Refractory inclusions

CAIs are composed of a variety of minerals, primarily hibonite, perovskite, melilite, spinel, aluminum- and titanium-rich diopside, anorthite, forsterite, and occasionally corundum or grossite. They also show significant enrichments in refractory trace elements. CAIs exhibit a host of isotopic anomalies inherited from incorporated presolar grains or from the early nebula itself. 

CAIs are especially abundant in carbonaceous chondrites, but they occur in lesser abundance in other chondrite groups as well. Most types of refractory inclusions occur in all groups, but their relative proportions and sizes vary. 

The amoeboid descriptor for amoeboid olivine aggregates refers to their irregular shapes. AOAs tend to be fine-grained and porous, and have comparable sizes to CAIs in the same meteorite. They consist mostly of forsterite and lesser amounts of iron-nickel metal, with a refractory component composed of anorthite, spinel, aluminum-rich diopside, and rarely melilite. The refractory component is sometimes recognizable as a CAI embedded within the AOA. The AOAs show no evidence of having been melted, but some contain CAIs that have melted. 

like CAIs, are depleted in volatile and moderately volatile elements, but to a lesser degree. AOAs are generally interpreted as aggregates of grains that condensed from nebula 

Two different kinds of metals are found in chondrites. Small nuggets composed of highly refractory siderophile elements (iridium, osmium, ruthenium, molybdenum, tungsten, rhenium) occur within CAIs. These refractory alloys are predicted to condense at temperatures above 1600 from a gas of solar composition. Except for tungsten, they are also the expected residues of CAI oxidation. 

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If 26A1 was present in the solar system at the time refractory inclusions formed, it must have been incorporated into the major minerals together with normal 27A1, initially with an abundance ratio (26A1/27A1)0. Subsequent decay of 26A1 to 26Mg produced... 

One of the difficulties in interpreting the isotopic data from refractory inclusions has been the lack of correlation with distinctive mineralogic and textural features. This report... 

Chromium. The isotopic heterogeneity is limited to this isotope which can be compared with the normal refractory inclusions of Allende. Both Cr dehcits and excesses are formd ranging from -7.6 e to +210 e (Fig. 8b). The fractions showing the highest enrichment in Cr with no correlated effects in Cr, Cr, Cr points towards a nucleosynthetic component, which is 99% pure in Cr. This component is probably the same as the component found in the CV3 inclusions, and which is produced in a neutron-rich nuclear statistical equilibrium in presupemova massive stars. 

Birck JL, Allegre CJ (1984) Chromium isotopic anomalies in Allende refractory inclusions. Geophys Res Lett 11 943-946... 

Hinton RW, Davis AM, Scatena-Wachel DE (1987) Large negative Ti anomalies in refractory inclusions from the Murchison carbonaceous chondrite evidence for incomplete mixing of neutron-rich supernova ejecta into the solar system. Astrophys J 313 420-428... 

Hutcheon ID, Steele IM, Wachel DES, MacDougall JD, Phinney D (1983) Extreme Mg fractionation and evidence of Ti isotopic variations in Murchison refractory inclusions. Lunar Planet Sci XIV 339-340 Hutcheon ID, Hutchison R (1989) Evidence from the Semarkona ordinary chondrite for A1 heating of small planets. Nature 337 238-241... 

MacPherson GJ, Bar-Matthews M, Tanaka T, Olsen E, Grossman L (1983) Refractory inclusions in the Murchison meteorite. Geochim Cosmochim Acta 47 823-839 MacPherson GJ, Wark DA, Armstrong JT (1988) Primitive material surviving in chondrites refractory inclusions. In Meteorites and the Early Solar System. Kerridge JF, Matthews MS (eds) University of Arizona Press, Tucson, p 746-807... 

Niederer PR, Papanastassiou DA, Wasserburg GJ (1981) The isotopic composition of titanium in the Allende and Leoville meteorites. Geochim Cosmochim Acta 45 1017-1031 Niederer PR, Papanastassiou DA (1984) Ca isotopes in refractory inclusions. Geochim Cosmochim Acta 48 1279-1293... 

Davis AM, MaePherson GJ (1996) Thermal processing in the solar nebula Constraints from refractory inclusions. In Chondrules and the Protoplanetary Disk. Hewins RH, Jones RH, Scott ERD (eds) Cambridge University Press, New York, p 71-76... 

Ireland TR, Fahey AJ, Zinner EK (1991) Hibonite-bearing microspherules a new type of refractory inclusions with large isotopic anomalies. Geochim Cosmochim Acta 55 367-379 Johnson CM, Beard BL (1999) Correction of instrumentally produced mass fractionation during isotopic analysis of Fe by thermal ionization mass spectrometry. Int J Mass Spect 193 87-99 Jungck MHA, Shimamura T, Lugmair GW (1984) Calcium isotope variations in Allende. Geochim Cosmochim Acta 48 2651-2658... 

