Chapter01.pdf

Geochemistry

W. M. White

Chapter 1: Introduction

1.1 Geochemistry

he term “geochemistry” was first used by the Swiss chemist Schönbein in 1838. You might guess,

merely from the etymology of the word, that the field of geochemistry is somehow a marriage of

the fields of geo logy and chemistry . That would be a good guess. But just how are chemistry and

geology combined within geochemistry; what is the relationship between them? Perhaps the best ex-

planation would be to state that in geochemistry, we use the tools of chemistry to solve geological problems;

that is, we use chemistry to understand the Earth and how it works . The Earth is part of a family of heavenly

bodies, our Solar System, that formed simultaneously and are closely related. Hence, the realm of geo-

chemistry extends beyond the Earth to encompass the entire Solar System. The goals of geochemistry

are thus no different from those of other fields of earth science; just the approach differs. On the other

hand, while geochemists have much in common with other chemists, their goals differ in fundamental

ways. For example, our goals do not include elucidating the nature of chemical bonding or synthesiz-

ing new compounds, although these may often be of interest and use in geochemistry. Though geo-

chemistry is a subdiscipline of earth science, it is a very broad topic. So broad in fact that no one can

really master it all; geochemists invariably specialize in one or a few aspects, such as atmospheric

chemistry, geochemical thermodynamics, isotope geochemistry, marine chemistry, trace element geo-

chemistry, soil chemistry, etc.

Geochemistry has flourished in the quantitative approach that grew to dominate earth science in the

second half of the twentieth century. This quantitative approach has produced greater advances in the

understanding of our planet in the last 50 years than in all of prior human history. The contributions of

geochemistry to this advance have been simply enormous. Much of what we know about how the

Earth and the Solar System formed has come from research on the chemistry of meteorites. Through

geochemistry, we can quantify the geologic time scale. Through geochemistry, we can determine the

depths and temperatures of magma chambers. Through geochemistry, mantle plumes were recognized.

Through geochemistry, we know that sediments can be subducted into the mantle. Through geochem-

istry, we know the temperatures and pressures at which the various metamorphic rock types form and

we can use this information, for example, to determine the throw on ancient faults. Through geochem-

istry, we know how much and how fast mountain belts have risen. Through geochemistry, we are

learning how fast they are eroding. Through geochemistry, we are learning how and when the Earth’s

crust formed. Through geochemistry, we are learning when the Earth’s atmosphere formed and how it

has evolved. Through geochemistry, we are learning how the mantle convects. Through geochemistry,

we are learning how cold the ice ages were and what caused them. The evidence of the earliest life, 3.8

gigayears (billion, or 10 9 years, which we will henceforth abbreviate as Ga), is not fossilized remains,

but chemical traces of life. Similarly, the tenuous evidence that life existed on Mars about the same

time is also largely chemical. Not surprisingly, instruments for chemical analysis have been key part of

probes sent to other heavenly bodies, including Venus, Mars, Jupiter. Geochemistry lies at the heart of

environmental science and environmental concerns. Problems such as acid rain, the ozone hole, the

greenhouse effect and global warming, water and soil pollution are geochemical problems. Addressing

these problems requires knowledge of geochemistry. Similarly, most of our non-renewable resources,

such as metal ores and petroleum, form through geochemical processes. Locating new sources of these

resources increasing requires geochemical approaches. In summary, every aspect of earth science has

been advanced through geochemistry.

