Meteorites oxygen

The systematic variations in oxygen isotopes provide an independent means of classifying chondrites that generates the same groups as the chemical compositions. The oxygen isotopes also work for classifying non-chondritic meteorites. Oxygen isotopic compositions are somewhat easier to obtain than detailed chemical data and so are often used to nail down a classification. 

Most non-chondrule solids in the inner Solar System experienced thermal processing (see Chapter 8) that could have modified their initial oxygen isotopic composition (Yurimoto Kuramoto 2004). The complicated structure of meteoritic oxygen isotopes is difficult to reproduce simply by physical mixing of different reservoirs. Apart from thermal processing (e.g. melting, vaporization, condensation), a large mass-independent chemical process is required. The exact mechanism for this likely photochemical process is yet unknown, but the available constraints leave only a few pathways open. 

Meteorites "map the Solar System by having specific oxygen isotopic compositions, a major element in all but the irons. Since its high chemical reactivity causes oxygen to form numerous compounds, it exists in many meteoritic minerals, even in silicate inclusions in iron meteorites. Elsewhere in this volume, Clayton (5) discusses meteoritic oxygen isotopic differences and processes that established these [cf (/)]. 

Environmental Aspects. Airborne particulate matter (187) and aerosol (188) samples from around the world have been found to contain a variety of organic monocarboxyhc and dicarboxyhc acids, including adipic acid. Traces of the acid found ia southern California air were related both to automobile exhaust emission (189) and, iadirecfly, to cyclohexene as a secondary aerosol precursor (via ozonolysis) (190). Dibasic acids (eg, succinic acid) have been found even ia such unlikely sources as the Murchison meteorite (191). PubHc health standards for adipic acid contamination of reservoir waters were evaluated with respect to toxicity, odor, taste, transparency, foam, and other criteria (192). BiodegradabiUty of adipic acid solutions was also evaluated with respect to BOD/theoretical oxygen demand ratio, rate, lag time, and other factors (193). 

Eig. 4. The bulk oxygen isotopic composition of different meteorite classes where (—) is the terrestial fractionation line. The 5 notation refers to the normalized difference between or ratios to those in standard mean ocean water (SMOW) in relative units of parts per thousand. The... 

The nuclei of iron are especially stable, giving it a comparatively high cosmic abundance (Chap. 1, p. 11), and it is thought to be the main constituent of the earth s core (which has a radius of approximately 3500 km, i.e. 2150 miles) as well as being the major component of siderite meteorites. About 0.5% of the lunar soil is now known to be metallic iron and, since on average this soil is 10 m deep, there must be 10 tonnes of iron on the moon s surface. In the earth s crustal rocks (6.2%, i.e. 62000ppm) it is the fourth most abundant element (after oxygen, silicon and aluminium) and the second most abundant metal. It is also widely distributed. 

Almost all aquatic organisms rely on the presence of dissolved oxygen for respiration. Although oxygen is nonpolar, it is very slightly soluble in water and the extent to which it dissolves depends on its pressure. We have already seen (in Section 4.2) that the pressure of a gas arises from the impacts of its molecules. When a gas is introduced into the same container as a liquid, the gas molecules can burrow into the liquid like meteorites plunging into the ocean. Because the number of impacts increases as the pressure of a gas increases, we should expect the solubility of the gas—its molar concentration when the dissolved gas is in dynamic equilibrium with the free gas—to increase as its pressure increases. If the gas above the liquid is a mixture (like air), then the solubility of each component depends on that component s partial pressure (Fig. 8.21). 

Stony meteorites are similar to igneous rocks (rocks formed by fire) on earth, but they have less silicon and oxygen, and more iron, cobalt, and trace metals. A third kind of meteorite, called pallasite, is somewhere between the other two types. It is about half iron and half stone, in a mixture not normal on earth. 

Broadly speaking the classification of meteorites follows the geological mineral classification and with 275 mineral species reported so far this quickly becomes complex some classes of meteorite have only one member. The mineral structure does convey essential information about the temperature at which the meteorite formed as well as the reduction-oxidation (redox) environment was the environment in which it formed rich in oxygen Meteorites have been classified into three broad classes ... 

The largest class of meteorite finds is stony meteorites, made principally of stone. The general stony classification is divided into three subclasses called chondrites, carbonaceous chondrites and achondrites, and it is at this level of distinction at which we will stop. Before looking at their mineral and isotopic structure in more detail, it is useful to hold the composition of the Earth s crust in mind here for comparison. The Earth s crust is 49 per cent oxygen, 26 per cent silicon, 7.5 per cent aluminium, 4.7 per cent iron, 3.4 per cent calcium, 2.6 per cent sodium, 2.4 per cent potassium and 1.9 per cent magnesium, which must have formed from the common origin of the solar system. 

