide information on the nature of fluid movement through rocks and on the origin of economic ore deposits.

Stable Isotopes

Fluids that circulate within the crust of the earth take part in chemical reactions involving an exchange of elements between the fluid and the host rock. Such reactions, referred to as fluid-rock interactions, are an important mechanism in the concentration of minerals in economically valuable ore deposits. A useful means of studying fluid-rock interactions is by measurement of the stable isotopic composition of rocks that have undergone such an exchange history.

Rocks are exposed to fluids that are diverse in their origin and composition. The most important volatile constituents of natural fluids are water, carbon dioxide, carbon monoxide, hydrogen fluoride, sulfur compounds, and light hydrocarbons such as methane. In addition, fluids contain dissolved solids derived from the crustal rocks through which they pass. As the list of volatiles makes clear, the dominant elements present in fluids are oxygen, hydrogen, carbon, and sulfur.

An element may be characterized by its atomic number and atomic weight. Atomic number refers to the number of protons present in the nucleus of an atom and is a constant value for each element. Atomic weight is determined by adding together the number of protons and neutrons contained in an atom. Because the number of neutrons present often varies within a limited range, atoms of a particular element may have several different atomic weights. These atoms are referred to as isotopes. Oxygen, with an atomic number of 8, occurs most commonly with eight neutrons but may have nine or ten. Therefore, three isotopes of oxygen occur in nature: oxygen-16, oxygen-17, and oxygen-18. Similarly, carbon occurs as carbon-12 or carbon-13. (Carbon-14 is a radioactive isotope that forms in the earth's upper atmosphere and will not be considered here.) Hydrogen contains only one proton; however, a small fraction of hydrogen atoms also contain a neutron, which doubles the mass of the atom. This heavy hydrogen isotope is called deuterium. Sulfur has four stable isotopes: sulfur-32, -33, -34, and -36.

Stable Isotope Ratios

For any given mineral or fluid, the relative concentrations of the isotopes of a particular element may be expressed as a stable isotopic ratio, or the ratio of the second most abundant isotopic species over the most abundant isotope. Any physical process that results in the enrichment or depletion of the concentration of a heavy isotope is referred to as fractionation. A common fractionation process is evaporation of water. Water molecules containing the lighter isotope of oxygen (oxygen-16) will preferentially evaporate so that the remaining liquid will be enriched in the heavier oxygen isotope (oxygen-18). Fractionation also occurs during the growth of minerals in either a magma or a water-rich solution. Some minerals, because of the nature of their chemical bonds and their crystal structure, tend to contain a greater number of heavy isotopes than do other minerals. Quartz, dolomite, and calcite are common examples of minerals that contain high concentrations of heavy oxygen, while oxides such as ilmenite and magnetite have very little of the heavy isotope. The effectiveness of fractionation during mineral growth is dependent upon temperature. Low temperatures permit minerals to be more selective in choosing atoms for growing crystal sites, resulting in large differences in isotopic ratios between different minerals. At high temperatures, the selection of atoms is a more random process, and differences between isotopic ratios become progressively smaller.

Because the heavier isotopes of elements naturally occur in such small concentrations, isotopic ratios of oxygen and carbon, for example, have numerical values that are very small and difficult to measure accurately. For this reason, isotopic ratios for a particular sample are presented as a relative enrichment or depletion of the heavy isotope of an element as compared with a defined standard. The difference between the sample and the standard is measured in parts per thousand or per million and is expressed by the Greek letter delta (δ). For carbon isotopic values, for example, the accepted standard is called PDB and is obtained from a belemnite fossil of the Cretaceous-age Pee Dee formation of North Carolina.

Exchange of Stable Isotopes

As fluids migrate through rocks, reactions occur that involve the exchange of stable isotopes between the fluid and the solid. The exchange process may be pervasive, where fluid movement is diffusive and affects the entire rock mass, or localized along specific fluid channelways, such as fractures, where only the wall of the rock is altered along the route of water movement. The degree to which the isotopic composition of the rock is altered depends on the initial composition of the rock and the fluid, the temperature at which isotopic exchange is occurring, and the amount of fluid present. Typically, the oxygen and hydrogen isotopic values for crustal water are light compared to those for most rocks. Therefore, as isotopic exchange proceeds, the isotopic composition of the rock becomes progressively lighter, while the water becomes increasingly enriched in the heavier isotopes. If the fluid-rock interaction has occurred under constant temperature conditions, the final isotopic value of the rock is proportional to the volume of water that has passed through the rock.

Reservoirs of Crustal Fluids

There are four principal sources, or reservoirs, of crustal fluids. Each of these fluid reservoirs contains stable isotopic ratios that reflect the fractionation mechanisms at work and the result of chemical reactions between the fluids and their host rocks.

