Correlated alteration effects in CM carbonaceous chondrites

Three parameters are proposed to determine the relative extent of alteration in CM chondrites. The mineralogic alteration index monitors the relative progress of coupled substitutions in the progressive alteration of cronstedtite to Mg-serpentine and increases with increasing alteration. To calculate values of this index, an algorithm has been developed to estimate the average matrix phyllosilicate composition in individual CM chondrites. The second parameter is the volume percent of isolated matrix silicates, which decreases with progressive alteration due to mineral hydration. Finally, the volume percent of chondrule alteration monitors the extent of chondrule phyllosilicate production and increases as alteration proceeds. These parameters define the first CM alteration scale that relies on multiple indicators of progressive alteration. The following relative order of increasing alteration is established by this model: Murchison ≤ Bells < Pollen ≤ Murray < Mighei < Nogoya < Cold Bokkeveld. The relative degree of aqueous processing Cochabamba and Boriskino experienced is less precisely constrained, although both fall near the middle of this sequence. A comparison between the mineralogic alteration index and literature values for the whole-rock chemistry of CM chondrites reveals several correlations. A positive, nearly linear correlation between bulk H content and progressive CM alteration suggests an approximately constant production rate of new phyllosilicates relative to the mineralogical transition from cronstedtite to Mg-serpentine. The abundance of trapped planetary 36Ar decreases systematically in progressively altered CM chondrites, suggesting the wholesale destruction of primary noble gas carrier phase (s) by aqueous reactions. Because low temperature fluid-rock reactions are generally associated with large isotopic mass fractionation factors, we also compared our model predictions with δ18O values for bulk CM samples. Although some of these data are poorly resolved, the order of increasing δ18O values approximates the order of increasing alteration predicted by our model parameters. Multiple correlations between diverse alteration parameters strongly suggest that (a) different CM chondrites experienced similar kinds of processes and conditions, and (b) CM materials experienced in situ alteration on the CM parent body or bodies.

[1]  D. Black.  On the origins of trapped helium, neon and argon isotopic variations in meteorites—II. Carbonaceous meteorites , 1972 .

[2]  R. Clayton,et al.  The oxygen isotope record in Murchison and other carbonaceous chondrites , 1984 .

[3]  J. Ferris.  The chemistry of life's origin. , 1984, Chemical and engineering news : "news edition" of the American Chemical Society.

[4]  R. Jones.  On the relationship between isolated and chondrule olivine grains in the carbonaceous chondrite ALHA77307 , 1992 .

[5]  Harry Y. McSween,et al.  Alteration in CM carbonaceous chondrites inferred from modal and chemical variations in matrix , 1979 .

[6]  S. Richardson,et al.  Textural evidence bearing on the origin of isolated olivine crystals in C2 carbonaceous chondrites , 1978 .

[7]  J. H. Reynolds,et al.  Rare-gas-rich separates from carbonaceous chondrites , 1976 .

[8]  D. J. Barber.  Matrix phyllosilicates and associated minerals in C2M carbonaceous chondrites , 1981 .

[9]  A. Brearley.  Aqueous alteration and brecciation in Bells, an unusual, saponite-bearing, CM chondrite , 1995 .

[10]  L. Schultz,et al.  Helium, neon, and argon in meteorites: A data collection , 1989 .

[11]  Ronald G. Prinn,et al.  The Atmospheres of Venus, Earth, and Mars: A Critical Comparison , 1987 .

[12]  Peter R. Buseck,et al.  MATRICES OF CARBONACEOUS CHONDRITE METEORITES , 1993 .

[13]  H. McSween,et al.  Water and the thermal evolution of carbonaceous chondrite parent bodies , 1989 .

[14]  R. Burns,et al.  Nanophase mixed-valence iron minerals in meteorites identified by cryogenic Mössbauer spectroscopy , 1994 .

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

[16]  M. Prinz,et al.  Petrologic study of the Belgica 7904 carbonaceous chondrite: Hydrous alteration, oxygen isotopes, and relationship to CM and CI chondrites , 1993 .

[17]  M. Zolensky,et al.  Mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites , 1993 .

[18]  D. Heymann,et al.  Light-dark structure and rare gas content of the carbonaceous chondrite Nogoya. , 1967 .

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

[20]  E. Anders,et al.  Chemical fractionations in meteorites—XI. C2 chondrites , 1980 .

[21]  J. B. Moody.  Serpentinization: a review , 1976 .

[22]  L. Grossman,et al.  Early chemical history of the solar system , 1974 .

[23]  T. E. Bunch,et al.  Carbonaceous chondrites. II - Carbonaceous chondrite phyllosilicates and light element geochemistry as indicators of parent body processes and surface conditions , 1980 .

[24]  P. Buseck,et al.  Indicators of aqueous alteration in CM carbonaceous chondrites: Microtextures of a layered mineral containing Fe, S, O and Ni , 1985 .

[25]  E. Anders,et al.  Chemical Evolution of the Carbonaceous Chondrites , 1962 .

[26]  M. Zolensky,et al.  Aqueous alteration on the hydrous asteroids - Results of EQ3/6 computer simulations , 1989 .

[27]  R. Clayton,et al.  Oxygen Isotope Classification of Carbonaceous Chondrites , 1989 .

[28]  J. Hayes,et al.  Chemical and petrographic correlations among carbonaceous chondrites , 1974 .

[29]  D. Stöffler,et al.  Accretionary dust mantles in CM chondrites: Evidence for solar nebula processes , 1992 .

[30]  M. Zolensky,et al.  CM chondrites exhibit the complete petrologic range from type 2 to 1. [Abstract only] , 1994 .

[31]  D. J. Barber,et al.  The Semarkona meteorite: First recorded occurrence of smectite in an ordinary chondrite, and its implications , 1987 .

[32]  Stephen M. Larson,et al.  Ferric Iron in Primitive Asteroids: A 0.43-μm Absorption Feature , 1993 .

[33]  T. Swindle.  Trapped noble gases in meteorites , 1988 .

[34]  JOHN S. Lewis,et al.  Chemistry of Primitive Solar Material , 1976 .

[35]  F. Wicks,et al.  Serpentine textures and serpentinization , 1977 .

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

[37]  H. McSween,et al.  Cosmochemical implications of the physical processing of cometary nuclei , 1989 .

[38]  John A. Wood,et al.  A chemical-petrologic classification for the chondritic meteorites. , 1967 .

[39]  P. Buseck,et al.  Phyllosilicates in the Mokoia CV carbonaceous chondrite: Evidence for aqueous alteration in an oxidizing environment , 1990 .

[40]  A. Tobi,et al.  A chart for judging the reliability of point counting results , 1965 .

[41]  H. McSween.  On the nature and origin of isolated olivine grains in carbonaceous chondrites , 1977 .

[42]  R. Clayton,et al.  Oxygen isotopes in separated components of CI and CM meteorites , 1994 .

[43]  D. Heymann,et al.  Noble gases in carbonaceous chondrites , 1970 .

[44]  H. McSween.  Aqueous alteration in carbonaceous chondrites - Mass balance constraints on matrix mineralogy , 1987 .

[45]  Robert N. Clayton.  Oxygen Isotopes in Meteorites , 1993 .

[46]  C. Pillinger,et al.  Determination of Sulphur-Bearing Components in C1 and C2 Carbonaceous Chondrites by Stepped Combustion , 1991 .