Differentiated meteorites

Ruzicka A., Fowler G. W., Snyder G. A., Prinz M., Papike J. J., and Taylor L. A. (1999) Petrogenesis of silicate inclusions in the weekeroo station HE iron meteorite differentiation, remelting, and dynamic mixing. Geochim. Cosmochim. Acta 63, 2123 -2143. 

Stony Irons. The stony iron meteorites are composed of substantial iron and siUcate components. The paHasites contain cm-sized ohvine crystals embedded ia a soHd FeNi metal matrix and have properties consistent with formation at the core mantle boundary of differentiated asteroids. The mesosiderites are composed of metal and siUcates that were fractured and remixed, presumably ia the near-surface regions of their parent bodies. 

Differentiated meteorites they come from asteroids which have been through a fusion process which led to a more or less clear separation into nucleus, mantle and crust. 

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. 

Substantial abundance anomalies occur among the heavy oxygen isotopes 170 and 180, which are underabundant by up to about 4 per cent relative to 160 in oxide grains of certain of the CAIs, compared with the bulk composition in which the isotope ratios are closer to a terrestrial standard. The intriguing feature of these anomalous ratios is that, in common with some other meteorites, but in contrast to terrestrial and lunar samples, the relative deviations of the two heavy isotopes are equal most normal fractionation processes would cause 180 to have twice the anomaly of 170, as indeed is observed in terrestrial samples and more differentiated meteorites, where the anomalies are also usually much smaller. While there has been speculation that there might be a substantial admixture of pure 160 from a supernova, there are fractionation mechanisms that may be able to account for the effect, e.g. photo-dissociation of molecules affected by selfshielding (R. Clayton 2002). In this case, it is possible that the terrestrial standard is enriched in the heavy O-isotopes while the inclusions have more nearly the true solar ratio. 

Figure 8. Figure (a) after Clayton et al. (1976, 1977). The scales are as in Figure 1. The O isotopic compositions of the different meteorite classes are represented ordinary chondrites (H, L, LL), enstatite chondrites (EFl, EL), differentiated meteorites (eucrites, lAB irons, SNCs) and some components of the carbonaceous chondrites. As the different areas do not overlap, a classification of the meteorites can be drawn based on O isotopes. Cr (b) and Mo (c) isotope compositions obtained by stepwise dissolution of the Cl carbonaceous chondrite Orgueil (Rotaru et al. 1992 Dauphas et al. 2002), are plotted as deviations relative to the terrestrial composition in 8 units. Isotopes are labeled according to their primary nucleosynthetic sources. ExpSi is for explosive Si burning and n-eq is for neutron-rich nuclear statistical equilibrium. The open squares represent a HNOj 4 N leachate at room temperature. The filled square correspond to the dissolution of the main silicate phase in a HCl-EIF mix. The M pattern for Mo in the silicates is similar to the s-process component found in micron-size SiC presolar grains as shown in Figure 7.

Caro G, Bourdon B, Birck JL, Moorbath S (2003) Sm- Nd evidence from Isua metamorphosed sediments for early differentiation of the Earth s mantle. Nature 423 428-432 Chen JH, Wasserburg GJ (1981) The isotopic composition of uranium and lead in Allende inclusions and meteoritic phosphates. Earth Planet Sci Lett 52 1-15... 

Eberhardt P, Jungck MHA, Meier FO, Niederer FR (1981) Aneon-E rich phase in Orgueil results obtained on density separates. Geochim Cosmochim Acta 45 1515-1528 El Goresy A, Zinner E, Marti K (1995) Survival of isotopically heterogeneous graphite in a differentiated meteorite. Nature 373 496-499... 

Mullane E, Russel SS, Gounelle M (2002) Iron isotope fractionation within a differentiated asteroidial sample suite. 65 Annual Meteoritical Society Meeting, abstract number 5157 Mullane E, Russel SS, Gounelle M, Mason TED (2003a) Iron isotope composition of Allende and Chainpur chondrules effects of equilibration and thermal history. Lunar Planet Sci Conf XXXIV, abstract number 1027... 

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. 

The interiors of planets, moons, and many asteroids either are, or have been in the past, molten. The behavior of molten silicates and metal is important in understanding how a planet or moon evolved from an undifferentiated collection of presolar materials into the differentiated object we see today. Basaltic volcanism is ubiquitous on the terrestrial planets and many asteroids. A knowledge of atomic structure and chemical bonding is necessary to understand how basaltic melts are generated and how they crystallize. Melting and crystallization are also important processes in the formation of chondrules, tiny millimeter-sized spherical obj ects that give chondritic meteorites their name. The melting, crystallization, and sublimation of ices are dominant processes in the histories of the moons of the outer planets, comets, asteroids, and probably of the Earth. 

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. 

We will now describe each of the various kinds of meteoritic samples available for cosmochemical investigation, progressing from primitive materials to samples from differentiated bodies. Presolar grains extracted from meteorites have already been described in Chapter 5, and interplanetary dust particles (IDPs) and returned comet samples will be described in Chapter 12. 

Possible heat sources for melting in meteorite parent bodies are discussed in Chapter 11. Differentiated meteorites generally have old radiometric ages (Chapter 9), which indicate igneous activity began and ended soon after their parent bodies accreted. 

Iron meteorites offer the unique opportunity to examine metallic cores from deep within differentiated bodies. Most of these samples were exposed and dislodged when asteroids collided and fragmented. Although irons constitute only about 6% of meteorite falls, they are well represented in museum collections. Most iron meteorites show wide variations in siderophile-element abundances, which can be explained by processes like fractional crystallization in cores that mimic those in achondrites. However, some show perplexing chemical trends that may be inconsistent with their formation as asteroid cores. 

Mesosiderites are a highly enigmatic group of differentiated meteorites. They are breccias composed of iron-nickel metal and silicate in roughly equal proportions. The metal represents molten material from the deep interior of an asteroid, whereas the silicate fraction consists of basalts and pyroxene cumulates similar to HED meteorites that formed near the surface. It is difficult to construct models that allow mixing of such diverse materials, but these disparate materials are generally thought to have been violently mixed by impact. 

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). 

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. 

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