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Weatherford

CBa, bencubbinite
standby for weatherford photo
Found 1926, recognized 1940
35° 30′ N., 98° 42′ W. A mass of 2 kg was plowed up in Custer County, Oklahoma and later recognized as a meteorite. Weatherford is a primitive, polymict, chondritic breccia containing cm-sized clasts of a metal host (~60 vol%) and a mafic silicate host (~40 vol%), along with ordinary chondrite, R-chondrite, carbonaceous, and other xenolithic clasts.

One ordinary chondrite xenolith classified petrologically as >3.5 is composed of 85% chondrules of a size closest to that of H chondrites, as well as an FeNi and FeS content very similar to that of H chondrites. However, the O-isotopic composition is unlike that of any other ordinary chondrite group or a chondritic clast that was found in CBa Bencubbin. The unique type 3 carbonaceous xenoliths have been characterized as dehydrated CM-like chondritic material. Very small olivine-rich clasts resembling Rumuruti-like material have also been found. These xenoliths are highly equilibrated to type 6 and devoid of chondrules.

While refractory inclusions have only been found in the HaH 237, QUE 94411, and Gujba bencubbinites, and the transitional member Isheyevo, cm-size chondrule fragments of barred olivine composition occur in all group members. The metal in Weatherford records the effects of a late shock event ~0.5 b.y. after accretion (~4.2 b.y. ago) in which recrystallization and minor impact-melting occurred. It is proposed that this silicate melt phase, which itself contains tiny immiscible blebs of FeNi-metal, thereafter infiltrated the space between the metal and silicate fragments leading to reheating of the metal particles to temperatures of ~400°C (Chappell et al., 2011; Srinivasan et al., 2013). A significant component of these kamacite grains (~40%) have both homogenous and exsolved (the latter from late-stage impact reheating to <600°C) Cr-rich sulfide inclusions occurring in arcuate textures along metal grain boundaries, rarely containing metal blebs (Srinivasan et al., 2014). Host metal elemental abundance ratios are correlated with those of the silicate host phase, providing evidence for a common nebular reservoir origin for the two host phases; the FeNi-metal blebs likely formed through nebular condensation or silicate reduction processes (possibly initiated through impact shock heating).

All of the bencubbinites are characterized by a significant enrichment in planetary-type rare gases and isotopically heavy nitrogen (15N). Contrary to other bencubbinites, the main N-carrier phase in Weatherford is located in the shock-melted veinlets. A lesser source of heavy nitrogen, along with rare gases such as radiogenic 40Ar, has been found inside µm-sized vesicles within the silicate melt phase. The high oxide content within this vesicle-containing melt phase is consistent with fractionation processes resulting from a high-temperature shock event. This chemically reactive environment could lead to the release of N, creating the N- and Ar-rich vesicles.

Bencubbinites have been divided into two petrologic subgroups, CBa and CBb, representing those with cm-sized metal and silicate chondrules, and those with mm-sized chondrules. The CB, CH, and CR chondrites constitute the CR clan, comprising groups which likely formed in the same isotopic reservoir under similar conditions in the solar nebula. The best current evidence supports an origin for these late-formed metal-rich carbonaceous chondrites in an impact plume generated by a hypervelocity collision between planetary embryos (Krot et al., 2009). The ‘Grand Tack’ model of Walsh et al. (2011) could have generated such high velocities; further details about the ‘Grand Tack’ scenario can be found in the Appendix Part III. Planetary modeling employed by Johnson et al. (2016) demonstrates that only during a relatively short timeframe within this migration period will impact velocities reach levels high enough (>18 [±5] km/s) to vaporize Fe in a planetesimal core. It is notable that the timing of the inward migration of Jupiter and Saturn is consistent with the timing of the accretion of CB chondrites from an impact-generated vapor plume, occurring ~4.8. m.y. after CAIs (Scott et al., 2018).

Following condensation of the various components, they were aerodynamically sorted according to their velocity, size, and density as they spread out into the local nebular gas in a typical fan-shaped pattern. It was calculated by Morris et al. (2012) that in ~1% of the impacts the host planetesimal would be propelled in the direction of the impact plume, sweeping up some of the aerodynamically sorted ejecta within a short time period measured in weeks. They reason that this re-accreted material would have been mixed with existing crustal components to form a layer many meters thick. See the HaH 237 page for a more detailed scenario of the CB group formation process ascertained by Fedkin et al. (2015) through kinetic condensation modeling.

The specimen shown above is a 2.54 g polished partial slice of Weatherford. This specimen was acquired from Steve Arnold (International Meteorite Brokerage) in 2000, who had obtained it through a trade with the Smithsonian Institution, and was subsequently traded to another collector in 2018. The photo below shows a larger 22 g slice of this rare bencubbinite in the J. Piatek Collection, acquired from the National Museum of Natural History, Smithsonian Institution. standby for weatherford photo
Photo courtesy of the J. Piatek Collection


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NWA 4025

CBa, bencubbinite standby for nwa 4025 photo
click on photo for a magnified view Purchased August 2005
no coordinates recorded A group of sixty-nine small, paired, stone fragments, having a combined weight of 745.5 g, were recovered in the Sahara Desert by a Moroccan hunter. Further searches of the area have yielded no additional fragments. All of these fragments were subsequently purchased by H. Strufe, and a sample was submitted for analysis to the Museum für Naturkunde, Humboldt University, Berlin, (A. Greshake). Northwest Africa 4025 was classified as a rare CBa bencubbinite.

