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 agreed upon and submitted to the Nomenclature Committee, Isheyevo exhibits mineralogical characteristics intermediate between CBb and CH chondrites.
Isheyevo consists primarily of two lithologies that transition smoothly between each anotherone 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 CH 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 ~6090 vol%, while the fewer metal-poor sections contain 720 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 0.21 mm, the metal-rich lithology contains a significantly lower abundance of chondrules, as low as 30 vol%, measuring 0.10.4 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, hibonitemelilite (± pyroxene and/or spinel), spinel-rich, and pyroxenehibonite (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 gassolid interaction including anorthite replacing melilite and the formation of WarkLovering rims (Ivanova et al., 2008). Along with sparse AOAs, a single grain of the refractory mineral osbornite (TiN) was found in Isheyevo, 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. 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 chondrulesa 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 solar nebula material rather than condensates from the impact vapor plume (Fedkin et al., 2015).
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 gasmelt 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 olivinepyroxene 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 ascertained 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 ~110 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.
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 CrS 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 13.05 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 CRCHCB 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 5×107; 2) those which have a less refractory nature and contain the "canonical" 26Al/27Al initial ratio estimated to be 5.17(±0.10)×105 (Yin et al. 2008). Oxygen and AlMg isotopic systematics for CH chondrites also indicate the existence of two populations of CAIs (Krot et al., 2008). It has been suggested by other investigators 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. 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.
Based on the KAr 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. A CRE age of 34 m.y. was ascertained, which is in close accord with the CRE age of Bencubbin but significantly older than most CH chondrites (Ivanova et al., 2008).
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 meteoriteup 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.
Photo courtesy of Dr. J. Piatek Meteorite Collection