Though we will rarely discuss it in this book, geochemistry, like much of science, is very much

driven by technology. Technology has given modern geochemists tools that allow them to study the

Earth in ways that pioneers of the field could not have dreamed possible. The electron microprobe al-

lows us to analyze mineral grains on the scale of microns in minutes; the electron microscope allows us

to view the same minerals on almost the atomic scale. Techniques such as X-ray diffraction, nuclear

magnetic resonance, and Raman and infrared spectroscopy allow us to examine atomic ordering and

September 9, 2009

Geochemistry

W. M. White

Chapter 1: Introduction

bonding in natural materials. Mass spectrometers allow us to determine the age of rocks and the tem-

perature of ancient seas. Ion probes allow us to do these things on micron scale samples. Analytical

techniques such as X-ray fluorescence and inductively coupled plasma spectrometry allow us to per-

form in minutes analyses that would take days using “classical” techniques. All this is done with

greater precision and accuracy than was possible just a few decades ago. Mega-computers with giga-

hertz of power and gigabytes of memory allow us to perform in seconds thermodynamic calculations

that would have taken years or lifetimes half a century ago; the tera-computers just around the corner

will offer us even more power. New instruments and analytical techniques now being developed

promise even greater sensitivity, speed, accuracy, and precision. Together, these advances will bring us

ever closer to our goal understanding the Earth and its cosmic environment.

1.2 This Book

The intent of this book is to introduce you to geochemistry and to further your understanding of the

Earth through it. To do this, we must first acquire the tools of the trade. Every trade has a set of tools.

Carpenters have their saws and T-squares; plumbers have their torches and wrenches. Psychologists

have their blot tests, physicians their stethoscopes, accountants their balance sheets, geologists have

their hammers, compasses, and maps. Geochemists too have a set of tools. These include not only a

variety physical tools such as analytical instruments, but interpretative tools that allow them to make

sense of the data these instruments produce. The first part of this book, entitled The Geochemical Tool-

box , is intended to familiarize you with the tools of geochemistry. These include the tools of thermody-

namics, kinetics, aquatic chemistry, trace element geochemistry, and isotope geochemistry. Once we

have a firm grip on these tools, we can use them to dissect the Earth in the second part of the book, enti-

tled Understanding the Earth . We begin at the beginning, with the formation of the Earth and the Solar

System. We then work our way upward through the Earth, from the mantle and core, through the

crust and hydrosphere, and finally into the atmosphere.

In filling our geochemical toolbox, we start with the tools of physical chemistry: thermodynamics

and kinetics. Thermodynamics is perhaps the most fundamental tool of geochemistry; most other tools

are built around this one. For this reason, Chapters 2, 3, and 4 are devoted to thermodynamics. Ther-

modynamics allows us to predict the outcome of chemical reactions under a given set of conditions. In

geochemistry we can, for example, predict the sequence of minerals that will crystallize from a magma

under given conditions of temperature and pressure. The mineral assemblage of the resulting igneous

rock, however, will not be stable at some other temperature and pressure. Thermodynamics allows us

to predict the new suite of minerals that replace the original igneous ones. Thus thermodynamics pro-

vides enormous predictive power for the petrologist. Since geologists and geochemists are more often

concerned with understanding the past than with predicting the future, this might seem to be a point-

less academic exercise. However, we can also use thermodynamics in the reverse sense: given a suite of

minerals in a rock, we can use thermodynamics to determine the temperature and pressure conditions

under which the rock formed. We can also use it to determine the composition of water or magma

from which minerals crystallized. This sort of information has been invaluable in reaching our un-

derstanding of how the Earth has come to its present condition. We can use this information to deter-

mine the amount of uplift experienced by a mountain range, the temperature at which an ore deposit

formed, or the composition of ancient seas.

Thermodynamics has an important limitation: it is useful only in equilibrium situations. The rate at

which chemical systems achieve equilibrium increases exponentially with temperature. Thermo-

dynamics will be most useful at temperatures relevant to the interior of the Earth, say 500° C and

above, because equilibrium will be more closely approached in those cases. At temperatures relevant

to the surface of the Earth, many geochemical systems will not be in equilibrium and are governed by

partly or largely by kinetics, the subject of Chapter 5. Kinetics deals with the rates and mechanisms of

reactions. In this chapter, we will also touch upon such topics as diffusion and mineral surfaces. We

will see that kinetics is intimately related to thermodynamics.

