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Simultaneous Accretion of Differentiated Or Metamorphosed Asteroidal Clasts and Chondrules

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Simultaneous Accretion of Differentiated Or Metamorphosed Asteroidal Clasts and Chondrules Simultaneous accretion of differentiated or metamorphosed asteroidal clasts and chondrules Anna K. SOKOL 1,3 , Addi BISCHOFF 1, Kuljeet K. MARHAS 2, Klaus MEZGER 3, and Ernst ZINNER 2 1Institut für Planetologie, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany 2McDonnell Center for the Space Sciences and Physics Department, Washington University, One Brookings Drive, St. Louis, MO 63130, U.S.A 3Institut für Mineralogie, Corrensstr. 24, 48149 Münster, Germany submitted to: Meteoritics & Planetary Science (October, 2006) 1 Abstract Recent results of isotopic dating studies ( 182 Hf-182 W, 26 Al-26 Mg) and the increasing number of observed igneous and metamorphosed fragments in (primitive) chondrites provide strong evidence that accretion of differentiated planetesimals pre-dates that of primitive chondrite parent bodies. Some very primitive chondrites (Adrar 003, Acfer 094), contain unusual fragments, that seem to have undergone high-grade thermal metamorphism; some clasts have igneous textures (granitoid and andesitic fragments in the ordinary chondrite breccias Adzhi-Bogdo and Study Butte). All these fragments indicate mixing of achondritic fragments and chondritic components. Here, we present Al- Mg isotopic data for several of these fragments. No clear evidence of radiogenic 26 Mg could be found in any of the studied fragments and 2 σ upper limits for 26 Al/ 27 Al range from 5.6 x10 -7 to 2.4 x 10 -5. The possibility that evidence for 26 Al was destroyed by parent-body metamorphism after formation is not likely, because these fragments and several other constituents of these primitive chondrites do not show any metamorphic features. Since final accretion of a planetesimal must have occurred after formation of its youngest components, formation of these parent bodies must thus have been relatively late (i.e. after most 26 Al had decayed). The absence of 26 Mg excess in the igneous inclusions does not exclude 26 Al from being a heat source for planetary melting. In large, early formed planetesimals, cooling below the closure temperature of the Al-Mg system may be too late for any evidence for live 26 Al (in the form of 26 Mg excess) to be retained. In summary, growing evidence exists that chondritic meteorites represent the products of a complex, multi-stage history of accretion, parent body modification, disruption and re-accretion. Introduction Chondrites are traditionally considered to represent the most primitive and oldest objects in the Solar System. However, it has also been suggested that certain meteorites could represent samples of “second-generation” parent bodies (daughter asteroids) resulting from collisional destruction of "grandparent planetary bodies" (e.g., Urey, 1959, 1967; Zook, 1980; Hutchison et al., 1988; Hutcheon and Hutchison, 1989; Hutchison, 1996; Sanders, 1996; Bischoff, 1998; Bischoff and Schultz, 2004; Bischoff et al., 2006; Kleine et al., 2005b). Recent studies show increasing evidence for the existence of planetesimals before the accretion of the parent asteroids of chondrites. Based on a comparison of the abundances of 182 W, the decay product of the short lived isotope 182 Hf (half-life = 9 Ma), in several iron meteorites and a Ca,Al-rich inclusion (CAI), Kleine et al. (2004, 2005a, b) concluded that most magmatic iron meteorites formed within less than ~1 Ma after the formation of CAIs. Combining age constraints derived from the Hf-W and Al-Mg systems Kleine et al. (2004, 2005a, b) argued that certain iron meteorites may have originated from the earliest accreted asteroids. This in turn implies that most chondrites have to derive from relatively late-formed bodies. Srinivasan et al. (1999), Baker et al. (2005) and Bizzarro et al. (2005) reported 26 Mg excesses resulting from the decay of 26 Al in bulk samples from some achondritic meteorites (angrites, eucrites, mesosiderites). Based on these results they argued that parent bodies of these meteorites melted while 26 Al and 60 Fe were still abundant to drive asteroid melting. Thermal modeling based on these heat sources shows that accretion of these differentiated bodies must have been completed within < 1 Ma after CAIs had formed. Since most chondrules postdate CAI formation by at least 2 Ma, accretion of some achondritic parent bodies must have predated accretion of chondrite parent bodies. Furthermore, Baker and Bizzarro (2005) reported 26 Mg deficits in meteoritic basaltic 2 material with Al/Mg ~ 0 (aubrites, ureilites and pallasites). Ages calculated from these deficits are older than those of most chondrules from chondritic meteorites. Again, these results support the conclusion that accretion of differentiated meteorites predates that of chondrule-bearing meteorites. A consequence of all these studies is that chondritic planetesimals accreted when 26 Al was no longer available in sufficient amounts to drive melting. These planetesimals may have formed