A SEM ATEM AND STABLE-ISOTOPE STUDY OF CARBONATES FROM THE HAUGHTON IMPACT CRATER, CANADA

Highly and intermediately shocked carbonate-rich fragments of the allo chtonous polymict breccia from the Haughton impact crater (Canada) wer e studied by Scanning Electron Microscopy (SEM), Analytical Transmissi on Electron Microscopy (ATEM) and analyses of carbon and oxygen stable isotopes (deltaC-13 and deltaO-18). In areas subjected to severe shoc k conditions, carbonates represent only about 10 vol% of the shocked s amples and they are located in holes and fractures within a matrix of SiO2-rich glass. Shock features are absent in these crystals. High-tem perature reactions have occurred between molten silicates and carbonat es, producing Ca-Mg-rich glasses, or crystalline phases such as augite and lamite (Ca2SiO4). The carbonates are dominated by calcite and the y generally have significantly positive deltaC-13, ranging up to +9 pa rts per thousand, with a weighted average value of +1.75 parts per tho usand. Their deltaO-18 values range between +15 parts per thousand and +20 parts per thousand and they are about 5 parts per thousand lower than in unshocked reference sediments, a trend consistent with that re sulting from silicate-carbonate reactions. The microstructures of the carbonates suggest that they did not undergo shock conditions but, ins tead, were produced by back-reactions between impact-released CO2 and highly reactive residual oxides. Such a process would introduce isotop e fractionations, which might explain the positive deltaC-13 values ob served. A simple kinetic fractionation model involving a Rayleigh dist illation process is used to estimate the CO2 fraction actually lost fr om the carbonates. It appears that this fraction is related to the amo unt of high-temperature carbonate-silicate reactions. Moderately shock ed fragments from other areas of the polymict breccia consist of 40-81 vol% carbonates. Their deltaC-13 values lie in the range of unshocked reference sediments between -2 parts per thousand and -4 parts per th ousand, whereas their deltaO-18 values are by about 5 parts per thousa nd lower than in the unshocked equivalents. No evidence for important decarbonatization is observed from C-13, and O-18 is again buffered by isotope exchange reactions between molten silicates and carbonate cry stals producing Ca and Mg enriched SiO2 glass and Ca-Mg silicate cryst als such as monoclinic pigeonite, which is indicative of fast cooling. This study indicates that significant evidence for outgassing is limi ted to a narrow zone in the centre of the crater, where peak shock-pre ssures 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-reaction s might be a general mechanism for retaining impact-produced volatiles during impact events.