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Sutter’s Mill

CM2.0/2.1 chondrite (Yamakawa and Yin, 2013)
Genomict Regolith Breccia
(with thermally metamorphosed (dehydrated) and Tagish Lake-like components)

Photo taken by Lisa Warren in Reno, Nevada
Fell April 22, 2012
38° 48′ 14′ N., 120° 54′ 29′ W. On Earth Day 2012, April 22 at 7:51 A.M., a fireball accompanied by a sonic boom was seen, heard, and even smelled by local residents as it streaked over California and Nevada moving in a generally western direction. As the approximately 70-ton, 3-m-sized object reached an altitude of 48 km at a speed of 28.6 (±0.7) km/second, it exploded with the release of energy equivalent to an ~4-kiloton explosion (Jenniskens et al., 2012). Fragments of ‘black gold’ fell within a strewn field encompassing the towns of Lotus and Coloma, including the location of the first discovery of California gold in 1848 at Sutter’s Mill.

Two days after the fall, the first charcoal-colored stone weighing 5.5 grams was recovered by meteorite hunter Robert Ward. Utilizing NEXRAD high-resolution Doppler weather radar data, along with seismic data, Marc Fries of the Planetary Science Institute constructed a more accurate map of the inferred strewn field. Because of rather strong winds aloft blowing towards the ENE, the approximately 4 × 2 mile strewn field has been depicted curving slightly to the north reflecting the drift of smaller, lighter fragments. Models predict that the largest fragments weighing perhaps 10–20 kg would have landed ~19 miles farther west of the known strewn field (Fries et al., 2012). A helium airship was employed by scientists from NASA and the SETI Institute to search for possible impact features, but none were reported. Over the next few weeks, meteorite hunters and locals together spent thousands of manhours searching the rattlesnake and poison oak infested strewn field collecting numerous small fragments. The largest single find weighing in at 205.2 g was made by Jeffrey Grant. Although many remain unofficially recorded, nearly 100 Sutter’s Mill fragments have been recovered having a combined weight of over 1,000 grams.

A consortium of investigators led by Dr. Peter Jenniskens of the SETI Institute has begun the long process of analysis and classification. Initial characterization of Sutter’s Mill conducted at Johnson Space Center by M. Zolensky (2012) indicated that this is a carbonaceous chondrite breccia showing many petrological similarities to CM chondrites. Sutter’s Mill was described as a highly comminuted regolith breccia by Kebukawa et al. (2013), consistent with the wide variety of components present in the matrix and the presence of solar-wind-implanted noble gases. Matrix components include chondrules similar in size to those in CM2.5 Murchison, isolated lithic fragments, aggregates of forsteritic olivine and low-Ca pyroxene, abundant CAIs, grains of the sulfides pyrrhotite, pentlandite, and oldhamite (CaS), and rare FeNi-metal; the low abundance of the latter being attributed to seismically-driven gravitational sorting/settling by Zolensky et al. (2013). The presence of oldhamite and Fe-Ni-Cr phosphides in Sutter’s Mill attests to the impact fragmentation of an E chondrite or aubrite (A. Rubin, UCLA), or alternatively, the oldhamite could have been formed during the dehydration process at temperatures of at least ~750°C as outlined by Haberle et al. (2013), or during impact-generated heating to >300°C as suggested by Beck et al. (2013).

Ott et al. (2013) found that diamond was present in an abundance of ~471 to ~1460 ppm, indicating the possible admixture of a ureilite component, while a lower limit was calculated for a presolar SiC content in Sutter’s mill of 3.8 (±0.4) ppm. Analyses of specimen SM2-5 by Kebukawa et al. (2014) led to their discovery of two relatively large diamond grains, considered to be xenolithic in origin, and to have likely formed through a chemical vapor deposition (CVD) process on a large parent body. In addition, Haberle et al. (2013) reported finding bluish-white grains within the matrix that have been identified as the first occurrence of portlandite (Ca(OH)2), thought to be a product of reduction of CaSO4 catalyzed by CO and CO2. Utilizing X-ray micro-tomography, Tsuchiyama et al. (2014) have found ubiquitous µm-scale solid inclusions present in all calcite grains studied, and they identified one calcite grain that likely harbors an ~2 µm-sized remnant spherical fluid inclusion incorporating a bubble with a solid particle inside. However, due to the small size of the bubble it could not be ascertained if it contains an aqueous fluid.

Advanced analyses of numerous Sutter’s Mill samples were conducted at the Center for Meteorite Studies (L. Garvie, 2013), leading to the conclusion that there are two distinct mineralogical classes present—one is rich in olivine and the other is rich in amorphous clays. The olivine-rich (75–80 wt%) material exhibits characteristics akin to the Belgica-group of thermally metamorphosed CM chondrites, while the clay-rich material is considered to be a strong match to the C2-ungrouped Tagish Lake. It was proposed that both of these disparate classes of chondritic material were independently incorporated into the Sutter’s Mill parent object, itself characterized as a rubble pile.

A detailed study of Sutter’s Mill by Beauford et al. (2012, 2013) revealed that it is a complex regolith breccia consisting of a primary accretional matrix containing two dominant clast lithologies present in approximately equal abundances in variable combinations and in breccia-in-breccia clasts, attesting to a history of impact mixing and regolith recycling. One clast type is a dark-colored chondrule-rich lithology (CRD) and the other is a light-colored chondrule-poor lithology (CPL), with the components of each expressing a different degree of aqueous alteration. Other minor lithologies reported include sub-mm-sized dark inclusions (DI), carbonate-rich clasts, and a xenolithic component consisting of enstatite, oldhamite, and phosphides likely derived from E chondrites or aubrites (Zolensky et al., 2014).

Similar to the dust mantles prevalent around other CM components, dark, fine-grained rims are present on many of the coarse-grained objects in Sutter’s Mill. These rims are considered likely to have formed and hardened during impact compaction processes, but they might be accretionary rims developed in the nebula, or possibly the result of a combination of both of these formation processes (Haack et al., 2012). These fine-grained rims were investigated by Beauford and Sears and it was found that their presence is limited to those primary CRD lithologies that experienced only limited aqueous alteration, and that their formation was restricted to the period prior to comminution and evolution of the CM regolith.

Nagashima et al. (2012) found that O-isotopic compositions of olivine from type-I and type-II chondrules and AOAs plot along the CCAM line, and they identified abundant coarse dolomite and calcite grains, the latter having O-isotopic compositions nearly identical to calcites in CM chondrites. Major and trace element analyses of three separate samples were consistent with those of CM chondrites (Yin et al., 2012; Friedrich et al., 2012). Grady et al. (2012) studied the abundance and isotopic composition of carbon and argon by stepped combustion in a Sutter’s Mill sample and found close similarities to carbonaceous chondrites, with the closest match demonstrated for C2-ung Tagish Lake. O-isotopic measurements conducted by Kohl et al. (2013) of acid-washed Sutter’s Mill material, thus eliminating carbonate mineral influence, led them to conclude that aqueous alteration increased the water/rock ratio and shifted the three-isotope plot away from the CCAM line towards the TFL.

Like CM chondrites, Sutter’s Mill is a breccia containing features indicative of both weak aqueous alteration and thermal metamorphism to 500–750°C affecting the chondrule mesostasis. Some clasts have been heavily aqueously altered to subtype 2.0, resulting in the replacement of some chondrule constituents with the phyllosilicates Fe-cronstedtite/tochilinite + Mg-serpentine (A. Rubin, UCLA). Notably, Howard et al. (2009) have argued that the phyllosilicate abundances among CM chondrites are within a few percent of each other, and thus reflect similar aqueous alteration processes. Other clasts (e.g., SM2-5, thought to represent a comminuted regolith breccia) exhibit secondary heating features consistent with Stage III, based on the scale of Nakamura (2005) (Zolensky et al., 2014). In these clasts, phyllosilicates have been converted to fine-grained olivine, tochilinite has been converted to troilite, and carbonates have been destroyed. See the Murchison page for further details on classification based on thermal metamorphism.