by re-accretion of debris produced during collisional disruption of old first-generation planetesimals. A detailed study of numerous chondrites has revealed evidence supporting the existence of differentiated precursor planetary bodies. Possibly old lithic components occurring as foreign clasts within primitive ordinary and carbonaceous chondrites can be found in several chondrites (Sokol and Bischoff, 2006). Based on their texture these fragments may represent clasts of precursor planetesimals and their Al-Mg isotope systematic can be used to obtain relative ages on these fragments. The short-lived radionuclide 26 Al (half-life = 0.73 Ma) is useful as a chronometer in establishing time differences between important events in the solar nebula differing by only a few million years. The former presence of 26 Al can now be observed as an excess of 26 Mg, its decay product. Given a homogeneous distribution of 26 Al in the solar nebula, differences in inferred initial 26 Al/ 27 Al ratios can give information about relative formation times. The Al-Mg isotope systematics were determined for different fragments in primitive chondrites (Adrar 003, Acfer 094) and in chondritic breccias having unequilibrated components (Study Butte, Adzhi-Bogdo). These fragments must have formed through igneous processes or were modified through thermal metamorphism. The thermal overprint (melting and metamorphism) of all these fragments must have occurred under much higher temperature than that experienced by their current matrix. Thus these fragments were part of bodies with an older high-temperature evolution prior to their incorporation into chondrites. Analytical Procedures Polished thin sections of all meteorites were studied by optical microscopy in transmitted and reflected light. A JEOL 840A scanning electron microscope (SEM) was used to resolve the fine- grained texture of the rocks as well as to locate grains of feldspar with sizes suitable for ion probe analysis. Mineral analyses were obtained with a JEOL JXA-8600 S electron microprobe operating at 15 kV and a probe current of 15 nA. All data were corrected using the ZAF correction procedures. Magnesium isotope ratios and Al/Mg ratios were measured by secondary ion mass spectrometry (SIMS), with the CAMECA IMS 3f instrument and the NanoSIMS ion microprobe at Washington University in St. Louis, Missouri. Feldspar grains in granitoidal fragments from Adzhi Bogdo have sizes that are suitable for analysis with the IMS 3f instrument. Two of these fragments were measured in this study. The primary ion beam of 16 O- impinged onto the sample surface with an energy of 17 keV. The current of the focused beam was ~3.0 nA and resulted in a primary beam spot of about 10 – 15 µm. The Mg ions were collected by electron multiplier (EM) in peak jumping mode. The 27 Al ions were collected by a Faraday cup sequentially with the Mg isotopes. A mass resolving power of a ≈3500 was sufficient to eliminate all significant interferences. Because of the small sizes of feldspar grains and/or the existence of cracks in these grains from all other inclusions (from Acfer 094, Adrar 003 and Study Butte) the NanoSIMS ion microprobe was 3 used. This instrument, with its high lateral resolution and its high transmission, is well suited for the analysis of feldspar grains (Zinner et al., 2002). However, the presence of many tiny Mg-rich grains in the plagioclase and surrounding Mg-rich phases complicated the analysis. In order to avoid the high Mg signal originating from inclusions and surrounding phases, ”clean” areas were selected, and Al and Mg ion images were observed before each analysis. A 16 O- primary ion beam of a few tens of picoamperes was used to generate positive secondary ions for the analysis. The typical beam diameter was 3 – 5 µm. The measurements were carried out at a mass resolving power ~ 4000. Magnesium isotopes were measured in a peak jumping mode, while Al + was detected simultaneously (multi-detection mode). Due to very high count rates of 27 Al + in some cases, 27 Al/ 24 Mg ratios were determined independently before and after the measurement of the Mg- isotope ratio. For both instruments, the reference value used for 25 Mg/ 26 Mg was 0.12663 and 0.13932 for 26 Mg/ 24 Mg (Catanzaro et al., 1966). The nonlinear excess in 26 Mg ( δ26 Mg) was calculated by assuming a linear mass fractionation law: 26 26 25 δ Mg=∆ Mg −(2 ×∆ Mg ) i i 24 i 24 where ∆ Mg = ([ Mg/ Mg] measured /[ Mg/ Mg] reference – 1) × 1000 [per mil]; (i =25, 26) The measured 27 Al/ 24 Mg ratios were corrected for the relative sensitivity factor determined from terrestrial plagioclase standards (Miakejima Plagioclase). RESULTS Characterization of the meteorites Acfer 094, Adzhi-Bogdo, Study Butte, and Adrar 003 and selected fragments Acfer 094 This is a unique, type 3 carbonaceous chondrite that was first classified as a CO(CM) chondrite (Bischoff et al., 1991; Wlotzka, 1991). Bischoff and Geiger (1994) suggested that it may be the first CM3 chondrite. However, chemical, mineralogical, and O-isotope characteristics are unable to distinguish unambiguously between a CO3 and CM2 classification.
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