Origin of compact type A refractory inclusions from CV3 carbonaceous chondrites |
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| Institution: | 1. Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois, USA;2. Enrico Fermi Institute, The University of Chicago, Chicago, Illinois, USA;1. WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53706, USA;2. Chemistry Division, Nuclear and Radiochemistry, Los Alamos National Laboratory, MSJ514, Los Alamos, NM 87545, USA;3. National Institute of Polar Research, Tokyo 190-8518, Japan;4. Kochi Institute for Core Sample Research, JAMSTEC, 200 Monobe-otsu, Nankoku, Kochi 783-8502, Japan;5. Fi Group, Direction Scientifique, 14 terrasse Bellini, 92800 Puteaux, France;6. Institute of Earth Sciences, Heidelberg University, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany;1. WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53706, USA;2. Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 200 Monobe-otsu, Nankoku, Kochi 783-8502, Japan;3. Chemistry Division, Nuclear and Radiochemistry, Los Alamos National Laboratory, MSJ514, Los Alamos, NM 87545, USA;4. Department of Earth and Planetary Science, Graduate school of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan;1. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA;2. Hawai′i Institute of Geophysics and Planetology, University of Hawai′i at Mānoa, Honolulu, HI 96822, USA;3. Geoscience Institute/Mineralogy, Goethe University Frankfurt, Germany;4. High Pressure Science and Engineering Center and Department of Geoscience, University of Nevada, Las Vegas, NV 89154, USA;5. Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA;6. Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA;7. Institut für Planetologie, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany;1. Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA;2. Chicago Center for Cosmochemistry, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA;3. Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA;4. Origins Laboratory, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA;5. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA |
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| Abstract: | Compact type A (CTA) inclusions are one of the major types of coarse-grained refractory inclusions found in carbonaceous chondrites. They have not been studied in a systematic fashion, leading to some uncertainties and unproven assumptions about their origin. To address this situation, we studied a total of eight CTAs from Allende, Efremovka and Axtell by scanning electron-microscopic and electron and ion-microprobe techniques. These inclusions are very melilite-rich, ranging from ～60 vol% to nearly monomineralic. Also present are Mg–Al spinel (5–20%), perovskite (trace–～3%) and, in some samples, Ti-rich (～17 wt% TiO2tot) fassaite (trace–～20%), and rhönite (≤1%). Melilite compositions are mostly between Åk15 and Åk40. Chondrite-normalized REE abundance patterns for melilite (flat at ～10 × CI with positive Eu anomalies) and fassaite (slight HREE enrichment relative to LREE and negative Eu anomalies) are like those for their counterparts in once-molten type B inclusions. The patterns for rhönite have positive slopes from La through Lu and abundances <10 × CI for La and 35–60 × CI for Lu. Features of CTAs that suggest that they were once molten include: rounded inclusion shapes; positively correlated Sc and V abundances in fassaite; radially oriented melilite laths at inclusion rims; and the distribution of trace elements among the phases. Fractional crystallization models show that, with one exception, the REE contents of perovskite and fassaite arose by crystallization of these phases from late, residual liquids that would have resulted from prior crystallization of the observed proportions of melilite and spinel from liquids having the bulk compositions of the inclusions. One Allende CTA (TS32), however, has several features (irregular shape, reversely zoned melilite, fassaite REE contents) that are not readily explained by crystallization from a melt. This inclusion may have undergone little melting and may be dominated by relict grains. |
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