Northwest Africa 4025 exhibits very close similarities to the type specimen Bencubbin, and has a shock stage of S3 and a weathering grade of W2/3. A visual comparison was conducted between NWA 4025 specimens and the previously found Saharan CBa chondrite NWA 1814; it was clearly demonstrated that they are not paired. Northwest Africa 4025 closely resembles the Bencubbin meteorite, while NWA 1814 manifests some characteristics of the CBa chondrite Gujba. Moreover, The degree of terrestrial alteration on the outer surface of NWA 4025 is generally higher than that on NWA 1814. An O-isotopic analysis indicates that a close relationship exists between this bencubbinite and the CH chondrites.

The CB, CH, and CR chondrites constitute the CR clan, comprising groups which likely formed in the same isotopic reservoir under similar conditions in the solar nebula. The best current evidence supports an origin for these late-formed metal-rich carbonaceous chondrites in an impact plume generated by a collision between planetary embryos (Krot et al., 2009). Following condensation of the various components, they were aerodynamically sorted according to their velocity, size, and density as they spread out into the nebular gas in a typical fan-shaped pattern. It was calculated by Morris et al. (2012) that in ~1% of the impacts the host planetesimal would travel in the direction of the impact plume, sweeping up some of the aerodynamically sorted ejecta within a short time period measured in weeks. They reason that this reaccreted material would have been mixed with existing crustal components to form a layer many meters thick. See the HaH 237 page for a more detailed scenario of the CB group formation process ascertained by Fedkin et al. (2015) through kinetic condensation modeling.

The specimen of NWA 4025 shown above is a 7.6 g prepared end section (half individual). Pictured below are two of the sixty-nine recovered fragments constituting this meteorite.

standby for nwa 4025 photo
Photo courtesy of the J. Piatek Collection


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NWA 1814

CBa, bencubbinite
standby for nwa 1814 photo
Found April 1999
no coordinates recorded A single stone of 156 g was found in Morocco between Taouz and Ouzina by a French team under the organization of Bruno Fectay and Carine Bidaut. Analysis of this bencubbinite was conducted at the Museum National d’Histoire Naturelle, Paris, and it was described as being very similar to Bencubbin, Gujba, and Weatherford.

Large clasts of FeNi-metal constitute ~60 vol% of this bencubbinite, while the remainder is composed of olivine and pyroxene silicates having barred to crytocrystalline textures. A plagioclase mesostasis is also present. The metal clasts are composed primarily of kamacite with very rare troilite inclusions, and they exhibit oriented, roughly hemispheric shapes, possibly the result of quenching upon contact with the asteroid surface while still in a plastic state (Perron and Leroux, 2004). The best current evidence supports an origin for these late-formed metal-rich carbonaceous chondrites in an impact plume generated by a collision between planetary embryos (Krot et al., 2009). Following condensation of the various components, they were aerodynamically sorted according to their velocity, size, and density as they spread out into the nebular gas in a typical fan-shaped pattern. It was calculated by Morris et al. (2012) that in ~1% of the impacts the host planetesimal would travel in the direction of the impact plume, sweeping up some of the aerodynamically sorted ejecta within a short time period measured in weeks. They reason that this reaccreted material would have been mixed with existing crustal components to form a layer many meters thick. See the HaH 237 page for a more detailed scenario of the CB group formation process ascertained by Fedkin et al. (2015) through kinetic condensation modeling.

An analysis of N, C, and H in NWA 1814 revealed that the highest N abundances are located within tetrataenite mostly associated with sulfides and carbides, with high concentrations occupying metal grain boundaries (Perron and Mostefaoui, 2007). The N, C, and H isotopic values are consistent with those of Bencubbin. The specimen of NWA 1814 shown above is a 1.02 g partial end section. The main mass is shown in the top photo below, while a 23.5 g cut section is shown at the bottom, courtesy of the J. Piatek Collection.

standby for nwa 1814 photo
Photo courtesy of B. Fectay and C. Bidaut—meteorite.fr
standby for nwa 1814 photo
Photo courtesy of the J. Piatek Collection


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Isheyevo

CH/CBb
(transitional between CH3 chondrite and CBb bencubbinite)
standby for isheyevo photo
Found October 2003
53° 37′ N., 56° 20′ E. A single fusion-crusted meteorite weighing 16.7 kg was found in a field by a farmer while operating a tractor. The location of the find was in the Ishimbai region of Bashkortostan, Russia, near the village of Isheyevo. In September of 2004, a sample was submitted for analysis to the Vernadsky Institute, Russian Academy of Sciences, Moscow (M. Ivanova), while additional analysis was conducted at Michigan State University (A. Ulianov). Although a classification of CBb was INITIALLY agreed upon and submitted to the Nomenclature Committee, Isheyevo exhibits mineralogical characteristics intermediate between CH and CBb chondrites and a classification of CH/CBb is recommended.

