Allende CV3 Carbonaceous Chondrite Meteorite

arguably the most scientifically important meteorite yet


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Above: two views of a sawn individual, showing the pale exterior and (right) a sawn and gently polished view of the interior of the stone. Classified as a CV3, the interior displays chondrules and a paucity of metal (the stone is not appreciably magnetic). The most notable feature, however, is the pale, mm-scale, irregular outlines of white aggregates of refractory minerals, mostly silicates, known as calcium-aluminium inclusions (CAI). The sample is 50x41x6 mm in size, weight 25.37 grams, bulk magnetic susceptibility, as estimated with the 50-mm coil on an SM-30 instrument, 2.6x10-3 SI units. This result is considered a minimum estimate for log(χ)=3.42. Immediate provenance: New England Meteoritical Services.

"Rock of the Month #136, posted for October 2012" ---

Allende is arguably the most-studied meteorite, despite its relatively recent fall in the Sonoran desert of Chihuahua state, northwestern Mexico, on 08 February 1969. The Allende strewnfield is huge: some 50 km long and approximately 300 km2 in area. It is certainly the most-cited meteorite in the literature I have seen, as shown in the following table. The number of citations is drawn from the MINLIB database, described elsewhere on this web site, and exemplified by the Canadian meteorite bibliography accessible from the meteorite index page.

Some of the Most-Researched meteorites

Meteorite Class Date of find or fall No. of references
Allende CV3 1969 555
Murchison CM2 1969 421
Canyon Diablo IA 1891 221
Orgueil CI 1864 196
ALH84001 SNC-orthopyroxenite 1984 166
Semarkona LL3.0 1940 155
Nakhla SNC-nakhlite 1911 139
Abee EH5 1952 132
Tagish Lake CI2 2000 132
Zagami SNC-shergottite 1962 115
Shergotty SNC-shergottite 1865 113
Toluca IA 1776 112
Gibeon IVA 1836 103
Chassigny SNC-chassignite 1815 100

Note that some meteorites have been available much longer than others... Of the 13 meteorites listed here, four are carbonaceous chondrites, three are large iron meteorites, one is an enstatite chondrite, and no less than five represent a modern favourite, the inferred martian meteorites (SNC achondrites).

Though whole shelf-fulls have been written on this spectacular fall, a few brief notes will suffice here. Factors in the phenomenal scientific interest in the meteorite included:

  • The remarkable mineralogy, especially of the CAI,
  • The isotopic chemistry, as revealed in ever-smaller presolar grains (see below)
  • Rapid recovery of a large number of samples soon after the fall (TKW is at least 2 tonnes)
  • Wide distribution of material, available for study worldwide
  • Perfect timing!

This last point is for real: the 1960s saw the rapidly expanding use of the electron microprobe, a powerful, essentially non-destructive method of analysing the major-element composition of materials down to the levels of microns (one-thousandth of a millimetre). Small grains could be analysed, traverses across grains could reveal evidence of compositional zonation, and inclusions within grains could be analysed. The following decades saw further expansion of in-situ analysis, with instruments such as the ion microprobe, proton microprobe and laser Raman microprobe, amongst others. Analysis of isotopic ratios of major elements (e.g., of magnesium in aluminium-rich minerals such as spinel) revealed evidence of a fierce dawn of the solar system bathed in the radiation from radioactive isotopes now largely extinct. An example is 26Al, the signature of its former presence and ultimate demise recorded in the isotope ratios of magnesium, since its decay adds to the quota of its decay product, 26Mg, in the host mineral. Ultimately, in Allende and other "primitive" meteorites, some grains of minerals such as diamond and graphite were identified as presolar, formed around stars that were mature, or even entering their death throes, while our Sun was yet an infant. Indeed, one of the greatest benefits of meteorite studies has been a clearer picture of the origins of the chemical elements ("nucleosynthesis"), predicted by the growing discipline of nuclear astrophysics since the 1950s.

That's a lot to chew upon! A few choice references are appended below, concerning on Allende, the science of "stardust", and something on Smithsonian Institution staff such as Roy Clarke and Brian Mason, who were closely involved with the initial excitement of Allende's fall, recovery and characterization.

Two brief examples of the importance of Allende: starting with cosmochemistry and the age of the solar system. A recent precise study dated Allende chondrules (separated from a 30-gram sample from the Royal Ontario Museum) at 4566.2±2.5 Ma (Amelin and Krot, 2007). The best overall estimate of the age of chondrule formation is 4566.6±1.0 Ma. Chondrule formation began at or soon after the time of formation of the CAI in CV chondrites, and overlapped with the time of formation of basaltic crust and the iron cores of differentiated asteroids.

Secondly: the importance to materials science, including mineralogy. As the chemical and mineralogical techniques become ever more refined, so re-examination of Allende and other meteorites brings more and stranger minerals to light. Thus no fewer than nine new mineral species have been found in Allende samples since 2007: allendeite (an oxide of scandium and zirconium), hexamolybdenum, monipite, tistarite, davisite, grossmanite, hibonite-(Fe), panguite and kangite (Simon, 2012).

References

Clarke,RS, Jarosewich,E, Mason,B, Nelen,J, Gomez,M and Hyde,JR (1970) The Allende, Mexico, meteorite shower. Smithsonian Contributions to the Earth Sciences 5, 1-53.

Mason,B (1972) The mineralogy of meteorites. Meteoritics 7, 309-326.

Mason,B and Taylor,SR (1982) Inclusions in the Allende Meteorite. Smithsonian Contributions to the Earth Sciences 25, 30pp.

Hutchison,R (1983) The Search for our Beginning. British Museum (Natural History), London / Oxford University Press, 164pp.

McSween,HY (1999) Meteorites and their Parent Bodies. Cambridge University Press, 2nd edition, 310pp.

Clarke,RS, Plotkin,H and McCoy,TJ (2006) Meteorites and the Smithsonian Institution. In `The History of Meteoritics and Key Meteorite Collections: Fireballs, Falls and Finds' (McCall,GJH, Bowden,AJ and Howarth,RJ editors), Geol.Soc. Spec.Publ. 256, 513pp., 237-265.

Amelin,Y and Krot,A (2007) Pb isotopic age of the Allende chondrules. Meteoritics & Planetary Science 42, 1321-1335.

Simon,SB (2012) The enduring legacy of the Allende meteorite. Elements 8 no.3, 174,176.

Graham Wilson, 07 August 2012, minor update on 06 October 2012

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