Oxygen isotopes mass-fractionation line

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. 

Oxygen isotopes in achondrites (above) and primitive achondrites (below). The 8 notation and units are explained in the caption for Figure 6.4. Most achondrites define mass fractionation lines parallel to, but slightly offset from the terrestrial line. Aubrites and lunar samples plot squarely on the terrestrial line. Primitive achondrites generally do not define oxygen mass fractionation lines, but are scattered and resemble their chondrite precursors. 

The oxygen isotopes of terrestrial materials mostly fall along with what is termed a mass-fractionation line (MFL). The MFL is defined by a slope of 0.5 passing through VSMOW as shown in Fig. 4.5. The oxygen isotopic composition is unique for different types of meteorites, which are broadly classified as chondrites and achondrites. 

Figure 7 Oxygen isotopic compositions in martian meteorites define a distinct mass fractionation line from that of terrestrial geologic materials. Open circles are mean values of Clayton and Mayeda (1996), and filled circles are mean values of Franchi et al. (1999).

Figure 3 The distribution of neon isotopes in mantle-derived rocks, indicating the presence of an atmospheric component, a radiogenic component adding Ne (produced by neutrons from uranium fission acting on oxygen and magnesium), and a solar component. It is this latter that indicates that gases in the mantle were derived from the capture of solar material in the early history of the Earth. M = MORB (midocean ridge basalts) P = plume or ocean island basalts (OIB) A = atmosphere. Solar neon is represented by the horizontal line at Ne/ Ne = 12.5 MFL is the mass fractionation line. The presence of solar neon in ocean basalts was first identified by Craig and Lupton (Craig H and Lupton JE (1976) Earth and Planetary Science Letters 31 369-385). (Reprinted with permission from Farley and Poreda (1993).

Figure 14.6 compares measured and calculated isotope fractionations for all 16 possible ozone isotopomers prepared from an enriched oxygen precursor. In this figure (160160160, 160160170, 160170160, 160160180, etc. are represented as 666, 667, 676, 677, 767, 668, 686, 678, 777, 688, 868, 778, 787, 788, 878, and 888). The calculations are those of Gao and Marcus described in sections below. They are in quantitative agreement with experiment. It is interesting that isotope fractionations observed in product ozone for the totally symmetric isotopomers, 8170 = 1000 ln(777/666) and 8lsO = 1000 ln(888/666), are negative they show the heavy isotope to be depleted. Moreover, these totally symmetric effects lie on the mass dependent fractionation line [ln(777/666)]/[ln(888/666)] 0.5. That... 

In principle, the three isotope method may be widely applied to new isotope systems such as Mg, Ca, Cr, Fe, Zn, Se, and Mo. Unlike isotopic analysis of purified oxygen, however, isotopic analysis of metals that have been separated from complex matrices commonly involves measurement of several isotopic ratios to monitor potential isobars, evaluate the internal consistency of the data through comparison with mass-dependent fractionation relations (e.g., Eqn. 8 above), or use in double-spike corrections for instrumental mass bias (Chapter 4 Albarede and Beard 2004). For experimental data that reflect partial isotopic exchange, their isotopic compositions will not lie along a mass-dependent fractionation line, but will instead lie along a line at high angle to a mass-dependent relation (Fig. 10), which will limit the use of multiple isotopic ratios for isobar corrections, data quality checks, and double-spike corrections. 

Oxygen 3-isotope plot showing excesses of 160 in minerals from calcium-aluminum inclusions (CAIs). All samples from Earth rocks plot along the terrestrial fractionation line. Mass-dependent fractionation processes cannot move a composition off of this line, so the excesses of 160 were clearly isotopic anomalies. After Clayton et al. (1977). 

The degree of equilibrium isotopic fractionation among phases depends on temperature, so the isotopic compositions of co-existing phases can be used for thermometry. Oxygen is widely used in this way. For example, Clayton and Mayeda (1984) found that the oxygen isotopic compositions of calcite and phyllosilicates from Murchison lie on a mass-dependent fractionation line and differ in 6180 by 22%o. This difference requires a temperature of around 0 °C, which is interpreted to be the temperature of aqueous alteration on the Murchison parent asteroid. Similar measurements for Cl chondrites indicate that aqueous alteration for these meteorites occurred at higher temperature, 50-150 °C (Clayton and Mayeda, 1999). 

Meteorites fall into different classes, and the distinct classes have distinct oxygen isotopic compositions. The slope 1/2 mass-dependentchemical fractionation line... 

Oxygen isotope variations in differentiated bodies—such as Earth, Moon, Mars—and the parent body of the HED group (for howardites, eucrites, and diogenites) show characteristic mass-dependent fractionation lines, with a slope of 0.52 on the three-isotope plot... 

Figure 10 Oxygen isotopic compositions of whole-rock meteorites from differentiated bodies HED, possibly asteroid Vesta SNC, possibly Mars and Moon. Each body produces a slope-1/2 mass-dependent fractionation line, with values of characteristic of the whole source planet. The isotopic compositions of...

FIGURE 2.10 (a) An oxygen isotope plot for chondritic meteorites showing the two main meteorite trends, relative to the terrestrial mass-dependent fractionation (MDF) trend (which passes through the oxygen isotope standard SMOW). The CCAM line (dashed) represents a mixing line defined by separated minerals from refractory CAI inclusions. Amoeboid olivines plot at the lower 160-rich end of this line. The aqueous alteration trend is shown in more detail in (b) below. 

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