The L type ordinary chondrites are the second most common group of , accounting for approximately 35% of all those catalogued, and 40% of the ordinary chondrites.[ NASA (YouTube) – Dr. David Kring – Asteroid Initiative Workshop Cosmic Explorations Speakers Session]
Name
Their name comes from their relatively low iron abundance (less than 10%) with respect to the
, which are about 20–25% iron by weight.
Historically, the L chondrites have been named hypersthene chondrites or olivine hypersthene chondrites for the dominant minerals, but these terms are now obsolete.
Chemical composition
Characteristic is the
fayalite content (Fa) in olivine of 21 to 25 mol%. About 4–10% iron–nickel is found as a free metal, making these meteorites magnetic, but not as strongly as the
.
Mineralogy
The most abundant minerals are
olivine and
hypersthene (an
orthopyroxene), as well as iron–nickel and
troilite.
Chromite, sodium-rich
feldspar and calcium
phosphates occur in minor amounts. Petrologic type 6 dominates, with over 60% of the L chondrites falling into this class. This indicates that the parent body was sizeable enough (greater than 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).]
Ordovician meteor event
Many of the L chondrite meteors may have their origin in the Ordovician meteor event, radioisotope dated with uranium-lead method at around million years ago. 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 impact has been dated via cosmic ray exposure at around million years ago.
Earlier argon dating placed the event at around 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. "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.]
Parent body
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
, whose composition is similar to that of
chondrite meteorites; and (4) the Flora family parent body was over in diameter.
The Massalia family is also a possible source of these meteorites.
See also
External links
