Impact melting in sedimentary target rocks?

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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

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Impact melting in sedimentary rocks?

• Kieffer Simonds (1980)
• Volume of impact melt documented
• 102 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

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Haughton impact structure
Ries impact structure

• Strat colums

Data from Thorsteinsson Mayr (1987)
Data from Schmidt-Kaler (1978)
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Crater fill impactites at Haughton
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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 (gt95 vol) of the
  matrix-forming glasses
• Si-rich glass particles - high totals (90-95
  wt)
• Rare, sometimes angular (early-formed melt?)

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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

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Anomalous calcite composition
Analysis 1 2 3 4 5
SiO2 1.0 3.2 2.1 1.8 -
Al2O3 0.2 0.5 0.6 7.9 -
FeO - - - - 0.2
MgO 0.8 3.3 2.7 - 0.7
CaO 55.9 47.5 48.0 49.1 54.9
SO3 - 0.9 0.7 - -
Cl 0.3 - 0.2 - -
Total 58.2 55.4 54.3 58.8 55.8
Ti, Mn, Na K were analyzed for but were below
detection for all analyses
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Ries impact structure, Germany
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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 lt1-2 wt
• Al2O3, K2O 1-6 wt
• Protolith
• L. Jurassic and Triassic sandstones
• gt350lt770 m pre-impact depth

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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 gt 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

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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
• lt350 m pre-impact depth

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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)

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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

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