Isheyevo consists primarily of two lithologies that transition smoothly between each another—one lithology that is metal-rich and most similar to the CBb chondrites HaH 237 and QUE 94411, and another that is metal-poor and most similar to the CH3 chondrites NWA 470 and Acfer 182. A comparison between the two lithologies reveals that the dominant metal-rich lithologies have an FeNi-metal content of ~60–90 vol%, while the fewer metal-poor sections contain 7–20 vol% (Ivanova et al., 2008; Krot et al., 2008); the average FeNi-metal content of the meteorite is ~60 vol%. Compared to the metal-poor lithology, which contains up to 90 vol% chondrules measuring 0.2–1 mm, the metal-rich lithology contains a significantly lower abundance of chondrules, as low as 30 vol%, measuring 0.1–0.4 mm. Moreover, the metal-rich lithology contains a greater percentage of non-porphyritic chondrules compared to the metal-poor lithology, and also contains six times the abundance of highly refractory CAIs (Ivanova et al., 2006).

There are two distinct populations of CAIs in Isheyevo which together constitute ~1 vol%. The CAIs show no mineralogical differences between the two lithologies. The majority population (~55%) of CAIs have igneous textures and show little interaction with a cooling nebula gas. They have highly refractory compositions and are similar to those found in CH and CB chondrites; among these are grossite-rich, melilite-rich, perovskite-rich, hibonite–melilitepyroxene and/or spinel), spinel-rich, and pyroxene–hibonite (Krot et al., 2006). These CAIs were isolated from the hot nebula region and experienced rapid cooling, and they typically show a depletion in 16O. This is a distinct population of CAIs compared to those found in all other carbonaceous chondrite groups. A smaller population (~45%) of less refractory CAIs are similar to those in other carbonaceous chondrite groups, with features indicative of an extended period of gas–solid interaction including anorthite replacing melilite and the formation of Wark–Lovering rims (Ivanova et al., 2008). There are sparse AOAs present in Isheyevo, and a single grain of the refractory mineral osbornite (TiN) was found in a sample, a mineral previously identified in the CH chondrite ALH 85085. Osbornite is among the earliest minerals that condensed from the solar nebula, forming under high temperatures and highly reducing conditions. In addition, a relatively large, sapphire-blue-colored hibonite (Ca,Ti-aluminate) inclusion, similar to the ‘Blue Angel’ inclusion from the CM chondrite Murchison, was captured in a photo of a slice of Isheyevo (see image below). standby for hibonite inclusion photo
Photos shown courtesy of Olga Dobrova—Rus Meteorites A unique group of 26Al-poor relict CAIs consisting of highly refractory spinel+grossite+hibonite has also been identified in Isheyevo. These are embedded in 26Al-poor, magnesian, porphyritic host chondrules within the metal-poor lithology (Krot et al., 2007). This was an early-formed population of CAIs that must have been extant in the chondrule-forming region prior to the formation of the host magnesian porphyritic chondrules—a region inferred to have been distant in time and/or proximity from that of all other chondrule-bearing chondrite groups. Since the O-isotopic values of the CAIs present in CB chondrites plot along the CCAM line instead of the CR trend line, they represent primordial 26Al-poor molecular cloud material rather than condensates from the impact vapor plume (Fedkin et al., 2015; Van Kooten et al., 2016). Schematic of the Evolution Scenario of the Early Solar System
A. Giant Molecular Cloud ⇒ B. Protostars ⇒ C. Proto-Sun and Protoplanetary Disk
standby for solar system evolution schematic
click on photo for a magnified view

Diagram credit: Van Kooten et al., PNAS, vol. 113, no. 8 (2016, open access link)
‘Isotopic evidence for primordial molecular cloud material in metal-rich carbonaceous chondrites’
(https://doi.org/10.1073/pnas.1518183113)
Chondrules in the two lithologies of Isheyevo are ferromagnesian and Al-rich, but the chondrule textures are mostly distinct between the two lithologies. The metal-rich lithology contains mostly magnesian cryptocrystalline (some oxidized, FeO-rich [type-II]) and skeletal olivine chondrules considered to have formed as gas–melt condensates in an impact-generated plume. This lithology contains a lower abundance of barred chondrules formed from a melt component of the plume. By contrast, the metal-poor lithology contains mostly various types of olivine and pyroxene porphyritic chondrules (primarily reduced, FeO-poor [type-I]) which formed by melting of precursor material in the solar nebula (Krot et al., 2006; Krot and Kazuhide, 2008). Some porphyritic olivine–pyroxene chondrules are zoned like those in CH chondrites (Ivanova et al., 2005), and the Al-rich chondrules which are present are also more like those in CH chondrites than in CB chondrites.