“Meteoric water” is a term applied to fluids that take part in the surface hydrologic cycle. Water that undergoes evaporation, precipitation, and runoff to lakes and to the ocean is capable of penetrating the earth's crust to a depth of several kilometers. This penetration is usually accomplished by fluid migration along weaknesses in the crust, such as faults or fracture systems. Natural hot springs are an example of meteoric water that has been heated deep in the crust and then reemerges at the surface. The oxygen and hydrogen isotopic ratios associated with meteoric water are controlled principally by the distillation effect of evaporation and precipitation. As a result of the general transport of air masses from the equator toward the poles, meteoric water isotopic values vary in a systematic manner, with heavier ratios found near the earth's equator and progressively lighter isotopic values occurring toward the poles.

Meteoric water that is trapped in the pore spaces of accumulating sediments is referred to as connate water. When loose sediments have lithified to form hard sedimentary rock, the enclosed pore fluids may become isolated for very long periods. Connate water reveals an isotopic trend similar to that of meteoric water, except that the oxygen values tend to be heavier because of the capacity of the lighter oxygen (oxygen-16) contained in the trapped pore fluids to be exchanged for some of the heavier oxygen (oxygen-18) of the host sedimentary rock. Isotopic exchange between connate water and the host sediments continues until equilibrium is achieved.

As rocks undergo increases in temperature and pressure associated with metamorphism, water is frequently released in a process called dehydration. The escaping water may be from pore spaces in the rock or from hydrous minerals such as micas or amphiboles. Because of their origin, these dehydration fluids are also referred to as metamorphic fluids. Dehydration water has a very wide range of isotopic compositions, which reflects the diversity of the original sediments.

Juvenile water, which originates in the upper mantle, escapes from ascending magma and represents the fourth important fluid source. Not all water derived from magma should be considered juvenile, as meteoric or connate water will frequently be present in sediments that undergo melting deep in the crust. True juvenile water has a very narrow range of isotopic compositions. Because of mixing of crustal fluids and exchange reactions with igneous rocks, samples of unaltered juvenile water are found very rarely.

Isotopic Compositions of Rocks

The isotopic compositions of rocks reflect the formation history of the particular rock type. Igneous rocks are controlled by the composition of their magmatic source area, which is usually in the lower crust or upper mantle. Other factors include the temperature of crystallization, the type of minerals, and the degree to which the magma remains isolated or mixes with other constituents. Unaltered igneous rocks typically have a narrow range of isotopic values as compared with natural fluids. Deep mantle rocks contain minerals that are low in oxygen-18 and therefore fall in a narrow range of isotopically light compositions. Crustal rocks, with a higher proportion of silicate minerals, which concentrate oxygen-18, are isotopically heavy. Sedimentary rocks have two distinct modes of formation. Clastic sedimentary rocks are made of transported particles of weathered material and therefore have isotopic ratios that reflect the individual components. Chemical sedimentary rocks precipitate directly and often involve biological activity. The oxygen isotopic values of these rocks are usually much heavier than those of fluids. Because of fractionation factors associated with organisms, carbon isotope ratios are highly variable. Rocks that have undergone metamorphism contain the widest range of isotopic compositions, as a result of chemical reactions in the presence of fluids that may be derived from any of the reservoirs previously described.

Mass Spectrometry

The most important analytical tool in the study of fluid-rock interactions is the stable isotope mass spectrometer. This instrument separates isotopes according to their mass differences, as determined by the deflection of charged ions within a magnetic field. Elements of interest are extracted from minerals through appropriate chemical reactions and then converted to a gas, which is entered into the mass spectrometer. The sample gas is bombarded by a stream of electrons, converting the gas molecules to positively charged ions. The ions are accelerated along a tube, where a powerful magnet deflects the charged molecules into curved pathways. Lighter particles are deflected more than heavier ones, so several streams of ions result, each with a particular mass. The relative proportions of each isotope are measured by comparing the induced current produced by each ion stream at a collector.

Research Involving Igneous Rocks

Studies of oxygen and hydrogen isotopes have been useful in research on water-rock interactions involving igneous rocks. Hydrous minerals, such as biotite and hornblende, are separated out of granitic rocks in order to extract hydrogen isotopic values. Oxygen isotopic compositions are usually measured from feldspars and quartz. Two areas of water-rock study have been of particular interest. The first concerns the origin and quantity of water responsible for the isotopic alteration of large igneous intrusions within the continental crust. The second area of investigation addresses the interaction of ocean water with ocean-crust basalt and the formation of associated sulfide ore deposits.

