CM chondrite


CM chondrites are a group of chondritic meteorites which resemble their type specimen, the Mighei meteorite. The CM is the most commonly recovered group of the 'carbonaceous chondrite' class of meteorites, though all are rarer in collections than ordinary chondrites.

Overview and Taxonomy

Meteorites mostly divide into Ordinary and 'Carbonaceous' chondrite classes; far fewer belong to lesser classes like Enstatites and Ureilites. The term 'chondrite' indicates that these contain chondrules in a matrix. Chondrules are cooled droplets of minerals, predating the meteorites themselves. The term 'carbonaceous' was assigned relative to the ordinary chondrites; some Enstatite and Ureilite meteorites may have more carbon than C-chondrites. Still, all C-chondrites are distinguished from ordinary chondrites by a non-trace carbon content, plus other volatiles, giving a lower density. After the classes were devised, a more rigorous definition was found: C-chondrites contain proportionally higher magnesium than ordinary chondrites.
The C-chondrites subdivide into CI, CM, CO, CV, CK, CR, and lesser groups. Specimens are formed into groups by their petrological and chemical qualities, and the group named for a salient example. These include the CI, CM, CO, etc. The CM group most resembles the CI and CO chondrites; a CM–CO is sometimes described. All three groups contain clearly anomalous 50Ti and 54Cr isotopes.
Though the C-chondrites are far rarer than ordinary chondrites, the CM group is "the most abundant type of" them. The latest Catalogue of Meteorites gives 15 CM falls, and 146 finds. By contrast, the next highest are the COs- 5 falls, 80 finds listed. These are in a class of 36 C-chondrite falls, 435 finds. If the CMs and COs are taken to be a clan, its dominance is even higher.

Petrologic types

C-chondrites in general, and CM chondrites among them, have low densities for meteorites. CMs are slightly more dense than the CIs, but less dense than CO and other C-chondrites. This is due to a combination of brecciation including porosities and inherently light constituent materials.
Based primarily on petrology, early scientists attempted to quantify different meteorites. Rose, then Tschermak devised early taxonomies. In the 1904 scheme of Brezina, today's CM chondrites would be "K". Wiik published the first recognizably modern system in 1956, dividing meteorites into Type I, II, and III. CMs fell within Wiik's Type II.
The CM chondrites are essentially all Type 2 in the petrographic scale of Van Schmus and Wood 1967; by that time, CI and CM recoveries were enough to define the 'left' end of the scale. The types 4 through 6 indicate increasing thermal alteration; Type 3 is assumed to be unaltered.
Van Schmus, Wood 1967; Sears, Dodd 1988; Brearley, Jones 1998; Weisberg 2006
The modern groups 'V' and 'O' were named by Van Schmus in 1969 as divisions of Type 3, as 'subclass ' and . Wasson then added C2M in 1974; since then, C2Ms have generally been shortened to simply 'CM', as have the other groups.
Group1234567
CI
CM
CR
CH
CB
CV
CO
CK

After Weisberg et al. 2006, Giese et al. 2019'' Note: lone CV2 specimen, Mundrabilla 012

Chondrules and similar

As Type 2 meteorites, CM chondrites have some remaining chondrules; others have been modified or dissolved by water. COs have more chondrules; CIs have either trace outlines of former chondrules or, some have argued, never contained any chondrules at all. Many CM chondrules are surrounded by either rims of accessory minerals, or haloes of water-altered chondrule material.
The chondrules of CM chondrites, though fewer, are larger than in COs. While CM chondrules are smaller than average in diameter, CO chondrules are exceptionally small. This may be a survivor bias: consider that the water which dissolves CM chondrules successfully eliminates those which are already small, while those which were large may remain to be observed, though with less of the original material. Similarly, CMs contain minor CAIs.

Matrix

The matrix of CMs has been described as "sponge" or "spongy."
Grains of olivine and pyroxene silicates, too, are fewer in CM meteorites than COs, but more than CIs. As with chondrules, these are water-susceptible, and follow the water progression of the petrographic scale. So, too, do grains of free metal. CO meteorites contain higher levels of free metal domains, where CIs have mostly oxidized theirs; CMs are in between.
Both free metal, and grains of olivine/pyroxene, have been largely or predominantly altered to matrix materials. A CM meteorite will consist of more matrix than a CO, but less than a CI.
In 1860, Wohler presciently or coincidentally identified matrix as serpentinite. Fuchs et al. 1973, unable to identify the constituent phyllosilicates, gave matrix as "poorly characterized phase". Cronstedtite was published by Kurat and Kracher in 1975.
Tomeoka and Buseck, identifying cronstedtite and tochilinite in 1985, gave matrix material as "FESON", as well as the backronym "partly characterized phase" for "PCP." Later authors would use the term TCI, tochilinite-cronstedtite intergrowths. Less common phyllosilicates include chlorite, vermiculite, and saponite.

