Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13e..04l&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P13E-04
Other
[6040] Planetary Sciences: Comets And Small Bodies / Origin And Evolution, [6240] Planetary Sciences: Solar System Objects / Meteorites And Tektites
Scientific paper
The presence of phyllosilicates and carbonates in the CM carbonaceous chondrites provides clear evidence for water-mediated crystallization very early in the history of the solar system [1]. The relatively coarse crystal size of the carbonates makes them amenable to electron- and ion-beam analysis and so they are potentially powerful tools for unraveling parent body histories. To date most studies have found a single carbonate generation within any one meteorite and together with the limited differences between samples these results are consistent with a brief period of carbonate mineralization within near-static parent body aqueous solutions. Using electron beam imaging and analysis techniques we have characterised the carbonates within a suite of CM meteorites with different degrees of alteration: Murchison, Pollen, Murray, Mighei, LON 94101, Nogoya, Cold Bokkeveld, QUE 93005 and SCO 06043. These meteorites contain five compositionally distinct carbonate minerals, namely aragonite, calcite, magnesian dolomite, calcian dolomite, and breunnerite. Ca-carbonates are dominant in the less altered CMs (e.g. Murchison and Murray) and most grains have crystallized as cements within equant pores in the matrix. Aragonite formed before calcite, and both minerals were subsequently partially or completely replaced by tochilinite and/or Mg-phyllosilicates. The patchy distribution of aragonite and scarcity of carbonate veins indicates low water/rock ratios, and the good preservation of otherwise unstable aragonite suggests that aqueous alteration was short-lived. The more highly altered CMs (e.g. QUE 93005 and SCO 06043) have evidence for four or even more phases of mineralization by Ca-carbonates and Ca-Mg-Fe-Mn-carbonates. These minerals occur within concentrically layered polymineralic grains and fill fractures produced by expansion of chondrules during hydration. Calcite has replaced calcian dolomite, and the Ca-Mg-Fe-Mn-carbonates have themselves been extensively replaced by Fe,Ni sulphides and Mg,Fe phyllosilicates. The good correspondence between the complexity of the record of carbonate, silicate and sulphide mineralization of CMs and their degree of aqueous alteration shows that the carbonates preserve a high fidelity record of parent body evolution. The greater variety of carbonate minerals present in the highly altered CMs shows that solution compositions changed significantly during alteration and as mineral sequences and compositions vary little on the centimeter scale, water/rock ratios must have been high. The challenge remains to reconcile such a dynamic aqueous system with evidence from bulk meteorite compositions for little or no fluid flow.
Lee Minchul
Lindgren Paula
Sofe M. R.
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