The initial isotopic composition of igneous rocks is predictable according to their mineral content and temperature of crystallization. Therefore, it is possible to recognize when igneous rocks have been influenced by exchange with a fluid. Many examples have been found of shallow igneous intrusions that have been depleted in oxygen-18 through interaction with large volumes of isotopically light meteoric water. Hydrogen isotope exchange is even more sensitive than with oxygen, because igneous rocks have so little hydrogen relative to the amount in water. A very small quantity of water may produce a large isotopic shift in a rock's hydrogen value, while the oxygen is largely unaffected.

Isotopes associated with the ocean crust have been studied through cores obtained from seabed drilling and through the analysis of ophiolites, which represent portions of ocean crust exposed on land. In the vicinity of the mid-ocean ridge, where temperatures exceed 300 degrees Celsius, ocean water circulates to a depth of 3 or 4 kilometers and is responsible for depleting oxygen-18 by one or two parts per thousand.

Study of Metamorphism and Precipitated Rock

Fluids associated with metamorphism have also been intensively studied by stable isotopic methods. Increasing metamorphic grade is associated with progressively lighter isotopic values for oxygen and hydrogen. Areas of regional metamorphism may show two end-member types of fluid behavior. Consistent depletion of oxygen and hydrogen isotopic values throughout a large terrain points to exchange with an external source of water that flows pervasively through the region. Alternatively, only the trapped connate water may be involved in exchange reactions, leading to higher isotopic composition values and to enrichment of deuterium in hydrous minerals. Because of the large difference in isotopic composition between magma and sedimentary rocks, contact metamorphism is particularly appropriate for study. Samples from intrusions indicate that the margin of the igneous rock is enriched in oxygen-18 through exchange with the country rock, while the interior of the body remains unaltered. The early stages of fluid-rock interaction are dominated by magmatic water, while meteoric water becomes increasingly important as cooling proceeds.

Sedimentary rocks that form by precipitation, such as limestone and chert, have heavy isotopic compositions as a result of the large fractionation between water and either calcite or quartz at low temperature. Clastic sandstones, which contain transported quartz grains, are characterized by the lighter isotopic values of the component particles. Sedimentary rocks selected from a large sample area frequently show a progressive change in the amount of isotopic depletion that has resulted from water-rock exchange. As water continues the exchange process, the isotopic composition of the fluid also shifts. During the next increment of water-rock exchange, the potential amount of depletion of the rock will not be as great. By plotting isotopic values of sedimentary rocks, it is possible to determine directions of fluid motion.

Search for Economic Resources

Water-rock interactions are important primarily for their role in the formation of economically vital ore deposits. These are localized regions where metals such as gold, silver, lead, copper, zinc, and tungsten occur in unusually high concentrations and can be extracted. Water, circulating within the crust of the earth, plays an important role in the formation of most ore deposits by leaching elements from rocks and concentrating them in zones of new mineral growth. During this process, the isotopic compositions of the host rock and fluid are progressively changed. Analysis of the resulting stable isotope ratios of the ore rocks allows geologists to understand the sources and quantity of the mineralizing fluids. Better understanding of the formation of ore deposits has led to greater success in the discovery of new mineral resources.

Stable isotope research into water-rock interaction is not limited to studying water-rich fluids associated with precious mineral deposits. Petroleum and natural gas flow from source rocks rich in organic material to porous reservoir rocks, where they may become trapped. Rocks through which hydrocarbons have migrated frequently show well-depleted carbon isotope values and thus preserve a record of fluid movement. Oil companies have used carbon data to track the migration history of hydrocarbons and to locate regions where petroleum leaks to the surface. The use of carbon isotopes has also proved successful in identifying source rocks associated with producing oil fields. This technology will become even more important as resources become increasingly scarce.

The source of fresh drinking water for almost half the population in the United States is subsurface groundwater. This crucial resource is jeopardized by contamination with common pollutants, such as pesticides, and by depletion through the withdrawing of water faster than it is replenished at recharge zones. Research involving stable isotope studies has become important in hydrology to track fluid movement within aquifers and identify sources of recharge water.

Hazardous Waste Storage

Another area of concern associated with water-rock interaction is the safe, long-term storage of toxic and nuclear waste. One of the most important criteria for the isolation of dangerous wastes is that the enclosing rocks be relatively dry and impermeable to water movement so that hazardous material is not transported into water supply aquifers. The record of water flow recorded by stable isotopes is sensitive to even very small fluid volumes and provides one of the means of assessing the risks associated with a toxic disposal site.