Cooper and Jenniskens (2012) and Dillon et al. (2013) measured soluble organic compounds in Sutter’s Mill and identified mono-carboxylic acids (e.g., formic acid and acetic acid) typically present in significant abundances in many CM chondrites; they were present in much lower abundances than in Murchison. In addition, large abundances of soluble inorganic compounds were found, particularly sulfate, which is common to CM chondrites. Using advanced methods to characterize soluble organic compounds, Schmitt-Kopplin et al. (2012) found that they were present in comparatively low abundances, comprising highly oxygenated species or organometallic compounds. An organic C component was determined to reside in both hollow and filled, 15N-rich nanoglobules that likely formed in the cold solar or presolar nebula (Nakamura-Messenger et al., 2013). They also observed that the constituents in the relatively anhydrous matrix component of Sutter’s Mill were mineralogically similar to the matrix of Acfer 094, a unique carbonaceous chondrite tentatively classified as a subgroup of the CO chondrites; Simon and Grossman, 2015). Utilizing X-ray spectroscopy in a study of specimen SM2-5, Kebukawa et al. (2014) determined that the matrix organic matter has a lower N/C ratio compared to other carbonaceous chondrites.

Pizzarello et al. (2012) determined that amino acids were scarce in Sutter’s Mill, and that they contain low-complexity hydrocarbons, mainly naphthalene. Analyses by Glavin et al. (2012, 2013) of both pre- and post-rain samples also revealed lower C2–C5 amino acid abundances (~660–9,500 ppb) compared to those in Murchison (~14,000 ppb). Similarly, analyses of one of the most pristine Sutter’s Mill specimens (SM2) by Burton et al. (2012) found a 20 × lower abundance of amino acids than measured in Murchison. These low levels are considered likely the result of significant parent body aqueous alteration and/or thermal (>150°C) metamorphism. Advanced infrared analyses by Flynn et al. (2013) indicated the presence of carbonates and associated organic matter. This organic matter consists in large part of aliphatic hydrocarbons, and it was determined to be compositionally different from organic matter identified in Murchison, but consistent with the type identified in Tagish Lake.

CT scans conducted at AMNH (Ebel et al.) provided density and porosity data for two Sutter’s Mill samples. From these it was determined that Sutter’s Mill has a bulk density of 2.23 g/cm³. Similarly, bulk density and grain density measurements were made by Britt et al. (2012) using conventional methods. The bulk density for one sample was determined to be 2.31 g/cm³; all sample density values are within the range of those for CM chondrites. A measurement of porosity showed that it is relatively high at 31 (±1.4) %, also similar to typical values for CM chondrites. Likewise, the magnetic susceptibility value is within the range for CM chondrites. Results of reflectance spectroscopy performed by Grady et al. (2013) was consistent with a CM classification.

An analysis of Mn–Cr systematics in Sutter’s Mill calcite and dolomite revealed a resetting of this chronometer ~4.563 b.y. ago, while a more recent resetting event within 1 b.y. ago was evident in the Re–Os system (Walker et al., 2013 and references therein). The ε54Cr value for Sutter’s Mill calculated by Yamakawa and Yin (2013) is identical to that of Murchison, indicating an origin for both meteorites from the same precursor. Similarly, the secondary carbonate mineral dolomite was utilized by Jilly et al. (2014) for Mn–Cr radiometric dating in Sutter ’s Mill. This short-lived (half-life = 3.74 m.y.) chronometer is well suited for that purpose in that Mn becomes sequestered in the precipitating carbonate, while Cr remains with the percolating aqueous fluid. This creates a measurable excess of 53Cr through the in situ decay of radioactive 53Mn, a value which is temporally related to the onset of secondary carbonate formation during aqueous alteration. This age for carbonate formation was determined to be 4.5637 (+0.001.1/–0.001.5) b.y., or 2.34–5.26 m.y. after CV3 CAIs, which is an absolute age anchored to the U-corrected Pb–Pb age of the D ’Orbigny angrite. Their study showed that carbonate formation occurred relatively early in Sutter’s Mill, as well as in other carbonaceous chondrite groups—a result of aqueous processing sustained by radiogenic heating of accreted ices.

Noble gas studies conducted by Hamajima et al. (2012) and Ott et al. (2013) established a very young CRE age for Sutter’s Mill that defines the low end of the range for the CM2 chondrite group (previously exhibiting two major peaks at 0.2 and 2.0 m.y.), reflecting a relatively recent ejection from its parent body ~19–51 t.y. ago. Cosmogenic radionuclide studies conducted by Nishiizumi et al. (2014) provide a similar very young CRE age of 82 (±8) t.y. Another noble gas study of Sutter’s Mill was conducted by Okazaki and Nagao (2017). Based on 21Ne and the estimated shielding depths of the samples, they calculated a CRE age of 59 (± 23) t.y. In a broad study of cosmogenic radionuclides in Sutter’s Mill and in a large number of CM group members, Nishiizumi et al. (2013) detected several major collisional clusters representing a mixture of both petrologic types 1 and 2. The pre-atmospheric size of the Sutter’s Mill meteorioid was also calculated by Nishiizumi et al. (2013, 2014), and it was demonstrated to have been a minimum of ~1 m in diameter based on a bulk density of 2.3 g/cm³, which is consistent with the estimate of 1–2 m based on other parameters.

Current studies suggest that both cometary dust and meteorites should be produced from the disruption of Jupiter-family comets which originate in the Kuiper belt. Studies have shown that Antarctic micrometeorites have a similar carbonaceous chondrite:ordinary chondrite ratio (~7:1) as the composition of zodiacal dust (M.M.M. Meier, 2014). Based on observational evidence and current modeling, it is thought that comets should be dark in color and have a low density and strength, a high porosity, a solar ratio of elements, an elevated ratio of C, H, O, and N, a high interstellar grain content, anhydrous and highly unequilibrated silicates, few to no chondrules, and a low cosmic-ray exposure age (<10 m.y.). Both the CI and CM groups of meteorites exhibit characteristics which are consistent with the above descriptions.

Orbital data obtained from several carbonaceous chondrites (e.g., the CI chondrite Orgueil [eyewitness plotting] and the CM chondrites Maribo and Sutter’s Mill [instrument recording] are a good match to the orbits expected from the disruption of Jupiter-family comets, but are unlike the orbits of ordinary chondrites and most other asteroidal objects (M.M.M. Meier, 2014). Both the orbital eccentricity and semimajor axis for Maribo is nearly identical to those of Comet Encke and the associated Taurid swarm of objects (Haack et al., 2011). On the other hand, a CRE age study of CM chondrites conducted by Meier et al. (2016) shows a possible relationship exists to the asteroid breakup event ~8.3 m.y. ago that formed the Ch/C/Cg-type members of the Veritas family. In addition to the large abundance of 3He-enriched interplanetary dust discovered in 8.2 m.y.-old deep-sea drill cores, ~1/6 of all CM meteorites have 21Ne-based CRE ages that are consistent with derivation from this catastrophic breakup, while others with significantly younger CRE ages could represent secondary collisions among the Veritas fragments.

Fragments from many Sutter’s Mill samples were generously donated to the University of Arizona and other institutions where studies will be conducted in preparation for the upcoming carbonaceous asteroid sample return mission, OSIRIS-REx. The 0.053 g specimen shown above is a portion from stone #SM14, as listed in Jeniskens’ official NASA database, a stone that impacted the garage door of Suzanne Matin and broke into fragments weighing together 11.5 g. A BBC News video captured the actual recovery of portions of this stone, and also features meteorite hunter Mike Farmer searching the strewn field with fellow hunters as he discusses the significance of this rare fall.


The photo above shows the broken fragments of a stone that impacted a parking lot, which were immediately recovered by Dr. Peter Jenniskens.

<!–
Video of the fall of the Sutter’s Mill meteorite, April 22, 2012
A brilliant fireball plunges through the sky of the Western United States, accompanied by several detonations.
note: reload page to repeat this video

–>

April 22nd Sutter Mills Meteor from Shon Bollock on Vimeo.
Only known video of the fall of the Sutter’s Mill meteorite, inadvertently caught on a GoPro by Shon Bollock

An X-ray computed tomography video of Sutter’s Mill 18. The image resolution is 14 micron/voxel and the field of view is ~2 × 3 cm².
Courtesy of Yin Lab at UC Davis.
note: reload page to repeat this video


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NWA 7821

C2-ung or CM2-an
standby for nwa 7821 photo
Purchased 2012
Eight fragments of a single meteorite having a combined weight of 38 g were found in the desert region of Northwest Africa. The pieces were sold at the 2013 Tucson Gem and Mineral Show to G. Hupé. A type sample was submitted for analyses and classification to the University of Washington at Seattle (A. Irving and S. Kuehner), and NWA 7821 was determined to be a new ungrouped C2 chondrite, or alternatively, an anomalous member of the CM chondrite group.

