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- Channon Visscher
- EPSc 571
- Fall 2003
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- Definition and overview
- Current inventory of fossil meteorites
- Importance of fossil meteorites
- Where to look for them
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- Meteorite with ancient terrestrial age
- Buried in sediment after fall event
- Incorporated into geologic record
- Not considered fossil meteorites:
- Meteorites embedded in ice (e.g. Antarctica)
- Objects associated with craters
- Cosmic spherules, tektites, etc.
- Objects found in surface sediments(?)
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- Small (sub-mm) objects that survive atmospheric entry
- If totally melted: cosmic spherules
- Typically iron, stony, or glassy
- Large numbers collected from deep sea sediments, etc.
- Large age range: kyr to several hundred Ma
- Present flux of ~30,000 tons per year
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5
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- Most recognizable; High Ni contents
- Oxidized partially or completely into limonite, etc.
- May preserve some of original structure
- Surface finds
- Technically not “fossil”, but shown to have old terrestrial ages
- Ider (3.1 Ma), Tamarugal (2.7 Ma) (53Mn/10Be)
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7
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- More difficult to preserve, recognize
- Original meteorite structures may be present
- Original minerals usually replaced by diagenitic minerals (e.g.
limestone chondrules)
- Chromite as a meteoritic marker (Schmitz et al. 2001):
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8
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- Polished slab of Ordivician limestone
- Found in 1952, recognized 1979
- 463 Ma terrestrial age; probably H-L chondrite
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- systematic search (1992-present) in limestone quarry
- 40 meteoritic objects recovered so far
- 480 Ma terrestrial age; probably L chondrites
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10
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11
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- 2.5 mm meteorite found in K-T boundary clays
- Original minerals replaced by hematite and clay
- 65 Ma, maybe a carbonaceous chondrite
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- Has flux changed over geologic time?
- Flux: ~28,000 objects > 0.1 kg per year on Earth (Halliday et al.
1989)
- Quantity – higher/lower flux in the past?
- Österplana suggests 10-100X higher flux
- asteroidal breakup of L-type parent body?
- Quality – similar distribution of groups?
- Irons are better preserved/more recognizable
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- Coals, shales, limestones, sandstones, clay deposits, evaporites
- Slow accumulation over long time period
- Favorable conditions for preservation
- Ongoing systematic searches:
- Swedish quarries
- PA and WY coal mines
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- Relatively rapid accumulation rates (~15 cm/kyr)
- 75 million tons of limestone products per year
- limestone: 2700 kg m3
- 75 million tons = 25 m thick deposit over 1 km2
- 25 m = 170,000 years of accumulation
- With present flux, can expect ~10 meteorites > 0.1 kg processed each
year in Missouri
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Notes