In practice, it is not sufficient for an object to have an isotopic composition that cannot be explained by radioactive decay or mass-dependent fractionation effects. The object must also have physical and chemical characteristics making it unlikely to be a product of solar system processes. For example, millimeter- to centimeter-sized refractory inclusions from primitive chondrites have been shown to contain small (parts in 103 to 104) isotopic anomalies in many elements. However, based on the size, composition, physical characteristics, and abundance of the inclusions, it is generally believed that these objects formed within the solar system. They preserve small isotopic anomalies because they did not form from a representative sample of the bulk solar system (see Chapters 7 and 14). So, isotopic anomalies can indicate either that an object is itself presolar or that it formed in the solar system from precursor material that was not fully homogenized in the solar system. As mass spectrometry has become more precise, small isotopic anomalies of the second type have shown up in a wide variety of chondritic materials. As we discuss below and in Chapter 7, these anomalies and bona fide presolar grains can be used as probes of processes in the early solar system. 

Cosmochemistry is the study of the chemical compositions of various solar system materials. Chondrites are the most abundant primitive samples. They are essentially sedimentary rocks composed of mechanical mixtures of materials with different origins (chondrules, refractory inclusions, metal, sulfide, matrix), which we will call components. Chondrites formed by the accretion of solid particles within the solar nebula or onto the surfaces of growing planetesimals. They are very old (>4.5 billion years, as measured by radioactive chronometers) and contain some of the earliest formed objects in the solar system. Chondrites have bulk chemical compositions very similar to the solar photosphere, except... 

The chemical compositions of individual chondrules have been determined by neutron activation of extracted samples or by electron microprobe analyses of chondrules in situ. Some, but not all chondrules are depleted in moderately volatile elements. There is a compositional continuum between the olivine-rich and aluminum-rich chondrules. Original concentrations of the short-lived radionuclide 26A1 in chondrules suggest they formed very early, before all of this isotope decayed, but as much as 2-5 million years after the formation of CAIs (see Refractory Inclusions, below). 

Another chondrite component is an optically opaque (in thin section) assortment of very fine-grained minerals that fills the spaces between the larger chondrules, refractory inclusions, and metal grains. This material is called matrix. Characterization of matrix minerals is hampered by their tiny particle sizes (as small as 50-100 nm). Moreover, the fine grain sizes, high porosity, and permeability of matrix make it especially susceptible to alteration during later heating or exposure to aqueous fluids. 

Chondrules, refractory inclusions, metal, troilite, and matrix have been assembled in varying proportions to form chondrites. Some microscopic images of chondrite thin sections are illustrated in Figure 6.2. 

Russell, S. S., Davis, A. M., MacPherson, G. J., Guan, Y. and Huss, G. R. (2000) Refractory inclusions from the ungrouped carbonaceous chondrites MacAlpine Hills 87300 and 88107. Meteoritics and Planetary Science, 35, 1051-1066. 

Thrane, K., Bizzarro, M. and Baker, J. A. (2006) Extremely brief formation interval for refractory inclusions and uniform distribution of Al-26 in the early solar system. Astrophysical Journal, 646, L159-162. 

In this chapter, we review what is known about the chronology of the solar system, based on the radioisotope systems described in Chapter 8. We start by discussing the age of materials that formed the solar system. Short-lived radionuclides also provide information about the galactic environment in which the solar system formed. We then consider how the age of the solar system is estimated from its oldest surviving materials - the refractory inclusions in chondrites. We discuss constraints on the accretion of chondritic asteroids and their subsequent metamorphism and alteration. Next, we discuss the chronology of differentiated asteroids, and of the Earth, Moon, and Mars. Finally, we consider the impact histories of the solar system bodies, the timescales for the transport of meteorites from their parent bodies to the Earth, and the residence time of meteorites on the Earth s surface before they disintegrate due to weathering. 

Bizzarre, M., Baker, J. A. and Haack, H. (2004) Mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions. Nature, 431, 275-278. 

Figure 12.17a shows lithophile element abundances, and Figure 12.17b shows sid-erophile and chalcophile element abundances in CM chondrites, normalized to Cl chondrites. Illustrated for comparison are the abundances in CO chondrites, which are the anhydrous carbonaceous chondrite group most closely allied to CM chondrites. As in other chondrites, the greatest differences are in volatile elements. The volatile and moderately volatile elements in CM chondrites are present at 50-60% of the abundances of the refractory elements. The volatile elements are primarily located in the matrix, and the matrix comprises 50-60% of CM chondrites. This implies that the matrix has essentially Cl abundances of all elements, while the chondrules and refractory inclusions have Cl relative abundances of refractory elements but are highly depleted in the volatile elements. The sloping transition in the region of moderately volatile elements indicates... 

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