Sub-Classification

The CM group is both numerous and diverse. Multiple attempts have been made to subdivide the group beyond the Van Schmus–Wood typing. McSween 1979 was an early proposal. After him, these add a suffix after the petrologic type, with 'CM2.9' referring to less-altered, CO-like specimens, and 'CM2.0' being more-altered, CI-like meteorites.
McSween 1979 graded the amount of matrix versus total amount, and the depletion of iron in the matrix, to quantify higher degrees of alteration.
Browning et al. 1996 devised a formula, quantified the amount of unaltered silicate grains, and graded the alteration level of chondrules to quantify alteration.
Rubin et al. 2007 added measurement of carbonates, with more dolomite and less calcite indicating higher alteration.
Howard et al. 2009, 2011 measured total abundance of phyllosilicates to quantify alteration.
Alexander et al. 2012, 2013 measured deuterium level, C/H, and nitrogen isotopes to quantify alteration.
This line of inquiry continues, as the systems have some disagreement on specimens. Murchison is consistently ranked as low-alteration, but authors differ on some more-altered meteorites.

Transitional examples

CM–CO
  • Paris – described as "the least altered CM chondrite so far" "that bridges the gap between CMs and COs"
  • ALHA77307
  • Adelaide
  • Acfer 094
  • MAC87300, MAC88107
CM–CI
The CI and CM chondrites are the "water rich" meteorites, CMs having 3–14 wt% water. Water is contained in tochilinite, cronstedtite, and others.
This water, not comets, was the likely origin of Earth's oceans via isotope tracing.

Fluid inclusions

containing meteorite water have long been reported; however, these claims were doubted due to, e. g., contamination by cutting fluids during sectioning. More modern claims have taken steps such as waterless preparation.

Chemistry

Carbonaceous chondrites, as the name suggests, contain appreciable carbon compounds. These include native carbon, simple compounds like metal carbides and carbonates, organic chains, and polycyclic aromatic hydrocarbons.
The elemental abundances of some C-chondrite groups have long been known to resemble solar abundance values. The CI chondrites, in particular, correspond "quite closely, more so than does any other type of meteoric or terrestrial matter"; called "somewhat miraculous". Of course, only gas giant planets have the mass to retain, explicitly, hydrogen and helium. This extends to most noble gases, and to lesser amounts the elements N, O and C, the atmophiles. Other elements- volatiles and refractories- have correspondences between CI chondrites and the solar photosphere and solar wind such that the CI group is used as a cosmochemical standard. As the Sun is 99% of the mass of the Solar System, knowing the solar abundance is the starting point for any other part or process of this System.
The solar correspondence is similar but weaker in CM chondrites. More-volatile elements have been somewhat depleted relative to the CIs, and more-refractory elements somewhat enriched.
A small amount of meteorite materials are small presolar grains. These are crystals of material which survives from interstellar space, since before the formation of the Solar System. PSGs include silicon carbide and micro-diamonds, as well as other refractory minerals such as corundum and zircon. The isotope levels of their elements do not match solar system levels, instead being closer to e. g., the interstellar medium. PSGs themselves may contain smaller PSGs.
As with other meteorite classes, some carbon content is as carbides and carbonates such as calcite and dolomite. Aragonite appears, where CIs contain little or none.
Total carbon compounds in CM chondrites are lower than in CI chondrites; however, more are aromatics. Isotope profiling indicates these are meteoritic, not terrestrial.
The organics of C-chondrites divide into soluble, and IOM. The soluble fraction would yield to the chemistry techniques of the mid-20th century, giving paraffin, naphthene and aromatics, with other contributions.
The IOM is, however, the clear majority of the organic component; in 1963, Briggs and Mamikunian could only give it as "very high molecular weight". IOM itself divides into two components: thermally labile, and refractory.