Volatile elements depletions

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... 

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... 

Plots of uranium versus lanthanum (two refractory elements), and potassium versus lanthanum (a volatile element and a refractory element) for terrestrial and lunar basalts, HED achondrites (Vesta), and Martian meteorites. All three elements are incompatible elements and thus fractionate together, so their ratios remain constant. However, ratios of incompatible elements with different volatilities ( /La) reveal different degrees of volatile element depletion in differentiated bodies. After Wanke and Dreibus (1988). 

Trace element measurements in lunar basalts also indicate that the Moon is depleted in highly volatile elements (Taylor et al., 2006a). Estimates of some of the Moon s volatile element concentrations are compared with the Earth in Figure 13.11 a. The absence of water in lunar basalts suggests that the mantle is dry. The Moon may also be enriched in refractory elements (Fig. 13.11b). Volatile element depletion and refractory element enrichment are expected consequences of the giant impact origin and subsequent high-temperature accretion of the Moon. 

To summarize, chondrules and CAIs formed by transient heating events that processed a large fraction of the matter in the accretion disk. These heating events appear to overprint the thermal processing that produced the volatile element depletions among chondrites. The exact nature of these events is unknown, although shock waves in the nebula and the X-wind model are currently receiving the most attention. 

Describe the building blocks that accreted to form the terrestrial planets, and explain how that may relate to their volatile element depletions. 

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... 

The bulk trace element abundance patterns in CAIs are generally agreed to reflect element volatility, with the most refractory elements enriched relative to solar (Cl chondrite) abundances, and volatile elements depleted. 

The volatile element depletion patterns of planetary size objects and the chemical and isotopic composition of numerous smaller objects such as chondrules and CAIs provide the motivation to consider evaporation and condensation process in the early solar system. The key point is that the processes that led to chondrules and planets appear to have occurred under conditions very close to equilibrium, whereas the processes that led to CAIs involved significant departures from equilibrium. 

Humayun M. and Clayton R. N. (1995) Potassium isotope cosmochemistry genetic implications of volatile element depletion. Geochim. Cosmochim. Acta 59, 2131-2148. 

Volatile-element depletion patterns in other (e.g., CV, CM, or CO) carbonaceous chondrites (Larimer and Anders, 1970 Palme et al., 1988 ... 

Figure 16 Comparison of observed (open) and calculated (solid) depletions of phosphorus, tungsten, cobalt, nickel, molybdenum, and rhenium (circles) together with those for gallium, tin, and copper (inverted triangles) (sources Righter and Drake, 1997, 1999, 2000). The calculated depletions utilize the partitioning expressions of Righter and Drake (1999) for conditions of 2,250 ( 300) K (1,973 °C), 27 ( 6) GPa, AIW = — 0.4 ( 0.3) between a hydrous peridotite (NBO/t = 2.65) magma ocean and metallic liquid. The observed depletions are those of McDonough and Sun (1995), but volatility corrected as described by Newsom and Sims (1991), where the correction is made based on comparisons to trends of lithophile volatile element depletions.

The HfrW ratio of a bulk planetary mantle must be inferred by comparing the W concentrations with another element that behaves similarly during silicate melting (i.e., has a similar incompatibility), tends to stay in the mantle and whose abundance relative to Hf is known. The latter two conditions are met by refractory lithophile elements (RLE) because their relative abundances in bulk planetary mantles are chondritic (see above). This is because they are neither fractionated by core formation (because they are lithophile) nor by volatile element depletion (because they are refractory). The Hf/W ratio of a bulk planetary mantle can thus be calculated as follows ... 

Huff GA, Satterfield CN (1984) Intrinsie kineties of the Fiseher-Tropseh synthesis on a reduced fused-magnetite catalyst. Ind Eng in Chem Prod Res Develop 23 851-954 Humayun M, Clayton R (1995) Potassium isotope cosmochemistiy—genetic-implications of volatile element depletion. Geoehim Cosmoehim Acta 59 2131-2148 Igunmov SA (1976) Sulfur isotope exchange between sulfide and sulfate in hydrothermal solutions. Geokhimiya 4 497-503... 

Holness MB (1997) The permeability of non-deforming rock. In Holness MB (ed) Deformation-Enhanced Fluid Transport in the Earth s Cmst and Mantle. Chapman and Hall, London, p 9-39 Humayun M, Clayton RN (1995a) Potassium isotope geochemistry Genetic implications of volatile element depletion. 59 2131-2148... 

Fig. 14. Uranium is refractory like the rare earths, so that K/U ratios are an analogue for K/La ratios (fig. 11). K and U data are available for a wide variety of solar system material, since both elements are readily determined by gamma-ray spectroscopy. Both are incompatible in igneous processes and so tend to preserve their bulk planetary ratios during differentiation. This diagram illustrates that substantial volatile element depletion was widespread in the inner solar nebula, so that similar variations to K/U in volatile element/rare earth ratios are lo be expected. (From Taylor 1987a.)...

Some short-lived radionuclides were sufficiently abundant at the start of the solar system to produce variations in the abundance of their daughter isotopes in early-formed objects (Table 10.2). The half-lives of these nuclides are between about 0.1 and 100 Ma (Table 10.2). Hence, the parent isotopes are no longer present today, but they were synthesized in stars shortly before solar system formation and therefore they were present in the early solar nebula. The isotopic record of these nuclides provides information about stellar nucleosynthetic sites active shortly before the birth of the solar system and the time scales over which the early solar system formed and first differentiated. Depending on half-life and chemical affinities of parent and daughter isotopes, extinct radionuclide systems can be used to date processes as diverse as the formation of CAIs and chondrules, volatile element depletion and planetary difierentiation (e.g., core segregation and differentiation of early silicate reservoirs). In particular, they are powerful tools to study the Earth s accretion and core formation [90-92],... 

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