Northwest Africa 7821 is a low-density carbonaceous chondrite containing scattered granular chondrules ranging in size from 0.1 to 0.4 mm. CAIs are present in a fine-grained matrix having a composition similar to that of CM chondrites. The meteorite has been shocked to S2 and terrestrially weathered to a grade of W2. On an oxygen three-isotope diagram, NWA 7821 plots along the CM trend line for CM chondrites but with a higher than normal 16O content (Carnegie Institution, Washington D.C. (D. Rumble, III; see O-isotopic diagram). The specimen of NWA 7821 shown above is a 0.056 g partial slice. The photo shown below is one of the larger slices photographed by G. Hupé.

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Photo courtesy of Greg Hupé—Nature’s Vault


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NWA 4765

CM1
(CM2.0; Rubin et al., 2007) standby for nwa 4765 photo
click on photo for a magnified view Purchased 2006
no coordinates recorded A single, partially crusted meteorite fragment weighing 42 g (19 g recorded for NWA-series designation) was found in Northwest Africa, and a portion was sold through A. Habibi to meteorite collector S. Ralew in Erfoud, Morocco. A type specimen was submitted for analysis and classification to the Museum für Naturkunde (A. Greshake and M. Kurz), and it was determined that NWA 4765 is a CM1 carbonaceous chondrite, the first of its kind to be found outside of Antarctica. An analysis of the O-isotopes was conducted by the Open University (Franchi and Greenwood).

This meteorite has been exposed to extensive parent-body aqueous alteration, and all mafic silicates have been converted to phyllosilicates. Chondrules have been severely altered and converted to pseudomorphs composed of phyllosilicate, while various secondary minerals such as carbonates and pyrrhotite are now abundant. Fine-grained accretionary rims, considered by some to be rims formed after parent body accretion through impact-compaction of fine-grained, porous matrix material (Trigo-Rodriguez et al., 2006), surround some discrete coarse-grained objects. Aqueous alteration processes also contributed to the replacement of melilite with phyllosilicate in refractory inclusions, and to the subsequent disruption of the inclusions (Rubin, 2007). Northwest Africa 4765 has experienced a low degree of shock and exhibits minor terrestrial weathering.

The degree to which various CM chondrites have been aqueously altered has been determined through such factors as the water/rock ratio, the temperature, and the duration of the alteration process. A new aqueous alteration sequence for CM chondrites was proposed by Rubin et al. (2005, 2007) due to the fact that chondrule pseudomorphs are present in the least altered CM members. The most altered members, previously classified as CM1, will become CM2.0, with the less altered members proportionately increasing in petrologic type up to CM2.6; lower degrees of aqueous alteration have not been identified in the CM group thus far. Based on eight major diagnostic parameters of progressive alteration, the team classified many CM chondrites while invoking a hypothetical precursor lithology having a petrologic type of 3.0. This precursor lithology is broadly similar to the anhydrous, ungrouped (probably CO-related; Simon and Grossman, 2015), type 3.0 Acfer 094, or perhaps the CO3.0 ALHA77307. Following are the major parameters developed by Rubin et al. (2007), Rubin (2007), and de Leuw et al. (2008) for estimating the degree of alteration of the CM chondrites:

early to intermediate stage alteration processes:

  1. hydration of fine-grained matrix material to form phyllosilicates, which gradually consists of Mg-rich serpentines
  2. conversion of primary igneous glass in chondrules to phyllosilicate
  3. production of large PCP clumps, now determined to be tochilinite–cronstedtite intergrowths (TCI)
  4. precipitation of sulfides within cavities

processes occurring throughout the alteration sequence:

  1. oxidation of FeNi-metal
  2. alteration of chondrule mafic phenocrysts
  3. compositional changes in TCI; e.g., S depletion
  4. formation of increasingly complex carbonates within sulfide-lined cavities
  5. compositional changes in sulfides
  6. replacement of primary melilite by secondary alteration products
  7. fragmentation/disintegration of refractory inclusions

An aqueous alteration sequence for some CM group members from most to least aqueously altered follows (Rubin et al., 2007): MET 01070 [2.0]
LAP 02277 [2.0]
QUE 93005 [2.1]
Cold Bokkeveld [2.2]
Nogoya [2.2]
QUE 99355 [2.3]
Mighei [~2.3]
Y-791198 [2.4]
Murray [2.4/2.5]
Murchison [2.5]
Kivesvaara [2.5]
QUE 97990 [2.6] Representatives of the earliest stages of aqueous alteration (2.9–2.7) on the CM parent body have not yet been discovered, although the unaltered, type CM3.0 precursor material was probably similar to the ungrouped (probably CO-related; Simon and Grossman, 2015) Acfer 094. See the Colony page for further details about a potential genetic relationship between the CM and CO groups.

Current studies suggest that both cometary dust and meteorites should be produced from the disruption of Jupiter-family comets which originate in the Kuiper belt. Studies have shown that Antarctic micrometeorites have a similar carbonaceous chondrite:ordinary chondrite ratio ((~7:1) as the composition of zodiacal dust (Meier, 2014). Based on observational evidence and current modeling, it is thought that comets should be dark in color and have a low density and strength, a high porosity, a solar ratio of elements, an elevated ratio of C, H, O, and N, a high interstellar grain content, anhydrous and highly unequilibrated silicates, few to no chondrules, and a low cosmic-ray exposure age (<10 m.y.). Both the CI and CM groups of meteorites exhibit characteristics that are consistent with the above descriptions.

Orbital data obtained from several carbonaceous chondrites (e.g., CI Orgueil [eyewitness plotting]; CMs Maribo and Sutter’s Mill [instrument recording]) are a good match to the orbits expected from the disruption of Jupiter-family comets, but are unlike the orbits of ordinary chondrites and most other asteroidal objects (Meier, 2014). Both the orbital eccentricity and semimajor axis for Maribo is nearly identical to those of Comet Encke and the associated Taurid swarm of objects (Haack et al., 2011). On the other hand, a CRE age study of CM chondrites conducted by Meier et al. (2016) shows a possible relationship exists to the asteroid breakup event ~8.3 m.y. ago that formed the Ch/C/Cg-type members of the Veritas family. In addition to the large abundance of 3He-enriched interplanetary dust discovered in 8.2 m.y.-old deep-sea drill cores, ~1/6 of all CM meteorites have 21Ne-based CRE ages that are consistent with derivation from this catastrophic breakup, while others with significantly younger CRE ages could represent secondary collisions among the Veritas fragments.

Based on reflectance spectra in the range of 0.4 to 2.4 μm captured by the OSIRIS-REx spacecraft, as well as utilizing meteorite analogs, it has been speculated that asteroid (101955) 1999 RQ36 is composed of CM1-like material. Further details of the formation of CM chondrites can be found on the Murchison page. The specimen of NWA 4765 shown above is a 0.8 g partial slice. A photo of the main mass is shown below.

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Photo courtesy of Chladni’s Heirs—S. Ralew & M. Altmann

Photos shown below courtesy of Aziz Habibi standby for nwa 4765 photo
click on photo for a magnified view
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Murchison

CM2.5
(Hydration subtype 1.6)
standby for murchison photo
Fell September 28, 1969
36° 37′ S., 145° 12′ E. On a Sunday morning around 10:45, a fireball exploded with loud detonations and hissing noises. Hundreds of stones rained down over Victoria, Australia covering 5 square miles, permeating the town with the odor of alcohol. Over 700 charcoal-colored stones with a total weight of over 100 kg were collected, the largest mass weighing 7 kg.

