CI chondrite

CI chondrites , also C1 chondrites , are stone meteorites that belong to the carbonaceous chondrites . Their importance lies in their composition, which of all the meteorites found so far comes closest to the elemental abundance distribution in the sun .

designation

The designation CI goes back to C for English Carbonaceous chondrites (carbonaceous chondrites) and I for the type locality Ivuna in Tanzania . The 1 in the designation C1 refers to type 1 of the Van Schmus Wood classification . Type 1 does not normally contain any chondrules .

Find history

Coal chondrites with a Tagish Lake find (a CI 2) in the middle

The previous finds of CI chondrites can be counted on one hand. The oldest find dates from 1806, when two meteorites fell near Alais in France (finds near Saint-Étienne and Valence ). Together they weighed 6 kilograms. In 1864 there was another decline in France near Orgueil near Montauban . The meteorite had disintegrated into 20 fragments that weighed a total of 10 kilograms. In 1911 a meteorite was sighted in Tonk , India . Several small fragments were found, but they weighed only 7.7 grams in total. At the type locality Ivuna in Tanzania, a meteorite fell to the ground in 1938 and shattered into three fragments weighing 705 grams. Another case occurred in 1965 in Revelstoke in British Columbia ( Canada ) with two fragments that were only 1 grams. In total, there are only about 17 kilograms of CI chondrites. On September 12, 2019, a racing car was spotted in parts of Belgium, the Netherlands, Germany and Great Britain . The following day a man in Flensburg discovered a stone weighing 24.5 grams in his garden, which turned out to be CI chondrite and is probably a remnant of the event from the previous day.

The discovery of a meteorite on the moon during the Apollo 12 mission (1969) is worth mentioning . The find was initially mistaken for a CI chondrite, but then turned out to be a closely related CM chondrite . A meteorite fell on Tagish Lake in the Yukon Territory in 2000 and is now classified as a CI chondrite. Amazingly, it contains chondrules and is therefore listed as CI 2.

description

CI chondrites are quite fragile, porous rocks that disintegrate very easily when passing through the atmosphere . That is why there are only relatively small fragments. In the Revelstoke case, a huge fireball was seen, but, as already mentioned, only two fragments under one gram were found. CI chondrites have a matt, black enamel crust that is often very difficult to distinguish from their uniform matrix inside. The opaque matrix is colored black due to the high content of carbon-rich material, fine-grain magnetite and (subordinate) pyrrhotite . It can also lead to white-colored, hydrous carbonates and sulfates .

The main feature of CI chondrites is the lack of distinct chondrules (exception: Tagish Lake). Tiny chondral fragments and calcium-aluminum-rich inclusions ( CAI ) occur, albeit very rarely.

Mineralogical composition

CI chondrites contain the following minerals in their matrix :

Olivine . Tiny, equally sized, idiomorphic grains of forsterite (with fayalite Fa _10-20 ). Developed in the high temperature range.
Clinopyroxene . Also tiny, equally sized, idiomorphic grains. Developed in the high temperature range.
Orthopyroxene . Tiny, equally sized, idiomorphic grains. Developed in the high temperature range.
Magnetite. Formed as framboids, spherulites and platelets.
Pyrrhotite.
Water-containing, clay-rich silicates such as montmorillonite and serpentine- like minerals.
Epsomit . As microscopic veins.

The ferromagnesium minerals occur only in isolation and surprisingly show no discernible transformation phenomena. In the case of montmorillonite and the serpentine-like ones, however, it is assumed that they emerged from magnesium-rich olivines and pyroxenes under water supply.

Chemical composition

CI chondrites contain between 17% and 22% water . The high porosity (up to 30%) is likely to be related to this fact. However, water only occurs in bound form in the water-containing silicates. The occurrence of epsomite indicates the presence of liquid water in the parent rock of the meteorite, which penetrated through cracks. The sulfates and carbonates were then also deposited on the cracks.

The iron (25 weight percent) in CI chondrites practically only in oxidized form, iron sulfides and elemental iron are practically non-existent. The Mg / Si ratio is quite high at 1.07. Only CV chondrites are even more enriched in Mg. The Ca / Si ratio is the lowest within the carbonaceous chondrites at 0.057. However, the δ ¹⁷ O and δ ¹⁸ O values are highest among the carbonaceous chondrites. The ratio of the two oxygen isotopes 17/18 corresponds to the values found in terrestrial samples.

