O isotopic exchange

An experiment that may be simpler in its reactions, more sensitive than the detection limit of O isotope exchange need to explain the discrepancy, and as free as possible from OH vibrational excitation, would be helpful to explore the issue further. On the other hand the isotope effect is very small and the system sufficiently complicated that the theoretical calculations themselves may be the source of the discrepancy. In either case we infer that the reaction itself is mass anomalous rather than mass-independent in the strict sense of the term, both experimentally and in terms of current theoretical insight. In concluding this section we comment in physical terms on the small isotope effects, about —5 to —10 per mil, observed for and in this system at low pressures. 

For high pressures, equation (1.6) indicates that the rate determining step for dissociation in the dissociation of highly excited NOf for recombination it is the association of O -1- NO to give NO. The question arises, whether (i) association in high pressure recombination and (ii) association as the first step of O isotope exchange with NO... 

Boden has investigated the mechanism of Pb oxidation and the role of water through isotope-exchange experiments [21]. He has used deuterium oxide instead of water. In the presence of D2O, the rate constant of the reaction of Pb oxidation diminishes almost twice. This indicates that water takes part in the reactions of lead oxidation. The bond D—O is stronger than the bond H—O. Isotope-exchange experiments have been conducted with H2O and H20 isotopes. It has been established that almost 50% of PbO obtained as a result of Pb oxidation contains O isotope. On grounds of these results, the following mechanism of Pb oxidation with involvement of H2O has been proposed ... 

V.A. Sobyanin, V.I. Sobolev, V.D. Belyaev, O.A. Mar ina, A.K. Demin, and A.S. Lipilin, On the origin of the Non-Faradaic electrochemical modification of catalytic activity (NEMCA) phenomena. Oxygen isotope exchange on Pt electrode in cell with solid oxide electrolyte, Catal. Lett. 18, 153-164 (1993). 

We have previously assumed that the principle diagenetic process is attributable to exchange or interaction of (depleted) browser apatite values with (enriched) sedimentary matrix values (Lee-Thorp and van der Merwe 1987). Another possibility is ionic or isotopic exchange with soil CO2 however in most cases soil CO2 values will be closely related to matrix carbonate values. If the former is the case, one would expect enriched grazer values (near 0%o)... 

Xenon [ F]difluoride was synthesized by isotopic exchange between XeF2 and H F, Si F4, or As F, and F-2DFG was prepared - by the action of this reagent on tri-O-acetyl-D-glucal (61). In this reaction, the combination of ethyl ether (as the solvent) and BFj (as the Lewis acid catalyst) was found to give the best result. ... 

We know, however, that isotope exchange in nature tends to be a slow process, especially at low temperature (e.g., O Neil, 1987). This knowledge comes from experimental study (e.g., Cole and Ohmoto, 1986) as well as from the simple observation that, unless they have reacted together, groundwaters and minerals are... 

We briefly review processes in which isotopic fractionations may be recorded in isotopically distinct reservoirs that are preserved in nature. These concepts have been extensively covered in the H, C, O, and S isotope literature, and we illustrate several examples for the non-traditional stable isotope systems discussed in this volume. One of the simplest processes that produces isotopically distinct reservoirs would be slow reaction of substance A to B, where A and B remain open to complete isotopic exchange during the process. This is commonly referred to as closed system equilibrium, and the changes in isotopic compositions that occur may be defined by the exact relation ... 

Figure 11. Determination of ferrous-ferric isotope exchange kinetics in dilute aqueous solutions using Fe-enriched tracer solutions. Measured 5 Fe values for ferrous (squares) and ferric (circles) Fe in solution versus time. Initial 5 Fe values for Fe(II), 0%o and Fe(III),q 331%o. The rapid convergence in Fe/ Fe ratios for the ferric and ferrous species indicates that isotopic equilibrium is attained within minutes. Adapted from Welch et al. (2003).

Figure 1. Three isotope plot of O isotopes in Allende inclusions. Deviations are plotted in 6 units which are %o deviations relative to the terrestrial SMOW standard. In a two stage model, normal inclusions had initially a composition close to point A and exchanged with a reservoir poorer in O in the region of point D (Clayton et al. 1973). FUN inclusions underwent an intermediate step along a fractionation line between point A and point C. Then each inclusion exchanged with the same O poor reservoir D (Clayton and Mayeda 1977).

Many more high-precision whole-rock analyses of both Mg and O isotope ratios for chondrite components would help to establish whether or not the mixing trends in Figure 15 are valid, or even if the concept of mixing is useful. Bulk objects are desirable because their isotopic compositions are not affected by secondary inter-mineral exchange reactions that are know to be important for both the Mg and O isotopic systems in CAIs and chondrules. 

Poulson RL, Beard BL, Johnson CM (2003) Investigating isotopic exchange between dissolved aqueous and precipitated iron species in natural and synthetic systems. Geochim Cosmochim Acta 67 A382 Reynolds BC, Frank M, O Nions RK (1999) Nd- and Pb-isotope time series from Atlantic ferromanganese crusts implications for changes in provenance and paleocirculation over the last 8 Myr. Earth Planet Sci Lett 173 381-396... 

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