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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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Saint Sauveur

Impact-melt breccia
(EHb5 in Weyrauch et al., 2018)

standby for saint-sauveur photo
Fell July 10, 1914
43° 44′ N., 1° 23′ E. Around 2:00 on a July afternoon, people in Haute Garonne, France heard detonations and observed the fall of a meteor. A meteorite weighing ~14 kg was recovered 1.5 km south of Saint-Sauveur. The owner of the field in which it fell, Antoine Esculie, donated the stone free of charge to the Museum of Toulouse (R. Mathieu). standby for saint-sauveur impact pit photo
Photo courtesy of Société d’Histoire Naturelle de Toulouse
Originally published in the Bulletin de la Société d’Histoire Naturelle de Toulouse, vol. 93 (1958), by G. Astre.
Pictured L–R: Gaston Astre, geologist and naturalist, director of the Museum of Toulouse (1944–1962); Guillaume Champagne, priest of Saint-Sauveur; unidentified neighbor; Barthélémy Cazemajou, mayor of Saint-Sauveur Saint-Sauveur is a member of the high-Fe group of enstatite chondrites, one of a very small number classified as petrologic type 5. It is considered to be an impact-melt breccia, and has been weakly shock metamorphosed to stage S3 corresponding to a shock pressure of ~10 GPa. Shock features include planar fractures and twinned clinoenstatite lamellae within orthopyroxene, and the occurrence of opaque veins of kamacite and troilite.

Enstatite chondrites are the most reduced meteorites among chondrites as evidenced by their extremely low FeO content, and by the presence of rare sulfide minerals such as oldhamite, daubréelite, and alabandite (EL) or niningerite (EH). Moreover, metal occurs primarily as low-Ni kamacite in both the EH and the EL groups. Surprisingly, it has been demonstrated by Macke et al. (2009) that these two groups do not actually differ in their iron content, and that they are indistinguishable in density, porosity, and magnetic susceptibility as well; however, differences in siderophile, chalcophile, and other mineralogical abundances can be employed to distinguish the two groups. The EH and EL groups are clearly resolved from each other based on compositional, textural, and mineralogical differences, as well as by O-isotopic data and formation intervals, indicating that they were derived from separate, but closely related parent bodies. In addition, both Fe- and Zn-isotopic compositions are fractionated to different degrees between EL and EH chondrites; EL chondrites are heavier than EH chondrites, indicating that EL chondrites experienced higher volatilization due to its formation closer to the Sun (Mullane et al., 2005), or alternatively, due to elemental fractionation during impact shock events (Rubin et al., 2009). The nonrefractory siderophile, chalcophile, and alkali elements in Saint-Sauveur clearly establish it as a member of the EH group.

Within the EH group, a distinction can be readily made between EH3 and EH4,5 petrologic types based on mineral compositions. One difference is evident in their respective Ni content in kamacite (EH3: 24–33 mg/g Ni; EH4,5: 65–79 mg/g Ni), which might be explained by the depletion of Ni by the formation of high-Ni perryite at the surface of kamacite grains in the EH3 chondrites. Perryite formation was induced through hot nebular exchange reactions in which metal was converted to FeS, thus freeing up Ni to form perryite. In contrast to the unmetamorphosed E chondrites, this mineral did not survive subsequent metamorphic heating in E chondrites of higher petrologic types. Since elemental abundances in E chondrites of petrologic types 4 and 5 are practically the same, it is only from observations of mineralogical changes, produced by varying degrees of thermal metamorphism, that a distinction can be made between them.

The Van Schmus–Wood (1967) scheme for petrographic type has been modified for enstatite chondrites, establishing both a textural type (3–7), reflecting peak metamorphic temperature, and a mineralogical type (α–δ), pertaining to the cooling history (Zhang and Sears, 1996; Quirico et al., 2011). Under this classification scheme, Saint-Sauveur has thermometers that give it a classification of EH5γ.

The possibility that the EH group is comprised of two distinct subgroups has been considered. Within these two subgroups the cooling rate and MnS content in niningerite are correlated. This correlation is not adequately explained by burial depth or impact-generated differences, and therefore, formation on two separate bodies has been suggested by some. The thermal history of Saint-Sauveur is consistent with inclusion in the subgroup that experienced fast cooling with a low MnS content in niningerite. Weyrauch et al. (2018) analyzed the mineral and chemical data from 80 enstatite chondrites representing both EH and EL groups and spanning the full range of petrologic types for each group. They found that a bimodality exists in each of these groups with respect to both the Cr content in troilite and the Fe concentration in niningerite and alabandite (endmembers of the [Mn,Mg,Fe] solid solution series present in EH and EL groups, respectively). In addition, both the presence or absence of daubréelite and the content of Ni in kamacite were demonstrated to be consistent factors for the resolution of four distinct E chondrite groups: EHa, EHb, ELa, and ELb (see table below).

