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Ancient Asteroids Enriched in Refractory Inclusions J Ancient Asteroids Enriched in Refractory Inclusions J. M. Sunshine, et al. Science 320, 514 (2008); DOI: 10.1126/science.1154340 The following resources related to this article are available online at www.sciencemag.org (this information is current as of April 25, 2008 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/cgi/content/full/320/5875/514 Supporting Online Material can be found at: http://www.sciencemag.org/cgi/content/full/1154340/DC1 A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/cgi/content/full/320/5875/514#related-content This article cites 25 articles, 6 of which can be accessed for free: http://www.sciencemag.org/cgi/content/full/320/5875/514#otherarticles on April 25, 2008 This article appears in the following subject collections: Planetary Science http://www.sciencemag.org/cgi/collection/planet_sci Information about obtaining reprints of this article or about obtaining permission to reproduce this article in whole or in part can be found at: http://www.sciencemag.org/about/permissions.dtl www.sciencemag.org Downloaded from Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2008 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS. REPORTS CV3 meteorites and more likely represent local- Ancient Asteroids Enriched in ized processing of minerals within restricted por- tions of the nebula (9). Refractory Inclusions To support our analysis of asteroid spectra, we collected representative spectra from CAIs 10 J. M. Sunshine,1* H. C. Connolly Jr.,2,3,4 T. J. McCoy,5 S. J. Bus,6 L. M. La Croix5,7 within the Allende CV3 meteorite (Fig. 1) ( ). Spectra of three type Bs, three FTAs, and one Calcium- and aluminum-rich inclusions (CAIs) occur in all classes of chondritic meteorites amoeboid olivine aggregate (AOA) were col- and contain refractory minerals predicted to be the first condensates from the solar nebula. lected (Fig. 2). The major minerals in FTAs are Near-infrared spectra of CAIs have strong 2-micrometer absorptions, attributed to iron melilite and spinel, often with abundant alteration oxide–bearing aluminous spinel. Similar absorptions are present in the telescopic spectra of phases such as nepheline and hedenbergite, several asteroids; modeling indicates that these contain ~30 ± 10% CAIs (two to three times whereas type Bs contain spinel, melilite, fassaite, that of any meteorite). Survival of these undifferentiated, large (50- to 100-kilometer diameter) anorthite, and minor amounts of alteration phases. CAI-rich bodies suggests that they may have formed before the injection of radiogenic 26Al into Both types have minor and varying amounts of the solar system. They have also experienced only modest post-accretionary alteration. Thus, these perovskite, hibonite, and metal phases. The spectra asteroids have higher concentrations of CAI material, appear less altered, and are more ancient than of both FTA and type B CAIs are dominated by m any known sample in our meteorite collection, making them prime candidates for sample return. absorptions at 2 m. However, the spectra of FTAs exhibit much stronger 2-mm absorption fea- tures than do the spectra of type Bs. This stronger alcium- and aluminum-rich inclusions abundant mafic minerals, CAIs can thereby be absorption is consistent with the higher FeO con- (CAIs) found within all chondritic mete- remotely identified on asteroid surfaces (4). centrations in aluminous spinels in FTAs: ~3 to 14 Corites are arguably the oldest rocks in our CAIs are generally classified, on the basis of weight percent (wt %) compared with <0.4 wt % collections and are used to define the start of the petrography and geochemistry, into three major in type Bs. Given that abundant FeO is ef- solar system (1). These millimeter-to-centimeter groups: type As, Bs, and Cs (5). Of these, only fectively excluded from CAI minerals during size objects contain refractory minerals that are fluffy type As (FTAs) were not melted by a tran- condensation, the FeO must have been intro- the first phases predicted to condense from a gas sient heating event before accretion (6). Type As duced into the CAIs during an alteration event. of solar (or enhanced solar) composition. The and related inclusions are found in all classes of Although some FeO can be introduced into type on April 25, 2008 highest abundance of CAIs is found in CO chon- chondrites and are therefore thought to have been A and B CAIs through gas/solid exchange within drites (<13 volume %), but CV3 chondrites contain well dispersed within the asteroid accretion zone. the nebula, the observed correlation between FeO the most diverse range of CAI types with abun- Refractory minerals similar to those found in content in spinel and the abundance of alteration dances of up to 10 volume % (2). Burbine