Impact melting in sedimentary target rocks? G.R. Osinski 1, J.G. Spray 1 & R.A.F. Grieve 2 1 Planetary and Space Science Centre University of New Brunswick, Fredericton NB, Canada 2 Earth Science Sector Natural Resources Canada, Ottawa ON, Canada

Impact melting in sedimentary rocks? Kieffer & Simonds (1980):  Volume of impact melt “documented”:  ~10 2 LESS 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

 Strat colums……… Haughton impact structureRies impact structure Data from Thorsteinsson & Mayr (1987)Data from Schmidt-Kaler (1978)

Crater fill impactites at Haughton

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-CO 2 -rich glasses - low totals (~60-65 wt %) Comprise the bulk (>95 vol%) of the matrix- forming glassesComprise the bulk (>95 vol%) of the matrix- forming glasses  Si-rich glass particles - high totals (~90-95 wt%) Rare, sometimes angular (early-formed melt?)Rare, sometimes angular (early-formed melt?)

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

Anomalous calcite composition Analysis12345 SiO Al 2 O FeO MgO CaO SO Cl Total *Ti, Mn, Na & K were analyzed for but were below detection for all analyses

Ries impact structure, Germany

SiO 2 -rich glasses  Ubiquitous in ‘fallout’ suevites (Osinski, 2003)  Occur as individual particles/clasts in the groundmass or as inclusions in other glass particles  Composition:  ~ wt% SiO 2  FeO, MgO, CaO, Na 2 O <1-2 wt%  Al 2 O 3, K 2 O ~1-6 wt%  Protolith:  L. Jurassic and Triassic sandstones  > <770 m pre-impact depth

Al-Ca-H 2 O-rich glasses  Recognized in 4 samples (Osinski, 2003)  Composition:  Low SiO 2 : wt%  High Al 2 O 3 (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

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

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)

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  No evidence 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