Volatile depletions

Figure 4. Correlation between the excesses of of the different classes of meteorites, as measured by their A 0 and their 5 Cu values. The volatile-depleted carbonaceous chondrites (CV) show a deficit in the heavier isotope Cu, which led Luck et al. (2003) to suggest that the observed correlations result from a nucleosynthetic excess of Cu.

Two types of models have been proposed that use this general picture as the basis for understanding volatile depletions in chondrites. Yin (2005) proposed that the volatile element depletions in the chondrites reflect the extent to which these elements were sited in refractory dust in the interstellar medium. Observations show that in the warm interstellar medium, the most refractory elements are almost entirely in the dust, while volatile elements are almost entirely in the gas phase. Moderately volatile elements are partitioned between the two phases. The pattern for the dust is similar to that observed in bulk chondrites. In the Sun s parent molecular cloud, the volatile and moderately volatile elements condensed onto the dust grains in ices. Within the solar system, the ices evaporated putting the volatile elements back into the gas phase, which was separated from the dust. Thus, in Yin s model, the chondrites inherited their compositions from the interstellar medium. A slightly different model proposes that the fractionated compositions were produced in the solar nebula by... 

Schonbachler, M., Carlson, R. W., Horan, M. E, Mock, T. D. and Hauri, E. H. (2008) Silver isotope variations in chondrites Volatile depletion and the initial 107Pd abundance of the solar system. Geochimica et Cosmochimica Acta, 72, 5330-5341. 

Volatilization depletion rate constant k = 45 d 1 from aqueous solution in a 26-cm or 67-cm purge vessel, (Girvin et al. 1997). 

This indicates that the volatility fractionations affecting silicates and metal occurred under different conditions. Since chondrites are sedimentary rocks consisting largely of chondrules that were melted and rapidly cooled in zero gravity, the volatility depletions in bulk chondrites almost certainly arose in the nebula, likely during the thermal processing events that melted chondrules (see Chapters 7 and 8). 

Figure 1 The estimated composition of the silicate portion of the Earth as a function of condensation temperature normalized to Cl values in Anders and Grevesse (1989). Open circles lithophile elements shaded squares chalcophile elements shaded triangles moderately siderophile elements solid diamonds highly siderophile elements. The spread in concentration for a given temperature is thought to be due to core formation. The highly siderophile element abundances may reflect a volatile depleted late veneer. Condensation temperatures are from Newsom (1995).

Perhaps the best current way to view the disk of debris from which the Earth accreted was as an environment of vigorous mixing. Volatile-depleted material that had witnessed very high temperatures at an early stage (CAIs) mixed with material that had been flash melted (chondrules) a few milhon years later. Then presolar grains that had escaped these processes rained into the... 

Co-accretion. This theory proposes that the Earth and Moon simply accreted side by side. The difficulty with this model is that it does not explain the angular momentum of the Earth-Moon system, nor the difference in density, nor the difference in volatile depletion (Taylor, 1992). 

Certain features of the Moon may be a consequence of the giant impact itself. The volatile-depleted composition of the Moon, in particular, has been explained as a consequence of... 

The relationships show the volatile depleted nature of the Moon relative to the Earth and the Moon and the Earth relative to primitive Cl chondrites. 

CO, and other carbonaceous chondrites (Table 10.1) relative to the Cl chondrites, which are thought to best approximate the composition of the bulk solar system, were not caused by processes involving partial evaporation, as this would generate larger differences in isotopic compositions due to kinetic isotope fractionation [136, 136a], The limited variability seen in Zn and Cd isotopic compositions rather indicates that the variable volatile depletion of carbonaceous chondrites (and potentially other planetary bodies) is most readily explained by partial condensation at near-equilibrium conditions or two-component mixing between volatile-rich and volatile-poor material [38, 127, 134]. 

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