Murchison is a breccia that contains mm- to cm-sized xenoliths of mostly type-3 carbonaceous chondrite provenance. Murchison has a shock stage of S1–2 and a weathering grade of W1–2. The CM chondrites have undergone extensive aqueous alteration on the parent body, the degree of which having been determined by such factors as water/rock ratio, temperature, and duration of the process. An early alteration sequence for CM chondrites was proposed by Ikeda (1983), by which most Murchison chondrules would be designated stage-II:

Aqueous Alteration Categories in CM Chondrites
Degree of Alteration
Stage-I unaltered or least altered
Stage-II weakly altered (affecting mesostasis)
Stage-III moderately altered (affecting Ca-poor pyroxene)
Stage-IV intensely altered (affecting olivine)

Utilizing petrography and mineralogy, Browning et al. (1996) observed several features correlated to an increase in hydration, among which was a progression from Fe-rich serpentine to Mg-rich serpentine. Based on these indicators they arranged a set of CM chondrite falls in a sequence of increasing relative aqueous alteration as follows: Murchison ≤ Bells < Pollen ≤ Murray < Nogoya < Mighei < Cold Bokkeveld Results of a petrological, geochemical, and reflectance spectral study (diversity in the 3 µm band center) involving a set of CM chondrites, conducted under dry and vacuum conditions, has led Takir et al. (2013) to establish three spectral groups corresponding to an increasing state of hydration, and which could be applied to the interpretation of spectroscopic observations of asteroids:

  • Group 1 (least aqueously altered) phyllosilicates composed of Fe-serpentine (cronstedtite) corresponding to subtypes 2.6–2.3
  • Group 2 (intermediate aqueous alteration with intermediate mineralogical phases)
  • Group 3 (most aqueously altered) phyllosilicates composed of Mg-serpentine (antigorite) corresponding to subtypes 2.2–2.1

Building upon and strengthening the basis for a unique scale reflecting the degree of aqueous alteration, Rubin et al. (2005, 2007) proposed a new alteration sequence for CM chondrites. Based on eight major diagnostic parameters of progressive alteration, they classified Murchison as a petrologic type 2.5, which, along with the other CM chondrites, was derived from a hypothetical precursor lithology having a petrologic type 3.0, broadly similar to the anhydrous, ungrouped (probably CO-related; Simon and Grossman, 2015), type-3.0 Acfer 094, or perhaps the CO3.0 ALHA77307. Following are the major parameters developed by Rubin et al. (2007) for estimating the degree of alteration of the CM chondrites:

early to intermediate stage alteration processes:

  1. hydration of fine-grained matrices to form phyllosilicates, which gradually consists of Mg-rich serpentines
  2. conversion of primary chondrule glass to phyllosilicate
  3. production of large PCP (tochilinite–cronstedtite intergrowths [TCI]) clumps

processes occurring throughout the alteration sequence:

  1. oxidation of FeNi-metal
  2. alteration of chondrule silicate phenocrysts
  3. compositional and size changes of TCI (e.g., depletion in S and ‘FeO’ [FeO in phyllosilicates, Fe+3 in cronstedtite, and Fe+2 in sulfide]; SiO2, TiO2, Cr2O3, MnO, and CaO enrichment; decrease in size)
  4. formation of increasingly complex carbonates (e.g., calcite => dolomite)
  5. compositional changes in sulfides

Howard et al. (2015) established an aqueous alteration scale for the CM chondrites (as well as CR and C2-ungrouped chondrites) based on the phyllosilicate fraction (total phyllosilicate/total anhydrous silicate + total phyllosilicate). Through modal measurments utilizing Position Sensitive Detector X-ray Diffraction (PSD-XRD), and defining the sub-types in increments of 5% of the abundance of phyllosilicate, they found that CM chondrites fall in the range of 1.7–1.2 in their classification scheme (see example below). However, King et al. (2015) reasoned that since alteration might continue even after all silicates have been hydrated, this scheme could be difficult to extend to CI chondrites in which members reflect similar phyllosilicate abundances but show variable degrees of alteration in other features.     type 3.0: phyllosilicate fraction of <0.05
    ⇓
    type 1.0: phyllosilicate fraction of >0.95

The scale of Rubin et al. (2005) can be converted to the scale of Howard et al. (2015) using the following equation:
(Rubin CM or Harju CR classification × 0.96) – 0.81 = Howard classification In their study, de Leuw et al. (2009, 2010) found that CM chondrites which have lower petrographic subtypes experienced longer durations of aqueous alteration. Analyzing carbonates in CM chondrites, they determined that correlations in the Mn–Cr systematics were indicative of in situ decay of 53Mn during carbonate formation. They were able to utilize the correlation between the degree of aqueous alteration and the age of carbonate formation to arrive at a duration for such alteration of at least 4 m.y. It was shown that the carbonates that are the least altered are the oldest and vice versa. It was also concluded that FeCO3 can be used as a taxonomic parameter. The least-altered CM chondrites (petrologic type ≥2.4) have more abundant FeCO3 contents than those that are more-altered (petrologic type ≤2.4). Features present in the FeNi-metal and sulfides in CM chondrites have been identified which reflect the heating stage as follows (Kimura et al., 2009; Nakamura, 2005):

<td align='center'

kamacite or martensite without plessite;
some pentlandite blebs in pyrrhotite

<td align='center'

serpentine decomposes;
pentlandite blebs in pyrrhotite common

<td align='center'

tochilinite and pentlandite decomposed to pyrrhotite,
kamacite, and Ni-rich metal; secondary olivine crystallizes

<td align='center'

secondary low-Ca pyroxene crystallizes;
dehydration to anhydrous minerals

<td align='center'

Murchison, Murray, Nogoyo, Cold Bokkeveld

<td align='center'

A-881334, A-881655, Y-793321, Y-86695

<td align='center'

A-881655, Y-82054, Y-82098 (or IV), Y-86029

<td align='center'

B-7904, Y-82162, Y-86720, Y-86789

Characteristic Features in CM Chondrites Reflecting Secondary Heating
Stage I Stage II Stage III Stage IV
Temperature <300°C 300–500°C 500–750°C >750°C
Major Features
Examples

Kimura et al. (2011) revised this classification scheme by assigning a heating stage category A–C to CM chondrites. Category A (unheated) corresponds to heating stage I above. Category B (300°C to 750°C) corresponds to heating stage II and III above. Category C (>500°C to >750°C) corresponds to heating stage III and IV above. The presence of primordial metal in the form of martensite attests to a low degree of secondary thermal alteration following aqueous alteration in the case of Murchison. The placement of Murchison in Category A is a reflection of the metal composition (Ni content) and the sulfide texture (abundance of pentlandite blebs in pyrrhotite) of this meteorite.

Murchison contains sparse chondrules with diameters between 0.1 mm and 0.5 mm, composed of individual phenocrysts of olivine and pyroxene. Porphyritic chondrules contain some FeNi-metal grains. The chondrule mesostasis has been altered to phyllosilicates—Fe- and Mg-serpentines. The black matrix constitutes ~48 vol% of the meteorite and is similar to that of the CI matrix, but contains less magnetite. Chondrule fragments and olivine crystals are abundant in the Murchison matrix, while the CAI content is low. Isotopic research indicates that the matrix and chondrules of pristine carbonaceous chondrites probably condensed during the same heating event (Nyquist et al., 2009). Carbonate grains that formed in situ under low temperature conditions of 0–25°C are present in amounts of a few vol%, which consist predominantly of calcite/aragonite with minor amounts of dolomite, along with low abundances of sulfides.

Interestingly, newly identified refractory inclusions in Murchison composed primarily of hibonite represent some of the earliest condensed solids or residues from the early, hot, solar nebula (Liu et al., 2009). These refractory grains comprise platy crystals (PLACs), spinel–hibonite spherules (SHIBs), and blue aggregates (BAGs). PLACs lack resolvable 26Mg-excesses and were formed within a timespan of ~100,000 years in an 16O-enriched, heterogeneous region (based on anomalous δ48Ca and δ50Ti isotopic signatures) prior to incorporation and mixing of short-lived nuclides such as 26Al into the solar nebula through injection of interstellar dust. Both PLAC and BAG formation occurred hundreds of thousands of years prior to the formation of CV CAIs. In contrast, SHIBs formed later by condensation of precursor material in a lower temperature environment than that of PLACs. They record in situ 26Al decay and ‘canonical’ initial levels of 26Al/27Al, and they are considered to have formed 100,000–300,000 years after the formation of CV CAIs. Some hibonite grains in Murchison have experienced Rayleigh fractionation through distillation/evaporation, while some also exhibit nuclear anomalies showing similarities to the FUN group of CAIs studied in CV3 Allende.

