Planetary differentiation

Differentiated (planetary) Achondrites Angrites Aubrites Brachinites HED meteorites Eucrites Howardites Diogenites Ureilites Stony-irons Pallasites... 

Without iron and stony-iron meteorites, our chances of ever sampling the deep interior of a differentiated planetary object would be next to nil. Although we live on a planet with a very substantial core, we will never be able to sample it. Fortunately, asteroid collisions provide us with a rich sampling of the deep interiors of differentiated asteroids. 

Jochum K. P., Seufert H. M., Spettel B., and Palme H. (1986) The solar system abundances of Nb, Ta, and Y. and the relative abundances of refractory hthophile elements in differentiated planetary bodies. Geochim. Cosmochim. Acta 50, 1173-1183. 

Figure 4. Hf/Wfractionation and W isotope anomalies of mantles and cores of differentiated planetary bodies. The Hf-W systematics of bulk planets is represented by chondrites. Data from (5, 16, 19, 20, 29, 31).

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. 

Evolved extraterrestrial materials are generally igneous rocks, which according to their thermal history can be discnssed analogonsly to terrestrial samples. To this category belong planetary bodies, differentiated asteroids, and achondritic meteorites. 

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... 

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... 

Some meteorites, and all planetary samples, have undergone melting and differentiation at some stage. Hence, the compositions of differentiated materials do not resemble solar system abundances. These samples can, however, tell us about various geochemical processes within asteroids and planets. 

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. 

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. 

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. 

Mostefaoui, S., Lugmair, G. W. and Hoppe, P. (2005) 60Fe a heat source for planetary differentiation from a nearby supernova explosion. Astrophysical Journal, 625,271-277. 

Ma (Fig. 9.16). The 40Ar-jyAr ages appear to be considerably older, but are compromised by the presence of trapped 40Ar from the Martian atmosphere and possibly inherited radio-genic Ar as well. In contrast to the mineral (internal) isochrons, whole-rock Rb- Sr and Pb-Pb isochrons for shergottites suggest a much older age of-4.1 Ga (Bouvier et al., 2008). Most workers interpret this age to reflect planetary differentiation that established mantle... 

Noble gases are most abundant in planetary atmospheres, although even there they are only minor components. They have been measured in the gas envelopes of Venus, Earth (of course), Mars, and Jupiter. We will consider their utility in understanding planetary differentiation and atmospheric evolution shortly, but first we will focus on their rather miniscule abundances in meteorites and other extraterrestrial materials. 

Cosmochemislry places important constraints on models for the origin of the solar nebula and the formation and evolution of planets. We explore nebula constraints by defining the thermal conditions under which meteorite components formed and examine the isotopic evidence for interaction of the nebula with the ISM and a nearby supernova. We consider how planetary bulk compositions are estimated and how they are used to understand the formation of the terrestrial and giant planets from nebular materials. We review the differentiation of planets, focusing especially on the Earth. We also consider how orbital and collisional evolution has redistributed materials formed in different thermal and compositional regimes within the solar system. 

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