Weyrauch et al., 2018
Troilite Cr <2 wt% Cr >2 wt% Cr <2 wt% Cr >2 wt%
(Mn,Mg,Fe)S Fe <20 wt% Fe >20 wt% Fe <20 wt% Fe >20 wt%
Daubréelite Abundant Missing Abundant Missing
Kamacite Ni <6.5 wt% Ni >6.5 wt% Ni <6.5 wt% Ni >6.5 wt%

A few other E chondrites with intermediate mineralogy have also been identified, including LAP 031220 (EH4), QUE 94204 (EH7), Y-793225 (E-an), LEW 87223 (E-an), and PCA 91020 (possibly related to LEW 87223). Studies have determined that these meteorites were not derived from the EH or EL source through any metamorphic processes, and some or all of them could represent separate E chondrite asteroids. The revised E chondrite classification scheme of Weyrauch et al. (2018) including selected examples from their 80-sample study can be found here. It was determined that Saint-Sauveur is a member of the EHb subgroup.

Enstatite chondrites have O-isotope compositions that plot along the terrestrial fractionation line, suggesting that they may have formed within the Mercury–Venus region in the inner Solar System, and that they were subsequently perturbed into the inner regions of the asteroid belt. In such a case, the strongly reducing conditions under which they were formed could have been promoted by an excess of H and C, maintained by a hot, dusty environment close to the Sun. Utilizing Mn–Cr isotope systematics, Shukolyukov and Lugmair (2004) concluded that the E chondrites formed at a location closer to the Sun—between at least 1 AU outward to 1.4 AU—than at the location within the asteroid belt they now occupy.

However, if the region between ~1.0 and 1.4 AU were truly the formation location of E chondrites, they should have highly elliptical orbits; but this is not what is observed. In fact, reflectance spectrometry has identified asteroids similar to E chondrites in stable orbits between 1.8 and 3.2 AU, suggesting that the inner asteroid belt is the actual location where they originated. In addition, a heliocentric distance of ~2.0–2.9 AU was calculated for two E chondrites on the basis of their implanted solar noble gas concentrations (Nakashima et al., 2004). Furthermore, an isotopically anomalous Xe-containing component, associated with an anomalous light N component, is found proportionately in both carbonaceous and enstatite chondrites, but not on Earth. Since this component is almost certainly of nucleosynthetic origin, it follows that the carbonaceous and enstatite chondrites should share a similar heliocentric formation location. In this case, the strongly reducing conditions under which E chondrites formed could have been promoted by the loss of refractory oxides prior to condensation from the local nebula.

Data from Rb–Sr systematics infer a formation age for Saint-Sauveur of 4.516 (±0.029) b.y., with indications of a high-temperature shock event occurring 60–200 m.y. after formation, consistent with the presence of the high-pressure silica polymorph cristobalite. This cristobalite was preserved through rapid cooling (Kimura et al., 2005). The occurrence in Saint-Sauveur of the mineral keilite, produced from melting of niningerite and troilite, is also indicative of an impact melting event accompanied by rapid quenching (Hill et al., 2014). The presence of fluor-richterite grains also attests to an impact-melt history. Cosmic-ray exposure ages are generally lower for EH chondrites than for EL chondrites, 0.5–7 m.y. and 4–18 m.y., respectively. More in-depth information on the complex thermal history of the EH chondrites can be found on the Sahara 97096 page.

