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These individuals set the stage that allowed geochemistry to flourish 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 60 years than in all of prior human history. The contributions of geochemistry to this advance has 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 have quantified the geologic time scale. Through geochemistry, we can determine the depths and temperatures of magma chambers. Through geochemistry, 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 geochemistry, 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 have learned how cold the Ice Ages were and what caused them. The evidence of the earliest life, at 3.8 gigayears (billion, or 10⁹ years, which we will henceforth abbreviate as Ga*), is not fossilized remains, but chemical traces of life. And through geochemistry, we have learned that the Earth's atmosphere first became oxidizing about 2.3 to 2.4 Ga but another billion and a half years would pass before multi-celled animals would emerge.

Not surprisingly, instruments for chemical analysis have been key part of probes sent to other heavenly bodies, including Venus, Mars, Jupiter, and Titan. Geochemistry lies at the heart of environmental science and environmental concerns. Problems such as acid rain, ozone holes, the greenhouse effect and global warming, water and soil pollution are geochemical problems. Addressing these problems requires knowledge of geochemistry. Similarly, most of our nonrenewable 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.

Although 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 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.