Fractionation planetary differentiation

Weyer S, Anbar AD, Brey GP, Miinker C, Mezger K (2005) Iron isotope fractionation during planetary differentiation. Earth Planet Sci Lett 240 251-264 White JWC (1989) Stable hydrogen isotope ratios in plants a review of current theory and some potential applications. In Stable isotopes in ecological research. Ecological Studies 68. Springer Verlag, New York, p. 142-162... 

Element fractionation resulting from planetary differentiation... 

Planetary differentiation is a fractionation event of the first order, and it involves both chemical fractionation and physical fractionation processes. Planetary crusts are enriched in elements that occur in silicate minerals that melt at relatively low temperatures. Recall from Chapter 4 that the high solar system abundances of magnesium, silicon, and iron mean that the silicate portions of planetesimals and planets will be dominated by olivine and pyroxenes. Partial melting of sources dominated by olivine and pyroxene ( ultramafic rocks ) produces basaltic liquids that ascend buoyantly and erupt on the surface. It is thus no surprise that most crusts are made of basalts. Remelting of basaltic crust produces magmas richer in silica, eventually resulting in granites, as on the Earth. 

We discussed two different types of radiochronometers. Those based on long-lived radionuclides for which a portion of the primordial abundance is still present provide absolute ages relative to the present time on suitable samples. Examples of how these chronometers are used to date individual objects (chondrules, CAIs, achondrites) and fractionation events (planetary differentiation, magma generation) were discussed. 

Bishunpur and Chainpur Nyquist et aL, 2001) and for planetary differentiates (whole-rock eucrites Lugmair and Shukolyukov, 1998). Plotted are measured values of e( Cr), the deviation of Cr/ Cr in a sample from the terrestrial standard value in parts per 10", as a function of Mn/ Cr. The correlation is interpreted as an isochron indicating the decay of Mn the slope for the eucrites (dashed line) corresponds to an initial Mn/ Mn = (4.7 0.5) X 10 and that for chondrules (solid line) indicates ( Mn/ Mn)o = (8.8 1.9) X 10 , implying that Mn/Cr fractionation in chondrule precursors preceded global fractionation of the eucrite parent body by approximately one half-life, or —3.5 Myr. All data are replotted from Lugmair and Shukolyukov (1998) and Nyquist et al. (2001a) 2(7 error bars are indicated and the datum for EET87520 is excluded from the fit for the eucrite whole-rock isochron. 

Early planetary differentiation included separation of Fe-Ni from mantle silicates to form the core. While a considerable fraction of some elements partitioned into coreforming material at this time, it is not known to what extent significant amounts of noble gases were transferred to the core in this way. The basic data for noble gas partitioning between silicates and metal at high temperatures and pressures is still scarce (see Porcelli and Ballentine 2002, this volume). 

Weyer, S., Anbar, A.D., Brey, G.P., Miinker, C., Mezger, K., and Woodland, A.B. (2005) Iron isotope fractionation dining planetary differentiation. Earth Planet. Sci. Lett., 240, 251-264. 

For many years, meteorites have provided the only means to determine the abundance of 3He in protosolar material. The values obtained by mass spectroscopy techniques in the so-called planetary component of gas-rich meteorites have been critically examined by Geiss (1993) and Galli et al. (1995). The latter recommend the value 3He/4He= (1.5 0.1) x 10-4. The meteoritic value has been confirmed by in situ measurement of the He isotopic ratio in the atmosphere of Jupiter by the Galileo Probe Mass Spectrometer. The isotopic ratio obtained in this way, 3He/4He= (1.66 0.04) x 10 4 (Mahaffy et al. 1998), is slightly larger than, but consistent with, the ratio measured in meteorites, reflecting possible fractionation in the protosolar gas in favor of the the heavier isotope, or differential depletion in Jupiter s atmosphere. 

Such a measurement can tell us about the chemical evolution of oxygen, such as whether the isotopes differentiated via a thermal cycle in which lighter leO fractionates from the heavier lsO, much as Vostok ice-core oxygen ratios reveal the Earth s prehistoric climate. From this fixed point of the Sun s oxygen ratios, we can then trace the history of water in other planetary bodies since their birth in the solar nebulae through the subsequent cometary bombardment [13]. In NASA s search for water on the Moon, important for the establishment of a future Moon base, such isotopic ratios will determine whether the water is a vast mother lode or just a recent cometary impact residue. 

The Earth and other planetary bodies have been heavily modified by planetary-scale differentiation, smaller scale melting and the resulting chemical fractionations, collisions that mix material with different histories, and other processes. Samples of these materials are thus not suitable for determining the solar system composition. More primitive objects, such as comets and chondritic meteorites, have compositions more similar to the composition of... 

Common igneous processes (partial melting and fractional crystallization) lead to element fractionations. Incompatible elements tend to be concentrated in melts and compatible elements in solids. Separation of partial melts from residual crystals as the melts ascend to higher levels, or accumulation of early-formed crystals from melts, ultimately produces rocks with compositions different from the starting materials. These processes account for the fractionations seen in differentiated meteorites and planetary samples. 

It is likely that the earliest events in the Earth s mantle were not the product of "normal" mantle convection but rather related to planetary processes such as fractionation within a magma ocean. In this way we can explain the very early differentiation of the Earth (pre-4.5 Ga), proposed on the basis of short-lived Nd-isotopes. Similarly, the extreme volatile element loss from the Earth might be explained in this way. These early processes are thought to have ceased within the first 100 Ma of Earth history (Yokochi Marty, 2005). 

The decay of Hf to is well suited to date core formation in planetary objects mainly for three reasons. First, owing to the Hf half-life of 9 Myr, detectable W isotope variations can only be produced in the first -60 Myr of the solar system. This timescale is appropriate for the formation of the Earth and Moon in particular and to planetary accretion and differentiation in general. Second, both Hf and W are refractory elements such that there is only limited fractionation of Hf and W in the solar nebula or among different planetary bodies (see above). The HfrW ratio of the bulk Earth therefore can be assumed to be chondritic and hence can be measured today. Third, Hf is a lithophile and W is a siderophile element such that the chondritic HfrW ratio of the Earth is fractionated internally by core formation. If core formation took place during the effective lifetime of Hf, the metal core (HfrW-O) will develop a deficit in the abundance of whereas the silicate mantle, owing to its enhanced Hf/W, will develop an excess of (7-P). 

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