Another group of highly zoned chondrules present in Isheyevo differ from those in CH chondrites in that they contain phyllosilicate rims (Ivanova and Lorenz (2006). These highly zoned chondrules likely formed in a multistage event begining with condensation of a Mg-rich core. Thereafter, lower-temperature and/or more oxidizing conditions ensued resulting in the formation of a more ferroan mantle. Incorporation of impact-generated water vapor, or alternatively, interaction with a highly oxidizing gas, led to the production of the phyllosilicate rims. Although these phyllosilicate rims are considered by some to be unrelated to the phyllosilicates of the hydrated matrix lumps, others have argued that these rimmed chondrules might have once been part of the hydrated matrix lumps (see description below), retaining a remnant phyllosilicate rim upon accretion to the Isheyevo parent body (Ivanova et al., 2009). The 15N-enriched hydrated matrix lumps were likely formed in an asteroidal or cometary setting and then accreted separately to the Isheyevo parent body (van Kooten et al., 2014). Accretion of all these components to a common Isheyevo/CH/CB parent body ensued. Based on O-isotopic analyses of the ferromagnesian and Al-rich chondrules in Isheyevo and CB chondrites, it was determined that many are unique among known carbonaceous chondrite groups and were formed in separate nebular regions and/or time periods (Krot and Kazuhide, 2008; Krot et al., 2009).

Isheyevo shows significant compositional and petrological variability, especially in the metal-rich lithology, which can be compared to that of the CH group. As with the CH group, Isheyevo incorporates both type-I and type-II POP chondrules. The fact that the individual components in Isheyevo (e.g., CAIs, chondrules, and metal grains) are the same in each of the two lithologies, and that no clasts consisting of a mixture of metal-rich and metal-poor lithologies are present, it was argued that Isheyevo was not formed from individual fragments of CBb and CH metal-rich chondrites (Krot et al., 2008). Like the CB and CH chondrites, Isheyevo comprises a wide diversity of components intergrown together, which led Krot et al. (2006; 2008) to suggest that these components may have formed over several generations involving multiple events and locations, including initial evaporation/condensation in the protoplanetary disk, late-stage condensation within a protoplanetary impact-generated melt and gas plume, and asteroidal aqueous alteration. See the HaH 237 page for a more detailed scenario of the CB group formation as determined by Fedkin et al. (2015) through kinetic condensation modeling.

A slice of Isheyevo was observed through CT imaging and electron microscopy by Chaumard et al. (2014). They described the texture as consisting of numerous poorly graded layers of well sorted silicates arranged in nearly parallel alignment. These layers have a variable thickness measuring ~1–10 mm and comprise both metal-rich and metal-poor compositions. This layered texture is thought to represent a sedimentary deposition process resulting from the parent body moving through a protoplanetary impact-generated vapor plume.

An investigation of these unique sedimentary laminations present in Isheyevo was conducted by Morris et al. (2015). They propose that Isheyevo represents a portion of a meters-thick layer from the surface of its parent body that re-accreted through a process of low-velocity sweep-up of aerodynamically size-sorted silicate and metal spheres following their condensation within an impact ejecta plume. They ascertained that the ejecta particles were slowed and size-sorted according to their respective compositions as they interacted with the primordial nebular gas. Their calculations constrain the timing of this event to the late-stage protoplanetary disk when the gas density was 10–11–10–12 g per cm–3, considered to be ~5 m.y. after CAIs as determined by isotopic dating. Based on assumptions derived from their studies, Morris et al. (2015) constructed a more detailed scenario for this event: sweep-up would have occurred ~70 hours after condensation of the particles (measured in days to weeks) while these particles were still within ~6,950 km of the asteroid as it moved through the plume at ~28 m/s (30–200 m/s; Garvie et al., 2017). An Isheyevo sample that was studied in-depth by Garvie et al. (2017) exhibits sedimentary laminations and features indicative of faulting and shearing due to compaction (see photo below). Home Page
mouseover to view diagrammatic image

Photos credit: Garvie et al., Icarus, vol. 292, pp. 36-47 (2017)
‘Sedimentary laminations in the Isheyevo (CH/CBb) carbonaceous chondrite formed by gentle impact-plume sweep-up’
(https://doi.org/10.1016/j.icarus.2017.03.021)
The unfractionated nature of the REEs in Isheyevo, as well as the near-chondritic ratios of refractory lithophile elements, led Pack et al. (2006) to conclude that Isheyevo was formed primarily from primitive, unfractionated nebular material. This should be contrasted with the proposition that the bencubbinites formed within a metal-enriched, impact vapor plume produced by the collision of a metal-rich chondritic body and a reduced silicate body. In support of an asteroidal model is the discovery by Uymina and Grokhovsky (2006) of intermingled zoned and unzoned metal grains. The zoned grains contain small spherical inclusions consisting of a Cr–S mineral which is associated with the hydrated boundary of these grains. Diffusion-induced, oriented gradients of Ni and Cr are present within zoned grains, and a heterogeneous mixture of several FeNi-metal alloys is present as well. Isheyevo contains a significant proportion of chemically-zoned, FeNi-metal grains that likely formed during condensation from a gas. Chromium-rich troilite occurs as inclusions in some metal grains, while fine-grained matrix material like that present in some CB chondrites is absent. All of these features are more consistent with a complex, multistage formation history for Isheyevo rather than a simple nebular condensation history.