Physical parameters

Because of their high porosity, CI chondrites only have a very low density of 2.2 g / cm ³ .

meaning

Of all the meteorites found so far, CI chondrites are most similar to the elemental abundance distribution in the original solar nebula . They are therefore also referred to as primitive meteorites . Except for the volatile elements carbon, hydrogen , oxygen and nitrogen , which occur depleted in CI chondrites, the frequencies are practically identical. Lithium is another exception; it is enriched in the meteorites opposite the sun (lithium is consumed during nucleosynthesis in the interior of the sun).

Of great importance is the fact that in CI chondrites with its very high carbon content in addition to the inorganic carbonates and graphite and organic carbon compounds are included - in particular in regard to the discussion about the origin of life (as well as found, for example amino acids ) .

normalization

Because of the similarities of the CI chondrites with the matter of the solar nebula, rock samples are normalized in petrology compared to CI chondrites. That is, the sample / chondrite ratio is established for the element in question. Ratios greater than 1 indicate an enrichment of the sample compared to solar matter, ratios less than 1 indicate a depletion. This method is mainly used in the so-called spider diagrams and mainly affects the lanthanoids ( rare earths ).

The element concentrations used in normalization are as follows:

Normalization of CI chondrites
	La	Ce	Sr	Nd	Zr	Sm	Eu	Gd	Ti	Dy	Y	He	Yb	V	Cr
CI chondrite ppm	0.2347	0.6032	7.8	0.4524	3.94	0.1471	0.056	0.1966	436	0.2427	1.56	0.1589	0.1625	56.5	2660

Compared to CI chondrites, earthly mantle and sub-crustal rocks enrich the rare earths with the exception of lanthanum . In return, CI chondrites can in turn have higher levels of the trace elements cerium , chromium , neodymium , strontium , vanadium and zirconium .

Place of origin

CI chondrites and the closely related CM chondrites are very rich in volatile substances, especially water. It is therefore assumed that they originated in the outer asteroid belt > 4 AU . For the former solar nebula, this represented a critical distance is, the so-called snow line (Engl. Snowline ), outside of which water at 160 K to ice condensed and remained thus obtained. In terms of their composition, CI chondrites actually show a resemblance to the icy moons of the outer solar system. Furthermore, there is a direct relationship to comets : how they also accreted the CI asteroids silicate minerals, ice and other volatiles, as well as organic compounds (see Comet Halley ).

literature

OR Norton: The Cambridge Encyclopedia of Meteorites . Cambridge University Press, 2002, ISBN 0-521-62143-7 .

Individual evidence

↑ Spiegel.de: Researchers discover extremely rare meteorites in Flensburg
^ The Meteoritical Society: Entry for Flensburg
^ RT Dodd: Meteorites: A Petrologic-Chemical Synthesis . Cambridge University Press, New York 1981, pp. 36-38 .
↑ M. Zolensky, HY McSween,: Aqueous alteration. Meteorites and the Early Solar System . University of Arizona Press, Tucson 1988, pp. 137 .
↑ B. Mason: Meteorites . John Wiley and Son Inc., New York 1962.
↑ H. Von Michaelis, IH Ahrens, JP Willis: The compositions of stony meteorites - II. The analytical data and an assessment of their quality . In: Earth and Planetary Scientific Letters . tape 5 , 1969, p. 387-394 .
^ WR Van Schmus, JM Hayes: Chemical and petrographic correlations among carbonaceous chondrites . In: Geochimica Cosmochimica Acta . tape 38 , 1974, p. 47-64 .
↑ E. Anders, N. Grevesse: Abundances of the elements: Meteoritic and solar . In: Geochim. Cosmochim. Acta . tape 53 , 1989, pp. 197-214 .

[1] Spiegel.de: Researchers discover extremely rare meteorites in Flensburg

[2] The Meteoritical Society: Entry for Flensburg

[3] RT Dodd: Meteorites: A Petrologic-Chemical Synthesis . Cambridge University Press, New York 1981, pp. 36-38 .

[4] M. Zolensky, HY McSween,: Aqueous alteration. Meteorites and the Early Solar System . University of Arizona Press, Tucson 1988, pp. 137 .

[5] B. Mason: Meteorites . John Wiley and Son Inc., New York 1962.

[6] H. Von Michaelis, IH Ahrens, JP Willis: The compositions of stony meteorites - II. The analytical data and an assessment of their quality . In: Earth and Planetary Scientific Letters . tape 5 , 1969, p. 387-394 .

[7] WR Van Schmus, JM Hayes: Chemical and petrographic correlations among carbonaceous chondrites . In: Geochimica Cosmochimica Acta . tape 38 , 1974, p. 47-64 .

[8] E. Anders, N. Grevesse: Abundances of the elements: Meteoritic and solar . In: Geochim. Cosmochim. Acta . tape 53 , 1989, pp. 197-214 .