Calcium chondrites

The largest class of meteorite finds is stony meteorites, made principally of stone. The general stony classification is divided into three subclasses called chondrites, carbonaceous chondrites and achondrites, and it is at this level of distinction at which we will stop. Before looking at their mineral and isotopic structure in more detail, it is useful to hold the composition of the Earth s crust in mind here for comparison. The Earth s crust is 49 per cent oxygen, 26 per cent silicon, 7.5 per cent aluminium, 4.7 per cent iron, 3.4 per cent calcium, 2.6 per cent sodium, 2.4 per cent potassium and 1.9 per cent magnesium, which must have formed from the common origin of the solar system. 

Huss GR, MacPherson GJ, Wasserburg GJ, Russell SS, Srinivasan G (2001) Aluminum-26 in calcium-aluminum-rich inclusions and chondrales from unequilibrated ordinary chondrites. Meteorit Planet Sci... 

MacPherson GJ, Huss GR, Davis AM (2003) Extinct Be in type A calcium-aluminum-rich inclusions from CV chondrites. Geochim Cosmochim Acta 67 3615-3179 Matthews GJ, Cowan JJ (1990) New insights into the astrophysical r-process. Nature 345 491-494 McCulloch MT, Wasserburg GJ (1978a) Barium and neodymium isotopic anomalies in the Allende meteorite. Astrophys J 220 L15-L19... 

Weber D, Zinner E, Bischoff A (1995) Trace element abundances and magnesium, calcium, and titanium isotopic compositions of grossite-containing inclusions from the carbonaceous chondrite Acfer 182. Geochim Cosmochim Acta 59 803-823... 

Broken surface of the Allende meteorite, a chondrite that fell in Mexico in 1969. Note the abundant round chondrules and white calcium-aluminum inclusions. 

The aubrites are the most reduced achondrites (Keil et al., 1989). Their silicates are essentially free of iron, and they contain minor metallic iron. A variety of unusual sulfides of calcium, chromium, manganese, titanium, and sodium - all usually lithophile elements -occur in aubrites. These unusual sulfides also characterize the highly reduced enstatite chondrites, which may have been precursors for these rocks. 

Srinivasan, G., Huss, G. R. and Wasserburg, G. J. (2000) A petrographic, chemical and isotopic study of calcium-aluminum inclusions and aluminum-rich chondrules from the Axtell (CV3) chondrite Meteoritics and Planetary Science, 35, 1333-1354. 

The peak temperatures that asteroids experienced can be estimated from chemical exchange reactions between minerals - so-called geothermometers. For example, the exchange of calcium between coexisting orthopyroxene and clinopyroxene in highly metamorphosed chondrites has been used to estimate their equilibration temperatures (Slater-Reynolds and McSween, 2005). For ordinary chondrites, these temperatures range up to -1175 K. The experimental conditions at which achondrites melt provide minimum temperatures for their parent bodies. Melting of achondrites typically requires temperatures of>1200 K. 

MacPherson, G. J., Simon, S. B., Davis, A. M., Grossman, L. and Krot, A.N. (2005) Calcium-aluminum-rich inclusions major unanswered questions. In Chondrites and the Protoplanetary Disk, ASP Conference Series, 341, eds. Krot, A. N., Scott, E. R. D. and Reipurth, A. San Francisco Astronomical Society of the Pacific, pp. 225-250. 

The most primitive chondrites consist of coarse-grained (mm-sized) mineral assemblages embedded in fine-grained (10 nm-5 pm) matrix material (see Fig. 1.2). The coarse-grained chondritic components are diverse in their composition and mineralogy and include calcium-aluminum-rich inclusions (CAIs), amoeboid olivine aggregates (AOAs), Al-rich chondrules, Fe-Mg chondrules, Fe-rich metals, and iron sulfides. The CAIs are composed largely of calcium, aluminum, and titanium... 

Calcium-aluminum-rich refractory inclusions (CAIs) in chondrites are the oldest Solar System solids (2-3 Myr older than chondrules, see Chapters 1 and 9). The mineralogy, petrographic, chemical, and isotopic characteristics of these primitive solids constrain the physical and chemical processes through which these materials have been processed (Zhu et al. 2001 Becker Walker 2003 Biz-zarro et al. 2004 Chaussidon et al. 2008). The primordial presolar grains were... 

Ma (Wadhwa et al. 2007 and references therein), which is actually the age of a group of inclusions within chondrites known as calcium-aluminum-rich inclusions (CAIs). The word primitive refers to the fact that the bulk compositions of all chondrites, within a factor of two, are solar in composition for all but the most volatile elements (Weisberg et al. 2006). This fact indicates that chondrites have not been through a planetary melting or differentiation process in their parent body, indicating that they have recorded the materials that were present and the processes that operated within the disk before or during planet formation. 

