ALBERT

Beschreibung / Inhaltsverzeichnis: Hydrogen may be the most abundant element in the universe, but in science and in nature oxygen has an importance that is disproportionate to its abundance. Human beings tend to take it for granted because it is all around us and we breathe it, but consider the fact that oxygen is so reactive that in a planetary setting it is largely unstable in its elemental state. Were it not for the constant activity of photosynthetic plants and a minor amount of photo dissociation in the upper atmosphere, we would not have an oxygen-bearing atmosphere and we would not be here. Equally, the most important compound of oxygen is water, without which life (in the sense that we know it) could not exist. The role of water in virtually all geologic processes is profound, from formation of ore deposits to igneous petrogenesis to metamorphism to erosion and sedimentation. In planetary science, oxygen has a dual importance. First and foremost is its critical role in so many fundamental Solar System processes. The very nature of the terrestrial planets in our own Solar System would be much different had the oxygen to carbon ratio in the early solar nebula been somewhat lower than it was, because elements such as calcium and iron and titanium would have been locked up during condensation as carbides, sulfides and nitrides and even (in the case of silicon) partly as metals rather than silicates and oxides. Equally, the role of water ice in the evolution of our Solar System is important in the early accretion and growth of the giant planets and especially Jupiter, which exerted a major control over how most of the other planets formed. On a smaller scale, oxygen plays a critical role in the diverse kinds of physical evolution of large rocky planets, because the internal oxidation state strongly influences the formation and evolution of the core, mantle and crust of differentiated planets such as the Earth. Consider that basaltic volcanism may be a nearly universal phenomenon among the evolved terrestrial planets, yet there are basalts and basalts. The basalts of Earth (mostly), Earth's Moon, Vesta (as represented by the HED meteorites) and Mars are all broadly tholeiitic and yet very different from one another, and one of the primary differences is in their relative oxidation states (for that matter, consider the differences between tholeiitic and calc-alkaline magma series on Earth). But there is another way that oxygen has proven to be hugely important in planetary science, and that is as a critical scientific clue to processes and conditions and even sources of materials. Understanding the formation and evolution of our Solar System involves reconstructing processes and events that occurred more than 4.5 Ga ago, and for which the only contemporary examples are occurring hundreds of light years away. It is a detective story in which most of the clues come from the laboratory analysis of the products of those ancient processes and events, especially those that have been preserved nearly unchanged since their formation at the Solar System's birth: meteorites; comets; and interplanetary dust particles. For example, the oxidation state of diverse early Solar System materials ranges from highly oxidized (ferric iron) to so reducing that some silicon exists in the metallic state and refractory lithophile elements such as calcium exist occur in sulfides rather than in silicates or carbonates. These variations reflect highly different environments that existed in different places and at different times. Even more crucial has been the use of oxygen 3-isotope variations, which began almost accidentally in 1973 with an attempt to do oxygen isotope thermometry on high-temperature solar nebula grains (Ca-, Al-rich inclusions) but ended with the remarkable discovery (see Clayton 2008) of non-mass-dependent oxygen isotope variations in high-temperature materials from the earliest Solar System. The presolar nebula was found to be very heterogeneous in its isotopic composition, and virtually every different planet and asteroid for which we have samples has a unique oxygen-isotopic fingerprint.

Beschreibung / Inhaltsverzeichnis: Minerals are intrinsically resistant to the processes that homogenize silicate liquids—their compositions thus yield an archive of volcanic and magmatic processes that are invisible at the whole rock scale. New experiments, and recent advances in micro-analytical techniques open a new realm of detail regarding the mineralogical record; this volume summarizes some of this progress. The alliance of the sub-fields reviewed in this volume bear upon fundamental issues of volcanology: At what depths are eruptions triggered, and over what time scales? Where and why do magmas coalesce before ascent? If magmas stagnate for thousands of years, what forces are responsible for initiating final ascent, or the degassing processes that accelerate upward motion? To the extent that we can answer these questions, we move towards formulating tests of mechanistic models of volcanic eruptions (e.g., Wilson, 1980; Slezin, 2003; Scandone et al., 2007), and hypotheses of the tectonic controls on magma transport (e.g., ten Brink and Brocher, 1987; Takada, 1994; Putirka and Busby, 2007). Our goal, in part, is to review how minerals can be used to understand volcanic systems and the processes that shape them; we also hope that this work will spur new and integrated studies of volcanic systems. Our review begins by tracing the origins of mineral grains, and methods to estimate pressures (P) and temperatures (T) of crystallization. Hammer shows how "dynamic" experiments (conducted with varying P or T) yield important insights into crystal growth. Chapters by Putirka, Anderson, and Blundy and Cashman review various igneous geothermometers and geobarometers and introduce new calibrations. Among these chapters are many familiar models involving olivine, amphibole, feldspar, pyroxene, and spinel. Blundy and Cashman introduce new methods based on phase equilibria, and in another chapter, Hansteen and Klügel review P estimation based on densities of entrapped fluids and appropriate equations of state. Rutherford's chapter returns to the issue of disequilibrium, with a review of methods to estimate magma ascent rates, and a summary of results. Our volume then moves to a review of melt inclusions. Kent shows how pre-mixed magma compositions can be preserved as inclusions, providing a window into pre-eruptive conditions. Métrich and Wallace review the volatile contents in basaltic melt inclusions and "magma degassing paths". Such methods rely upon vapor saturation pressures, which are derived from experimentally calibrated models. Chapters by Moore and Blundy and Cashman test two of the most important models, by Newman and Lowenstern (2002) (VolatileCalc) and Papale et al. (2006). Moore provides a guide to the appropriate use of these models, and their respective errors. The next four chapters document insights obtained from isotopic studies and diffusion profiles. Ramos and Tepley review developments of micro-analytical isotope measurements, which now have the potential to elucidate even the most cryptic of open system behaviors. Cooper and Reid examine the time scales for such processes through U-series age dating techniques, and Bindeman reviews oxygen isotopes and their uses as tracers of both magmas and crystals. Costa then reviews yet another means to estimate the rates of magmatic processes, using mineral diffusion profiles, with important implications for magma processing. In the next two chapters, Streck reviews an array of imaging methods and mineral textures, and their potential for disentangling mixed magmas, and Armienti takes a new look at the analysis of crystal size distributions (CSD), with applications to Mt. Etna. Our volume concludes with a chapter by Bachmann and Bergantz summarizing compositional zonations and a review of the thermal and compositional forces that drive open system behavior. Finally, descriptions of many of the most common analytical approaches are also reviewed within these chapters. Analytical topics include: secondary ion mass spectrometry (Blundy and Cashman; Kent); electron microprobe (Blundy and Cashman; Kent; Métrich and Wallace; laser ablation ICP-MS (Kent; Ramos and Tepley); Fourier transform infrared spectroscopy (Moore; Métrich and Wallace); microsampling and isotope mass spectrometry (Ramos and Tepley); U-series measurement techniques (Cooper and Reid); Nomarski differential interference contrasts (Streck); micro-Raman spectroscopy (Métrich and Wallace); back-scattered electron microscopy, and cathodoluminescence (Blundy and Cashman). As noted, our hope is that integrated studies can bring us closer to understanding how volcanic systems evolve and why eruptions occur. Our primary goal is to review how minerals can be used to understand volcanic systems; we also hope that this review might spur new and integrated studies of volcanic systems.