Raman spectra have identified the first occurrence in a carbonaceous chondrite of several high pressure phases, located within barred olivine fragments and in matrix components of the CB chondrite Gujba. These phases include majorite garnet, majorite-pyrope solid solution, and wadsleyite, along with minor grossular-pyrope solid solution and coesite (Weisberg and Kimura, 2010). These high pressure phases formed either through solid-state transformation of pyroxene, or through crystallization from an impact-melt during a heterogeneous, planetesimal wide impact-shock event reaching minimum pressures of ~19 GPa and temperatures of ~2000°C. The investigators argue that these high pressure phases are inconsistent with the subsequent formation of chondrules within an impact plume since at such high temperatures these phases would be rapidly back-transformed to their low-temperature polymorphs. Moreover, the measured cooling rate of chondrules (ave. 100K/hr) is much slower than that at which shock veins with high pressure polymorphs would be expected to survive (~1000K/hr). Therefore, they determined that the barred chondrules and metal in CB chondrites were formed prior to the impact event which produced the high-pressure polymorphs in Gujba.

Three groups of hydrous matrix lumps, or lithic clasts, have been identified in a multi-component study of Isheyevo (Bonal et al., 2008, 2010). Primary mineralogical features distinguishing the three groups are i) total hydration, ii) presence of anhydrous silicates, and iii) magnetite-free, FeNi-metal bearing. The anhydrous silicates present in the second group of clasts experienced the lowest aqueous alteration, lack magnetite and carbonates, and have the lowest degree of thermal metamorphism. Cryptocrystalline microchondrules and a microCAI have been identified in this group of lithic clasts. Moreover, this clast group shows isotopic evidence consistent with that of pristine chondrule fragments associated with the high-temperature component of metal-rich chondrites; i.e., they were derived from a planetesimal that was in some manner involved in the collisional disruption event that created the gas-melt plume which was the precursor to the formation of the CH/CB chondrites.

Hydrous lithic clasts are primarily composed of the phyllosilicate serpentine and the carbonates dolomite and magnesite, and they are comparable to petrologic type 1–3.05 chondrites. Hydrous lithic clasts can contain magnetite, sulfides, FeNi-metal, and even small amounts of olivine and pyroxene. These clasts exhibit various degrees of structural order in their polyaromatic carbonaceous matter component, which corresponds to low degrees of thermal metamorphism and dehydration occurring prior to incorporation of these clasts into the Isheyevo planestesimal. These hydrated clasts are chemically similar to metamorphosed CM matrix material, although some highly hydrated clasts are more similar to CI phyllosilicates (Ivanova et al, 2009). However, the mineralogy and O-isotopic composition of the carbonates in the Isheyevo clasts are distinct from those of any known aqueously altered carbonaceous chondrite including those of the CM, CI, and Tagish Lake groups, as well as from ordinary and HED meteorites (Bonal et al., 2010). The hydrous lithic clasts exhibit a wide range of isotopic compositions, all occurring together in a non-aqueously altered meteorite, and represent previously unsampled material derived from at least three unique parent bodies: Group I experienced a high degree of aqueous alteration; Group II contains anhydrous silicates; and Group III lacks magnetite and contains FeNi-metal. Briani et al. (2010) studied the structural order of the macromolecular organic matter present in five different lithic clasts, and demonstrated that each clast was derived from a separate parent body that experienced a range of alteration histories. These diverse clasts eventually accreted together with the other high-temperature components to form the Isheyevo parent body.

The bulk O-isotopic composition of Isheyevo plots in the range of CH chondrites, along the CR–CH–CB mixing line. The vast majority of chondrules and CAIs in Isheyevo are 16O-enriched just as in CH-group components, and they plot along the CCAM line instead of the CR trend line; therefore, they represent solar nebula material rather than condensates from the impact vapor plume (Fedkin et al., 2015). The remaining 16O-depleted CAIs likely experienced remelting and isotopic exchange during accretion of the Isheyevo parent body (Krot et al., 2007). It has also been recognized that two generations of CAIs can be distinguished among the 16O-enriched group: 1) those which formed earliest, contain the most highly refractory minerals, and have minimal values of 26Al/27Al estimated to be 5×10–7; 2) those which have a less refractory nature and contain the ‘canonical’ 26Al/27Al initial ratio estimated to be 5.17(±0.10)×10–5 (Yin et al., 2008).<!–It has been suggested by some investigators that a 'supra-canonical' value (5.8–7.0×10–5) represented the original CAI value prior to the thermal processing which occurred over a period of ~350,000 years (Young and Shahar, 2007). However, this ‘supra-canonical’ value has not been reproduced by any other laboratory and is probably not correct. –> Oxygen and Al–Mg isotopic systematics for CH chondrites also indicate the existence of two populations of CAIs (Krot et al., 2008). The O-isotopic differences that exist between the CAIs and the magnesian cryptocrystalline and skeletal chondrules in Isheyevo indicate that each of these components formed under distinct conditions. On the other hand, the similarity in O-isotopic composition between these chondrule types in Isheyevo and the same chondrule types present in other CH and CBb chondrites suggests a genetic connection (i.e., same parent body), or at least formation within a common reservoir.