In Chapter 6, we see how tools of physical chemistry are adapted for use in dealing with natural so-

lutions, the subject of aquatic chemistry. Much of the Earth’s surface is covered by water, and water

usually is present in pores and fractures to considerable depths even on the continents. This water is

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Geochemistry

W. M. White

Chapter 1: Introduction

not pure, but is instead a solution formed by interaction with minerals and atmospheric gases. In

Chapter 6, we acquire tools that allow us to deal with the interactions among dissolved species and

their interactions with the solids with which they come in contact. These interactions include phenom-

ena such as dissolution and precipitation, complexation, adsorption and ion exchange. Clays are often

the products of water-rock interaction and they have some very interesting chemical properties, so we

will have a particularly close look at this group of minerals. The tools of aquatic chemistry are essential

to understanding processes such as weathering and precipitation of sedimentary minerals, as well as

dealing with environmental problems.

In Chapter 7, we move on to trace element geochemistry. In this chapter we will see that trace ele-

ments have provided remarkable insights into the origin and behavior of magmas. Without question,

their value to geochemists far outweighs their abundance. There are several reasons for this. Their

concentrations vary much more than do those of the more abundant elements, and their behavior tends

often to be simpler and easier to treat than that of major elements (a property we will come to know as

Henry’s Law). Geochemists have developed special tools for dealing with trace elements; the objective

of Chapter 7 is to become familiar with them.

Chapters 8 and 9 are devoted to isotope geochemistry. In Chapter 8, we learn that radioactive decay

adds the important element of time; radioactivity is nature’s clock. By learning to read this clock, we

now know the age of the Earth and the continents, and we have gained some perspective on the rate

and manner of evolution of the Earth. We can also use the products of radioactive decay, “radiogenic

elements”, as tracers. By following these tracers much as we would dye in fish tank, we can follow the

evolution of a magma, the convection pattern of the mantle, and the circulation of the oceans. The iso-

topes of another set of elements vary not because of radioactive decay, but because of subtle differences

in their chemical behavior. These “stable isotopes” are the subject of Chapter 9. The subtle differences

in isotopic abundances of elements such as H, C, N, O, and S have, among other things, revealed the

causes of the ice ages, provided insights into the composition of the ancient atmosphere, and reveal the

diets of ancient peoples. Stable isotope geochemistry is the last of our geochemical tools.

With our toolbox full, we examine the Earth from the geochemical perspective in the second part of

the book. We begin in Chapter 10 by looking at “the big picture”: the cosmos and the Solar System. We

learn how the chemical elements were formed, and how they, in turn, formed our Solar System and the

Earth. We will find the tools of thermodynamic and isotope geochemistry particularly valuable in this

Chapter. We will focus particularly closely on meteorites, because the chemistry of these objects pro-

vides the best record of the early history of the Solar System. Meteorites also provide essential informa-

tion about the composition of the Earth as a whole, which will in turn be valuable to us in the following

chapter.

In Chapter 11, we begin our inside-out geochemical tour of the Earth. First, we consider the compo-

sition of the Earth as a whole, then see how the Earth has differentiated into two major reservoirs: the

mantle and core. We pay particular attention to the mantle. Though remote, the mantle is hardly ir-

relevant. It is important for several reasons. First, it constitutes 1/2 of the mass of the Earth. Second,

the reservoirs we are most familiar with, the crust, the hydrosphere, and the atmosphere, have all

formed from the mantle. Third, most geologic processes are ultimately a result of processes occurring

within the mantle, processes such as convection and melting. In Chapter 12, we return to more familiar

territory: the Earth’s crust. We will find that geochemistry has provided much of our knowledge of

how the crust has formed and how it has differentiated. We will find the tools of isotope and trace

element geochemistry particularly useful in our examination of the solid Earth.

