Modal abundances of CAIs: Implications for bulk chondrite element abundances and fractionations

Modal abundances of Ca,Al-rich inclusions (CAIs) are poorly known and reported data scatter across large ranges. CAIs are Poisson distributed, and if only small areas (<1000 mm2) are studied, the data are probably not representative of the true CAI modal abundances, explaining their reported large scatter in a single chondrite group. We combine reported CAI modal abundances and our own set, and present a complete list of CAI modal abundances in carbonaceous chondrites. This includes (in area%): CV: 2.98, CM: 1.21, Acfer 094: 1.12, CO: 0.99, CK/CV (Ningqiang and Dar al Gani [DaG] 055): 0.77, CK: 0.2, CR: 0.12 and CB: 0.1. CAIs are Poisson distributed and if only small areas are studied, the data are probably not representative of the true CAI modal abundances, Carbonaceous chondrites have excess bulk Al concentrations when compared to the CI-chondritic value. We find a correlation between this excess and CAI modal abundances and conclude that the excess Al was delivered by CAIs. The excess Al is only a minor fraction (usually ~10 rel%, but 25 rel% in case of CVs) of the bulk chondrite Al and cannot have contributed much 26Al to heat the chondrite parent body. Ordinary, enstatite, R and K chondrites have an Al deficit relative to CI chondrites and only very low CAI modal abundances, if any are present at all. Carbonaceous chondrites also had an initial Al deficit if the contribution of Al delivered by CAIs is subtracted. Therefore all chondrites probably lost a refractory rich high-T component. Only minor amounts of CAIs are present in the matrix or have been present in the chondrule precursor aggregates. Most CAI size distributions contain more than one size population, indicating that CAIs from within a single meteorite group had different origins.

[1]  A. Rubin.  Correlated petrologic and geochemical characteristics of CO3 chondrites , 1998 .

[2]  G. Huss,et al.  Petrogenesis of Al-rich chondrules: Evidence from bulk compositions and phase equilibria , 2005 .

[3]  N. Takaaki.  Petrology and mineralogy of CK chondrites: Implications for the metamorphism of the CK chondrite parent body , 1993 .

[4]  K. Lodders.  Solar System Abundances and Condensation Temperatures of the Elements , 2003 .

[5]  A. Rubin,et al.  The compositional classification of chondrites: V. The Karoonda (CK) group of carbonaceous chondrites , 1991 .

[6]  E. Jarosewich,et al.  The Allende, Mexico, meteorite shower , 1971 .

[7]  K. McKeegan,et al.  Calcium‐aluminum‐rich inclusions and amoeboid olivine aggregates from the CR carbonaceous chondrites , 2002 .

[8]  S. Simon,et al.  A preferred method for the determination of bulk compositions of coarse-grained refractory inclusions and some implications of the results , 2004 .

[9]  K. McKeegan,et al.  MINERALOGY, PETROGRAPHY AND OXYGEN-ISOTOPIC COMPOSITIONS OF Ca, Al-RICH INCLUSIONS AND AMOEBOID OLIVINE AGGREGATES IN THE CR CARBONACEOUS CHONDRITES , 2001 .

[10]  H. Palme,et al.  The conditions of chondrule formation, Part I: Closed system , 2007 .

[11]  A. Rubin,et al.  Chondrules, matrix and coarse-grained chondrule rims in the Allende meteorite: Origin, interrelationships and possible precursor components , 1987 .

[12]  M. Kimura,et al.  Hydrous and anhydrous alterations of chondrules in Kaba and Mokoia CV chondrites , 1998 .

[13]  D. Sears,et al.  The Colony meteorite and variations in CO3 chondrite properties , 1985 .

[14]  H. McSween.  Carbonaceous chondrites of the Ornans type - A metamorphic sequence , 1977 .

[15]  A. Kearsley,et al.  THE RELATIONSHIP BETWEEN CK AND CV CHONDRITES: A SINGLE PARENT BODY SOURCE? , 2004 .

[16]  Bruce Fegley,et al.  The Planetary Scientist's Companion , 1998 .

[17]  J. Armstrong,et al.  Primitive material surviving in chondrites: refractory inclusions. , 1988 .

[18]  Lutz Nasdala,et al.  Origin of SiO2-rich components in ordinary chondrites , 2006 .

[19]  S. Richardson,et al.  The composition of carbonaceous chondrite matrix , 1977 .

[20]  H. McSween.  Petrographic variations among carbonaceous chondrites of the Vigarano type , 1977 .

[21]  A. Rubin.  Relatively Coarse-Grained Chondrule Rims in Type 3 Chondrites , 1984 .

[22]  Dominik C. Hezel,et al.  Constraints for chondrule formation from Ca–Al distribution in carbonaceous chondrites , 2008 .

[23]  G. Wasserburg,et al.  An isotopic and petrologic study of calcium-aluminum-rich inclusions from CO3 meteorites , 1998 .

[24]  E. Jarosewich,et al.  Chemical analyses of meteorites: A compilation of stony and iron meteorite analyses , 1990 .

[25]  J. Wood,et al.  Petrography and classification of Ca, Al‐rich and olivine‐rich inclusions in the Allende CV3 chondrite , 1984 .

[26]  J. Shelley,et al.  Chondrules with Peculiar REE Patterns: Implications for Solar Nebular Condensation at High C/O , 2004, Science.

[27]  J. Zipfel,et al.  Bulk chemistry and mineralogy of a new "Unique" metal-rich chondritic breccia, Hamadah Al Hamra 237 , 1998 .

[28]  A. Davis,et al.  Calcium-Aluminum-rich Inclusions: Major Unanswered Questions , 2005 .

[29]  Susanne Klerner.  Materie im frühen Sonnensystem: Die Enstehung von Chondren, Matrix und refraktären Forsteriten. , 2001 .

[30]  A. Kearsley,et al.  Volatile fractionation in the early solar system and chondrule/matrix complementarity. , 2005, Proceedings of the National Academy of Sciences of the United States of America.