Geochemistry. William M. White

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СКАЧАТЬ 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 allows 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 bonding in natural materials. Mass spectrometers allow us to determine the age of rocks and the temperature of ancient seas. Ion probes allow us to do these things on micron scale samples. Analytical techniques such as X-ray fluorescence, inductively coupled plasma spectrometry, and laser ablation allow us to perform in minutes analyses that would take days using “classical” techniques. All this is done with ever-increasing precision and accuracy. Computers with gigahertz of power and terabytes of memory allow us to perform in seconds thermodynamic calculations that would have taken years or lifetimes half a century ago and the future promises even more computational power. This makes possible ab initio computation, that is, from the first principles governing atomic interactions, of, for example, mineral structures at the enormous pressures in the Earth's deep interior and chemical equilibrium everywhere, something not possible half a century ago even though we knew those principles. 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 of understanding the Earth and its cosmic environment.

      Before we begin our study of geochemistry, we will review 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 atoms of different elements, how the properties of the elements vary, and how these atoms interact to form compounds. Finally, we review a few fundamental aspects of the Earth. Following that we will preview what will come in subsequent chapters.

1.4 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 certainly 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, clothe, and shelter the 7 billion people living 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 philosophy 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 biologist, for example, it might at first seem that they have little in common. Among other things, their vocabularies 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 scientific method.

      1.4.1 Building scientific understanding

      Science deals in only two quantities: observations and theories. The most basic of these is the observation. Measurements, data, analyses, and experiments are all observations in the present sense. An observation might be as simple as 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 knowledge, 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, or laws. Scientific understanding is achieved by constructing and modifying theories to explain observations. Theories are merely 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 correct, 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 often be modified without rejecting them entirely when they prove inconsistent with new observations. And as Carl Sagan once said, extraordinary claims require extraordinary proof.

      Occasionally, new observations are so inconsistent with a well-established theory that it must be discarded entirely and a new one developed to replace it. Scientific “revolutions” occur when major theories are discarded in this manner. Rapid progress in understanding generally accompanies these revolutions. Such was the case in physics in the early twentieth century when the quantum and relativity theories supplanted Newtonian theories (Lindley, 2001). The development of plate tectonics in the 1960s and 1970s is an excellent example of a scientific revolution in which old theories were replaced by a new unifying 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 sea floor, that could be tested. Thus scientific understanding 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, because 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 imagining that the Sun orbits the Earth as vice versa. Ancient astronomers could, for example, predict eclipses. However, the motions of the planets in the sky are quite complex and require a very complex theory if we assume that 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 heliocentric one for precisely this reason. This principle of simplicity, СКАЧАТЬ