Substantial abundance anomalies occur among the heavy oxygen isotopes 170 and 180, which are underabundant by up to about 4 per cent relative to 160 in oxide grains of certain of the CAIs, compared with the bulk composition in which the isotope ratios are closer to a terrestrial standard. The intriguing feature of these anomalous ratios is that, in common with some other meteorites, but in contrast to terrestrial and lunar samples, the relative deviations of the two heavy isotopes are equal most normal fractionation processes would cause 180 to have twice the anomaly of 170, as indeed is observed in terrestrial samples and more differentiated meteorites, where the anomalies are also usually much smaller. While there has been speculation that there might be a substantial admixture of pure 160 from a supernova, there are fractionation mechanisms that may be able to account for the effect, e.g. photo-dissociation of molecules affected by selfshielding (R. Clayton 2002). In this case, it is possible that the terrestrial standard is enriched in the heavy O-isotopes while the inclusions have more nearly the true solar ratio. 

R. C. Wiens, G. R. Huss, and D. S. Burnett. The Solar Oxygen Isotopic Composition Predictions and Implications for Solar Nebula Processes. Meteoritics and Planetary Science, 34(1995) 99-108. 

Stable isotope analysis of Earth, Moon, and meteorite samples provides important information concerning the origin of the solar system. 8lsO values of terrestrial and lunar materials support the old idea that earth and moon are closely related. On the other hand three isotope plots for oxygen fractionation in certain meteoric inclusions are anomalous. They show unexpected isotope fractionations which are approximately mass independent. This observation, difficult to understand and initially thought to have important cosmological implications, has been resolved in a series of careful experimental and theoretical studies of isotope fractionation in unimolecular kinetic processes. This important geochemical problem is treated in some detail in Chapter 14. 

Oxygen Isotope Fractionation in Earth, Moon, and Meteorite Samples... 

The assignment of the earth/moon system to one precursor nebular reservoir, and meteorites to a second, while still logically possible, was shown experimentally to be unnecessary by Thiemens and coworkers (reading list). In the early 1980s these workers studied isotope fractionation during the synthesis of ozone from molecular oxygen in an electric discharge operated at low pressure. The product ozone was... 

Romanek CS, Grossman EL, Morse JW (1992) Carbon isotopic fractionation in synthetic aragonite and calcite Effects of temperature and precipitation rate. Geochim Cosmochim Acta 56 419-430 Rowe MW, Clayton RN, Mayeda TK (1994) Oxygen isotopes in separated components of Cl and CM meteorites. Geochim Cosmochim Acta 58 5341-5347... 

Refractory materials in primitive meteorites were investigated first as they have the best chance of escaping homogenization in the early solar system. Inclusions in C3 carbonaceous chondrites exhibit widespread anomalies for oxygen and the iron group elements. Only a few members, dubbed FUN (for Fractionated and Unknown Nuclear effects), also display anomalous compositions for the heavy elements. Anomalies in inclusions have generally been connected with explosive or supernova nucleosynthesis. 

Clayton RN, Onuma N, Mayeda TK (1976) A classification of meteorites based on oxygen isotopes. Earth Planet Sci Lett 30 10-18... 

Hashimoto A, Hinton RW, Davis AM, Grossman L, Mayeda TK, Clayton RN (1986) A hibonite-rich Murchison inclusion with anomalous oxygen isotopic composition. Lunar Planet Sci XVII 317-318 Heydegger HR, Foster JJ, Compston W (1979) Evidence of a new isotopic anomaly from titanium isotopic ratios in meteoritic material. Nature 278 704-707... 

Yin Q, Jacobsen SB, Yamashita K, Blichert-Toft J, Telouk P, Albarede F (2002) A short time scale for terrestrial planet formation from the Hf-W chronometry of meteorites. Nature 418 949-951 Young ED, Russell SS (1998) Oxygen reservoirs in the early solar nebula inferred from an Allende CAL Science 282 452-455... 

Clayton DD (1999) Radiogenic iron. Meteor Planet Sci 34 A145-A160 Clayton RN (1993) Oxygen isotopes in meteorites. Ann Rev Earth Planet Sci 21 115-149 Clayton RN, Mayeda TK, Hurd JM (1974) Loss of oxygen, silicon, sulfur, and potassiimi from flie limar regolith. Proc Lunar Sci Conf 5 1801-1809... 

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