et al. FTAs have also been identified in a Stardust sam- phases between CAI-types imply that the FeO en- (3) identified CAIs on two dynamically related ple collected from comet 81P/Wild 2, providing richment is dominantly post-accretionary. asteroids, 387 Aquitania and 980 Anacostia, that evidence that these materials were widely distrib- To verify the identifications of Burbine et al. they argued contain CAI abundances of 5 to 10%, uted throughout the solar system (7). Recent (3) and locate additional spinel-rich asteroids, we similar to that observed in CV3 meteorites. Be- models of the evolution of protoplanetary disks combined visible spectra (11, 12)andnear- cause the early solar nebula evolved quickly and predict widespread, outward transport of such infrared data obtained with the SpeX instrument www.sciencemag.org was spatially heterogeneous, it is reasonable to high-temperature materials around the midplane at the NASA Infrared Telescope Facility (13). assume that asteroids with higher concentrations (8). In contrast, type Bs and Cs are restricted to SpeX data of Aquitania and Anacostia are of CAIs should exist. Here, we provide evidence for the existence of several such CAI-rich aster- oids from multiple parent bodies. Near-infrared telescopic spectral surveys have widely been used to identify mafic silicates on Downloaded from the surfaces of asteroids and to infer the compo- sition of the asteroid population as a whole. The only mineral contained in CAIs that has strong absorptions in the 1- to 2-mm region is aluminous spinel (MgAl2O4, sensu stricto). If aluminous spinel (in solid solution with hercynite, FeAl2O4) contains as little as tenths of a weight percent of FeO, its spectrum includes a strong, characteristic 2-mm absorption feature. In the absence of 1Department of Astronomy, University of Maryland, College Park, MD 20742, USA. 2Department of Physical Sciences, Kingsborough Community College of the City University of New York, Brooklyn, NY 11235, USA. 3Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA. 4Department of Earth and Planetary Sciences, American MuseumofNaturalHistory,NewYork,NY11024,USA. 5 Department of Mineral Sciences, National Museum of Natural Fig. 1. Smithsonian sample 3509 of the CV3 meteorite Allende sawed into ~1-cm-thick slabs (a History, Smithsonian Institution, Washington, DC 20560, USA. “ ” 6Institute for Astronomy, University of Hawaii, Hilo, HI 96720, centimeter-sized cube marked T is included for scale). Refractory inclusions suitable for both thin USA. 7Department of Geological Sciences and Engineering, sectioning and crushing into powders were identified as shown here for slabs 5 and 6. After recording University of Nevada, Reno, NV 89557, USA. their locations, CAIs were cored out from slabs, and both thin sections and thick butts were produced. Part *To whom correspondence should be addressed. E-mail: of the remaining sample was carefully excavated to avoid contamination from matrix materials and [email protected] crushed into powders for spectral measurements. 514 25 APRIL 2008 VOL 320 SCIENCE www.sciencemag.org REPORTS dominated by a strong 2-mm absorption feature tions, supporting the hypothesis of a breakup of a Although they favored a link between spinel- and confirm the previous data (Fig. 3A). On the larger body. rich asteroids and CAIs, Burbine et al.(3) could basis of their unique compositions and general Visible data also suggest a second group of not definitively rule out an alternative spectral location, it was suggested that these two asteroids strongly sloped asteroids, the Henan family (15), interpretation: that the spinel was igneous in ori- (100- and 85-km diameter, respectively) are which was originally identified dynamically (16). gin and due to the spinel group mineral chromite genetically related (3). Shortward of 0.75 mm, Near-infrared SpeX data of ~10 much smaller (FeCr2O4). Spectra of FeO bearing aluminous the spectra of these two asteroids are also strongly (10 to 15 km), and thus fainter, Henan family spinels are distinct from that of chromites (18, 19). red-sloped, as are a larger number of nearby asteroids also contain 2-mm absorptions (Fig. 3B), Chromite spectra include a strong absorption at objects (14). This cluster of asteroids, including suggesting the existence of a second spinel-rich 1.3 mm, and the reflectance is lower from 2 to Aquitania and Anacostia, was proposed as the asteroid family. In addition, we have identified a 2.5 mm than it is for FeO-bearing aluminous Watsonia family (15). We measured the near- third spinel-rich body, the 45-km diameter as- spinels. Spectra of Watsonia and Henan family infrared spectra of other members of the Watsonia teroid 234 Barbara (17). The spectrum of Barbara members and Barbara lack strong 1.3-mm fea- family.
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