Os: isotopic composition

The choice of isotopic ratios for reporting data increases, of course, for elements with three or more isotopes. Although O isotope compositions are almost exclusively reported in terms... 

The O isotopes show signihcant heterogeneity between the different meteorite classes (Fig. 8a Clayton et al. 1976, 1977). Differences are small, but, each chondrite group has a distinct bulk O isotopic composition. O isotopes also indicate the close ties between the Earth and the Moon. O therefore can be used to identify members of a family that formed from a common reservoir, which is the definition of a tracer. Such differences are also formd between chondrules within the same meteorites related to their size (Gooding et al. 1983). This is a survival of the initial isotopic heterogeneity in already high temperature processed materials like chondrules. 

Figure 8. Figure (a) after Clayton et al. (1976, 1977). The scales are as in Figure 1. The O isotopic compositions of the different meteorite classes are represented ordinary chondrites (H, L, LL), enstatite chondrites (EFl, EL), differentiated meteorites (eucrites, lAB irons, SNCs) and some components of the carbonaceous chondrites. As the different areas do not overlap, a classification of the meteorites can be drawn based on O isotopes. Cr (b) and Mo (c) isotope compositions obtained by stepwise dissolution of the Cl carbonaceous chondrite Orgueil (Rotaru et al. 1992 Dauphas et al. 2002), are plotted as deviations relative to the terrestrial composition in 8 units. Isotopes are labeled according to their primary nucleosynthetic sources. ExpSi is for explosive Si burning and n-eq is for neutron-rich nuclear statistical equilibrium. The open squares represent a HNOj 4 N leachate at room temperature. The filled square correspond to the dissolution of the main silicate phase in a HCl-EIF mix. The M pattern for Mo in the silicates is similar to the s-process component found in micron-size SiC presolar grains as shown in Figure 7.

CAI s that were once molten (type B and compact type A) apparently crystallized under conditions where both partial pressures and total pressures were low because they exhibit marked fractionation of Mg isotopes relative to chondritic isotope ratios. But much remains to be learned from the distribution of this fractionation. Models and laboratory experiments indicate that Mg, O, and Si should fractionate to different degrees in a CAI (Davis et al. 1990 Richter et al. 2002) commensurate with the different equilibrium vapor pressures of Mg, SiO and other O-bearing species. Only now, with the advent of more precise mass spectrometry and sampling techniques, is it possible to search for these differences. Also, models prediet that there should be variations in isotope ratios with growth direction and Mg/Al content in minerals like melilite. Identification of such trends would verify the validity of the theory. Conversely, if no correlations between position, mineral composition, and Mg, Si, and O isotopic composition are found in once molten CAIs, it implies that the objects acquired their isotopic signals prior to final crystallization. Evidence of this nature could be used to determine which objects were melted more than once. 

Lead, Sr, C and O isotope compositions were measured in carbonates of the dolomitic unit and in chalcocite concentrate. U-Pb and K-Ar radiometric dates were obtained from basement rocks and mafic dykes. The analysis were undertaken and The University of Sao Paulo under supervision of Professor C. Tassinari (Tassinari 2002). 

Different nebular isotopic reservoirs must have existed, since there are distinct differences in bulk meteoritic O-isotope composition. The carbonaceons chondrites display the widest range in oxygen isotope composition of any meteorite group (Clayton and Mayeda 1999). The evolntion of these meteorites can be interpreted as a progression of interactions between dust and gas components in the solar nebula followed by solid/fluid interactions within parent bodies. Yonng et al. (1999)... 

In general, it is probable that Tertiary and older volcanic rocks will exhibit O-isotope compositions that have been modified to higher ratios from... 

Continental basalts tend to be enriched in relative to oceanic basalts and exhibit considerably more variability in O-isotope composition, a feature attributed to interaction with 0-enriched continental crust during magma ascent (Harmon and Hoefs 1995 Baker et al. 2000). 

Most hot spring waters have deuterium contents similar to those of local precipitation, but are usually emiched in as a result of isotopic exchange with the country rock at elevated temperatures. The magnitude of the oxygen isotope shift depends on the O-isotope composition of both water and rock, the mineralogy of the rock, temperature, water/rock ratio, and the time of interaction. 

Fig. 3.12 C- and O-isotope compositions of calcites and siderites from the Bad Grund and Lautenthal deposits, Harz, Germany (after Zheng and Hoefs, 1993)...