It has been suggested by some investigators (e.g., Van Kooten et al. 2016) that the CAIs having the lowest abundances of 26Al might reflect a formation prior to 26Al injection into the solar nebula, rather than loss through radioactive decay and evaporation during multiple open system melting events in the early Solar System. In their isotopic study of the CR clan meteorites, Van Kooten et al. 2016) concluded that the precursor source material of these meteorites incorporated a significant volume (up to 50%) of primordial molecular cloud matter from the outer Solar System that was depleted in µ26Mg* and enriched in µ54Cr relative to CI chondrites; this is demonstrated in the two diagrams below. µ26Mg* and µ54Cr Compositions of Metal-rich Chondrites
(µ notation denotes deviation from terrestrial standards in parts per million)
standby for mg vs. cr diagram
standby for mg vs. cr diagram
Diagrams credit: Van Kooten et al., PNAS, vol. 113, no. 8 (2016, open access link)
‘Isotopic evidence for primordial molecular cloud material in metal-rich carbonaceous chondrites’
(https://doi.org/10.1073/pnas.1518183113)
Based on the K–Ar dating system, an age of ~4.3 b.y. was estimated for Isheyevo. Although this age typically represents the last degassing event, there is a strong likelihood that late thermal disturbances could have affected this chronometer. Van Kooten et al. (2016) employed 53Mn–53Cr chronometry to date the formation of secondary carbonate minerals in several hydrated lithic clasts, now constituents of the Isheyevo meteorite. This chronometer indicates that the source parent body of the hydrated lithic clasts had experienced aqueous activity 1.54 (+0.92, –1.11) m.y. later than it occurred on either the CI or CM chondrite parent bodies. This is consistent with a later accretion of the Isheyevo daughter body from an impact-generated vapor plume, while incorporating the exogenous hydrated lithic clasts. Ivanova et al. (2008) ascertained a CRE age of 34 m.y. for Isheyevo, which is in close accord with that determined for Bencubbin but significantly older than for most CH chondrites.

Although the bulk meteorite is unshocked (S1), olivine grains in some chondrules exhibit features of moderate shock to stage S4 including mosaicism, planar fractures, and planar deformation features (PDFs). As with other bencubbinites and CH chondrites, Isheyevo contains an abundance of isotopically heavy N, and it contains the highest average matrix δ15N value of any meteorite—up to +1500‰ (the average composition of the solar nebula is δ15N ~ –300‰). In addition, hotspots have been identified in Isheyevo with values as high as δ15N +4000 (±1500)‰. The 15N-enrichment of the lithic clasts predates the clast accretion to the Isheyevo parent body. The main carrier phase(s) of 15N in the lithic clasts is not carbide or taenite as was once thought, but instead the 15N may have been remobilized during a strong shock-melting event (Sugiura et al., 2000; Ivanova et al., 2007). In their in-depth study of Bencubbin, Perron et al. (2008) proposed that water and 15N-bearing organic compounds were degassed from the hydrated lithic clasts during the impact of a chondritic object(s). These hydrated lithic clasts later agglomerated onto the CB/CH/Isheyevo parent body during the initial accretionary stage.

Conversely, in their scanning ion study of Isheyevo lithic clasts, Bonal et al. (2008) have resolved the presence of isotopically anomalous organic compounds, suggesting that this may be the source of the abundant 15N. In a similar conclusion based on studies of a hydrated lithic clast, Leitner et al. (2010, 2011) identified a presolar silicate grain of a type associated with a core-collapse supernova which was found to contain 15N-rich material (δ15N = 1400‰ and higher). The presolar grains were injected ~100 AU from the solar nebula (Sugiura and Fujiya, 2011), at an abundance determined to be ~10 ppm. These findings support the theory of surviving protosolar cloud material as the source of the 15N-anomaly. Other studies attribute the source of the heavy N to N2 self-shielding or low-temperature ion-molecule reactions in the protosolar molecular cloud or the protoplanetary disk. The heavy N may have been carried in a phyllosilicate layer or in amorphous ferrihydrite and redistributed/diluted by aqueous alteration processes to produce Group I clasts (Bonal et al. (2010).

Isheyevo has undergone only minor terrestrial weathering (W1). This meteorite demonstrates characteristics of a transitional member of the CR clan, which allows mineral and chemical comparisons to be made between all of the members. The specimen of Isheyevo shown above is a 2.1 g slice (photography courtesy of Sergey Vasiliev). The photo below shows the detailed cut face of a 253 g slice, courtesy of the J. Piatek Collection. standby for isheyevo photo
Photo courtesy of Dr. J. Piatek Meteorite Collection


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HaH 237

CBb, bencubbinite
standby for hammadah al hamra 237 photo
Found October 18, 1997
28° 36.56′ N., 13° 02.95′ E. A single mass of 3,173 g was found in the Libyan Sahara Desert in the fall of 1997. This breccia consists of a mechanical mixture of silicate and metal fragments that are similar to those in Bencubbin, but smaller in size. While the FeNi-metal abundance in HaH 237 is exceptionally high (>70 vol%), the opposite is true for the abundance of fine-grained matrix. Compositionally and isotopically the bencubbinites are most similar to carbonaceous chondrites, thought to have formed in a vapor plume resulting from a collision between two CR-like parent bodies.

