Other
Scientific paper
Feb 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994e%26psl.121..559m&link_type=abstract
Earth and Planetary Science Letters, Volume 121, Issue 3-4, p. 559-574.
Other
23
Scientific paper
Highly and intermediately shocked carbonate-rich fragments of the allochtonous polymict breccia from the Haughton impact crater (Canada) were studied by Scanning Electron Microscopy (SEM), Analytical Transmission Electron Microscopy (ATEM) and analyses of carbon and oxygen stable isotopes (δ13C and δ18O). In areas subjected to severe shock conditions, carbonates represent only about 10 vol% of the shocked samples and they are located in holes and fractures within a matrix of SiO2-rich glass. Shock features are absent in these crystals. High-temperature reactions have occurred between molten silicates and carbonates, producing Ca-Mg-rich glasses, or crystalline phases such as augite and larnite (Ca2SiO4). The carbonates are dominated by calcite and they generally have significantly positive δ13C, ranging up to +9‰, with a weighted average value of +1.75‰. Their δ18O values range between +15‰ and +20‰ and they are about 5‰ lower than in unshocked reference sediments, a trend consistent with that resulting from silicate-carbonate reactions. The microstructures of the carbonates suggest that they did not undergo shock conditions but, instead, were produced by back-reactions between impact-released CO2 and highly reactive residual oxides. Such a process would introduce isotope fractionations, which might explain the positive δ13C values observed.
A simple kinetic fractionation model involving a Rayleigh distillation process is used to estimate the CO2 fraction actually lost from the carbonates. It appears that this fraction is related to the amount of high-temperature carbonate-silicate reactions. Moderately shocked fragments from other areas of the polymict breccia consist of 40-81 vol% carbonates. Their δ13C values lie in the range of unshocked reference sediments between -2‰ and -4‰, whereas their δ18O values are by about 5‰ lower than in the unshocked equivalents. No evidence for important decarbonatization is observed from 13C, and 18O is again buffered by isotope exchange reactions between molten silicates and carbonate crystals producing Ca and Mg enriched SiO2 glass and Ca-Mg silicate crystals such as monoclinic pigeonite, which is indicative of fast cooling.
This study indicates that significant evidence for outgassing is limited to a narrow zone in the centre of the crater, where peak shock pressures reached 50-60 GPa. Moreover, we suggest that, within this area, a large fraction of the shock-produced gas recombines with the highly reactive residual oxides and, in consequence, that such back-reactions might be a general mechanism for retaining impact-produced volatiles during impact events.
Agrinier Pierre
Javoy Marc
Martinez Isabelle
Scharer Urs
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