Refractory inclusions CAIs

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... 

Chondrites, the most primitive of all meteorites, formed in dynamic energetic, dust-rich zones in the solar nebula. In this environment, dust/gas ratios were constantly changing, temperatures fluctuated through 1,000 K, with multiple cycles of melting, evaporation, condensation, and aggregation. In addition there were influxes of matter from the interstellar dust and the periodic removal of batches of chondritic material to small planetesimals. In this section we explore how the most primitive materials of the solar system were formed and what they can tell us about processes during the condensation of the solar nebula. These materials include chondrules, refractory inclusions (CAIs), and amoeboid olivine aggregates (AOAs), the oldest component parts of chondritic meteorites. 

The oldest matter of the solar system is found as refractory inclusions (CAIs) and chondrules in carbonaceous chondrite meteorites. The oldest refractory inclusions from the Allende meteorite have been dated at 4,567 Ma (Amelin et al., 2002) and formed over an interval of less than a million years. T0 therefore is 4,567 Ma. The oldest chondrules formed at the same time, at 4,567 Ma but their period of formation lasted longer, until about 4,563 Ma (Amelin et al., 2004 Haack et al, 2004). 

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). 

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 most common type of refractory inclusions are the calcium-aluminum-rich inclusions (CAIs) whose mineralogy is consistent with the minerals that are... 

Shortly after the condensation of the first minerals to form within the Solar System, thermal processing of refractory inclusions began. The overall processing of these materials into igneous rocks was short-lived, perhaps only 100000 years (Bizzarro et al. 2007). The critical constraints on the thermal histories of these objects is essentially limited to Type-B CAIs (Connolly Desch 2004), which have been generated by the experimental reproduction of these objects in the laboratory. Type-B CAIs are composed mostly of melilite, anorthite, aluminous spinel, and a titanium-rich pyroxene known in the parlance as fassaite (for a summary of minerals common to the Solar System see Appendix 1). The general consensus is... 

Chondrites consist of four major components chondrules, FeNi-metal, refractory inclusions (Ca-Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)), and fine-grained matrix material. It is generally accepted that the refractory inclusions, chondrules, and FeNi-metal are formed in the solar nebula by high-temperature processes that included condensation and evaporation. Many CAIs and most chondrules and FeNi-metal were subsequently melted during multiple brief heating episodes. Matrix, some CAIs, and metal in some chondrites (e.g., CH and CB) appear to have escaped these high-temperature nebular... 

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. 

Figure 5 Backscattered electron images of refractory inclusions with clear condensation signatures (a) CAI in Adelaide (unique carbonaceous chondrite), (b) CAI in Efremovka (CV3), and (c) AOA in Acfer 094 (unique carbonaceous chondrite).

Calcium-41 decays by electron capture to with a half-life of only 103 kyr. It has the distinction of being the shortest-lived isotope for which firm evidence exists in early solar system materials, and this fact makes it key for constraining the timescale of last nucleosynthetic addition to solar system matter (in the external seeding scenario). It also makes " Ca exceedingly difficult to detect experimentally, because it can only be found to have existed in the oldest materials and then in only very small concentrations. Fortunately, its daughter potassium is rather volatile and calcium is concentrated in refractory minerals (the C in CAI) leading to large fractionations. Hutcheon et al. (1984) found hints for " Ca in Allende refractory inclusions, but could not clearly resolve excesses above measurement uncertainties. 

The existence of a canonical ( Al/ Al)o value was previously based on analyses of CAIs only from carbonaceous chondrites refractory inclusions from ordinary and enstatite chondrites are rare and often very small, and thus few had been discovered and none analyzed. There are now data for four CAIs from unequilibrated ordinary chondrites (Russell et al., 1996 Huss et al., 2001) and for 11 hibonite-bearing inclusions from enstatite chondrites (Guan et al., 2000) all are consistent with ( Al/ Al)o in the range (3.5-5.5) X 10 , except for 4 of the (very small) hibonite grains for which Mg could not be resolved. Thus, the same canonical value characterizes CAIs from all major meteorite classes. The possible meaning of this confirmation in terms of nebular chronology based on Al is not completely straightforward, however. 

The refractory inclusions provide the best samples since they incorporated all three of these radioisotopes, as well as °Be. It has already been mentioned that " Ca and A1 are highly correlated in CAIs and hibonite grains (Figure 2). At face value, this would imply the same source for both these refractory elements. A problem with "Ca, however, is that its abundance is only marginally above detection limits and it decays very quickly, so that there is essentially no chance to test for concordant decay between the " Ca and A1 systems. This is not the case for A1 and Be, which exist in much higher abundances and which have half-lives that differ by only a factor of two. 

There are other refractory inclusions, e.g., grossite-bearing CAIs from CH chondrites (Weber et aL, 1995), that do not fit this model since they lack calcium and titanium isotopic anomalies as well as Al. One interpretation of such objects could be that CAI formation lasted several million years, but this is not supported by any independent evidence and there could well be other reasons for the lack of both short-lived radioactivity and large isotopic anomalies (aside from excesses Sahijpal et al., 1999) in these inclusions. Circumstantial arguments against a long time period for CAI formation are that it... 

CAFs - refractory inclusions, or Ca-Al-Inclusions, up to 2 cm across, enriched in Si-poor, Ca-Al-rich minerals. The most abundant source of CAIs is the Allende meteorite, which fell in 1969. 

Probably the most informative objects in meteorites are the refractory, calcium-aluminum-rich inclusions (CAIs). They are sub-millimeter- to centimeter-sized objects found in all types of primitive (chondritic) meteorites. On the basis of their uranium/lead radiometric ages, they are believed to be the first-formed rocks in the Solar System 4). Their chemical compositions are consistent with equilibrium condensation as solids from a gas of solar composition at high temperatures 1700 K). The major mineral phases are spinel (MgAl204), pyroxene (Mg, Ca, Al, Ti silicate), melilite (another Mg, Ca, A1 silicate), and anorthite (CaAl2Si20s). They are enriched in refractory (less volatile) trace elements, such as the rare-earth elements, by a factor of 15-20 (5), reflecting their high temperature of condensation. The abundances of the three stable isotopes of oxygen exhibit a pattern not seen in any terrestrial rocks (6). On earth, ratios of abundances of isotopes, such as and vary by... 

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