The bencubbinites have been divided into two petrologic subgroups, CBa and CBb, representing those with cm-sized metal and silicate chondrules, and those with mm-sized chondrules, respectively. Hammadah al Hamra 237 is a member of the CBb subgroup of the bencubbinites, with an especially close relationship to QUE 94411 (paired with QUE 94627). HaH 237 is a metal-rich chondritic breccia formed from a combination of two separate nebular condensates; i.e., the collisional debris from two planetary embryos. These highly primitive components underwent a size-sorting process within a nebular region enriched in siderophile elements (9.6x that of lithophiles, relative to solar composition), leading to equilibrium condensation at variable pressures, temperatures, and cooling rates, and within distinct local environments containing variable dust enrichments, particularly Si/gas and Ni/gas ratios. The resulting zoned and unzoned metal grains, silicate chondrules, and other condensation components accreted to form the bencubbinites and CH chondrites. (Fedkin et al., 2015).

Silicates are present in the form of mm-sized cryptocrystalline (CC) and barred olivine (BO) chondrules and chondrule fragments, similar to those found in members of the CH group such as Acfer 214. In light of their non-igneous textures, absence of relict grains, depletion in volatiles, unfractionated REE patterns, and absence of FeNi-metal, the chondrules in HaH 237 are thought to represent first generation chondrules that condensed directly from an impact vapor plume. Large polycrystalline, chondrule-like metal spheres (up to 5 mm) and their fragments are also present. The nearly solar Ni/Co ratio and the strong compositional zoning in some metal grains (60–70%) is indicative of a volatility-based condensation origin in an impact vapor plume. This was followed by diffusion outward from the refractory siderophile-rich core at a total pressure of only 10 Pa (one ten-thousandth of a bar) (Campbell et al., 2005). It was initially ascertained that formation of zoned metal grains in CBb chondrites occurred at high temperatures during a temperature interval of 1092°C to 987°C, and then experienced rapid cooling. The unzoned metal grains were considered to have formed at lower temperatures, cooled more slowly, and then underwent minimal low-temperature metamorphism with little if any reduction. The observation of a sub-grain microstructure exhibiting deformation in areas remote from the core indicates a limited heating event occurred following the condensation/diffusion phase (Duffy et al., 2008).

Based on kinetic condensation modeling, Fedkin et al. (2015) ascertained a more detailed scenario for the formation of the bencubbinites. They determined that the chemical and isotopic compositions of all of the components, including zoned and unzoned metal grains, and cryptocrystalline (CC) and barred olivine (BO) chondrules, can be explained by equilibrium condensation in a vapor plume caused by the collision of two differentiated CR-type chondritic planetesimals, each composed of a core (~29 wt%), a CaO-, Al2O3-poor mantle (~57 wt%), and a CaO-, Al2O3-rich crust (~14 wt%), in addition to the presence of significant hydrous materials. Different fractions of each of these lithologies were sampled by the various components in the bencubbinites, each forming within distinct local regions of the impact vapor plume. They ascertained that formation of the unzoned metal grains occurred by equilibrium condensation as a liquid under one of two probable conditions: 1) 0.01 bar pressure and an enrichment in the Ni:gas ratio of 3,000 relative to solar composition, or 2) 0.001 bar pressure and an enrichment in the Ni:gas ratio of 30,000 relative to solar composition. By similar means, they showed that the BO chondrules could have formed under these same equilibrium condensation conditions, but with additional constraints including enrichment in the Si:gas ratio of 500 relative to solar composition, a water abundance of 20 wt% of the total vaporized silicate, and with formation occurring in a region sampling 40–70 wt% of the vaporized mantle lithology (or ≤40 wt% if sequestration of refractory condensates had occurred). Likewise, the CC chondrules could have formed under the same conditions as the unzoned metal grains, but with unique additional constraints including enrichment in the Si:gas ratio of 300 relative to solar composition, a water abundance of 15 wt% of the total vaporized silicate, and with formation occurring in a region sampling ≤40 wt% of the vaporized mantle lithology after sequestration of refractory condensates. In contrast, the calculations show that the zoned metal grains had to have formed in a separate region of the vapor plume, where condensation occurred in the solid state under significantly lower pressure involving a lower enrichment in the Ni:gas ratio (2,500 relative to solar composition), and cooling proceeded at a high rate under conditions of rapidly decreasing pressure.

<!– Silicates are present in the form of mm-sized cryptocrystalline (CC) and barred olivine (BO) chondrules and chondrule fragments, similar to those found in members of the CH group such as Acfer 214. In light of their non-igneous textures, absence of relict grains, depletion in volatiles, unfractionated REE patterns, and absence of FeNi-metal, the chondrules in HaH 237 are thought to represent first generation chondrules that condensed directly from a vaporized region of the solar nebula, or from a vapor plume created by the collision of two planetesimals, at pressures low enough to allow silicate to condense before metal. As an illustration of this scenario, some FeNi-metal grains contain inclusions of cryptocrystalline chondrules. After the condensation of chondrules and metal grains, they were radially transported by the solar wind to a colder, isolated region of the nebula prior to the condensation of volatile elements.

