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. [ [http://internt.nhm.ac.uk/jdsml/research-curation/projects/metcat/metsPerGroup.dsml Natural History Museum, meteorite catalogue] ]

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. [D. Nesvorný et al "The Flora Family: A Case of the Dynamically Dispersed Collisional Swarm?", Icarus, Vol. 157, p. 155 (2002).]

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. [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. [http://www.ingentaconnect.com/content/arizona/maps/2007/00000042/00000001/art00009 "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.] 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.

References

External links

* [http://www.meteorite.fr/en/classification/ordinarychon.htm All About Meteorites]
* [http://internt.nhm.ac.uk/jdsml/research-curation/projects/metcat/metsPerGroup.dsml The Catalogue of Meteorites]
* [http://www.meteorite.fr/en/classification/ordinarychon.htm www.meteorite.fr]