LINK<\/strong><\/a><\/p>\n\n\n\n“Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC3). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70\u201380% are ureilites (achondrites) and 20\u201330% are various types of chondrites. Asteroid 2008 TC3 was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC3. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 \u03bcm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal\u2010sulfide, as well as chondrules (~130\u2013600 \u03bcm) and chondrule fragments. The C1 material consists of fine\u2010grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28\u201042), an unidentified Ca\u2010rich silicate phase, Fe,Ni sulfides, and minor Ca\u2010phosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (\u03b418O = 13.53\u2030, \u03b417O = 8.93\u2030) is unlike that of any known chondrite, but similar to compositions of several CC\u2010like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75\u201088), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal\u2010sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and\/or CC, based on mineral and oxygen isotope compositions. Some of the metal\u2010sulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04\u20130.05) and relatively featureless in VNIR, and have an ~2.7 \u03bcm absorption band due to OH\u2212 in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the F\u2010type spectrum of the asteroid, suggests that 2008 TC3 consisted mainly of ureilitic and AhS 91A\u2010like materials, with as much as 40\u201370% of the latter, and <10% of OC, EC, and other meteorite types. The bulk density of AhS 91A (2.35 \u00b1 0.05 g cm\u22123) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7\u20132.2 g cm\u22123). Its porosity (36%) is near the low end of estimates for the asteroid (33\u201350%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 TC3 in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when a CC\u2010like body impacted into already well\u2010gardened ureilitic + impactor\u2010derived debris. AhS 91A bulk samples do not show a solar wind component, so they represent subsurface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5\u20139 Ma) than previously studied AhS stones (11\u201322 Ma). The spread in CRE ages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 \u03bcm absorption bands.”<\/p>\n","protected":false},"excerpt":{"rendered":"
Cyrena Anne Goodrich, Michael E. Zolensky, Anna Maria Fioretti, Muawia H. Shaddad, Hilary Downes, Takahiro Hiroi, Issaku Kohl, Edward D. Young, Noriko T. Kita, Victoria E. Hamilton, My E. I. Riebe, Henner Busemann, Robert J….<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[53,9,10,7,39,47],"tags":[460,1837,1854,357,1809,1849,2660,3515,359,245],"_links":{"self":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/20563"}],"collection":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=20563"}],"version-history":[{"count":1,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/20563\/revisions"}],"predecessor-version":[{"id":20564,"href":"https:\/\/karmaka.de\/index.php?rest_route=\/wp\/v2\/posts\/20563\/revisions\/20564"}],"wp:attachment":[{"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=20563"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=20563"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/karmaka.de\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=20563"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}