A xenolith that was found in the carbonaceous chondrite Kaidun, named Kaidun III, has been determined to be an EH5 inclusion, one that underwent hydration on the Kaidun parent body. Interestingly, another clast found in a Kaidun sample is a rare EL3, named Kaidun IV. In addition to three Antarctic EH5 members, the St. Mark’s meteorite is the only other non-Antarctic EH5 sample in our collections. The specimen of Saint-Sauveur shown above is a 1.34 g partial slice obtained in a trade with the Muséum National d’Histoire Naturelle, Paris, France, by International Meteorite Brokerage. The photo below is the 14 kg main mass of Saint-Sauveur in the Muséum de Toulouse. saint-sauveur main mass
click on photo for a magnified view
Photo courtesy of Didier Descouens—Muséum de Toulouse

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NWA 7135

F3/4 (forsterite chondrite)
(highly reduced)
standby for northwest africa 7135 photo
Purchased Oct 2010
no coordinates reported A single 51.3 g stone exhibiting a chondritic texture and lacking fusion crust was found in the Sahara Desert. This meteorite became just another ‘typical’ ordinary chondrite placed into a bag with many others that was purchased by Fabien Kuntz at the 2010 Munich Mineral Show. Upon closer examination, this small stone appeared to him to be an interesting unequilibrated stone, and on behalf of the Planetary Studies Foundation in Galena, Illinois, a sample was sent for analysis and classification to the University of Washington in Seattle (A. Irving and S. Kuehner). Northwest Africa 7135 was initially determined to be a unique ungrouped chondrite, potentially representing a new chondritic parent body (see the LPSC abstract).

Northwest Africa 7135 contains relatively small chondrules with olivine compositions that are more magnesian (Fa4–6) than chondrules in ordinary chondrites. At the same time, the chondrules are embedded in a metal-rich matrix that constitutes a larger volume of the meteorite (~30 vol%) than matrix in ordinary chondrites (~10–15 vol%), but is similar to that in CO chondrites (Irving and Kuehner, 2015; Weisberg et al., 2006). Of significant importance is the presence of highly reduced sulfide phases in NWA 7135, including pyrrhotite, pentlandite, daubréelite, oldhamite and djerfisherite, along with the phosphide schreibersite; most of these reduced sulfides are unknown from other ordinary chondrites, but some are observed in E chondrites, winonaites, and in the unequilibrated forsteritic inclusions present in the Cumberland Falls aubrite. Through studies of the chondritic inclusions in Cumberland Falls, the petrologic type was ascertained by Binns (1969) to be mostly type 3 and 4, and by Kuehner et al. (2016) to be type 6. By comparison, the F chondrites Acfer 370, NWA 7135 are petrologic type 3/4, while the most recently characterized El Médano 301 (see photo) is petrologic type 4. Photo of a chondritic inclusion in the Cumberland Falls aubrite
National Museum of Natural History, Smithsonian Institution
standby for cumberland falls photo
Photo courtesy of Dr. Martin Horejsi Oxygen-isotope values from six samples of NWA 7135 were obtained at the University of New Mexico (K. Ziegler), and the corresponding plots occupy a distinct space above the TFL on an oxygen three-isotope diagram (see below). These oxygen isotope values provide further evidence for a unique parent body unrelated to the known ordinary chondrites.
Diagram courtesy of the Meteoritical Bulletin: Oxygen Isotope Plots Direct Link A forsterite (F) chondrite group was proposed in 1977 by Graham et al. to include the four meteorites Kakangari, Mount Morris, Pontlyfni, and Winona, along with certain inclusions in the Cumberland Falls aubrite. Subsequent chemical and O-isotope data from the Winona, Pontlyfni, and Mount Morris (Wisconsin) meteorites, and the Cumberland Falls inclusions, led to the establishment of the Winonaite (and ‘W chondrite’) group, with evidence that its members have a genetic relationship with the IAB complex irons (e.g., Davis et al., 1977). At the same time, continued research also determined that Kakangari represents the type specimen of a unique K chondrite grouplet.

