Cosmochemistry is the study of extraterrestrial materials aimed at understanding the nature of Solar System bodies, including the planets, their natural satellites, and small bodies. An important goal is to increase our understanding of the chemical origin of the Solar System and the processes by which its planets and small bodies have evolved to their present states. Research in cosmochemistry covers a wide range of disciplines and techniques, including mineralogy, petrology, major and trace element chemistry, isotope compositions, radiometric ages, magnetism, and radiation-exposure effects. These studies provide a wealth of data about the processes of stellar evolution, planetary system formation, alteration in asteroidal and cometary interiors, and the accretion history of the Earth, including the origin of Earth’s volatile and organic material.
Iron is the fourth most abundant element at the Earth’s surface. As an essential nutrient and electron source/sink for the growth of microbial organisms, it is metabolically cycled between reduced and oxidized chemical forms. This flow of electrons is invariably tied to the reaction with other redox-sensitive elements, including oxygen, carbon, nitrogen, and sulfur. The end result of these interactions is that iron is intimately involved in the geochemistry, mineralogy, and petrology of modern aquatic systems and their associated sediments, particulates, and pore waters. In the geological past, vast iron sediments, the so-called banded iron formations, suggest that iron played an even greater role in marine geochemistry, and these deposits are now being used as proxies for understanding the chemical composition of the ancient oceans and atmosphere. This issue will explore not only the modern expression of iron cycling but also its record in Earth’s history.
The term water resources refers to natural waters (vapor, liquid, or solid) that occur on the Earth and that are of potential use to humans. These resources include oceans, rivers, lakes, groundwater, and glaciers. The Earth has plenty of water, over 1.4 × 109 km3. However, 97% of global water is saline seawater. Of the 3% that is freshwater, nearly 70% is locked in the polar icecaps and glaciers. The majority of nonglacier freshwater is groundwater (98%). Surface freshwater (rivers and lakes), which has historically served most human needs, constitutes only a small fraction of the Earth’s water resources. Water interacts with minerals, soils, sediments, and rocks, and hence studies of Earth materials have a direct bearing on water resources. Studies of the acquisition, mobility, and fate of elements and isotopes in water provide valuable signatures for tracking water cycles at regional and global scales and are essential for the development of remediation technologies for contaminated water.
Partial melting is the most important process affecting the continental crust. It is responsible for the large-scale compositional and density structure that has stabilized the crust over geological time. Partial melting occurs extensively in the deep crustal roots of mountain ranges that form where continents collide. The thin film of melt that develops on the edges and faces of mineral grains results in a substantial weakening of the crust, which concentrates deformation into the melt-bearing rocks and allows them to deform faster. This issue of Elements deals with the source of the heat responsible for widespread melting and the information that can be retrieved from mineral assemblages and microstructures in lower crustal rocks. It also explores the mechanisms of melt transfer and the large-scale geodynamic consequences of melting the crust as it deforms.
From the Vikings’ sunstone to a modern piezometric pressure sensor, tourmaline is an intriguing mineral with a new degree of significance. Tourmaline was considered by 18th century physicists as the key to a grand unification theory relating heat, electricity, and magnetism, but new studies define its role as an indicator of Earth’s processes. With its plethora of chemical constituents and its wide stability range, from near-surface conditions to the pressures and temperatures of the mantle, tourmaline has become a valuable mineral for understanding crustal evolution. Tourmaline encapsulates a single-mineral thermometer, a provenance indicator, a fluid-composition recorder, and a geochronometer. Although also prized as a gemstone, tourmaline is clearly more than meets the eye.
Since the dawn of civilization, humankind has been extracting metals and minerals for the production of goods, energy, and building materials. These mining activities have created great wealth, but they have also produced colossal quantities of solid and liquid wastes, known collectively as “mine wastes.” Mine wastes represent the greatest proportion of waste produced by industrial activity. In fact, the quantity of solid mine wastes and the quantity of Earth materials moved by fundamental global geological processes are of the same order of magnitude—approximately several thousand million tons per year. Therefore, the large-scale production, secure disposal, and sustainable remediation of mine wastes represent problems of global significance. Over the past 10–15 years, novel geochemical, mineralogical, microbiological and toxicological techniques have led to a much better understanding of the character, weathering mechanisms, long-term stability, ecotoxicology, and remediation of mine wastes. This issue of Elements will bring readers up to date with these current findings and will highlight new frontiers for mine waste research.