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Tucson

Iron, ungrouped
(silicated, possibly unique member of CR chondrite clan)
standby for tucson photo
Found 1845
31° 51′ N., 110° 58′ W. Two masses of the Tucson meteorite were found, the ring-shaped Irwin–Ainsa mass and the paired, slab-shaped, Carleton mass. No fusion crust or heat-affected zone remains on either mass. The meteorites consist of a refractory and reduced mixture of fine-grained, Si–Cr-bearing FeNi-metal (92 vol%) and nearly alkali-free silicates (8 vol%), both having a nebular rather than an igneous origin; the FeNi-metal is considered to have condensed first.

The small (0.1–2 mm) silicate inclusions occur as curvy-linear arrangements suggestive of a flow alignment. They consist primarily of forsteritic olivine (66.4%) with both pure and high-Al enstatite (30.2%), with minor low- and high-Al pyroxene (diopside, 2.7%), pure anorthite and mesostasis glass (0.7%), and trace spinel and brezinaite (Nehru et al., 1982 and references therein). It was determined that the olivine crystallized from a liquid, of which the latter is now present as glass inclusions within olivine grains.

Tucson is thought to have formed from a metal–silicate mixture that was co-precipitated from the solar nebula at high temperatures (~1500°C) and high pressures (~ >1 bar) under highly reducing and turbulent conditions. This was followed by rapid cooling (~1,000°C/m.y.) and annealing as evidenced by the high-Al pyroxene and the presence of quenched clear glasses, thus preserving the early-condensed, chondrule-like form of the silicate inclusions. The occurrence of rare Ca-rich plagioclase in Tucson instead of the alkali feldspar present in most other silicated irons is consistent with volatile loss during high temperature conditions (Ruzicka, 2014 and reference therein). Due to this rapid cooling, no Thomson (Widmanstätten) structure is present upon etching, and Tucson is classified structurally as an ataxite. Tucson is highly reduced and might be related to the similarly reduced and Ge-depleted meteorites Santiago Papasquiero or Nedagolla. Nehru et al. (1982) consider the likely precursor material of Tucson to have been a unique forsterite–enstatite silicate assemblage thus far unsampled as a meteorite, or perhaps it was a more forsterite-rich E chondrite-like body, although it has also been more recently proposed that Tucson may represent the most metal-rich and volatile-element-poor member of the CR chondrite clan.

A study of glass inclusions within and between olivines was conducted by Varela et al. (2008, 2010). Olivine inclusions exhibit rounded surfaces in contact with metal, and crystal faces in contact with glass. Some investigators interpret the flow-like arrangement of the inclusions as indicative of a metallic melt intruding a silicate assemblage as a result of impact forces. On the other hand, ballistic aggregation is considered by some to be responsible for the elongated shapes and preferred direction of the silicates (Kurat et al., 2010). In their studies of Tucson based on new petrographic evidence, Kurat et al. (2010) found that metal and the Ca–Al–Si-rich liquid are early nebular condensates, which precipitated prior to the formation of the silicates. The silicates formed later from the Ca–Al–Si-rich liquid by vapor–liquid–solid condensation, in accord with the ‘primary liquid condensation’ model (Varela et al., 2005; Varela and Kurat 2006, 2009). The glass phase is consistent with rapid quenching from the liquid phase, with a mineralogical composition consistent with derivation from carbonaceous chondrite material, especially CR chondrites; the composition is much different from that of enstatite chondrites.

In a similar manner, the O-isotopic composition of Tucson silicates and glass is similar to the CR clan and to Kakangari, and the low volatile element abundances are also consistent with the CR clan. The Ca–Al–Si-rich glass in Tucson is also similar in trace element contents to those of carbonaceous chondrites, and they show unfractionated REE patterns. In addition, Tucson glass inclusions show many similarities to the glass in C chondrites and CR chondrites in particular. Beyond that, the metal component in Tucson has a highly refractory nature, in many ways similar to that of the IVA irons (Humayun, 2010). It is basically unfractionated and has probably inherited its trace element abundances and highly depleted volatile element content by direct condensation from an early solar nebula gas, such as with CB and CH chondrites (Kurat et al., 2010). A high-temperature nebular condensation origin is considered most plausible by investigators. A possible formation relationship between Tucson glasses and the glasses in IIE irons has also been conjectured.

