L chondrite
The L type ordinary chondrites are the second most common type of meteorite, accounting for approximately 35% of all those catalogued, and 40% of the ordinary chondrites.[1]
Their name comes from their relatively (L)ow iron abundance, with respect to the H chondrites, which are about 20–25% iron by weight. Characteristic is the fayalite content (Fa) in olivine of 21 to 25 mol%. About 4–10% nickel-iron is found as a free metal, making these meteorites magnetic, but not as strongly as the H chondrites.
The most abundant minerals are olivine and hypersthene (an orthopyroxene), as well as nickel-iron metal and troilite. Chromite, Na-rich feldspar and Ca-phosphates occur in minor amounts. Petrologic class 6 dominates, with over 60% of the L chondrites falling into this class. This indicates that the parent body was sizeable enough (greater than 100 km in diameter) to experience strong heating.[2]
Compared to other chondrites, a large proportion of the L chondrites have been heavily shocked, which is taken to imply that the parent body was catastrophically disrupted by a large impact. This event has been radioisotope dated to around 470±6 million years ago.[3][4] This caused the Middle Ordovician Meteor Shower of 467.3±1.6 million years ago.
The parent body/bodies for this group are not known, but plausible suggestions include 433 Eros and 8 Flora, or the Flora family as a whole. 433 Eros has been found to have a similar spectrum, while several pieces of circumstantial evidence for the Flora family exist: (1) the Flora family is thought to have formed about 1,000 to 500 million years ago; (2) the Flora family lies in a region of the asteroid belt that contributes strongly to the meteorite flux at Earth; (3) the Flora family consists of S-type asteroids, whose composition is similar to that of chondrite meteorites; and (4) the Flora family parent body was over 100 km in diameter.
Historically, the L chondrites have been named hypersthene chondrites or olivine hypersthene chondrites for the dominant minerals, but these terms are now obsolete.
[edit] References
- ^ Natural History Museum, meteorite catalogue
- ^ D. Nesvorný et al. The Flora Family: A Case of the Dynamically Dispersed Collisional Swarm?, Icarus, Vol. 157, p. 155 (2002).
- ^ H. Haack et al. Meteorite, asteroidal, and theoretical constraints on the 500-Ma disruption of the L chondrite parent body, Icarus, Vol. 119, p. 182 (1996).
- ^ Korochantseva et al. "L-chondrite asteroid breakup tied to Ordovician meteorite shower by multiple isochron 40Ar-39Ar dating" Meteoritics & Planetary Science 42, 1, pp. 3-150, Jan. 2007.
