Metal oxide hypothesis, sulfide oxidation

Another difficulty with the metal oxide hypothesis is that the observed sum of the vertical electron equivalent gradients of Mn(II) and Fe(II) is much less than that of sulfide (Figure 7). In a simple vertical, steady-state system where the upward flux of Mn(II) and Fe(II) results in oxidized particulate metal oxides, which in turn settle to oxidize sulfide, the electron gradients of Mn(II) + Fe(II) would equal that for sulfide. The fact that they do not equal it suggests that the vertical flux of Mn(II) + Fe(II) would not produce sufficient particulate metal oxides. This problem would be solved if the particulate oxides were produced primarily at the boundaries and transported into the interior (40). 

Thus core-mantle equilibration can be excluded as the source of the HSEs in the Earth s mantle. It is more likely that a late accretionary component has delivered the HSEs to the Earth s mantle, either as single Moon-sized body which impacted the Earth after the end of core formation or several late arriving planetesimals. The impac-tors must have been free of metallic iron, or the metallic iron of the projectiles must have been oxidized after the collision(s) to prevent the formation of liquid metal or sulfide that would extract HSEs into the core of the Earth. The relative abundances of the HSEs in the Earth s mantle are thus the same as in the accretionary component, but may be different from those in the bulk Earth. The late addition of PGE with chondritic matter is often designated as the late veneer hypothesis (Kimura et al., 1974 Chou, 1978 Jagoutz et al., 1979 Morgan et al., 1981 O Neill, 1991). This model requires that the mantle was free of PGE before the late bombardment established the present level of HSEs in the Earth s mantle. 

Figure 8 shows the typically observed pit initiation sites. For steel A, whatever the pH or the chloride content, pits initiate either around A1 oxides (Fig. 8a), where some MnS is located, or on MnS inclusions (Fig. 8b) around Nb(C,N) or (seldom) isolated. This confirms the preceding hypothesis. For steel B in NaCl (0.5 M) solution, pitting generally occurs at the TiN boundary (Fig. 8c), where Ti sulfides are present. This shows clearly that for such chloride concentrations, Ti sulfides act as pitting sites and becomes unstable when the potential increases. For lower chloride concentration (0.2 M NaCl), the situation is not so clear and pits could initiate directly on the metallic matrix, with no direct relation to nonmetallic inclusions, or in some cases on titanium nitrides. In the two situations, however, Ti sulfides do not seem to act as pitting sites, which is consistent with the absenee of any pitting potential pH dependenee. 

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