Assuming that the H- and O-isotope compositions and temperatures of ancient ocean waters are comparable to present day values, the isotopic composition of ancient meteoric waters may have been governed by relations similar to those existing presently. However, given the local complexities, the application of this relation-... 

Fig. 3.43 Latitudinal distribution of O-isotope composition of planktonic foraminifera and yearly averaged temperature at sea surface and 250 m water depth (after Mulitza et al. 1997)...

Regional Variation of Trace Element and Sr-Nd-Pb-O Isotopic Compositions... 

Kobayashi et al. 2003 Sakamoto etal. 2007 Seto etal. 2008), and a few tenths of a percent among planets and bulk meteorites (although Earth and the Moon have essentially identical O isotopic compositions). These variations are used to probe a wide variety of Solar System processes and provide one of the criteria used to classify meteorites. It seems likely that isotopic variations in O arose owing to photochemical effects in the solar nebula (e.g. Clayton 2002 Yurimoto Kuramoto 2004 Lyons Young 2005). 

Staudigel H., Davies G. R., Hart S. R., Marchant K. M., and Smith B. M. (1995) Large-scale isotopic Sr, Nd, and O isotopic composition of altered oceanic crust at DSDP/ODP Sites 417/418. Earth Planet. Sci. Lett. 130, 169-185. 

Mauersberger (1981), utilizing in situ mass spectrometric measurements, demonstrated that ozone possesses a large 0 enrichment. The O isotopic composition was not determined and the mass-independent isotopic composition could not be detected. As reviewed by Thiemens (1999), Weston (1999), and Johnston and Thiemens (2003), there now exists an extensive literature on stratospheric ozone isotopic measurements obtained by different techniques. Measurements by Mauersberger (1987) confirmed that stratospheric ozone was mass-independently fractionated as displayed in the 1983 laboratory experiments of Thiemens and Heidenreich. Return ozone isotopic analysis by Schueler et al. (1990) demonstrated that stratospheric ozone possessed an isotopic composition entirely consistent with laboratory observations. Tropospheric ozone has also been studied for its 5 0 isotopic... 

Bach W. and Humphris S. E. (1999) Relationship between the Sr and O isotope compositions of hydrothermal fluids and the spreading and magma-supply rates at oceanic spreading centers. Geology 27, 1067-1070. 

Calcite speleothems are deposited in a cave when downward-percolating, carbonate-saturated grormdwaters, with a partial pressure of CO2 exceeding that of the cave atmosphere, enter a cave and become supersaturated with respect to calcium carbonate. If the degassing of CO2 from solution is slow and isotopic equilibrium is maintained between aqueous carbon phases and deposited carbonate, then two climatically-dependent variables (a fractionation effect and a water effect) will determine the O-isotopic composition of the speleothem calcite deposited in a cave. 

The second variable determining the 0-isotope composition of speleothem derives from the dependence of the O isotopic composition of cave seepage water (dw), 5 0dw, on climate and the environment at the site of speleothem deposition. It is a complex function of at least four interrelated factors. These are ... 

By assuming that the 5 Oct value of the modern material is representative of the O-isotopic composition of speleothem calcite formed under interglacial conditions, both at present and in the past, then a record of 5 Oct variation for a fossil speleothem can be interpreted in terms of its climatic significance (e.g. Fig. 5). However, only a few recent speleothem paleoclimate studies have taken this approach to estimating the sense of 5 Oct variation for fossil speleothem calcite as a function of climate change. 

Figure 4. Axial 5 0ct-time profile for a stalgamite from Aven d Orgnao, France deposited from o. 130,000 to 90,000 years B. P. (after Duplessey et al., 1971)., O-isotope compositions are given relative to the PDB standard. Times of enrichments of speleothem calcite in have been interpreted as episodes of warm climate (Duplessy et al., 1971) and as episodes of cold climate (Emiliani, 1971). See text for discussion.

Table 3. Interpretation of the temperature dependence of changes in fossil speleothem O-isotopic composition. Data from the references cited in Figure 1.

These observations also have important paleoclimatological as well as hydrological implications. Since cave temperatures throughout the Ingleborough cave system were closely similar (and equal to the average surface temperature), the speleothem calcite precipitated over the interval of the study from such drip waters sampled would exhibit the same difference in O-isotopic composition as the parent seepage waters. This would correspond to an apparent maximum difference of >6°C between contemporaneously deposited speleothem calcite. 

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