X-ray diffraction techniques and Mössbauer spectroscopy have been used by Bland et al. (2004) to determine the modal mineralogy of several carbonaceous chondrites including Murchison. They were also able to quantify the compositional range of the olivine phases. In addition, the grain density can be readily estimated from the mode data, and therefore, in combination with the calculated bulk density, the porosity can be determined. The modal mineralogy (vol%) and other physical properties of Murchison were determined to be as follows:

  • Olivine
    • Fo100 ——————– 6.8
    • Fo80 ——————— 1.9
    • Fo50 ——————— 1.6
  • Clinoenstatite (En98) ——— 1.9
  • Pyrrhotite ———————- 1.8
  • Pentlandite ——————— 0.3
  • Magnetite ———————– 0.2
  • Serpentine ——————— 26.2
  • Calcite ————————— 1.2
  • Tochilinite/Cronstedtite —- 58.1
  • TOTAL ———————– 100.0
  • grain density = 2.92 g/cm³
  • bulk density = 2.20 g/cm³
  • porosity = 24.7 vol%

A more technologically advanced determination of the Murchison modal (vol%) mineralogy was conducted by Howard et al. (2009) with results as follows:

  • Olivine —————————15.0
  • Clinoenstatite (En98) ———- 8.3
  • Pyrrhotite ———————– 1.2
  • Pentlandite ——————— 0.65
  • Magnetite ———————– 1.1
  • Serpentine ——————— 22.0
  • Calcite ————————— 1.2
  • Cronstedtite —————— 50.0
  • TOTAL ———————— 99.45

Dark, fine-grained (<1 µm) material (DFM) which forms opaque mantles surrounding chondrules, refractory inclusions, and matrix silicates, are believed by some to be accretionary features from solar nebula dust reservoirs unrelated to the silicate cores on which they are found. However, results of an investigation by Trigo-Rodriguez et al. (2006) of the variable porosities within these dark rims indicate that they were formed on the parent body through impact-compaction of fine-grained, porous matrix material, followed by aqueous alteration and the deposition within the mantle of what was historically termed ‘poorly characterized phases’ (PCP). This phase has now been determined to be tochilinite–cronstedtite intergrowths (TCI). The TCI are present in two distinct forms as described by Pignatelli et al. (2016) in the least altered CM chondrite Paris:

  1. type-I: rounded- or irregular-shaped secondary alteration phase composed primarily of sulfur-rich tochilinite with lesser abundance of cronstedtite and magnetite; usually observed as zoned rims around precursor FeNi-metal beads within chondrules, in which cronstedtite increases in proportion to tochilinite progressing outwards from the rim.
  2. type-II: round-, anhedral-, or euhedral-shaped secondary alteration phase located within the matrix, constituting two concentric zones of variable thickness; the outer zone is composed of variable proportions of tochilinite and cronstedtite, while the inner zone is composed of a porous, fibrous-textured, Fe-hydroxide mineral amakinite.
  3. Type-I TCI formed when precursor kamacite particles reacted with ionized, S-bearing, alkaline water at low temperatures of ~50–100°C under reducing conditions involving a stable temperature and pressure (Peng et al., 2007). Type-II TCI formed as a complex secondary alteration phase by a step-wise dissolution/reprecipitation process through interaction with an evolving fluid (high-S ⇒ low-S + high-Si ⇒ low-S + low-Si), ultimately replacing olivine, and to a lesser extent, pyroxene grains, in a localized and/or transient, micro-scale aqueous environment (Pignatelli et al., 2016).

    Unlike the results of previous studies, it was demonstrated that the DFM is not confined to rims around discrete objects, but instead, can form indistinct boundaries blending seamlessly from the objects into the matrix. Moreover, the dark, fine-grained material can be found extending beyond its associated object, comprising isolated patches within matrix space, forming single rims around multiple objects, and surrounding post-aqueous altertion phases. Many smaller discrete objects have no mantles at all, inconsistent with a common nebular origin. Where DFM composes mantles around discrete objects, it forms a layered structure in which the lowest porosities adjacent to the enclosed objects reflect a high degree of compaction. Because agglomeration modeling shows that such a high degree of compaction is not attainable through nebular processes, the investigators argue that this provides supporting evidence for a parent body origin for DFM mantling through multiple impact-compaction events.

    Notably, a microfabric consisting of foliations in some dark inclusions, as well as the preferred orientation of chondrules and matrix phyllosilicates, is likely the result of impact-induced deformation (Lindgren et al., 2012). In a study of deformed chondrules in Murchison using X-ray computed tomography, Hanna et al. (2014) observed both foliation (planar alignment) and lineation (much longer in one dimension than in the other two) fabrics produced by impact stress. Utilizing a combination of high-resolution X-ray computed tomography and 3-D software to study fine-grained rims on CM chondrules, Hanna and Ketcham (2015) found that a significant positive correlation exists between rim thickness and the size of the enclosed chondrule; this result supports the theory that the rims accreted in the solar nebula rather than formation through parent body processes.

    Following the agglomeration and impact-induced compaction of the various components in Murchison, the material experienced high degrees of aqueous alteration at temperatures of 20–35°C within a zone ~100–250 m thick at a depth of 1–1.8 km (Guo and Eiler, 2007; Hanna et al., 2014). Studies of CM chondrite REE patterns indicate the water had a pH of 6–8 during the alteration process (Inoue et al., 2009). Low-temperature reactions involving thin fluid films promoted precipitation–dissolution processes in which Fe- and S-bearing phyllosilicates replaced host minerals. Nanotubes have been identified in both Murchison and Mighei (Zega et al., 2004) which are believed to have condensed from low-temperature, S-rich, aqueous solutions. They are composed of a new serpentine phase intermediate between cronstedtite and chrysotile. It was found by Rubin (2012) that the CM chondrites composed of higher-porosity material were affected by higher degrees of shock deformation and fracturing and exhibit stronger petrofabrics, features which facilitated a subsequent commensurate higher degree of aqueous alteration, i.e., lower petrologic subtype. Contrariwise, probably due to their lower porosities, lower matrix abundances, and less bulk water content, the CR-group chondrites do not exhibit a similar correlation between shock features and degree of aqueous alteration (Rubin and Harju, 2012).

    As shown above, Rubin et al. (2007) consider Murchison to be one of the least altered CM chondrites, and it can be placed within an aqueous alteration sequence with other CM members from most to least aqueously altered as follows: MET 01070 [2.0], QUE 93005 [2.1], Cold Bokkeveld [2.2], QUE 99355 [2.3], Y-791198 [2.4], Murray [2.4/2.5], Murchison [2.5], and QUE 97990 [2.6]. The CM chondrite Paris is a newly discovered representative reflecting a lower degree of aqueous alteration and mild thermal metamorphism. It contains an abundance of well-preserved metal with little magnetite, and has large unaltered zones and less matrix component than other CM members (Zanda et al., 2010). While initially having a CI-like composition, the matrix of CM chondrites reflect a S/Si ratio that decreases as aqueous alteration increases. This pattern demonstrates an independent accretion within diverse regions of the protosolar disk and a subsequent exchange between two components—a high-temperature fraction and a CI-like fraction—rather than accept the pre-accretion complementarity model involving these two components (Zanda et al., 2011).

    The unaltered precursor material of the CM group is considered to be type CM3.0. Although Paris does contain some phyllosilicates, most of its features, such as a high content of FeS in PCPs and a high chromium oxide content, indicate a tentative petrologic assignment for hydrothermal alteration of type 2.7/2.8, and a petrologic assignment for thermal metamorphism of A/B (Blanchard et al., 2011). A new investigation of Paris by Rubin (2015) revealed that some regions are even less altered and should be classified as type 2.9. This assignment is similar to that of the ungrouped (probably CO-related; Simon and Grossman, 2015) Acfer 094 and the CO3.0 chondrite ALHA77307. Paris exhibits many intermediate characteristics to the CO chondrite group (Bourot-Denise et al, 2010). Interestingly, Paris shows signs of pre-terrestrial magnetism, possibly internally generated, attesting to a former convecting metallic core (Cournede et al., 2011).

    Based on advanced analytical techniques in which the modal mineralogy of a suite of CM chondrites (Mighei, Nogoya, Murchison, Murray, and Cold Bokkeveld) was quantified to a high degree of accuracy, Howard et al. (2009) more accurately resolved the degree of aqueous alteration experienced by these CM chondrites based on their total phyllosilicate abundances, measuring Mg-serpentine + Fe-serpentine (cronstedtite); Fe-rich serpentine transitions to Mg-rich serpentine as aqueous alteration progresses. The investigators found that a narrow range exists in the modal abundances of most CM chondrites with regard to phyllosilicates, anhydrous silicates, and other phases. They showed that the inverse relationship which exists between the anhydrous silicates and the phyllosilicates is reliable evidence that the latter formed from the former through aqueous alteration processes. In addition, the inverse relationship apparent between the abundance of Mg-serpentine and Fe-serpentine (cronstedtite) supports an aqueous alteration process as well; cronstedtite loses Fe and recrystallizes to form Mg-serpentine in the presence of water.

    At the same time, since this alteration process is controlled by variable abundances of anhydrous silicates within different CM samples, the initial composition of each CM chondrite may have been variable, rendering it impossible to determine the actual extent of aqueous alteration by this method. In light of the fact that accurate measurements of phyllosilicate abundances among the CM samples have now been obtained by Howard et al. (2009), and that these abundances have been ascertained to be the same within a small range (73–79%), it may be inferred that each CM chondrite has experienced an equal degree of aqueous alteration, and the subtypes proposed by Rubin et al. (2007) (as shown above) are not accurately defined. As a result, the team argues that it is not necessary to resolve the subtype of the CM group further than CM2.

