The oxygen isotope evolution of parent body aqueous solutions as recorded by multiple carbonate generations in the Lonewolf Nunataks 94101 CM2 carbonaceous chondrite

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Lee, M.R. ; Sofe, M.R. ; Lindgren, P. ; Starkey, N.A. ; Franchi, I.A. (2013)
  • Publisher: Elsevier Ltd.
  • Journal: Geochimica et Cosmochimica Acta, volume 121, pages 452-466 (issn: 0016-7037)
  • Related identifiers: doi: 10.1016/j.gca.2013.07.010
  • Subject: Geochemistry and Petrology

The CM2 carbonaceous chondrite LON 94101 contains aragonite and two generations of calcite that provide snapshots of the chemical and isotopic evolution of aqueous solutions during parent body alteration. Aragonite was the first carbonate to crystallize. It is rare, heterogeneously distributed within the meteorite matrix, and its mean oxygen isotope values are δ18O 39.9±0.6‰, Δ17O -0.3±1.0‰ (1σ). Calcite precipitated very soon afterwards, and following a fall in solution Mg/Ca ratios, to produce small equant grains with a mean oxygen isotope value of δ18O 37.5±0.7‰, Δ17O 1.4±1.1‰ (1σ). These grains were partially or completely replaced by serpentine and tochilinite prior to precipitation of the second generation of calcite, which occluded an open fracture to form a millimeter-sized vein, and replaced anhydrous silicates within chondrules and the matrix. The vein calcite has a mean composition of δ18O 18.4±0.3‰, Δ17O -0.5±0.5‰ (1σ). Petrographic and isotopic results therefore reveal two discrete episodes of mineralization that produced Ca-carbonates with contrasting δ18O, but whose Δ17O values are indistinguishable within error. The aragonite and equant calcite crystallized over a relatively brief period early in the aqueous alteration history of the parent body, and from static fluids that were evolving chemically in response to mineral dissolution and precipitation. The second calcite generation crystallized from solutions of a lower Δ17O, and a lower δ18O and/or higher temperature, which entered LON 9410 via a fracture network. As two generations of calcite whose petrographic characteristics and oxygen isotopic compositions are similar to those in LON 94101 occur in at least one other CM2, multiphase carbonate mineralization could be the typical outcome of the sequence of chemical reactions during parent body aqueous alteration. It is equally possible however that the second generation of calcite in formed in response to an event such as impact fracturing and concomitant fluid mobilisation that affected a large region of the common parent body of several CM2 meteorites. These findings show that integrated petrographic, chemical and isotopic studies can provide new insights into the mechanisms of parent body alteration including the spatial and temporal dynamics of the aqueous system.
  • References (64)
    64 references, page 1 of 7

    Alexander C. M. O'D., Bowden R., Fogel M. L. and Howard K. T. (2013) Carbonate abundances and isotopic compositions in chondrites. Lunar Planet. Sci. 44. Lunar Planet. Inst., Houston. #2788 (abstr.).

    Antarctic Meteorite Newsletter (1995) vol. 18, no. 2. JSC, NASA Johnson Space Center, Houston, TX (August).

    Balthasar U., Cusack M., Faryma L., Chung P., Holmer L. E., Percival I. G. and Popov L. E. (2011) Relic aragonite from Ordovician-Silurian brachiopods - implications for evolution of calcification. Geology 39, 967970.

    Barber D. J. (1981) Matrix phyllosilicates and associated minerals in C2M carbonaceous chondrites. Geochim. Cosmochim. Acta 45, 945970.

    Benedix G. K., Leshin L. A., Farquhar J., Jackson T. and Thiemens M. H. (2003) Carbonates in CM2 chondrites: Constraints on alteration conditions from oxygen isotopic compositions and petrographic observations. Geochim. Cosmochim. Acta 67, 15771588.

    Bland P. A., Jackson M. D., Coker R. F., Cohen B. A., Webber B. W., Lee M. R., Duffy C. M., Chater R. J., Ardakani M. G., McPhail D. S., McComb D. W. and Benedix G. K. (2009) Why aqueous alteration in asteroids was isochemical: High porosity - high permeability. Earth Planet. Sci. Lett. 287, 559-568.

    Bonal L., Huss G. R., Krot A. N. and Nagashima K. (2010) Chondritic lithic clasts in the CB/CH-like meteorite Isheyevo: Fragments of previously unsampled parent bodies. Geochim. Cosmochim. Acta 74, 25002522.

    Bots P., Benning L. G., Rickaby R. E. M. and Shaw S. (2011) The role of SO4 in the switch from calcite to aragonite seas. Geology 39, 331-334.

    Brearley A. J. (1998) Carbonates in CM carbonaceous chondrites: Complex zoning revealed by high resolution cathodoluminescence studies. Lunar Planet. Sci. 29. Lunar Planet. Inst., Houston. #1246 (abstr.).

    Brearley A. J. (2006) The role of microchemical environments in the alteration of CM carbonaceous chondrites. Lunar Planet. Sci. 37. Lunar Planet. Inst., Houston. #2074 (abstr.).

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