tool set to deal with solutions. These tools 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 or which should replace them in weathering reactions at the Earth's surface. Thus, thermodynamics provides 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 pointless 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 temperature and composition of water or magma from which minerals crystallized. This sort of information has been invaluable in reconstructing the past and understanding how the Earth has come to its present condition.
Thermodynamics has an important limitation: it is useful only in equilibrium situations. The rate at which chemical systems achieve equilibrium increases exponentially with temperature. Thermodynamics will be most useful at temperatures relevant to the interior of the Earth, but at temperatures relevant to the surface of the Earth, many geochemical systems will not be in equilibrium and instead be governed by kinetics, the subject of Chapter 5. Kinetics deals with the rates and mechanisms of reactions. Reactions can occur only when reactants are brought together. Unlike gas phase reactions or ones within a solution, this often requires the reactants be transported across an interface. So in this chapter, we will also touch on such topics as diffusion and mineral surfaces.
In Chapter 6, we see how tools of physical chemistry are adapted for use in dealing with natural aqueous solutions. 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 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 phenomena such as dissolution and precipitation, complexation, adsorption and ion exchange. 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 elements, which comprise most of the periodic table, 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 because the rate at which a radioactive nuclide decays is absolutely constant and independent of all external influences. We can read this clock by measuring the build-up of radiogenic daughter elements, for example 206Pb produced by decay of 238U. In this way we have established the age of the Solar System and the continents, and we have placed firm ages alongside the relative geologic time scale developed in the nineteenth century. Importantly, radiogenic isotope geochemistry has provided some perspective on the rate and manner of evolution of the Earth, and the evolution of our own species by answering questions such as how old are those bones and when were those cave paintings done? We can also use the products of radioactive decay, radiogenic elements, as tracers. By following these tracers much as we would dye in a fish tank, we can follow the evolution of a magma, the convection pattern of the mantle, and the circulation of the oceans, and determine from where sediments were derived. Radiogenic isotopes allow us to distinguish magmas produced by melting of the crust from those produced by melting of the mantle and to distinguish a number of distinct chemical reservoirs in the mantle; for example, magmas erupted by oceanic island volcanoes come from different reservoirs than those erupted at mid-ocean ridges.
The isotopes 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 has provided insights into the evolution of the Earth's atmosphere and reveal the diets of ancient peoples. One of the most significant contributions of stable isotope geochemistry has been to establish temperature changes of the oceans associated with the Pleistocene glacial cycles. Together with radiogenic isotope geochronology, the timing of these cycles and demonstrated that the ultimate driver of them has been small variations in the Earth's orbit and rotation, known as Milankovitch variations. Stable isotope geochemistry is the last of our geochemical tools.
With our toolbox full, we are ready to examine the Earth from the geochemical perspective in the second part of the book. Where else to start but at the beginning? The Earth today is the product of its long history, and of all the events in that history, none set the stage more for what Earth would become than its formation.
In Chapter 10 we'll begin by looking at “the big picture”: the cosmos and the Solar System. The cosmic beginning was some 13.8 billion years ago. The Big Bang, time's opening act, produced a universe of hydrogen and helium and very little else. Only once stars and galaxies had formed, perhaps half a billion years later, did the universe begin to be seeded with heavier elements. Stars the size of the Sun and larger synthesize the principal elements of life, carbon, nitrogen, and oxygen in their geriatric “red giant” phase and blow them back out into the cosmos in enormous stellar winds. That, however, is not enough to create a planet like Earth, or support life for that matter, both of which require heavier elements as well such as magnesium, silicon, phosphorous, and iron. These are synthesized during the death throes of giant stars and expelled into the cosmos in spectacular explosions called supernovae, which can radiate more energy than an entire galaxy.
Some 9.5 billion years later, part of a vast cloud of gas and dust, not unlike the Great Nebula in Orion visible in the northern hemisphere night sky in winter, began to collapse in on itself, spinning ever more rapidly as it did so like a skater pulling in her arms. The Sun formed in the center of this swirling mass and planets formed in the surrounding disk. The idea that the Solar System formed in this way is an old one: Immanuel Kant postulated it in 1755. But what are the details? We'll find that the details are revealed in leftovers from the process: chondritic meteorites. These meteorites consist of aggregations the dust from which the solar system is formed, although some were metamorphosed in their asteroidal parent bodies. Among other things, they reveal that this nebula, at least in the inner part, was so hot that almost all the dust had evaporated to gas. The first materials to condense, so-called calcium–aluminum inclusions, have been dated with exquisite precision by the decay of U to Pb at 4568.22 ± 0.17 million years. These meteorites also once contained short-lived radioactive nuclides that must have been synthesized within a million years or less of solar system formation, products of nucleosynthesis in our galactic neighborhood. The decay of these radionuclides resulted in the build-up of their daughter products, and we can put our tools of isotope geochemistry to good use to see how this can be used to produce a chronology of events in the young solar system. As samples of the solar system nebular dust, these meteorites provide an inventory of the elements available to build the Earth and in