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Meteorite Comet Impacts

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					       Meteorite composition
• Major groupings:
  – Stony meteorites
    • Chondrites
    • Achondrites
  – Iron meteorites
    • Octahedrites, hexahedrites, ataxites
  – Stony-iron meteorites
    • Pallasites
    • Mesosiderites
•   Chondrites
•   Carbonaceous chondrite class
     –   CI chondrite (Ivuna-like) group
     –   CM-CO chondrite (mini-chondrule) clan
           •   CM chondrite (Mighei-like) group
           •   CO chondrite (Ornans-like) group
     –   CV-CK chondrite clan
           •   CV chondrite (Vigarano-like) group
                  –   CV-oxA chondrite (oxidized, Allende-like) subgroup
                  –   CV-oxB chondrite (oxidized, Bali-like) subgroup
                  –   CV-red chondrite (reduced) subgroup
           •   CK chondrite (Karoonda-like) group
     –   CR chondrite clan
           •   CR chondrite (Renazzo-like) group
           •   CH chondrite (Allan Hills 85085-like) group
           •   CB chondrite (Bencubbin-like) group
                  –   CBa chondrite subgroup
                  –   CBb chondrite subgroup
•   Ordinary chondrite class
     –   H chondrite group
     –   L chondrite group
     –   LL chondrite group
•   Enstatite chondrite class
     –   EH chondrite group
     –   EL chondrite group
•   Other chondrite groups, not in one of the major classes
     –   R chondrite (Rumuruti-like) group
     –   K chondrite (Kakangari-like) grouplet (a grouplet is a provisional group with <5 members)
                   Chondrites
• Stony meteorites – most common meteorites and
  represent the oldest solids that are the building
  blocks of the solar system
• Parent bodies were small-medium asteroids, not
  planetoids!
• Contain up to 80% chondrules, which were freely
  floating molten drops in space

                          • Few mm to 1 cm spheres
                          • Mostly olivine and
                            pyroxene, also feldspathic
                            glass or crystals with minor
                            troilite (FeS), chromite,
                            phosphates
                          • 15 groups of chondrites
              Chondrite groups
• Ordinary – make up 80% of the meteorites and
  90% of chondritic meteorites, abundant chondrules,
  variable Fe-Ni lead to H, L, and LL chondrites
• Carbonaceous chondrites –less the 5% of
  chondritic meteorites, few chondrules, more
  lithophile elements (Ca, Mg, K, Cr, Al, Cl,…), high
  levels of water and organic compounds - many
  types based on characteristic specimen
• Enstatite Chondrites – 2% of chondritic meteorites,
  very chemically reduced – enstatite-rich chondrules
  and abundant metal and sulfide minerals
               Peekskill Meteorite
 Peekskill Meteor: October 9, 1992. This famous fireball was seen and
   filmed across several eastern states. It broke up into many
   fragments, one of which hit the trunk of Michelle Knapp's 1980
   Chevy Malibu. When Ms. Knapp investigated a crash sound outside
   her Peekskill, NY home, she discovered the damaged trunk and
   found a warm 12-kg meteorite lying beside the car.


12-kg Ordinary
   Chondritic
   meteorite
Car – $10K
Meteorite – $75K
                             Achondrites
• Stony meteorite similar to terrestrial basalts or
  plutonic rocks, represent 8% of meteorites grouped
  on the basis of Fe/Mn and 17O/18O ratios
  characteristic of the parent body
• 2/3 of these meteorites are HED type, originating
  from one asteroid, Vesta 4 - there is a large impact
  crater observed on this asteroid
• Also includes Martian and lunar meteorites




The Johnstown Diogenite.   Lunar Meteorite Allan Hills 81005
             Martian Meteorites
• All are igneous, lherzolitic to basaltic composition, and
  some contain hydrated carbonates and sulfates,
  evidence of liquid water
• ALH84001 is 4.5 ga, contains carbonate veins 3.6 – 4
  ga, Organic matter (PAH), aligned magnetite crystals,
  proposed nanofossils (careful!!) – very controversial…




                        ALH84001 – round
ALH84001                   carbonate grains   Nahkla Meteorite
                   Iron Meteorites
• 5% of meteorites, but 90% of the mass of recovered
  meteorites and all of the largest meteorites (66 tons is the
  record) - primarily composed of iron-nickel alloy
  intergrowths:
   – Kamacite – Fe-Ni alloy at 90:10 to 95:5 Fe:Ni
   – Taenite – Fe-Ni alloy at 80:20 to 45:65 Fe:Ni
   – Widmanstätten pattern – finely interweaved Kamacite and Taenite
     (Octahedrites)
• Classified on Ni content:
  Ataxites>Octahedrites>Hexahedrites
       Stony-iron Meteorites
• Mix of iron-nickel alloy and silicate
  minerals (mostly olivine), 1% of meteorites
  – Pallasites are thought to form a the core-
    mantle boundary of differentiated asteroids
  – Mesosiderite – equal parts metal alloy and
    silicate with a breccia texture
       Identifying a meteorite
• Fusion crust – dull black to dark brown,
  often soft, can be weathered to red (but
  can flake off)
• Density – generally dense
• Chondrules are specifically meteoritic
• Never porous, but can be ‘dimpled’ with
  surface depressions
• 99% of meteorites are magnetic
• Unusually high Ni content
• Fusion crust forms on entry through the
  atmosphere
    Meteorite/ Comet Impacts
• P/T space??
      Discovering the K/T impact

• Iridium elevated
  in very thin layer
  worldwide
• Alvarez and his
  students/
  postdocs
  postulated this in
  late 1979…
   Chicxulub, Yucatan Peninsula,
              Mexico
• K/T event at 64.98 ma, formed from a 6-12 mile
  diameter asteroid impact (50 megaton blast)
Barringer Meteor Crater, Arizona, formed from Can
                Impact Craters
• Many famous impact craters,
  hundreds known on earth
  (why might there have been
  plenty more??)
  – Sudbury, Ontario (250km
    diameter, 1.85 ga)
  – Chesapeake Bay (90km
    diameter, 35 ma)
  – Manson, Iowa (35km, 74ma)
  – Barringer, Arizona (1.2km, 49
    ka)
                                    Clearwater lakes, Quebec –
  – Serpent Mound, Ohio (8km,          36+26km diameter, 290
    320 ma)                            ma
           Megatsunamis
• Meteorite impact in the ocean displaces
  huge quantities of water instantly
• The Chixculub crater impact generated a
  megatsunami 150-300 feet high
        Energy of an impact
• Kinetic energy – going from
  very fast to stopping is a BIG
  change in energy

• What happens to that
  energy??

• Impactite – any mineral formed   Shatter cone
  as a result of this impact
  Materials indicating Meteorite
             Impacts
• Tektites
• Glass formed from impact
           Diaplectic Glass
• Glass formed through fusion of different
  minerals – not melted, but fused…
 What Happens to minerals that
   are there but not melted?
• Shock Quartz
• Lamellae retaining evidence of impact
               Diamonds
• Very small diamonds can form from
  impacts and are found in meteorite impact
  craters around the world
• Diamonds can also form in meteorites-
  these can be gray to black
                  • Lonsdalite - Hexagonal
                    allotrope of diamond,
                    specific to meteorite
                    impacts, thought to
                    form from graphite in
                    meteorites
         Quartz Polymorphs
• Coesite and Stishovite found associated with
  impact craters

				
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