The magnesian silicates in Northwest Africa 7135 are very similar to the xenolithic forsterite (Fa0.7) inclusions in Cumberland Falls (Kuehner et al., 2016), and it shares many characteristics with the anomalous chondrite Acfer 370 (see Moggi-Cecchi et al., #5421, 2009) and the ungrouped chondrite El Médano 301 (see Pourkhorsandi et al., #6176, 2016). In light of the current compositional and isotopic data, Kuehner et al. (2015, 2016) along with Pourkhorsandi et al. (2016, 2017) have recognized that NWA 7135, Acfer 370, El Médano 301, and the inclusions in Cumberland Falls (and in the ALHA78113 aubrite) represent a ‘new’ F chondrite grouplet. They demonstrated that NWA 7135 and the other members of this F chondrite grouplet are clearly resolved from other ordinary chondrite reservoirs and form a cluster on an oxygen three-isotope diagram. Values for NWA 7135 and the other forsteritic meteorites and inclusions overlap and establish a unique trend line between the ordinary chondrites and the TFL (see diagrams below). standby for f chondrite plot
Diagram credit: Kuehner et al., 78th MetSoc, #5238 (2015)
standby for o-isotopic diagram
Diagram credit: Kuehner et al., 47th LPSC, #2304 (2016)
standby for o-isotopic diagram
Diagram credit: Pourkhorsandi et al., GCA, vol. 218, p. 109 (2017)
‘The ungrouped chondrite El Médano 301 and its comparison with other reduced ordinary chondrites’
An early Solar System history for the F chondrite parent body was suggested by Neal and Lipschutz (1981). Their scenario involves formation of both the F chondrite and enstatite planetesimals in close proximity within the solar nebula. Thereafter, a severe collision between these two objects occurred, during which time F chondrite material became incorporated in the regolith of the enstatite planetesimal. This scenario is supported by their identification of shock-generated plagioclase and jadeitic pyroxene grains in the Cumberland Falls inclusions. In addition, Cumberland Falls exhibits shock effects from this collision, which include the presence of miniscule blebs of metallic FeNi and sulfide dispersed in the silicate grains producing silicate darkening, undulose to mosaic extinction with planar fractures in olivine, impact-melt clasts, and a shock stage of S2–S3 (A. Rubin, 2010). Ultimately, this enstatite parent body experienced another impact event through which ejecta eventually became the Cumberland Falls aubrite. Other collisional ejecta eventually crossed paths with the Earth, which was recovered and subsequently classified as E- and F-group chondrites and aubrite meteorites. Northwest Africa 7135 exhibits features of moderate weathering (W2) and very weak shock (S2).

Notably, a forsterite chondrite/achondrite clast with EH3 affinities has been discovered in the EH3 chondrite Sah 97158, paired with Sah 97096 (Boyet et al., 2011, #5120). The specimen of NWA 7135 shown above is a 0.58 g partial slice. The photos below show the complete meteorite as found and the cut face of the main mass, courtesy of Fabien Kuntz. standby for northwest africa 7135 photo
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Photos courtesy of Fabien Kuntz

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NWA 5958

C2-ungrouped (CM-like)
(C3.0-ung [2012], C2-ung in MetBull 99)
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Purchased September 2009 Numerous fragments of a fusion-crusted carbonaceous chondrite having a combined weight of 286 g were found in Algeria and purchased by G. Hupé in Morocco. A sample was submitted to Northern Arizona University (T. Bunch and J. Wittke) and the University of Washington at Seattle (A. Irving and S. Kuehner) for analysis and classification. Northwest Africa 5958 was designated an ungrouped carbonaceous chondrite of petrologic type 3.0, with a low shock stage of S1 consistent with other carbonaceous chondrites.

In their study of magnetic susceptibility, Elmaleh et al. (2012) identified abundant Fe-rich phyllosilicates such as the serpentine cronstedtite, indicative of a low degree of parent body hydrothermal alteration. Moreover, the observation of partially altered chondrules led them to consider revising the petrologic type from 3.0 to 2.9. Although the meteorite experienced low terrestrial weathering (W1), NWA 5958 has experienced significant loss or gain of some elements during its residence in the desert, with more extreme alteration observed toward the surface (Ash et al., 2011). Subsequent analyses of the magnetic properties of NWA 5958 conducted by Jacquet et al. (2016) indicate the presence of significant magnetite (1.2 wt%) compared to kamacite (0.42 wt%), which attests to a degree of parent body aqueous alteration. Utilizing infrared spectroscopy to investigate the hydrous phases in NWA 5958, Jacquet et al. (2016) identified a significant abundance of phyllosilicates indicative of weak to moderate aqueous alteration, presenting a spectral range that matches well that of CM2 LEW 85311.