The next three chapters focus on processes at the surface of the Earth. Here water is the dominant

substance, and the tools of thermodynamics, kinetics, and aquatic chemistry will be of great use. In

Chapter 13, we will take a close look at the interaction between water and the Earth’s surface, and proc-

esses such as weathering and soil formation. We will see how these processes control the chemistry of

streams, rivers, and lakes. Life is also an important force in shaping the face of our planet. The chemis-

try of living organisms is part of biochemistry and not geochemistry, so we will treat intracellular proc-

esses only very briefly. However, organisms produce a vast array of chemicals that find their way into

the physical environment. In Chapter 14, we will examine the role these organic chemicals play in

aquatic chemistry. We will also see how these chemicals are transformed into substances of great geo-

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Geochemistry

W. M. White

Chapter 1: Introduction

logical and societal interest: oil, gas, and coal. Most of the water at the surface of the Earth is in the

oceans, so we devote Chapter 15 to marine chemistry. We will find that oceans are a fascinating exam-

ple of an “open” geochemical system, with material constantly flowing both into and out. We will see

that in the face of this constant change, geological and biological processes together produce a solution

with very uniform concentrations of the major species, but highly variable concentrations of the minor

ones.

Before we begin our study of geochemistry, we will devote the remainder of the chapter to review-

ing some “fundamentals”. First, we briefly examine the philosophy and approach that is common to all

science. Then we review the most fundamental aspects of chemistry: how matter is organized into at-

oms and how these atoms interact to form compounds. Finally, we review a few fundamental aspects

of the Earth.

1.3 The Philosophy of Science

This book will concentrate on communicating to you the body of knowledge we call geochemistry.

Geochemistry is just part of a much larger field of human endeavor known as science. Science is cer-

tainly among humanity’s greatest successes; without it, our current civilization would not be possible.

Among other things, it would simply not be possible to feed, cloth, and shelter as many people as live

today. This phenomenal success is due in large part to the philosophy of science.

Science consists of two parts: the knowledge it encompasses and the approach or philosophy that

achieves that knowledge. The goal of all science is to understand the world around us. The arts and

humanities also seek understanding. Science differs from those fields as much by its approach and phi-

losophy as by its body of knowledge.

This approach and philosophy unite the great diversity of fields that we collectively call science.

When one compares the methods and tools of a high energy physicist with those of a behavioral biolo-

gist, for example, it might at first seem that they have little in common. Among other things, their vo-

cabularies are sufficiently different that each would have difficulty communicating his or her research

to the other. In spite of this, they share at least two things. The first is a criterion of “understanding”.

Both the physicist and the behavioral biologist attempt to explain their observations by the application

of a set of rules, which, by comparison to the range of phenomena considered, are both few and simple.

Both would agree that a phenomenon is understood if and only if the outcome of an experiment related

to that phenomenon can be predicted beforehand by applying those rules to measured variables*. The

physicist and biologist also share a common method of seeking understanding, often called the “scien-

tific method”.

1.3.1 Building Scientific Understanding

Science deals in only two quantities: observations and theories . The most basic of these is the ob-

servation . Measurements, data, analyses, experiments, etc. are all observations in the present sense. An

observation might be as simple a measurement of the dip and strike of a rock formation or as complex

as the electromagnetic spectrum of a star. Of course, it is possible to measure both the dip of rock strata

and a stellar spectrum incorrectly. Before an observation becomes part of the body of scientific knowl-

edge, we would like some reassurance that it is right. How can we tell whether observations are right

or not? The most important way to verify an observation is to replicate it independently . In the strictest

sense, ‘independent’ means by a separate observer, team of observers, or laboratory, and preferably by

a different technique or instrument. It is not practicable to replicate every observation in this manner,

but critical observations, those which appear to be inconsistent with existing theories or which test the

predictions of newly established ones should be, and generally are, replicated. But even replication

does not guarantee that an observation is correct.

Observations form the basis of theories . Theories are also called models, hypotheses, etc. Scientific

understanding is achieved by constructing and modifying theories to explain observations. Theories are merely

* Both would admit that chance, or randomness, can affect the outcome of any experiment (though the effect might be

slight). By definition, the effect of this randomness cannot be predicted. Where the effects of randomness are large,

one performs a large collection, or ensemble, of experiments and then considers the average result.