A sub-mm-sized xenolithic achondrite clast from the CM chondrite Mukundpura was analyzed by Ebert et al. (2018). The clast has an O-isotopic composition and a REE pattern with HREE enrichment similar to silicate inclusions in the Tucson iron, and it is considered that they might share a common parent body (see diagram below). standby for tucson vs. cm oxygen diagram
click on image for a magnified view

Diagram credit: Ebert et al., 81st MetSoc, #6246 (2018) A History Revealed

Each of the Tucson masses has a unique convoluted history. The first recovered and the largest of the two is the 1,400 pound (688 kg) ring-shaped mass, alternately called the Ring, Signet, Ainsa, and Irwin–Ainsa Meteorite at various times in history. The other mass, originally weighing 633 pounds (287 kg), is named the Carleton Meteorite for the Civil War general who appropriated the piece for public display.

The first written description of the Ring dates back to 1845. It was written in Spanish by a respected official of Sonora, Mexico, named José Velasco. From a section of his treatise concerning the state of Sonora, titled Mines of Iron, Lead, Copper, and Quicksilver, he described a mountain pass (known today as Box Canyon) within the Sierra de la Madera range (now the Santa Rita Mountains). This pass, located between Tucson and Tubac, contained many large masses of pure iron, lying at the foot of the mountains. He wrote of a medium-sized mass that was taken to Tucson, a journey of over thirty rugged miles, where it had resided for many years [before 1845], serving as an anvil for the garrison armorer/blacksmith.

Writing in his diary for May 31, 1849, the ’49er A. Clarke clearly described the find circumstances and provided details of the appearance of the meteorite anvil used by the shoer of his mule. Shortly thereafter, in his article of 1852, Notice of Meteoric Iron in the Mexican Province of Sonora, Dr. John LeConte described the appearance and recovery information of two meteoric anvils being used by blacksmiths in Tucson. That same year, in his diary entry for July 17, boundary commissioner John Bartlett described the origin and dimensions of the Ring mass and alluded to a second large mass located within the garrison in Tucson. He also made a detailed sketch of the celestial anvil, brought to light only in 1978.

Perhaps the most thorough description of the two masses was written by John Parke, lieutenant in charge of a survey expedition. He indicated that with much effort some small samples were acquired and sent to the east for analysis. An analysis was performed by Dr. Charles Shepard and published in 1854 in the American Journal of Science. He reported the lack of crust and the oxidized nature of the meteorite sample, along with its chemical composition.

It was a blacksmith named Ramón Pacheco, who recovered the slab-like mass on or about 1850, and put it to use as an anvil in Tucson. In 1856, the other blacksmith anvil, the Ring, was abandoned leaving all the blacksmith duties to Pacheco and his anvil. In 1862, Colonel James Carleton confiscated the Pacheco anvil and had it shipped to San Francisco where permission was obtained to saw off a specimen for analysis. The mass remained on display at the Society of California Pioneers until 1939 when it was purchased by the Smithsonian to be displayed alongside the Ring mass.

During the year 1860, a medical officer named Bernard Irwin found the abandoned Ring mass and took possession of it on behalf of the Smithsonian. The following year, the meteorite was contracted to begin its journey from Arizona to Washington D.C. via Guaymas by Augustin Ainsa. He took two years to haul the mass to the coast, where his brother, Santiago Ainsa, took over the remaining leg to New York. Santiago was primarily interested in glorifying the family name and contrived a false history of the Ring mass in correspondence with the Smithsonian. In part, he claimed the mass was recovered by his famous great grandfather, Juan Bautista de Anza, in 1735 at a known location, and transported to Tucson. This legend, along with his other claims, have been proven to be totally fabricated; but not before the credit for the presentation of the Ring Meteorite to the Smithsonian was given to the Ainsas, including naming the Ring meteorite the Ainsa Meteorite.

When Irwin learned of this appalling turn of events, he sent a letter to the Smithsonian, debunking Ainsa’s fabricated story and protesting their choice of names for the mass. He stated he would rather they rename it the Tucson Meteorite rather than honor the fraudulent claims of Santiago Ainsa. After all, the Ainsas had only contracted to carry it to Washington for Irwin, the original donator to the Smithsonian. The name was subsequently changed to the Irwin–Ainsa Meteorite, but Irwin was intent on removing the name of Ainsa from the meteorite and publishing the correct history of the mass. It took twelve years for the name to be changed at Irwin’s insistence to the Tucson Meteorite.

The 3.6 g specimen pictured above was originally part of the inner nodule of the ring mass, and shows a polycrystalline structure with flow patterns of silicate inclusions. The Tucson Ring can be viewed today at the Smithsonian National Museum of Natural History in the Hall of Geology, Gems, and Minerals.