Principal Terms

connate fluids: fluids that have been trapped in sedimentary pore spaces

dehydration: the release of water from pore spaces or from hydrous minerals as a result of increasing temperature

exchange reaction: the exchange of isotopes of the same element between a rock and a liquid

fractionation: a physical or chemical process by which a particular isotope is concentrated in a solid or liquid

isotopes: atoms of the same element with identical numbers of protons but different numbers of neutrons in their nuclei

juvenile water: water that originated in the upper mantle, also called magmatic water

mass spectrometer: a laboratory instrument that separates isotopes of a particular element according to their mass difference

meteoric water: water that takes part in the surface hydrologic cycle

volatiles: dissolved elements and compounds that remain in solution under high-pressure conditions but would form a gas at lower pressures

Bibliography

Albarede, Francis. Geochemistry: An Introduction. 2d ed. Boston: Cambridge University Press, 2009. A good introduction for students looking to gain some knowledge in geochemistry. Covers basic topics in physics and chemistry; isotopes, fractionation, geochemical cycles, and the geochemistry of select elements. Also includes water-rock reactions and ratios.

Blatt, Harvey, and Robert J. Tracy. Petrology: Igneous, Sedimentary, and Metamorphic. 3rd ed. New York: W. H. Freeman, 2005. Undergraduate text in elementary petrology for readers with some familiarity with minerals and chemistry. Thorough, readable discussion of most aspects of water-rock interactions. Abundant illustrations and diagrams, good bibliography, and thorough indices.

Bowen, Robert. Isotopes and Climates. London: Elsevier, 1991. Bowen examines the role of isotopes in geochemical phases and processes. This text does require some background in chemistry or the earth sciences but will provide some useful information about isotopes and geochemistry for someone without prior knowledge in those fields. Charts and diagrams help clarify difficult concepts.

Brantley, Susan, James Kubicki, and Art White, eds. Kinetics of Rock-Water Interactions. New York: Springer, 2007. Written by experts in the field of rock-water interactions. This text covers rates of reactions, transition state theory, the mineral water interface, mineral dissolution, and much more. Chapter 12 focuses on water-rock interactions.

Faure, Gunter. Isotopes: Principles and Applications, 3rd ed. New York: John Wiley & Sons, 2004. Originally titled Principles of Isotope Geology. A college-level text that covers both radioactive and stable isotopes. The first five chapters are introductory in nature and include a good historical review of the development of isotope geology and mass spectrometry. The last unit covers stable isotopes and includes figures reproduced from class research papers. Each chapter includes a detailed reference list.

Gregory, Snyder A., Clive R. Neal, and W. Gary Ernst, eds. Planetary Petrology and Geochemistry. Columbia, Md.: Geological Society of North America, 1999. A compilation of essays written by scientific experts, this book provides an excellent overview of the field of geochemistry and its principles and applications. The essays can get technical at times and are intended for college students.

Hoefs, Jochen. Stable Isotope Geochemistry. 6th ed. New York: Springer-Verlag, 2009. Suitable for an advanced college student who seeks a detailed discussion of isotope fractionation, sample preparation, and laboratory standards. The material is introduced in three sections. The first chapter provides theoretical principles; the second chapter is a systematic description of the most common stable isotopes; and the third summarizes the occurrence of stable isotopes in nature. An extensive list of references is included at the end of the book.

Krauskopf, Konrad B. Introduction to Geochemistry. 3rd ed. New York: McGraw-Hill, 2003. A comprehensive advanced text that covers most aspects of the chemistry of natural fluids. Radioactive and stable isotopes are briefly treated, along with a discussion of ore-forming solutions. This resource is particularly useful for students who seek detailed information on the chemistry and interaction of crustal water. Suggestions for further reading are provided at the end of each chapter.

Oelkers, Eric H., ed. Thermodynamics and Kinetics of Water-Rock Interaction: Reviews in Mineralogy and Geochemistry. Mineralogical Society of America, 2009. Contains multiple chapters on water-rock interactions. Each chapter complete with references.

O'Neil, J. R. “Stable Isotope Geochemistry of Rocks and Minerals.” In Lectures in Isotope Geology, edited by Emilie Jäger and Johannes C. Hunziker. New York: Springer-Verlag, 1979. This source provides a brief and clear introductory section on stable isotope nomenclature. The remainder of the chapter outlines major conclusions drawn from isotope analysis of igneous, metamorphic, sedimentary, and ore deposit rocks. Examples are provided from pioneering research studies. Although the text is oriented toward the college level, high school students interested in the results of isotope studies will find this chapter useful.

Smith, David G., ed. The Cambridge Encyclopedia of Earth Sciences. New York: Crown Publishers, 1981. Organized as a compilation of high-quality and authoritative scientific articles rather than a typical encyclopedia. Chapter 8, “Trace Element and Isotope Geochemistry,” is a brief, well-illustrated summary of the occurrence of trace elements, stable isotopes, and radiogenic elements. The chapter emphasizes how trace element and isotope studies have enhanced understanding of processes such as the generation of magma and the occurrence of ore deposits. The discussion of water-rock interaction associated with the ocean crust would be accessible to advanced high school students. Few additional references are offered.