February 2013 - One Hundred Years of Isotope Geochemistry
In 1913, Frederick Soddy’s research on the fundamentals of radioactivity led to the discovery of “isotopes.” That same year, Arthur Holmes published his now famous booklet The Age of the Earth. Combined, these two landmark events established the field of science we know as “isotope geochronology.” Today, isotope geochronology underpins much of our knowledge of the absolute age of minerals and rocks, and the records they contain. This field is constantly evolving, reflecting and responding to scientific drivers that require more highly resolved timescales, the microscopic analysis of smaller zoned minerals, or the generation of robust data sets in novel materials. This series of articles provides perspectives on the state of the art in the field of radioisotope dating—from the challenges of dating the Solar System’s oldest materials to resolving the record of Quaternary climate change, and the four and a half billion years in between.
Serpentinites, primarily composed of serpentine minerals and formed by hydration of peridotites, increasingly attract the attention of a wide range of scientists, including geophysicists, structural geologists, engineers, and astrobiologists. As serpentinites have wide stability fields, they form in a variety of environments, from the Earth’s surface to the interior of the mantle. They are important as reservoirs of water in the deep mantle and in the recycling of elements in subduction zones. Because of their physical properties, serpentinites play important roles in seismic activity and geodynamics, including earthquakes in subduction zones, rifting, oceanic spreading, strike-slip faulting, and the exhumation of deeply subducted rocks. Serpentinites are also economically important because obducted serpentinites contain more than half the world’s reserves of nickel. The formation of serpentinites is accompanied by the production of hydrogen and methane, producing unique ecosystems on the ocean floor. The generation of hydrocarbons during serpentinization is the essential first step in the origin of life on Earth and possibly other planets.
Reactions occurring at mineral–water interfaces are central to geochemical processes. They affect a wide range of important environmental issues, such as the composition of natural waters, weathering and soil formation, element cycling, biomineralization (including minor-element incorporation), acid mine drainage, and nuclear waste disposal. Recent studies using state-of-the-art spectroscopic and microscopic techniques have characterized the molecular structure of mineral surfaces, the distribution of fluids near surfaces, and dynamic processes such as dissolution, growth, and mineral replacement. These studies provide insights into the kinetics and mechanisms of reactions occurring at mineral surfaces, and they test the validity of predictions based on theory. These recent advances constitute the central theme of this issue of Elements. Modeling approaches used in mechanistic studies are also introduced. Such approaches complement direct, in situ, molecular-scale observations of processes occurring at mineral–water interfaces.
The discovery of diamond and coesite in crustal rocks is compelling evidence that continental material has experienced pressures that can be achieved only at mantle depths. The classical idea that continents are too buoyant to subduct has given way to the notion of density changes driving deep subduction during the collision process, thus enabling some crust to be exhumed to the surface and the rest to sink into the mantle. Over twenty localities of unequivocal continental crust containing diamond or coesite are now recognized around the globe, and their study constitutes a new field in petrology, dubbed ultrahigh-pressure metamorphism. Using microscopic observations, phase equilibrium modeling, geochronology, and geodynamic modeling, we track the journey of ultrahigh-pressure rocks to the mantle and back. Continental ultrahigh-pressure terranes impact our understanding of plate tectonics through time, crustal recycling and mantle geochemistry, melting in subduction zones, and collisional processes in general.
October 2013 - Nitrogen and Its (Biogeoscosmo) Chemcial Cycling
Nitrogen is the most abundant element in Earth’s atmosphere and a key component of the biosphere. It is also a critical part of the surface/near-surface cycling of nutrients, thus directly impacting our lives. Changes in the biogeochemical cycling of nitrogen through Earth’s history could reflect fundamental changes in its pathways from inorganic to biological reservoirs in response to change in the environment (e.g. oxygen fugacity in the atmosphere and oceans). Recognition of the importance of nitrogen to life on Earth, and likely elsewhere in the Solar System, has led to the mantra “Follow the Nitrogen” as one vehicle for focusing efforts in the search for extraterrestrial life. Nitrogen serves as a useful tracer of the transfer of “organic” signatures into the deep Earth (in records preserved in metamorphic and igneous rocks and in volcanic gases and rocks). It has been speculated that biological fixation of nitrogen and storage in rapidly forming continental crust has led to drawdown of nitrogen from the early-Earth atmosphere, strongly influencing the chemical evolution of the atmosphere and related surface conditions.
December 2013 - Garnet: Common Mineral, Uncommonly Useful
Garnet is among the most studied—and most beloved—minerals, owing to its commonality in diverse geologic contexts, its often large euhedral crystals, its sometimes dazzling colors, and its propensity for preserving information about its growth history. Chemically zoned garnet represents a remarkable tool for deciphering metamorphic conditions and the evolving tectonic processes that drive garnet growth over many millions of years. In the deep Earth, garnet is a key rock-forming mineral, influencing the physical properties of the mantle and the composition of mantle-derived magmas. Garnet has been sought for ages as a semiprecious gemstone (the birthstone of January) and has been mined or synthesized (including nonsilicate garnet) for industrial purposes, including laser, magnetic, and ion-conductor technology. This issue of Elements will emphasize the most recent innovations in thermodynamic, geochemical, geochronologic, and industrial applications of garnet, while providing perspective on decades of garnet-related research.