In the first compositional anaysis conducted by Bunch et al. (2011), it was found that NWA 5958 consists of a wide variety of small objects (0.05–2.5 mm) set in a dark, porous, fine-grained matrix. Notably, matrix material constitutes 76 vol% of the meteorite, a higher abundance than in most CM chondrites, and it contains a relatively low abundance of chondrules composing 19 vol% (Jacquet et al., 2016). Silicates, phyllosilicates (e.g., the Fe-serpentine cronstedtite), and partially equilibrated Fe-sulfides were observed in the matrix by Stroud et al. (2014). Similar to CM chondrites, relatively small chondrules of mostly Mg-rich POP, enstatite PP, and forsterite PO types are present and host multiple accretion rims (up to five), with each successive layer composing material of larger grainsizes. In their study, Jacquet et al. (2016) identified accessory metal, sulfides, and poorly characterized phases (PCP, now determined to be tochilinite-cronstedtite intergrowths [TCI]). The TCI composition in NWA 5958 reflects a relatively high mean ‘FeO’/SiO2 ratio of 5, which is consistent with very minimal aqueous alteration compared to other CM group members (Rubin et al., 2007). Contrariwise, the relatively low mean S/SiO2 ratio (0.05) for NWA 5958 is consistent with a high degree of aqueous alteration. At the same time, the low abundance of FeNi-metal (< ~0.2 vol%) is more consistent with a moderate degree of aqueous alteration comparable to petrologic type 2.2–2.5 in the Rubin et al. (2007) scheme.

Bunch et al. (2011) reported µm-sized hexagonal carbon grains present in some chondrules with larger C aggregates in the matrix and olivine in NWA 5958. They found only a few small CAIs in the meteorite (2 vol%), and it was ascertained that these do not contribute significantly to the unique 16O-rich composition of the bulk meteorite (Ash et al., 2011). A larger number of AOAs were observed in the meteorite by Jacquet et al. (2016). Rare fine-grained, carbonaceous chondrite xenoliths were only observed by Bunch et al. (2011).

An oxygen 3-isotope diagram based on the initial values determined by Bunch et al. (2011) is shown below. Given these values, NWA 5958 falls along an extension of the carbonaceous chondrite anhydrous mineral (CCAM) line of slope-0.94, but is distinct from other C chondrites in having values even closer to initial solar values exemplified by the slope-1 line (D. Rumble III, CIW; see O-isotope plot). The CCAM line represents a linear array on an oxygen three-isotope diagram that is defined by the ratio plots for a mixture of all minerals that constitute CAIs. It was determined by Young and Russell (1998) that the most primitive Solar System materials defined a linear array with a slope of 1.00 (Y&R line). These primary materials initially had heterogeneous 16O contents, but later mass fractionation and oxygen exchange processes resulted in material with higher 17O and 18O contents, generally evolving towards the CCAM line. standby for oxygen isotope diagram
Diagram credit: Bunch et al., 42nd LPSC, #2343 (2011) Further analyses of NWA 5958 were conducted by Jacquet et al. (2016), and they obtained O-isotopic values (Δ17O = –4.26‰) different from those determined previously (Δ17O = –7‰). A new oxygen 3-isotope diagram based on these new values is shown below. It demonstrates that NWA 5958 plots near some C2-ungrouped meteorites such as Acfer 094 rather than with C3-ungrouped meteorites. Together with other similar meteorites, NWA 5958 could sample a separate CM-like parent body (Jacquet et al., 2017). standby for nwa 5958 oxygen isotope diagram
click on image for a magnified view

Diagram credit: Jacquet et al., MAPS, vol. 51, #5, p. 862 (2016)
‘Northwest Africa 5958: A weakly altered CM-related ungrouped chondrite, not a CI3’ ( Sanborn et al. (2015) presented a Δ17O vs. ε54Cr coupled diagram in their analyses of NWA 5958. Utilizing the initial Δ17O value determined by Bunch et al. (2011), along with the 54Cr value determined by Göpel et al. (2013) of +0.973 (± 0.153)—this value being close to the mean for all carbonaceous chondrites—they demonstrated that the meteorite plots in a distinct region (bottom orange circle in diagram below). However, given the new O-isotope value determined by Jacquet et al. (2016) of Δ17O = –4.26‰, the meteorite is within the trend of other carbonaceous chondrite groups (top orange circle in diagram below); NWA 5958 plots slightly below the ungrouped achondrite (CV-clan-related) NWA 782217O = ~ –4‰). standby for cr-o isotope diagram
Diagram adapted from Sanborn et al., 46th LPSC, #2259 (2015) The initial bulk chemical and trace element compositions calculated for NWA 5958 were found to resemble that of CI chondrites, including a high volatile content, with the 187Os/188Os ratios having the largest value (the most radiogenic) of any measured carbonaceous chondrite (Ash et al., 2011). Subsequent bulk chemical and trace element compositional analyses were conducted by Jacquet et al. (2014). After accounting for the typical altered signature caused by an extended residence in a desert environment, NWA 5958 shows very close similarities to the CM chondrite Paris. Measurement of the Cr2O3 content in chondrule olivine for NWA 5958 is comparable to that for the CO3.03 ALHA77307 and the CM2.7–2.9 Paris, which is indicative of very limited thermal metamorphism at temperatures <300°C.

