Oxygen isotopes reservoirs

Young, E.D. and Russell, S.S., 1998. Oxygen isotope reservoirs in the early solar nebula inferred from Allende CAI. Science, 282, 452-5. 

Figure 7.1 Natural oxygen isotope reservoirs. Data from Taylor (1974), Onuitia et al. (1972),. Sheppard (1977), Graham and Harmon (1983) and Hoefs (1987).

This implies that the lithosphere and the hydrosphere of the parent body formed two distinct oxygen isotopic reservoirs. 

One of the main applications of hydrogen and oxygen isotope thermometry in geochemistry is the estimation of the reservoir temperatures of active geothermal systems or the evaluation of the ruling T conditions during deposition or alter-... 

Figure 2.18 sununarizes the naturally observed oxygen isotope variations in important geological reservoirs. 

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)... 

Another experimental method to investigate diffusion is the so-called half-space method, in which the sample (e.g., rhyolitic glass with normal oxygen isotopes) is initially uniform with concentration C, but one surface (or all surfaces, as explained below) is brought into contact with a large reservoir (e.g., water vapor in which oxygen is all 0). The surface concentration of the sample is fixed to be constant, referred to as Cq. The duration is short so that some distance away from the surface, the concentration is unaffected by diffusion. Define the surface to be X = 0 and the sample to be at x > 0. This diffusion problem is the so-called halfspace or semi-infinite diffusion problem. 

The drawback of tracer methodologies that use stable isotope compositions of water is that water-rock interaction will eventually modify the original oxygen isotopic composition of the re-injected water, if it remains in the reservoir for a long time. Moreover, isotopic re-equilibration may take place between the water and the gas species when the injected fluid comes into contact with large quantities of reservoir gas. Finally, the isotopic composition... 

Figure 4.5 The oxygen isotopic composition of various terrestrial and lunar reservoirs.

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. 

On the basis of analyses of bulk CAIs or separated minerals, the inheritance of 0-rich condensates from supemovae appears plausible. However, the magnitude of the isotopic difference between the putative dust and gas reservoirs is a free parameter, so that the model has no predictive power. Furthermore, the observed oxygen isotope anomalies in presolar oxide grains are best understood as resulting from processing in red giant stars, and do not show 0-excesses. 

Differences between the bulk compositions of chondrites, planets and asteroids can be attributed to accretion from different batches of CAIs, chondrules and other components, which are spread along variation lines on the standard three-isotope plot. The preservation of oxygen isotopic anomalies shows that there were numerous oxygen reservoirs for the manufacture of chondrules and CAIs that were quite separate. 

Figure 9 Oxygen-isotopic compositions of minerals in the Efremovka CAIE104 (a) and Vigarano CAI1623-2 (b) and their forsterite-rich accretionary rims (AR) and Wark-Lovering rims (WLR) showing that both formed in an 0-rich reservoir (after Krot et al, 2002b).

Figure 19 Oxygen-isotopic plot showing fields of analyses of large chondrules in chondrite groups. The data show that chondrules are derived from many iso-topically distinct reservoirs (Rubin (2000) reproduced by permission of Elsevier from Earth ScL Rev., 2000,50, p. 5).

Two parent body models have been proposed to explain the oxygen isotopic composition of carbonates in CM chondrites (i) a closed system, two reservoir model (Clayton and Mayeda, 1984, 1999) and (ii) a fluid-flow model (Young et al., 1999 Young, 2001 Cohen and Coker, 2000). Current oxygen-isotopic data are generally most consistent with the closed-system model, but can also be reconciled with the fluid-flow model if the CM chondrites sample a restricted region of the CM asteroid (Benedix et al., 2003), just downstream of the model alteration front proposed by Young (2001). 

SNC meteorites define an oxygen isotope mass fractionation line displaced from that of terrestrial samples (Figure 7), indicating that oxygen in martian materials is isotopically distinct. In fact, oxygen isotopes are commonly used to classify a meteorite as martian. Some other stable isotopic systems in these meteorites suggest that Mars has interior reservoirs that are very different from the outgassed martian atmosphere. 

---