The terrestrial planets and the Moon are differentiated, with dense iron-rich cores and rocky mantles. The uncompressed densities of Earth and Venus are similar. Mercury has a high density which suggests it has relatively large core. Conversely, the Moon has a low density, indicating a very small core. There is little observational evidence that asteroids are differentiated except for Vesta and Ceres (Thomas et al. 2005). However, iron meteorites from the cores of differentiated asteroids are quite common, and the irons found to date come from several dozen different parent bodies (Meibom Clark 1999). Most meteorites come from asteroids that never differentiated. These chondritic meteorites consist of intimate mixtures of heterogeneous material millimeter-sized rounded particles that were once molten, called chondrules, similarly sized calcium-aluminum-rich inclusions (CAIs), and micrometer-sized matrix grains. 

The MIF phenomenon was first observed by Clayton in 1973 for the isotopic oxygen content in the earliest solids in the solar system, the so-called calcium-aluminum-rich inclusions (CAIs) in carbonaceous chondritic meteorites [1]. The slope of versus plot for the CAIs was close to unity, the CAIs being equally deficient in the heavy O isotopes, deficient in the S notation sense, while the ozone is equally enriched in those isotopes in that sense, as in Figure 2.2. Both are examples of an MIF. Interest in this striking phenomenon for the CAIs is motivated by what it may reveal about the formation of the early solar system. Standard reaction rate transition state theory [3], and behavior of oxygen an other isotope fractionation in many other systems, would have led, instead, to the slope... 

Besmehn A. and Hoppe P. (2001) Silicon- and calcium-iso-topic compositions of presolar sihcon nitride grains from the Indarch enstatite chondrite. In Lunar Planet. Sci. XXXII, 1188.TheLunarandPlanetary Instimte,Houston(CD-ROM). 

In Figure 3, aluminum is representative of refractory elements in general and the Al/Si ratios indicate the size of the refractory component relative to the major fraction of the meteorite. It is clear from this figure that the Al/Si ratio of Cl meteorites agrees best with the solar ratio, although the ratios in CM (Type 2 carbonaceous chondrites) and even OC (ordinary chondrites) are almost within the error bar of the solar ratio. The errors of the meteorite ratios are below 10%, in many cases below 5%. A very similar pattern as for aluminum would be obtained for other refractory elements (calcium, titanium, scandium, REEs, etc.), as ratios among refractory elements in meteorites are constant in all classes of chondritic meteorites, at least within —5-10%. The average Sun/CI meteorite ratio of 19 refractory lithophile elements (Al, Ca, Ti, V, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er, Lu, see Table 2) is... 

New phosphoms and titanium XRF data for Cl chondrites were reported by Wolf and Palme (2001). The change in phosphorus is significant. The new Cl abundance is 926 ppm, which is much lower than the values in the older compilations, 1,105 ppm in Palme and Beer (1993) and 1,200 ppm in Anders and Grevesse (1989), respectively. The new phosphorus contents are considered to be more reliable. The changes in titanium are small. Wolf and Palme (2001) also reported major element concentrations of Cl meteorites and other carbonaceous chondrites. Their magnesium and silicon contents were almost identical to those given by Palme and Beer (1993) 10.69% versus 10.68% (for silicon) in Palme and Beer (1993) and 9.60% versus 9.61 % (for magnesium) in Palme and Beer (1993). The aluminum, calcium, and iron concentrations... 

Based on the bulk chemistry, IDPs are divided into two groups (i) micrometer-sized chondritic particles and (ii) micrometer-sized nonchondritic particles. A particle is defined as chondritic when magnesium, aluminum, silicon, sulfur, calcium, titanium, chromium, manganese, iron, and nickel occur in relative proportions similar (within a factor of 2) to their solar element abundances, as represented by the Cl carbonaceous chondrite composition (Brownlee et al., 1976). Chondritic IDPs differ significantly in form and texture from the components of known carbonaceous chondrite groups and are highly enriched in carbon relative to the most carbon-rich Cl carbonaceous chondrites (Rietmeijer, 1992 Thomas et al., 1996 Rietmeijer, 1998, 2002). 

Fagan T. J., McKeegan K. D., Krot A. N., and Keil K. (2001) Calcium, aluminum-rich inclusions in enstatite chondrites (2) oxygen isotopes. Meteorit. Planet. Sci. 36, 223 —230. 

Guan Y., Huss G. R., MacPherson G. J., and Wasserburg G. J. (2000) Calcium-aluminum-rich inclusions from enstatite chondrites indigenous or foreign Science 289, 1330—1333. 

RusseU S. S. (1998) A survey of calcium—aluminum-rich inclusions from Rumuritiite chondrites impUcations for relationships between meteorite groups. Meteorit. Planet. Sci. 33, A131-A132. 

There are two types of refractory inclusions calcium- and aluminum-rich inclusions (this section) and amoeboid olivine aggregates (Section 1.07.5.3). Since the mineralogy, chemistry and isotope chemistry of refractory inclusions were reviewed by MacPherson et al. (1988), many new analyses have been made of CAIs in CV, CM, CO, CR, CH, CB, ordinary and enstatite chondrites that provide important constraints on physicochemical conditions, time, and place of CAI formation. CAIs are addressed in detail in Chapter 1.08, the role of condensation and evaporation in their formation in Chapter 1.15, and their clues to early solar system chronology in Chapter 1.16. 

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