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  CM2

TKW 4 kg. Observed fall 30 Jun3 1879, Entre Rios, Argentina.


   

Steve writes:

An observed fall, the Nogoya meteorite landed in Argentina on June 30th, 1879. It was only the second witnessed CM fall (Cold Bokkeveld – or Koue Bolleveld, meaning "cold buck shrubland" in Afrikanns – fell over Western Cape Province in South Africa just over four decades earlier in 1838). Interestingly, the namesake for the CM class of carbonaceous chondrites is the Ukrainian Mighei meteorite, yet that one fell in 1889 – a full decade after Nogoya.

The two thin sections I have of this meteorite, like most CM2 carbonaceous chondrites, aren’t all that visually interesting and in fact were poorly prepared (the best of the two is shown in Photo 1). So I thought I’d include instead two collages of some of their more interesting areas (Photos 2 and 3 show four different features each at various magnifications).

CM chondrites are chemically-primitive solar system materials that have undergone high degrees of aqueous alteration in an environment that gradually changed in composition, resulting in the formation of secondary minerals that include carbonates (which at 1.4% in Nogoya are its second most abundant class of carbon-bearing phases – organic matter being the first at around 2%). But Nogoya’s carbonates have a distinctive composition compared with other CM carbonaceous chondrites. And Nogoya contains two lithologies: a dominant CM2.2 one containing no aragonite, and a secondary aragonite-bearing one more similar to a CM2.3 in its degree of alteration.

At 4 kg, Nogoya was not a particularly small or large fall, but it is a meteorite that is difficult for the collector to obtain. Conversely, it has been well-studied, usually in comparison with similar carbonaceous chondrites such as Murchison and Murray.

As an example, a 1965 article described using electronic paramagnetic resonance signals to study free organic radicals in Nogoya and Mighei; the fact that their results were sufficiently similar with coal suggested to the authors the existence of extra-terrestrial biogenic activity. In addition, the scientists concluded that carbonaceous chondrites were never exposed to peak metamorphic temperatures (PMTs) over 300 – 400 degrees Celsius (a result that was confirmed later by different research using Raman spectroscopy to study organo-carbonate relationships in several CM carbonaceous chondrites – research that also suggested the CM parent bodies have experienced at least two different kinds of metamorphism).

As an aside, the findings regarding the low PMTs played a part in the design of NASA’s OSIRIS-Rex hardware – since the asteroid Bennu is thought to essentially be a carbonaceous chondrite, the return capsule and heat shield were specifically designed to keep the acquired regolith samples below 80 degrees Celsius so as to avoid thermochemical decomposition.

Like other carbonaceous chondrites, Nogoya contains water, as well as amino acids and other complex organics. Some of its amino acids have been found to exhibit strange isotopic signatures that to some scientists suggest interstellar matter from nebulae or possibly a solar system outside of our own – matter that was trapped in Nogoya more than 4.5 billion years ago when its parent body formed.

Besides chondrules, Nogoya’s phyllosilicate and magnetite matrix also contains light-colored high-temperature inclusions – inclusions lacking in CI carbonaceous chondrites whose matrix is otherwise similar. In addition, some of the coarse olivine in Noyoga’s chondrules and isolated in its matrix have been replaced by serpentine, but because its composition is different than the serpentine replacement observed in some of the other CM2 chondrites, scientists believe there are multiple CM2 parent bodies (with Nogoya’s being more highly evolved toward Mg-rich solutions).

Laser-spot analysis on Nogoya and three other CM carbonaceous chondrites shows rims around some of their chondrules that are rich in primordial noble gases, suggesting these gases accreted on chondrules even before their parent bodies formed. Further, trapped solar gases in Nogoya and other carbonaceous chondrites suggest that gas implantation must have occurred between 1 and 8 AUs from the sun, indicating their parent bodies likely reside within the asteroid belt, where they had to remain long enough to develop a substantial regolith (these results essentially rule out a cometary origin for any of the gas-rich stony chondrites).

Finally, multiple analyses of elemental abundances and isotopic compositions of carbon and nitrogen in carbonaceous chondrites show no systematic differences between falls such as Nogoya and finds, suggesting terrestrial exposure has little effect on bulk carbon and nitrogen chemistry in these meteorites.