    Be that as it may, Howard and Alexander (2013) have proceded to establish an aqueous alteration classification scale (hydration scale) distinct from the thermal metamorphic classification scale, one that is based on the total abundance of phyllosilicates in a meteorite and that does not imply an evolutionary progression from lower to higher degrees of aqueous alteration. In their scheme, phyllosilicate abundance increases along the type sequence: 3.0 (0%) ⇒ 2.0 (50%) ⇒ 1.0 (100%), where the phyllosilicate abundance increases in 0.1 (5%) increments. This method allows for a consistent scale to be applied across all carbonaceous chondrite groups. A hydration indexed chart of 40 carbonaceous chondrites, comprising CI, CM, and CR group members resolved by phyllosilicate abundance, was published in an abstract by Howard and Alexander (2013), while the classifications of 54 meteorites was published in an article by Alexander et al. titled ‘The classification of CM and CR chondrites using bulk H, C and N abundances and isotopic compositions’, GCA, vol. 123 (2013), which was followed by an article by Howard et al. titled ‘Classification of hydrous meteorites (CR, CM and C2 ungrouped) by phyllosilicate fraction: PSD-XRD modal mineralogy and planetesimal environments’, GCA, vol. 149 (2015).

    CLASSIFICATION BASED ON HYDRATION
    Adapted from Alexander et al. (2013)
    ALH 83100 1.1 DOM 03183 1.6
    ALH 84034 1.1 GRA 98074 1.6
    DOM 08003 1.1 GRO 95566 1.6
    Nagoya II 1.1 LAP 02336 1.6
    SCO 06043 1.1 LAP 03718 1.6
    ALH 84029 1.2 LEW 88001 1.6
    ALH 84042 1.2 LEW 90500 1.6
    ALH 84044 1.2 Mighei 1.6
    MET 01070 1.2 Murchison 1.6
    SCO 06014 1.2 Nogoya 1.6
    SCO 06043 II 1.2 Banten 1.7
    Cold Bokkeveld 1.3 GRA 98005 1.7
    EET 96006 1.3 LAP 02239 1.7
    LAP 03785 1.3 MAC 88100 1.7
    LEW 87148 1.3 MET 00432 1.7
    ALH 85013 1.4 QUE 97990 1.7
    EET 96016 1.4 DNG 06004 1.8
    LEW 87022 1.4 DOM 08013 1.8
    MAC 88101 1.4 LEW 85312 1.8
    MAC 88176 1.4 LON 94102 1.8
    LAP 02277 1.4 MCY 05230 1.8
    QUE 93005 1.4 PCA 91084 1.8
    LAP 02333 1.5 Bells (W) 1.9
    LEW 87016 1.5 LEW 85311 1.9
    MET 01072 1.5 TIL 91722 1.9
    MET 01075 1.5 Bells (C) 2.3

    In further studies of hydrous alteration in carbonaceous chondrite groups, Howard et al. (2013) ascertained that neither the relative abundances of Fe- and Mg-rich serpentines, nor the bulk O-isotopic compositions, show any correlation to the degree of aqueous alteration. The investigators suggest that the Fe:Mg ratios in serpentines, as well as the final bulk O-isotopic compositions, were controlled instead by factors related to the anhydrous precursor material, e.g., the initial chondrule:matrix ratio, where the matrix is intrinsically more Fe-rich. Another controlling factor they invoke is the temporal abundance of water ice.

    Murchison contains water-bearing minerals including serpentines. These phyllosilicates provide a high water content of 4–18 wt%, water that initially condensed at a distance of 4 AU from the Sun (Eiler and Kitchen, 2004). A total of 2.8 wt% of this water is adsorbed or trapped inside pores (Yoldi-Martinez et al., 2011). In Murchison, Mg-rich serpentines as well as Fe-rich cronstedite are found to contain –OH. Fe-rich aureoles are among the products resulting from in situ aqueous alteration processes. Additionally, Murchison contains carbon (2–2.5 wt%) and nitrogen (0.09–0.16 wt%) as constituents of free organic matter, diamond, and soluble complex macromolecular organic compounds (10–15 ring polycyclic aromatic hydrocarbons [PAHs]), including at least 80 amino acids (~14,000 ppb for C2–C5) and the bases that make up the biological coding elements of RNA and DNA. All of these organic compounds have a nonbiogenic origin, and were formed by such processes as irradiation of interstellar organic ices by cosmic rays. A listing of amino acids identified in Murchison before 1991 can be found on the Murchison organics page.

    Further studies at the University of Bremen in Germany have led to the identification of seven diamino acids, which are the building blocks of peptide nucleic acids considered to have preceded RNA and DNA in the genesis of life. Also having an abiotic origin, a suite of over fifty monocarboxylic acids have been identified in Murchison (Huang et al., 2004), some of which have also been found in the C2 ungrouped chondrites Tagish Lake (Herd, University of Alberta, 2009) and EET 96029, the latter containing a high abundance of formic acid. Studies of Murchison at the University of California at Davis (D. Deamer) have revealed the existence of lipid-like organic chemicals able to self-assemble into a membrane-like film enclosing a fluid, an analog to a cell membrane. Although CM chondrites with lower petrologic types (extensive aqueous alteration) contain significantly less abundant amino acids compared to those with higher petrologic types, the finding of similar relative abundances among them suggests that a common parent body link may exist (Botta et al, 2007).

    The case has been made, based on O-isotopic compositions, for the pre-terrestrial production of water-soluble sulfate by the oxidation of sulfides in the presence of water (Airieau, et al., 2005). This process occurred as fluid flowed through unaltered rock that still preserved a component of its original oxygen ratio. This sulfate is isotopically stable, and preserves the oxygen isotopic signature of the water that was present at the time of sulfate formation, and to some degree may also reflect the signature of the water that was present during aqueous alteration processes of the meteorite.

    The isotopic composition of Murchison and other CM chondrites reflects contributions from two primary reservoirs: 1) an anhydrous silicate component similar to the primitive CO3.0 chondrite ALHA77307, and 2) an aqueous component manifest as phyllosilicates formed through parent body processes (Clayton and Mayeda, 1999). A parent body origin for the phyllosilicates is revealed by the heavy carbon (13C) content of the carbonates, which is heavier than nebular C gas. Rather than a mass fractionation process, the heavy C could have been derived from presolar carbide grains, or as proposed by Guo and Eiler (2007), through the production and escape of 13C-depleted methane during aqueous alteration. A low water/rock ratio employing isotopically-heavy water at temperatures near 0°C is considered the most likely parent body alteration environment. See the Colony page for information regarding a possible common parental source object for the CM and CO groups.

    Of possible historic significance is the recent discovery of a variety of sugar compounds, collectively known as polyols, within Murchison and the similar CM2 Murray. These compounds are constituents of RNA and DNA, and serve a role in cellular chemistry. Equally remarkable, fatty acids have been isolated from Murchison that independently form boundary membranes in alkaline conditions creating a rudimentary cell structure which could theoretically lead to self-replication. Two of the known nucleobases, which when associated with a sugar and a phosphate group constitute the nucleic acids that compose the genetic code, have been identified as indigenous components in Murchison (Martinsa et al, 2009): the one-ring pyrimidine, uracil, is present as a natural component of RNA, while the two-ring purine, xanthine, is instrumental in the synthesis of other purine nucleotides.

    Besides these discoveries, isotopic studies suggest that organic sulfur compounds within Murchison may have been created by interaction of carbon-disulfide molecules with light in the low-temperature, pre-planetary environment of interstellar space. Similarly, it was found that UV photolysis of interstellar ice could lead to the formation of the amino acids glycine, alanine, and serine, and could explain the existence of an L-enantiomer bias. In light of the fact that these naturally synthesized ingredients necessary for life are present in asteroidal material, the question arises: what influence did meteorite accumulation on Earth have on the genesis of terrestrial life? Interestingly, an ultrahigh-resolution analysis of the extraterrestrial organic matter in Murchison has revealed that its indigenous chemical diversity encompasses tens of thousands to millions of different molecular compositions, exhibiting a chemical complexity that is high compared to biological systems on Earth (Schmitt-Kopplin, 2010).

