Shaw meteorite, Lincoln County, Colorado, USAi

Regional Level Types
Shaw meteorite	Meteorite Fall Location
Lincoln County	County
Colorado	State
USA	Country

Ordinary chondrite, breccia (L6/7)
Found 1913; 17.5 kg, 3 stones

The unusual Shaw meteorite was one of many meteorites collected by Harvey H. Nininger in his trips through the midwest. Two stones of total mass 3.735 kg were initially found with a third stone found in 1967. The meteorite is a well-equilibrated ordinary chondritic mass with almost no trace of its original chondrule population. Composition of the dominant silicates, olivine (Fa~23) and orthopyroxene (Fs19.5), are characteristic of L chondrites (relatively low in total iron). Minor amounts of albitic plagioclase, troilite, and Fe-Ni metal are present plus very small amounts of accessory chromite and two phosphates. At least three pyroxenes are present with diopside, frequently found as exsolution lamellae in orthopyroxene, one of several indicators that the stone may have experienced an extended period of high temperature metamorphism. Limonite staining and small dolomite grains are indicative of terrestrial weathering which complicates compositional studies.

There are, in fact, a number of additional complexities. Three lithologies (dark, light, and intermediate) have been identified which exhibit different textures — textures suggestive of significantly different histories for different portions of the meteorite. Fe-Ni metal is found in a number of irregularly distributed morphologies — with some indications of shock-induced melting. The actual amount of Fe-Ni metal appears to be unusually low for an L chondrite so that bulk iron (~17 wt%) is more suggestive of an LL chondrite (very low in iron) rather than a normal L chondrite. The suggestion has been made that the the meteorite has lost a significant fraction of it original metal content — due either (1) to an extended period of metamorphism on the original parent body or (2) to additional heating from a subsequent [?] impact event. However, it is unclear the extent to which the two processes can be distinguished. A case can be made that both processes were important. A possible resolution of such issues may be connected to Shaw's complex cosmic ray exposure history. Understanding the implications of two apparent impacts on the pre-impact meteoroid at ~10 Ma and ~ 0.2 Ma before earth impact may provide essential clues.

While the L group of ordinary chondrites account for roughly 40% of all meteorite falls [the largest chondrite group], petrologic type 'L6/7' meteorites are quite rare. Shaw is the 2nd most massive of the very small group of known L6/7 meteorites. Nine records of L6/7 'meteorites' are currently listed with the Meteoritical Society Bulletin Database (early 2016). As four of these with tiny masses [<10 g] were found near other L6/7 stones the actual number of separate L6/7 falls may be as small as six or seven.

Nota bene: As there are some unresolved disagreements among the experts about the proper terminology for various phases and even the proper classification of the Shaw meteorite one may need to consult the original sources to explore the implications of the mineral list. As the meteorite itself is rather heterogeneous at several scales this is no mean task.

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References