In a study of presolar grains present in NWA 5958 matrix conducted by Nittler et al. (2012), a small abundance of 13C-rich grains (45 ppm) and O-anomalous grains (~100 ppm) were identified. The low abundance of O-rich presolar silicates (~50 ppm) observed by Stroud et al. (2014), including an Al,Mg-spinel and an enstatite grain, is thought to be due to loss as a result of parent body hydrothermal alteration. These values suggest a slightly higher degree of hydrothermal metamorphism compared to typical type 3.0 carbonaceous chondrites, more consistent with a type 2. Northwest Africa 5958 is a unique primitive sample from the early solar system, having many characteristics intermediate between CM and CO chondrites. The specimen of NWA 5958 shown above is a 1.62 g crusted fragment, while the image below shows an excellent interior close-up, shown courtesy of Greg Hupé. standby for nwa 5958 close-up photo
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Photo courtesy of Greg Hupé—Nature’s Vault

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Fell June 9, 1866
48° 54′ N., 22° 24′ E. Following detonations, a shower of stones numbering more than 1000, and weighing ~500 kg, fell at 5:00 P.M. in the Ukraine, USSR. A strewnfield with dimensions of 2 × 0.75 miles was delineated. The nearly spherical mass of Knyahinya had an estimated pre-atmospheric diameter of 90 cm, corresponding to a mass of ~1,300–1,400 kg, before it was broken into two nearly symmetrical sections upon impact; the largest section weighed 293 kg. The CRE age of Knyahinya as inferred from cosmic-ray track data and cosmogenic Ne ratios is 38 m.y. Knyahinya has proven to be instrumental in studies of cosmogenic nuclide production systematics due to its simple single-stage exposure history and its simple shielding geometry, allowing exact depth profiles to be determined.

Although Knyahinya has an accepted classification of L5 based on its trace element composition, it is partially resolved into an intermediate L/LL chondrite group based on studies involving the Co abundance in matrix kamacite and on its bulk metallic iron content similar to that of an LL chondrite. Contrariwise, the Ir/Au atomic ratio of Knyahinya falls within the H range (3.4–3.8) rather than the LL–L range (2.6–3.5).

It was demonstrated by Szurgot (2016) that the mean atomic weight (Amean) of meteorites can be used to resolve the OC groups, including the intermediate groups L/LL and H/L. Amean values can also be predicted through various equations based on other parameters such as atomic Fe/Si ratio, grain density, and magnetic susceptibility, and these Amean values all consistently resolve these groups into the ordered sequence LL < L/LL < L < H/L < H. Knyahinya has Amean values of 23.24 (chemical composition), 23.21 (Fe/Si atomic ratio), and 23.54 (grain density). Furthermore, it was demonstrated that Amean values are lower for unequilibrated type 3 samples than for equilibrated samples within each OC group due to the presence of water; Amean values for petrologic types 4–6 are indistinguishable within each group. standby for amean diagram
Diagram credit: M. Szurgot, 47th LPSC, #2180 (2016)
Amean based on chemical composition (Eq. 1), Fe/Si atomic ratio (Eq. 2), and grain density (Eq. 3) Members of the L/LL chondrite group include L/LL3 Esperance, L/LL4 Seemore Downs, Bjurböle, and Cynthiana, L/LL5/6 Sahara 97021, and L/LL6 Acfer 041, as well as the partially resolved L/LL3 Inman and L/LL6 Holbrook. In a study of the shock stage of Knyahinya conducted by Fürj et al. (2009), mosaicism in olivines and twinning in pyroxenes were detected consistent with a shock stage of S4. The specimen of Knyahinya shown above is a 1.93 g partial slice. The photo below shows a print depicting the Knyahinya fireball—from a book published in Denmark a few years after the fall (click image for title page). [A description posted online: ‘Mr. Kolbay’s drawings of what he and Mr. Rainer saw from near Eperies (today Pre šov, Slovakia), ~92 km from the Knyahinya strewnfield, as presented by W. R. v. Haidinger in 1866.’]

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Photo courtesy of Lars Pedersen

The largest mass of Knyahinya weighs 293 kg, which is curated at the Natural History Museum, Vienna.