    Murchison also contains insoluble organic compounds. One component (20%) consists of presolar diamonds in concentrations of 1000 ppm, while another (10%) consists of grains of presolar graphite and SiC. Croat et al. (2008) discovered that the graphite in Murchison is present as both ordered (onion types) and disordered (platy types and scaly types) morphologies. Some platy graphite grains contain internal refractory grains of oxides (e.g., eskolaite and magnetite), carbides, and RuFe-metal, similar to constituents found in onion type graphites. These platy grains also show enrichments in 12C. The combined isotopic and compositional evidence indicates that both the onion and platy graphite grains formed during s-process nucleosynthesis (in which neutrons are slowly added to nuclei over thousands of years) in one or more AGB carbon stars. It was further determined by the research team that the most disordered grain type, the scaly graphite, has O-isotopic ratios and other features more consistent with having originated during r-process nucleosynthesis occurring in supernovae.

    Nguyen et al. (2007) ascertained that the SiC grains in Murchison are comprised of a predominant mainstream type (from low-mass, C-rich AGB stars) plus the rare types A + B (from J-type carbon stars), X (from type II supernovae), and Y + Z (from low-metallicity AGB stars). The CRE ages of some large SiC grains range up to 1.5 b.y. The largest insoluble organic component (~70%) is a kerogen-like material (Mao, 2005). These components were mostly produced during s-process nucleosynthesis in aging, medium-sized carbon stars (TP-AGB phase), or very rarely in supernovae. TiC crystals thought to have been produced in supernovae, and rutile grains of variable composition thought to have been produced in AGB star outflows (Croat, 2007), along with rare SiC grains of uncertain origin (Hynes and Croat, 2007) have become internal constituents of later formed graphite grains. While one SiC grain has been determined to be from a nova, the origin of another anomalous grain highly enriched in 30Si has yet to be established. Additionally, it was discovered that the isotopic composition of some X-Type SiC grains is enhanced in heavy elements, which has given rise to the new formation theory of neutron burst nucleosynthesis. All of these interstellar grains, including aluminum oxide, spinel, and silicon nitride are remnants of the dust cloud from which our Solar System was formed.

    The CM chondrites may be linked by spectral properties to the C-type, G-class asteroids 19 Fortuna and 13 Egeria, both of which are located near resonances that should be supplying fragments to Earth. On the other hand, Rubin and Bottke (2007) argue that the prevalence of CM xenolithic clasts in ordinary chondrites and HED breccias is consistent with a recent collisional fragmentation event. They believe the source to be an ~170-km C-type asteroid located in the inner asteroid belt which had been disrupted ~160 m.y. ago, and which now comprises the Baptistina asteroid family. The close proximity of this asteroid family to the ν6 resonance, as well as the similar orbits of the OC and HED parent bodies, support such a scenario. Nonetheless, contradictory spectroscopic data have been obtained by Reddy et al. (2009), including analysis of absorption features and albedo values, suggesting that 298 Baptistina is more consistent with an S-type asteroid, perhaps similar to the LL chondrites, and is inconsistent with a carbonaceous chondrite.

    Interestingly, the recorded fall of the CM chondrite Maribo may provide a link to Comet Encke within the Taurid complex. The very high eccentricity derived for the pre-atmospheric orbit of Maribo, as well as the location of the semimajor axis within the main asteroid belt, are nearly identical to those of Comet Encke (Haack et al., 2011). In addition, significant similarities exist in the D/H ratio between the water present in certain comets, such as Halley and Hyakutake, and that observed in carbonates and phyllosilicates present in CM condrites. Similarities also exist in the mineralogy of CM chondrites and the samples returned from Comet Wild 2, while high annealing temperatures are required in both to produce their crystalline silicate component. However, a large disparity in CRE ages exists between CM chondrites and that conjectured for the Taurid complex comets—the Murchison meteoroid experienced a simple one-stage exposure history in transit to Earth, calculated to be 1.8 (±0.3) m.y., while the the Taurid complex comets are most consistent with a much more recent disruption event. Further investigations are necessary to better resolve this issue.

    On the other hand, a CRE age study of CM chondrites conducted by Meier et al. (2016) shows a possible relationship exists to the asteroid breakup event ~8.3 m.y. ago that formed the Ch/C/Cg-type members of the Veritas family. In addition to the large abundance of 3He-enriched interplanetary dust discovered in 8.2 m.y.-old deep-sea drill cores, ~1/6 of all CM meteorites have 21Ne-based CRE ages that are consistent with derivation from this catastrophic breakup, while others with significantly younger CRE ages could represent secondary collisions among the Veritas fragments.

    A significantly larger CM chondrite sampling (110) was utilized by Zolensky et al. (2017) in a CRE age and petrographic study. They contend that these samples fall into four separate CRE age groupings which may represent separate collisional events (see diagram below). They found that the CM chondrites with the youngest CRE ages have experienced the highest degrees of aqueous alteration, and that those with the oldest CRE ages are composed of a single lithology (monomict). Plausible explanations for these findings were presented considering both one- and two-parent body scenarios. standby for cm cre age diagram
    Diagram credit: Zolensky et al., 48th LPSC, #2094 (2017) and references therein Notably, a light-colored clast with affinities to R chondrites was identified in a Murchison sample (Isa et al., 2013, 2014). Further analyses of this clast were conducted by Bischoff et al. (2018). The clast has a recrystallized texture with 120° triple junctions, and it was determined that both the geochemical and O-isotopic composition was similar to CM chondrites rather than to the R chondrite or brachinite groups (see diagram below). A precise determination of the parental origin for this clast is still underway. standby for cm clast image
    Image and diagram credit: Bischoff et al., 81st MetSoc, #6217 (2018) The specimen of Murchison shown above is a 3.1 g cut fragment exhibiting a portion of fresh black fusion crust.


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Dho 225


CM-anom or ung
Possible ‘CY’
Chondrite
(thermally metamorphosed/dehydrated)

standby for dhofar 225 photo
Found January 15, 2001
18° 21.6′ N., 54° 11.3′ E. A fresh, black, carbonaceous chondrite weighing just 90 g was found in the desert of Oman. Dhofar 225 has textural characteristics similar to typical CM chondrites, but differs from members of that group in mineralogy, bulk composition, and O-isotopic composition (Ivanova et al., 2010). The chromium oxide content of Dhofar 225 indicates a petrologic type below 3.0.

Dhofar 225 has an O-isotopic composition that is enriched in heavy oxygen (18O, 17O), and has a plot very close to the C2-ungrouped Tagish Lake (oxygen isotope plot) and to the CM/CI-like, thermally metamorphosed/dehydrated Antarctic meteorite grouplet termed ‘CY’ by Y. Ikeda (1992 [Consortium Summary]) which consists of Belgica 7904 (C2-ung), Y-82162 (C1/2-ung), and Y-86720 (C2-ung) (see MetBull oxygen isotope plot, and diagram below). standby for cy group ox diagram
Diagram credit: King and Russell, 50th LPSC, #1386 (2019) Other possibly related dehydrated CM-like meteorites include Y-86789 (C2-ung, likely paired with Y-86720), WIS 91600 (CM2), EET 96010 (CM2), PCA 02012 (CM2). In addition, two meteorites classified as CI, Y-86029 and Y-980115, were determined by King and Russell (2019) to have similar mineralogical and chemical similarities to the CY group. In addition, Nakamura (2006) identified two regolith breccias containing solar-wind-implanted noble gases which belong to this dehydrated group, Y-793321 (CM2) and A-881458 (CM2), while M.M.M. Meier (2014) found that the meteorite Diepenveen (CM2-an) also contains similar trapped solar gases. Another ungrouped C2 meteorite with a similar O-isotopic composition is Dhofar 1988 (oxygen isotope plot; photo [courtesy of Marcin Cimala]), which was found by M. Cimala in 2011. In addition, the ungrouped C chondrite Dhofar 2066 also has a heavy oxygen isotope composition similar to the CY group (oxygen isotope plot) as do Y-86737 and Y-980134 which are both classified as CI1 (King and Russell, 2019). Notably, Dhofar 225 has many features and an oxygen isotopic composition similar to the anomalous CM chondrite Dhofar 735 (oxygen isotope plot; photo), which along with Belgica 7904 and PCA 02012, have experienced the highest temperatures (~900°C) over a brief time interval (PCA 02012 estimated at tens of hours; Nakato et al., 2013) compared to other members belonging to the CY group.