–> A clear, isotropic glass component is found within some chondrules, reflecting the unequilibrated type-3 nature of the meteorite. Other shock-melted silicate glass (5–20 GPa) containing miniscule Fe–Ni–S metallic blebs occurs between metal and silicate fragments, similar to that present in Bencubbin and Weatherford. This shock melt glass is considered by some to be the transformed matrix material, now preserved as sparce hydrated lithic clasts (see following paragraph). Refractory inclusions are a minor constituent in HaH 237, QUE 94411, and Gujba, but none have yet been found in Bencubbin or Weatherford. The CAIs present in the CH-group segment of the CR clan contain the most refractory minerals, providing evidence that they condensed from a hotter nebular region than those in the CR and CB groups, and that they experienced only very low degrees of alteration. The 16O-depleted, pyroxene-rich CAIs present in metal-rich chondrites are unique, and they have textural and mineralogical characteristics that exclude them from an origin on the CR parent asteroid. <!–According to Krot et al. (2012), the 16O-depleted CAIs in the metal-rich chondrites could have resulted from both remelting of CAIs and evaporation of chondrule melts during the presumed planetary-scale collision.–>

Similar to the CAIs, hydrated lithic clasts (or ‘matrix lumps’) are present in low abundance in the CBb group as well as in the CH and CR groups, but none have been identified in the CBa group. These clasts consist of magnetite, sulfides, and carbonates embedded within a hydrous phyllosilicate matrix composed of serpentine and minor smectite. These hydrated lithic clasts are very similar in composition to carbonaceous chondrite matrix material of types 1 and 2, and they were formed independently of the anhydrous CB components. Following aqueous alteration, the lithic clasts were accreted together with the high-temperature components in a cooler region of the Solar System, or through regolith gardening on the CB parent body.

Subsequent shock-lithification fused the porous, fine-grained matrix material that initially constituted the CBa chondrites (Meibom et al., 2004). The shock wave resulted in higher temperatures in this hydrated, porous material than in the denser metal and silicate components, which served to weld the latter components together (Meibom et al., 2005). Heating was localized and cooling was rapid, consistent with the low degree of chondrule melting and shock effects observed. Both the metallic and silicate chondrules in HaH 237 and several other CB members (QUE 94411, Bencubbin, Weatherford, and Gujba) exhibit preferential orientation, presumably resulting from this deformational event.

As with all bencubbinites, HaH 237 contains an abundance of isotopically heavy N. The main N carrier phase in this meteorite is molten metal, possibly residing in sub-microscopic carbide and nitride within kamacite. Another N carrier is taenite, or less often, carbide present around Cr-rich sulfide. More rarely, silicate glass and gas within vesicles are also found to contain heavy N. The hydrated lithic clasts are also being investigated as a carrier of heavy N (see the Bencubbin page for details).

Extraterrestrial amino acids (0.2–2 ppm) were found to be present in a sampling of CB chondrites studied by Burton et al. (2013), abundances of which are slightly lower than those found among aqueously altered type-1 carbonaceous chondrites. The types of amino acids are different from those identified in other carbonaceous chondrite groups, and were likely synthesized through different chemical pathways under different environmental conditions (e.g., degree of aqueous alteration).

Initial studies based on Pb-isotope systematics revealed that the silicates in Gujba (CBa) and HaH 237 (CBb) formed simultaneously ~4.5627 b.y. ago. Subsequently, high precision isotopic studies of HaH 237 conducted by Pravdivtseva et al. (2015, 2016) led them to suggest a refinement in the absolute I–Xe age for the Shallowater standard of 4.5624 (±0.0002) b.y. Based on this new refinement, the age of HaH 237 relative to Shallowater was ascertained to be 4.5621 (±0.0003) m.y., which is consistent with the U-corrected Pb–Pb age determined for Gujba chondrules by Bollard et al. (2015) of 4.56249 (±0.00021) b.y., as well as that determined for HaH 237 silicates by Krot et al. (2005) of 4.5619 (±0.0009) b.y. Agreement in the ages for these various components reflects the simultaneous closure of these chronometers following chondrule formation within a late-stage protoplanetary impact-generated plume.

The CB, CH, and CR chondrites constitute the CR clan, comprising groups which likely formed in the same isotopic reservoir under similar conditions in the solar nebula; current evidence argues for an origin of the metal-rich carbonaceous chondrites in a common collision between planetary embryos (Krot et al., 2009). The CRE age of HaH 237 is calculated to be greater than 3 m.y. The specimen of HaH 237 shown above is a 1.1 g thin partial slice, while that pictured below is a grand 76.33 g complete slice, courtesy of the J. Piatek Collection.

standby for hammadah al hamra 237 photo
Specimen size ~ 123mm by 63mm
Photo courtesy of the J. Piatek Collection


For additional information on the petrogenesis of HaH 237 and the CB chondrites, read the PSRD article by G. Jeffrey Taylor: ‘Little Chondrules and Giant Impacts‘, Oct 2005.