Posts Tagged ‘June 2020’
Primitive Meteorite Contains Cometary Surprise
Comets, and some primitive asteroids, preserve the microscopic remnants of the basic building blocks of rocky bodies in our solar system. Analyzing these building blocks can provide important clues both to the source of the water for Earths’ oceans and to the inventory of organic matter delivered to the early Earth. In general, samples from comets preserve a greater abundance of organic matter and primordial presolar dust grains from the protosolar molecular cloud, and generations of stars older than the sun, than do samples from asteroids. Meteorites, which come from asteroids, generally have less organic matter and preserve fewer presolar grains. This is because asteroids and comets formed at different places in the solar nebula, and, thus sampled different distributions of organic and mineral components. Furthermore, the warmer conditions on asteroids allowed for liquid water and greater subsequent physical and chemical alteration of the accreted material.
Read MoreBook Review — Thermodynamics in Earth and Planetary Sciences
Thermodynamics is a vast subject with a long and complex history. It is now about two hundred years since the general acceptance of the ideal gas equation of state (PV = nRT) (e.g., Biot 1816) and the discovery of limiting behaviour in the high temperature heat capacity of elemental solids (Petit and Dulong 1819). Since then, empirical thermodynamic laws and statistical thermodynamic models have revolutionised our understanding of a myriad of physical and chemical processes and material properties. Thermodynamics underpins much of our modern lifestyle and our understanding of the natural world. It plays, in the words of Russian Nobel laureate in chemistry Ilya Prigogine, “a fundamental role far beyond its original scope” (Prigogine 1977).
Read Morev16n3 From the Editors
When we started finalizing this issue for publication the prospect of a pandemic seemed very distant. In the intervening three months, COVID-19 has come to dominate everything: our conversations, news broadcasts, our working patterns, and our social lives. For many, this has been a tragic time, and we extend our condolences to all those readers of Elements who have lost loved ones and colleagues to COVID-19. For scholars, this is an uncertain time, as universities and research organizations take stock of the impact of the pandemic on their activities, and their financial well-being. The dramatic drop in student mobility across the world is already starting to take a toll on university income and may yet pose an existential threat. On a brighter note, it is hard to overlook the benefits of having cleaner air, happier wildlife, and lower global emissions due to our traveling less.
Read MoreA Symphony of Electrons
The concept of oxidation as the process that turns iron metal into rust is familiar to all of us. We might be equally familiar with reduction, the “reverse” of oxidation, by which iron metal is produced by heating iron ore with coke in a blast furnace. Rusting and smelting of iron are just two examples of reduction–oxidation (“redox”) reactions. As one species (e.g., the iron ore) becomes reduced, so the other (e.g., the coke) becomes oxidised. In redox, there is always something being oxidised and something else being reduced; it’s the yin and the yang of geochemistry, as the guest editors of this issue of Elements refer to it (cover).
Read MoreBiogeochemical Controls on the Redox Evolution of Earth’s Oceans and Atmosphere
Download Article (PDF) June 2020 Issue Table of Contents Editorial A Symphony of ElectronsFrom the EditorsMeet the AuthorsThematic ArticlesSociety NewsBook Review Thermodynamics in Earth and Planetary Sciences 2nd Ed.CalendarCosmoElements Primitive Meteorite Contains Cometary SurpriseDigital Edition The redox state of Earth’s atmosphere has undergone a dramatic shift over geologic time from reducing to strongly oxidizing, and…
Read MoreElectron Transfer Drives Metal Cycling in the Critical Zone
Electron transfer in the critical zone is driven by biotic and abiotic mechanisms and controls the fate of inorganic and organic contaminants, whether redox-sensitive or not. In these environments, Fe- and Mn-bearing minerals, as well as organic matter, are key compounds. They interact with each other and constitute important electron shuttles. As a result, not only their solubility but also their structure controls the mobility of many essential and toxic elements. In addition, microorganisms that form hot spots and are widespread in environmental systems are also primordial players in electron transfer processes by acting as a catalyst between an electron donor and an acceptor, and through their contaminant detoxification metabolism.
Read MoreVolcanic and Geothermal Redox Engines
The redox (reduction–oxidation) potential is an essential variable that controls the chemical reactions of fluids in magmatic and associated geothermal systems. However, the evolution of the redox potential is difficult to trace from a magma’s source at depth to the surface. The key is knowing that electron transfer is the twin face of the acid–base exchanges that drive charge transfer in the many reactions that occur in multiphase and chemically complex systems. The deduced redox reactivity can reveal many features about the evolution of a system’s composition and the external factors that control it. As such, redox potential analysis is an important geochemical tool by which to monitor volcanoes and to explore geothermal systems.
Read MoreMagmas are the Largest Repositories and Carriers of Earth’s Redox Processes
Magma is the most important chemical transport agent throughout our planet. This paper provides an overview of the interplay between magma redox, major element chemistry, and crystal and volatile content, and of the influence of redox on the factors that drive igneous system dynamics. Given the almost infinite combinations of temperature, pressure, and chemical compositions relevant to igneous petrology, we focus on the concepts and methods that redox geochemistry provides to understand magma formation, ascent, evolution and crystallization. Particular attention is paid to the strong and complex interplay between melt structure and chemistry, and to the influence that redox conditions have on melt properties, crystallization mechanisms and the solubility of volatile components.
Read MoreRedox Processes in Early Earth Accretion and in Terrestrial Bodies
The Earth is a unique rocky planet with liquid water at the surface and an oxygen-rich atmosphere, consequences of its particular accretion history. The earliest accreting bodies were small and could be either differentiated and undifferentiated; later larger bodies had formed cores and mantles with distinct properties. In addition, there may have been an overall trend of early reduced and later oxidized material accreting to form the Earth. This paper provides an overview—based on natural materials in our Earthbound sample collections, experimental studies on those samples, and calculations and numerical simulations of differentiation processes—of planetary accretion, core–mantle equilibration, mantle redox processes, and redox variations in Earth, Mars, and other terrestrial bodies.
Read MoreThe Redox Boundaries of Earth’s Interior
The interior of the Earth is an important reservoir for elements that are chemically bound in minerals, melts, and gases. Analyses of the proportions of redox-sensitive elements in ancient and contemporary natural rocks provide information on the temporal redox evolution of our planet. Natural inclusions trapped in diamonds, xenoliths, and erupted magmas provide unique windows into the redox conditions of the deep Earth, and reveal evidence for heterogeneities in the mantle’s oxidation state. By examining the natural rock record, we assess how redox boundaries in the deep Earth have controlled elemental cycling and what effects these boundaries have had on the temporal and chemical evolution of oxygen fugacity in the Earth’s interior and atmosphere.
Read More