The case for a distinct CY group as proposed by Y. Ikeda (1992) is strengthened by a mineralogical comparison conducted by King and Russell (2019). The significantly higher modal sulfide content in the CY-group chondrites Y-980115 and Y-82162 compared to that of average CM and CI chondrites is difficult to reconcile with an attribution to hydration/dehydration processes, but is instead more consistent with a difference in primary mineralogy (see diagram below). standby for cy group ox diagram
Diagram credit: King and Russell, 50th LPSC, #1386 (2019) Differences exist between Dhofar 225 and Dhofar 735 on one hand, and the Belgica-like grouplet on the other. In contrast to the FeNi-metal grains present among Belgica-like meteorites, those in Dhofar 225 and Dhofar 735 are not enriched in Cr and P (Ivanova et al., 2010). Moreover, the bulk chemistry between the Dhofar and Belgica metamorphosed meteorites are different. Similar to the Belgica grouplet, but unlike typical CM chondrites, Dhofar 225 exhibits considerable but incomplete dehydration of matrix phyllosilicates (<2 wt% water), Fe and S depletions, and contains tiny grains of tetrataenite within the matrix—all features consistent with a higher thermal metamorphism than that experienced by typical CM group members. However, sharp zoning profiles of olivine in the chondrule-like objects of Dhofar 225 severely constrain the maximum temperature of metamorphism. In particular, zoning of olivine grains observed in Dhofar 735 and Belgica 7904 indicates a short heating duration that negates the theory of heating by decay of radioactive elements (Nakato et al., 2011). Aqueously altered carbonaceous chondrites that have experienced thermal metamorphism have been classified according to their degree of heating and corresponding phyllosilicate dehydration. Estimates of dehydration temperatures are shown below (Nakamura, 2005):

Dehydration Temperature
Stage I <300°C
Stage II 300–500°C
Stage III 500–750°C
Stage IV >750°C

Chondrules in Dhofar 225 are sparse (24 vol%), and similar in size (0.3 mm) to those of CM chondrites. Olivine is forsteritic and commonly occurs as aggregates up to 0.6 mm in size, and as chondrule-like objects. The matrix constitutes 70 vol% and is primarily composed of phyllosilicates (serpentine), with minor sulfides, phosphides, phosphates, FeNi-metal, and chromite, with only rare CAIs (2 vol%). A previously unknown mineral phase, Ca,Fe-oxysulfide, was identified in the matrix, possibly an oxidation product of a primary sulfide phase (Ivanova et al., 2010). Tochilinite, characteristically abundant in CM chondrites, has been mostly thermally decomposed to troilite and oxides in both Dhofar 225 and Dhofar 735, as well as in the Belgica-like grouplet. The similarly thermally unstable P-rich oxysulfides only occur in very low abundances (Ivanova et al., 2005). Other rare minerals identified include eskolaite and Cr-barringerite.

In contrast to the low-Ni, low-Co content of the metal within chondrules of Dhofar 225, the composition of the matrix metal is high-Ni, high-Co taenite and tetrataenite. The Fe/Si matrix ratio of Dhofar 225 is consistent with that of the CM chondrite group. The absence of Cr and P in the metal of Dhofar 225 is similar to that in the metamorphosed meteorites Belgica 7904 and Y-86720. Although the matrix of Dhofar 225 is compositionally similar to CI chondrites, especially Y-82162, as well as to the metamorphosed-CM chondrite Y-86720, only Dhofar 225 has retained moderate abundances of tochilinite-cronstedtite intergrowths (TCI; formerly PCP or ‘poorly characterized phases’). This specific mineralogy suggests that the grouplet experienced a period of variable aqueous alteration followed by a low level heating/dehydration phase, probably caused by impacts (Choe et al., 2010). A later episode of aqueous alteration might have affected Dhofar 225 resulting in its extant tochilinite.

While this group of metamorphosed carbonaceous chondrites could have been derived from normal CM chondrites, in accord with their many common characterisics, some researchers consider it more likely that they originated from one or more separate parent bodies. This scenario can explain the significant difference in O-isotopic compositions between the metamorphosed Dhofar meteorites (and the Belgica-like grouplet) and typical CM chondrites (Choe et al., 2010). Furthermore, geochemical variations that exist between the two Dhofar meteorites and the Belgica meteorites attest to the fact that their source material was not exactly the same. It was accepted that these metamorphosed meteorites could not have been derived from typical CM2 material through dehydration processes, but rather were formed in a similar O-reservoir (Ivanova et al., 2010).

Continued research by Ivanova et al. (2012, 2013) has demonstrated that the differences observed in the O-isotopic composition between the metamorphosed carbonaceous chondrites of the Dhofar and Belgica-like groupings and typical CM chondrites are consistent with multiple cycles of hydration–dehydration on a common parent body. Following aqueous alteration of silicates involving a source of water enriched in 18O, the resulting phyllosilicate (primarily serpentine) was also enriched in 18O by ~10%. Moreover, subsequent dehydration processes led to a further enrichment in 18O by ~7%. They reasoned that a low degree of heating at some distance from an impact crater would result in melting of extant water ice, which was then utilized in the hydration of silicate rock—then followed burial, metamorphism, and dehydration of this rock. They propose that this hydration–dehydration cycle may have occurred multiple times to produce the isotopic and geochemical differences observed among these meteorites.

Heating experiments were conducted by Nakato et al. (2014, 2016) in which samples of the C2-ungrouped Tagish Lake, a meteorite that shares many characteristics with metamorphosed carbonaceous chondrites, were exposed to a varying temperatures and heating durations. They demonstrated that heating at a high temperature of 900°C for 1–96 hours caused progressive reduction and dehydration, resulting in mineralogical and textural changes similar to those observed in the Belgica group of thermally metamorphosed carbonaceous chondrites (e.g., fibrous textured phyllosilicates, reduction of magnetite to form FeNi–metal+troilite assemblages). In addition, both Tagish Lake and the Belgica group meteorites have Si-rich matrix compositions compared to typical low-temperature CM chondrites. The degree of change in the O-isotopic composition of these heated samples is yet to be established.

Current studies suggest that both cometary dust and meteorites should be produced from the disruption of Jupiter-family comets which originate in the Kuiper belt. Studies have shown that Antarctic micrometeorites have a similar carbonaceous chondrite:ordinary chondrite ratio (~7:1) as the composition of zodiacal dust (M.M.M. Meier, 2014). Based on observational evidence and current modeling, it is thought that comets should be dark in color and have a low density and strength, a high porosity, a solar ratio of elements, an elevated ratio of C, H, O, and N, a high interstellar grain content, anhydrous and highly unequilibrated silicates, few to no chondrules, and a low cosmic-ray exposure age (<10 m.y.). Both the CI and CM groups of meteorites exhibit characteristics which are consistent with the above descriptions.

Orbital data obtained from several carbonaceous chondrites (e.g., CI Orgueil [eyewitness plotting]; CMs Maribo and Sutter’s Mill [instrument recording]) are a good match to the orbits expected from the disruption of Jupiter-family comets, but are unlike the orbits of ordinary chondrites and most other asteroidal objects (M.M.M. Meier, 2014). Both the orbital eccentricity and semimajor axis for Maribo is nearly identical to those of Comet Encke and the associated Taurid swarm of objects (Haack et al., 2011). On the other hand, a CRE age study of CM chondrites conducted by Meier et al. (2016) shows a possible relationship exists to the asteroid breakup event ~8.3 m.y. ago that formed the Ch/C/Cg-type members of the Veritas family. In addition to the large abundance of 3He-enriched interplanetary dust discovered in 8.2 m.y.-old deep-sea drill cores, ~1/6 of all CM meteorites have 21Ne-based CRE ages that are consistent with derivation from this catastrophic breakup, while others with significantly younger CRE ages could represent secondary collisions among the Veritas fragments.

In consideration of the young CRE age of all of the Belgica group meteorites, a near-Earth asteroid is favored as the common source object. One possible candidate is the binary asteroid 1998 ST27, which appears to match the required spectrographic characterisics of these meteorites. Moreover, its binary nature is consistent with the likelihood for disruption and injection of material into an Earth approaching orbit. Other source asteroids, such as Phaethon, Icarus, and 2008 FF5, are considered by Ivanova et al. (2013) as potential sources for these meteorites; i.e., the heat source for their metamorphism may be associated with their perihelion close to the Sun. Notably, the C-type asteroid 162173 Ryugu, from which a sample return is planned for 2020 by the spacecraft Hayabusa2, has some spectral similarity to experimentally-heated hydrated carbonaceous chondrites which may be analogous to those of the CY group (Matsuoka et al., 2018; King and Russell, 2019). The specimen of Dhofar 225 shown above is a 0.69 g partial end section.