Investigating the variations in carbon and nitrogen isotopes in carbonaceous chondrites

The carbonaceous chondrites contain significant amounts of carbon- and nitrogen-bearing components, the most abundant of which is organic matter. Stepped combustion data of whole rock and HF/HCl residues of carbonaceous chondrites reveal that the organic material can be subdivided operationally into three components: (1) free organic matter (FOM), which is readily extractable from whole-rock meteorites and is enriched in 13C and 15N; (2) labile organic matter (LOM), which has a macromolecular structure but is liberated by hydrous pyrolysis; LOM is the parent structure for some FOM and is also enriched in 13C and 15N; and (3) refractory organic matter (ROM), which is also macromolecular but is virtually unaffected by hydrous pyrolysis and is relatively depleted in 13C and 15N. The macromolecular entities (LOM and ROM) are by far the most abundant organic components present, and as such, the relative abundances of the 13C- and 15N-enriched LOM and the 13C- and 15N-depleted ROM will have a major influence on the overall isotopic composition of the whole-rock meteorite. Laboratory experiments designed to simulate the effects of parent body aqueous alteration indicate that this form of processing removes LOM from the macromolecular material, allowing ROM to exert a stronger influence on the overall isotopic compositions. Hence, aqueous alteration of macromolecular materials on the meteorite parent body may have a significant control on the stable isotopic compositions of whole-rock carbonaceous chondrites. The enstatite chondrites are also carbon rich but have been subjected to high levels of thermal metamorphism on their parent body. Stepped combustion data of HF/HCl residues of enstatite chondrites indicate, that if they and carbonaceous chondrites inherited a common organic progenitor, metamorphism under reducing conditions appears to incorporate and preserve some of the 13C enrichments in LOM during graphitisation. However, when metamorphism is at its most extreme, the 15N enrichments in LOM are lost.

[1]  C. Pillinger,et al.  Carbon components and their isotopic compositions in the Allende meteorite , 1982 .

[2]  R Shipp,et al.  Isotopic characterisation of kerogen-like material in the Murchison carbonaceous chondrite. , 1987, Geochimica et cosmochimica acta.

[3]  O. Matsubaya,et al.  Carbon isotopic compositions of Antarctic carbonaceous chondrites with relevance to the alteration and existence of organic matter , 1997 .

[4]  E. Zinner.  Interstellar cloud material in meteorites , 1988 .

[5]  M. Barlow,et al.  Detection of carbonates in dust shells around evolved stars , 2002, Nature.

[6]  F. Robert,et al.  The concentration and isotopic composition of hydrogen, carbon and nitrogen in carbonaceous meteorites☆ , 1982 .

[7]  E. Zinner.  Trends in the study of presolar dust grains from primitive meteorites , 1998 .

[8]  C. Pillinger,et al.  High-precision determination of nitrogen stable isotope ratios at the sub-nanomole level , 1988 .

[9]  H. McSween.  Are carbonaceous chondrites primitive or processed? A review , 1979 .

[10]  C. Pillinger,et al.  Isotopically distinguishable carbon phases in the Allende meteorite , 1982, Nature.

[11]  Ian A. Franchi,et al.  Light dement geochemistry of the Tagish Lake CI2 chondrite: Comparison with CI1 and CM2 meteorites , 2002 .

[12]  C. Pillinger,et al.  Isotopic compositions of individual polycyclic aromatic hydrocarbons from the Murchison meteorite , 1994 .

[13]  R. Clayton,et al.  Carbon isotopes and light element abundances in carbonaceous chondrites , 1986 .

[14]  N. Grevesse,et al.  Abundances of the elements: Meteoritic and solar , 1989 .

[15]  Peter R. Buseck,et al.  Matrix mineralogy of the Orgueil CI carbonaceous chondrite , 1988 .

[16]  E. Anders,et al.  Interstellar grains in meteorites: I. Isolation of SiC, graphite and diamond; size distributions of SiC and graphite , 1994 .

[17]  I. Kaplan,et al.  Endogenous Carbon in Carbonaceous Meteorites , 1970, Science.

[18]  C. Pillinger,et al.  Compositional differences in enstatite chondrites based on carbon and nitrogen stable isotope measurements , 1986 .

[19]  G. Huss,et al.  Noble gases in presolar diamonds I: Three distinct components and their implications for diamond origins , 1994 .

[20]  E. Anders,et al.  Interstellar Grains in Primitive Meteorites: Diamond, Silicon Carbide, and Graphite , 1993 .

[21]  C. Pillinger,et al.  Separation of planetary noble gas carrier from bulk carbon in enstatite chondrites during stepped combustion , 2002 .

[22]  G. Boato.  THE ISOTOPIC COMPOSITION OF HYDROGEN AND CARBON IN THE CARBONACEOUS CHONDRITES , 1954 .

[23]  A. Kearsley,et al.  Clay mineral‐organic matter relationships in the early solar system , 2002 .

[24]  M. Sephton,et al.  Pyrolysis–gas chromatography–isotope ratio mass spectrometry of macromolecular material in meteorites , 2001 .

[25]  J. Bada,et al.  Geochromatography on the parent body of the carbonaceous chondrite Ivuna , 1991 .

[26]  Mark A. Sephton,et al.  Small-scale hydrous pyrolysis of macromolecular material in meteorites , 1998 .

[27]  S. Pizzarello,et al.  Isotopic and molecular analyses of hydrocarbons and monocarboxylic acids of the Murchison meteorite. , 1992, Geochimica et cosmochimica acta.

[28]  B. Mason.  The enstatite chondrites , 1966 .

[29]  Sherwood Chang,et al.  Organic matter in meteorites: molecular and isotopic analyses of the Murchison meteorite. , 1993 .

[30]  E. Zinner.  STELLAR NUCLEOSYNTHESIS AND THE ISOTOPIC COMPOSITION OF PRESOLAR GRAINS FROM PRIMITIVE METEORITES , 1998 .

[31]  C. Pillinger,et al.  The carbon and oxygen isotopic composition of meteoritic carbonates , 1988 .

[32]  K. Kvenvolden,et al.  Organic Matter in Meteorites , 1972 .

[33]  C. Pillinger,et al.  A carbon and nitrogen isotope study of diamond from primitive chondrites , 1996 .

[34]  C. T. Pillinger,et al.  Aromatic moieties in meteoritic macromolecular materials: analyses by hydrous pyrolysis and δ13C of individual compounds , 2000 .

[35]  J. Kerridge.  Carbon, hydrogen and nitrogen in carbonaceous chondrites: abundances and isotopic compositions in bulk samples. , 1985, Geochimica et cosmochimica acta.

[36]  C. Pillinger,et al.  The origin of chondritic macromolecular organic matter: A carbon and nitrogen isotope study , 1998, Meteoritics & planetary science.

[37]  Mark A. Sephton,et al.  δ13C of free and macromolecular aromatic structures in the murchison meteorite , 1998 .

[38]  E. Anders,et al.  Meteorites and the Early Solar System , 1971 .