Transcript Oxygen
Oxygen, noble gases Oxygen (O) • Universe: 10000 ppm (by weight) • Sun: 9000 ppm (by weight) • Carbonaceous meteorite: 4.1 x 105 ppm • Atmosphere: 2.095 x 105 ppm • Earth's Crust: 4.74 x 105 ppm Oxygen is magmatic processes It is principally as a lithophile element, occurring preferentially in the crust and mantle. It is the second most abundant element (30 wt%) in the bulk Earth and the most abundant element in the crust and mantle ( 62.55 and approximately 60 atom%, respectively). Thus, oxygen is a major constituent of nearly all minerals. It is the basis of the tetrahedral [SiO4 ]4- ion that polymerizes to form all silicates, as well as the anionic building blocks (e.g. CO3, SO4, PO4, or OH ions) of the most common non-silicate minerals. Oxygen Oxygen is extremely reactive, and forms compounds with every element except He, Ne, and Ar. The most abundant compounds of oxygen are oxides: binary compounds with oxygen in the -2 oxidation state. Transition metals generally combine with oxygen to form basic oxides such as Fe2O3. Although elemental oxygen occurs principally in the rocky solid Earth, its considerable abundance in the atmosphere also places it among the atmophile elements in the classification of Goldschmidt. Oxygen Dioxygen comprises 20.946 vol% of the bulk atmosphere at sea level. It is formed principally as a product of photosynthesis, which began with the evolutionary appearance on Earth of prokaryotic cyanobacteria (bluegreen algae) in the early Archean eon, about 3.5-3.8 Ga ago, drastically changing what would previously have been a virtually anoxic atmosphere formed of volcanic emissions. Oxygen is an important species in controlling redox conditions in natural waters: biologically mediated reduction of O2 to H2O is the predominant oxidizing reaction in the aqueous environment. Helium Universe: 2.3 x 105 ppm (by weight) Sun: 2.3 x 105 ppm (by weight) Atmosphere: 5.2 ppm Earth's Crust: 0.008 ppm Seawater: 7 x 10-6 ppm Helium After hydrogen, helium is the most abundant element in the universe (~6.5%). The particles emitted from uranium, thorium radium and polonium are doubled ionized 4He nuclei. Due to its extremely low abundance. 4He is formed by the radioactive decay of U and Th. On Earth we can roughly distinguish four different reservoirs of helium, each with different isotopic signatures: air, continental crust, upper mantle and lower mantle. Using these different isotopic signatures, helium can be a valuable tracer to determine the origin of erupted rocks and various terrestrial fluids, in particular due to its low atmospheric abundance and the small risk of air contamination. Argon Universe: 200 ppm (by weight) Sun: 70 ppm (by weight) Atmosphere: 9300 ppm Earth's Crust: 1.2 ppm Seawater: 0.45 ppm Argon Argon is the most abundant noble gas in the Earth's atmosphere. It is generally accepted that the terrestrial atmosphere was degassed from the Earth's mantle. Recent observations on basalt glasses show that the lower mantle (below 670 km depth) is less outgassed than the upper mantle. In the lithosphere it forms from radioactive decay of 40K isotope. This processes is the basic of potassiumargon radiometric age method. Neon Universe: 1300 ppm (by weight) Sun: 1000 ppm (by weight) Atmosphere: 14 ppm Earth's Crust: 3 x 10-3 ppm Seawater: 1.2 x 10-4 ppm Krypton Universe: 0.04 ppm (by weight) Atmosphere: 1.14 ppm Earth's Crust: 1 x 10-5 ppm Seawater: 2.1 x 10-4 ppm Noble gases in magmatic rocks (g/t): He – 0,003 Ne – 0,00007 Ar – 0,04 Nobel gases in troposphere (vol.%): He – 5,24 . 10-4 Ne – 1,8 . 10-3 Ar – 0,93 Kr – 1 . 10-4 Xe – 8 . 10-6 Radon (Rn) Universe: xxx (by weight) Atmosphere: xxx ppm Earth's Crust: xxxx 10-5 ppm Seawater: xxx 10-4 ppm Radon Rn occur in nature, one each in the 238U, 235U and 232Th decay series. In closed systems such as the interior of well crystallized minerals, radon usually comes to radioactive equilibrium withits parents so that its abundance varies with uranium or radium. In poorly crystalline or fine-grained materials such as soils and recent sediments, newly formed Rn atoms recoiling from alpha emission can travel a few tens of nanometers through the host solid and emanate into pore space or cracks. The concentrations of Rn in soil gas depends on the abundance of U and Ra in the soil. The reason of high Rn concentration can be some monazite or xenotime clasts in the soil. Actinides Actinium: terrestrial occurrence arises from the decay of 235U. This metal is found in trace quantities in uranium minerals (ng/g of pitchblende – the mixture of U-oxides). Radium: it occurs in the 238U, 235U and 232Th decay series. However, it may substitute for K+ in minerals because of its large radius. Radium may become at least partly separated from its parents in young igneous rocks, weathered and altered minerals, recent sediments, the biosphere and natural water. In weathering and soil formation, Ra is slightly to moderately enriched in plants and in soil organic matter relative to its parents U and Th, which tend to be more concentrated in the Fe oxides of the soil.