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Geochemistry

W. M. White

Chapter 1: Introduction

the products of the imagination of scientists, so we also need a method of sorting out ‘correct’ theories

from ‘incorrect’ ones. Good theories not only explain existing observations, but also make predictions

about the outcome of still unperformed experiments or observations. Theories are tested by performance of

these experiments and comparison of the results with the predictions of the theory. If the predictions are cor-

rect, the theory is accepted and the phenomenon considered to be understood, at least until a new and

different test is performed. If the predictions are incorrect, the theory is discarded or modified. When

trying to explain a newly discovered phenomenon, scientists often reject many new theories before

finding a satisfactory one. But long-standing theories that successfully explain a range of phenomena

can usually be modified without rejecting them entirely when they prove inconsistent with new obser-

vations.

Occasionally, new observations are so inconsistent with a well-established theory that it must be dis-

carded entirely and a new one developed to replace it. Scientific ‘revolutions’ occur when major theo-

ries are discarded in this manner. Rapid progress in understanding generally accompanies these revo-

lutions. Such was the case in physics in the early 20 th century when the quantum and relativity theories

replaced Newtonian theories. The development of Plate Tectonics in the 1960’s and 1970’s is an excel-

lent example of a scientific revolution in which old theories were replaced by a single new one. A

range of observations including the direction of motion along transform faults, the magnetic anomaly

pattern on the sea floor, and the distribution of earthquakes and volcanoes were either not predicted

by, or were inconsistent with, classical theories of the Earth. Plate tectonics explained all these and

made a number of predictions, such as the age of the seafloor, that could be tested. Thus scientific un-

derstanding progresses through an endless cycle of observation, theory construction and modification,

and prediction. In this cycle, theories can achieve “acceptance”, but can never be proven correct, be-

cause we can never be sure that it will not fail some new, future test.

Quite often, it is possible to explain observations in more than one way. That being the case, we

need a rule that tells us which theory to accept. When this occurs, the principle is that the theory that

explains the greatest range of phenomena in the simplest manner is always preferred. For example, the

motion of the Sun across the sky is quite simple and may be explained equally well by imaging that the

Sun orbits the Earth as visa versa. However, the motions of the planets in the sky are quite complex

and require a very complex theory if we assume they orbit the Earth. If we theorize that the Earth and

the other planets all orbit the Sun, the motions of the planets become simple elliptical orbits and can be

explained by Newton’s three laws of motion. The geocentric theory was long ago replaced by the he-

liocentric theory for precisely this reason. This principle of simplicity, or elegance, also applies to

mathematics. Computer programmers call it the KISS (Keep It Simple, Stupid!) Principle. In science,

we can sum it up by saying: don’t make nature any more complex than it already is .

1.3.2 The Scientist as Skeptic

Though we often refer to “scientific facts”, there are no facts in science. A fact, by definition, cannot

be wrong. Both observations and theories can be, and sometimes are, wrong. Of course, some obser-

vations (e.g., the Sun rises each morning in the East) and theories (the Earth revolves around the Sun)

are so oft repeated and so well established that they are not seriously questioned. But remember that

the theory that the Sun revolves around the Earth was itself once so well established that it was not se-

riously questioned.

One of the ways science differs from other fields of endeavor is that in science nothing is sacred . It is

best to bear in mind the possibility, however remote, that any observation or theory can be wrong.

Conversely, we must also accept the possibility that even the wildest observations and theories might

be correct: in quantum physics, for example, there is a great range of well-replicated observations that

can only be labeled as bizarre (see, for example, Gribbin, 1984). ‘Intuition’ plays a greater role in sci-

ence that most scientists might be willing to admit, even though scientific intuition is often very useful.

Nevertheless, our intuition is based largely on our everyday experience, which is very limited com-

pared to the range of phenomena that science attempts to understand. As a result, our intuition often

deceives us. Sometimes we must put it aside entirely. That a clock will run slower if it moves faster, or

that an electron can behave both as a wave and a particle, or that continents move great distances are all

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