Impact melting in sedimentary target rocks?

1. Impact meltingin sedimentary target rocks? G.R. Osinski1, J.G. Spray1 & R.A.F. Grieve2 1Planetary and Space Science Centre University of New Brunswick, Fredericton NB, Canada 2Earth Science Sector Natural Resources Canada, Ottawa ON, Canada

2. Impact melting in sedimentary rocks? Kieffer & Simonds (1980): • Volume of impact melt “documented”: • ~102LESS than for crystalline target rocks in comparably sized impact craters • Volume of target material shocked to pressures sufficient for melting: • NOT significantly different in sedimentary or crystalline rocks • ANOMALY attributed to “unusually” wide dispersion of shock-melted sedimentary rocks by expansion of sediment-derived vapour

3. Haughton impact structure Ries impact structure • Strat colums……… Data from Thorsteinsson & Mayr (1987) Data from Schmidt-Kaler (1978)

4. Crater fill impactites at Haughton

6. Nature of the groundmass • Unshocked microcrystalline CALCITE, generally occurring as irregular blebs and globules (~20-90 vol%) • Silicate-rich GLASS (~5-40 vol%): • Si-Mg-Al-rich glasses yielding relatively high (~85 wt%) totals • Si-Mg-Al-CO2-rich glasses - low totals (~60-65 wt %) • Comprise the bulk (>95 vol%) of the matrix-forming glasses • Si-rich glass particles - high totals (~90-95 wt%) • Rare, sometimes angular (early-formed melt?)

8. Evidence for shock melting of carbonates • Carbonate-silicate liquid immiscible textures • Anomalous calcite compositions • Calcite spheres in the matrix • Carbonate overgrowths on dolomite clasts • Assimilation of dolomite clasts • Infiltration of calcite and silicate-rich matrix phases into clasts • Ca-Mg silicates

11. Anomalous calcite composition *Ti, Mn, Na & K were analyzed for but were below detection for all analyses

13. Ries impact structure, Germany

17. SiO2-rich glasses • Ubiquitous in ‘fallout’ suevites (Osinski, 2003) • Occur as individual particles/clasts in the groundmass or as inclusions in other glass particles • Composition: • ~85-100 wt% SiO2 • FeO, MgO, CaO, Na2O <1-2 wt% • Al2O3, K2O ~1-6 wt% • Protolith: • L. Jurassic and Triassic sandstones • >350<770 m pre-impact depth

19. Al-Ca-H2O-rich glasses • Recognized in 4 samples (Osinski, 2003) • Composition: • Low SiO2: 50-53 wt% • High Al2O3 (17-21 wt%) and CaO (5-7 wt%) • Oxide totals ~83-88% => substantial volatile contents • Protolith: • Clay-rich sedimentary rocks (shales, claystones etc.) from lowermost part of sed. sequence • High CaO content may suggest a component of marls in the melt zone

22. Evidence for shock melting of carbonates • Calcite occurs as globules in silicate-rich glasses and in the groundmass • Unequivocal evidence for liquid immiscibility (Graup, 1999; Osinski, 2003) • Protolith: • U. Jurassic Malm limestones • <350 m pre-impact depth

23. Modeling • No modeling carried out at Haughton to date • Ries impact structure (Stoffler et al., 2002): • Modeling suggests shock melting of sandstones – confirmed by our analytical SEM studies (Osinski, 2003) • Modeling invokes shock degassing of carbonates – NOT supported by optical and analytical SEM studies (Graup, 1999; Osinski, 2003)

24. Conclusions • Carbonate-rich crater-fill deposits at Haughton are carbonate-rich impact melt breccias • Shocked-melted sedimentary rocks preserved in proximal “ejecta” from the Ries impact structure • Noevidence for decomposition and degassing of carbonates from Haughton or Ries • Shock melting of sedimentary rocks occurred during the Haughton and Ries (and Chicxulub) impact events • Agreement with theoretical studies which suggest that impacts into sedimentary targets should produce as much melt as impacts into crystalline targets