Chondritic meteorites volatile element depletion

The volatile element depletions among the various classes of chondrites were once considered to be the result of equilibrium condensation, with accretion of the different classes of meteorites taking place at different temperatures before the missing volatiles could... 

The process of condensation of minerals in the early solar nebula has long been invoked to explain the chemistry and mineralogy of primitive chondritic meteorites (e.g. Cameron 1963). Their observed bulk compositions show volatile-element depletions that are clearly smooth functions of calculated condensation temperature in a gas of solar composition (Davis 2006). Despite this success in explaining the bulk composition of chondrites, the diverse mineralogy of these bodies is not reproduced well in the condensation sequence calculations. To date, there is no incontrovertible evidence for direct condensation of rocky meteoritic material in the... 

Copper in meteorites is depleted in the heavier 65 isotope with respeet to the Earth (Luek et al. 2003 Russell et al. 2003). Luck et al. s (2003) study of the four main groups of carbonaceous chondrites CI-CM-CO-CV showed that Cu depletion is maximum (-1.5%o) for the C V chondrites (e.g., Allende) for which the depletion of volatile elements is strongest, which indicates that volatilization does not accormt for the observed isotopic heterogeneity (Fig. 4). Luck et al. (2003) found that 8 Cu in CI-CM-CO classes correlates with O excess, but this does not seems to be the case for CV (Luck et al. 2003) nor for the CR, CB, and the particularly Cu-depleted CH-like classes (Russell et al. 2003). In contrast, chondritic Zn is relatively heavy with 8 Zn up to 1 %o (Luck et al. 2001). The rather high 5 Zn values of iron meteorites (up to 4%o)is reminiscent of a similar fractionation of Fe isotopes between metal and silicates (Zhu et al. 2002). 

The geochemistry of angrites is characterized by strong silica undersaturation, by which we mean that there is not enough SiC>2 to combine with various cations to form common silicate minerals. The result is the formation of silica-poor minerals like kirschsteinite and nepheline. These meteorites also show strong depletions in moderately volatile elements. They are thought to have formed as partial melts of a chondritic source under oxidizing conditions. 

Chondrites can be described as conglomerate rocks characterised by a overall chemical composition similar to the composition of the sun (with a depletion of hydrogen, helium and some other highly volatile elements). This latter characteristic is an easy way to distinguish clearly between stony meteorites and terrestrial stones. 

Their abundances are in most cases below solar, i.e., they have lower element/silicon ratios than the Sun or Cl chondrites, they are depleted (see below). In Figure 2, abundances of moderately volatile elements in CVS meteorites relative to those in Cl meteorites are plotted. Increasing depletions correlate with decreasing condensation temperatures but are independent of the geochemical properties of the elements. Depletions of moderately volatile elements in meteorites are produced by incomplete condensation. The amount and the relative abundances of these elements in meteorites are probably the result of removal of volatiles during condensation (Palme et al., 1988). 

Itqiy is distinct from chondritic meteorites in bulk composition. Aluminum, FREE, europium, sodium, potassium, vanadium, chromium, and manganese are aU depleted. Itqiy has La/Yb of 0. lOxCI, and Eu/Sm of 0.16 X Cl. Refractory siderophile elements are enriched —2-3 X Cl, while moderately volatile siderophile elements are at roughly Cl abundances. The bulk rock Mg/Si and Fe/Si ratios are greater than those of EH or EL chondrites. 

The bulk compositions of the terrestrial planets and all meteorite parent bodies except that of the Cl chondrites are depleted in volatile elements to various degrees. These depletions are reasonably smooth functions of 50% condensation... 

Not only is there a shortage of nebular gas in the Earth and terrestrial planets today but the moderately volatile elements also are depleted (Figure 1) (Gast, 1960 Wasserburg et al, 1964 Cassen, 1996). As can be seen from Figure 2, the depletion in the moderately volatile alkali elements, potassium and rubidium in particular, is far greater than that found in any class of chondritic meteorites (Taylor and Norman, 1990 Humayun and Clayton, 1995 Halliday and Porcelli, 2001 Drake and Righter, 2002). The traditional explanation is that the inner terrestrial planets accreted where it was hotter. 

Most chondritic IDPs have chondrite-like trace-element compositions (Arndt et al., 1996). Abundances in individual chondritic IDPs generally scatter from —0.3 X Cl to —3 X Cl and that enrichments are more common than depletions (Flynn and Sutton, 1992a,b,c). Volatile elements tend to be enriched relative to Cl meteorites (Ganapathy and Brownlee, 1979 Sutton, 1994). Enrichments of bromine measured in some IDPs probably reflect stratospheric contamination (Van der Stap et al., 1986 Elynn, 1994a Hynn et al., 1996), and zinc depletions probably reflect loss of... 

The concentrations of four typical moderately volatile elements—manganese, sodium, selenium, and zinc—in the various classes of chondritic meteorites are shown in Figure 12, where elements are normalized to magnesium and CI-chondrites. Again there is excellent agreement between solar abundances and Cl-meteorites. A characteristic feature of the chemistry of carbonaceous chondrites is the simultaneous depletion of sodium and manganese in all types of carbonaceous chondrites, except Cl. Ordinary and enstatite chondrites are not or only slightly... 

Figure 12 Moderately volatile/Mg ratios in various types of chondritic meteorites. All groups of chondritic meteorites are depleted in moderately volatile elements, none is enriched. The two elements Mn and Na are depleted in carbonaceous chondrites and in the Earth but not in ordinary and enstatite chondrites. The Earth is also depleted...

Figure 16 Mn/Na versus Mn/Al in chondritic meteorites. The two moderately volatile elements Na and Mn have the same ratio in all chondritic meteorites and in the primitive Earth s mantle, here designated as BSE. The low Mn/Al content of the Earth s mantle reflects enrichment of A1 and depletion of Mn. Because of the chondritic Mn/Na ratio of the Earth s mantle, it is unlikely that a significant fraction of the Earth s inventory of Mn is in the core (source...

The atmophile elements hydrogen, carbon, nitrogen, and the rare gases are strongly depleted in the Earth compared to chondritic meteorites. Pepin (1989) concluded that it appears that simple veneer scenarios in which volatiles are supplied from sources resembling contemporaneous meteorite classes cannot explain the observed isotopic compositions. It is, therefore, often assumed that the isotopic compositions of these elements were affected by the process... 

The compositions of the planets in the solar system and those of chondritic meteorites provide a guide to the bulk Earth composition (see Chapter 2.01). However, the rich compositional diversity of these bodies presents a problem insofar as there is no single meteorite composition that can be used to characterize the Earth. The solar system is compositionally zoned planets with lesser concentrations of volatile elements are closer to the Sun. Thus, as compared to Mercury and Jupiter, the Earth has an intermediate uncompressed density (roughly a proportional measure of metal to rock) and volatile element inventory, and is more depleted in volatile elements than CI-chondrites, the most primitive of all of the meteorites. 

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