Portions above excerpted from The Tucson Meteorites by Richard R. Willey (1987) standby for tucson photo
A large part slice of Tucson.
Photo courtesy of the J. Piatek Collection
standby for tucson photo
Tucson Ring and Carleton masses on display at the Smithsonian.
Photo courtesy of M. Horejsi


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Renazzo

CR2.4
standby for renazzo photo
standby for renazzo photo
Fell January 15, 1824
44° 46′ N., 11° 17′ E. At 8:30 P.M. local time in Renazzo, Italy, a bright light was seen and three detonations heard by residents as several stones fell. Three individual stones were recovered having a combined weight of 10 kg, the largest weighing ~5 kg. Renazzo is a brecciated meteorite with a shock stage ranging from S1 to S3. This meteorite is a preserved fall with a unique volatile element and N-isotopic composition that distinguishes it from the other carbonaceous chondrite groups. It serves as the type specimen of the Renazzo-type chondrites, which are considered to be among the least thermally altered of the known meteorite groups.

While Renazzo ranks among the most primitive meteorites under study, new Cr2O3 abundance data obtained by Schrader et al. (2015) indicates that EET 96259 might represent the least thermally altered, most pristine sample of the CR parent body presently known (subtype 3.00; see diagram below). Other pristine CR chondrites with very low thermal alteration include MIL 090657 (2.7, based on bulk isotopic composition, presolar grain abundance, and Cr content of ferroan olivine; Davidson et al., 2015), QUE 99177 [2.8], MET 00426 [2.8], and LAP 02342 [2.8]. Details of a new petrographic-based aqueous alteration scale for CR chondrites proposed by Harju et al. (2015) are presented farther down the page.
Diagram adapted from Schrader et al. (and references therein), MAPS, vol. 50, #1, p. 37 (2015)
‘The formation and alteration of the Renazzo-like carbonaceous chondrites III: Toward understanding the genesis of ferromagnesian chondrules’
(http://dx.doi.org/10.1111/maps.12402)
The matrix component in Renazzo, which includes igneous fragments, isolated mineral grains, CAIs, AOAs, dark inclusions, and metal grains, accounts for ~35–40 vol% of the bulk composition (Bayron et al., 2014). The abundance of chondrules and chondrule fragments in Renazzo is ~50–60 vol%. Multilayered, FeO-poor (type-I) chondrules (mostly porphyritic attesting to partially melting) constitute the vast majority of the chondrules, with the remainder consisting of sulfide-rich, FeO-rich (type-II) chondrules and chondrule fragments. Rare relict type-I grains (and/or compositionally similar precursor material) have been identified within some type-II chondrules (Schrader et al., 2015). Al-rich chondrules have also been identified in minor abundances. The chondrules, metal, and matrix formed under variable conditions from a common nebula reservoir. This is attested by the complimentary chemical composition of these components, as well as by the enrichment and depletion in metal and silicate (respectively) of the same presolar s-process carrier phase (observed as a correlation in nucleosynthetic Mo and W isotope anomalies) (Budde et al., 2018). All of these components ultimately agglomerated in the outer Solar System (beyond Jupiter) at approximately the same heliocentric distance as the CI chondrites.

Based on the initial 26Al/27Al ratios calculated for chondrules from unequilibrated ordinary chondrites and CO3.0 chondrites, they are considered to have formed 1–2 m.y. after CV CAIs (age = ~4.567 b.y.). By contrast, the lower initial 26Al/27Al ratios inferred for chondrules in CR chondrites are consistent with a relatively late formation of ~2.5 m.y. after CAIs (Scott et al., 2007; Nagashima et al., 2008). A similar age of 2.5 (±0.9) m.y. after CAIs was determined by Amelin et al. (2002) utilizing the Pb–Pb dating method, which was translated by Connelly et al. (2012) to 3.66 (±0.63) m.y. after CAIs by employing a corrected 238U/235U ratio. The Pb–Pb chronometer was also applied to the CR chondrite NWA 6043, and it revealed a wide range of ages—from the oldest known chondrule at 4.5673 (±0.0010) b.y., to chondrules as much as 3.64 m.y. younger (Bollard et al., 2014). They reasoned that the latest formation of chondrules 4.56366 (±0.00091) b.y. ago establishes a benchmark for the time of dissipation of the solar accretion disk. In a comprehensive study of CR chondrules by Schrader et al. (2016) that included Al–Mg systematics, they recognized three distinct populations of chondrules dated at 2.2 (+0.1/–0.2), 2.9 (+0.2/–0.2), and 4.4 (+0.7/–0.4) m.y. after CV CAIs; the latter oldest age represents the largest population in the study. They concluded that accretion of the CR parent body continued for at least ~4.0 m.y. after formation of the earliest solids in the solar nebula as represented by CAIs in CV chondrites. Moreover, their study indicates that 26Al was uniformly distributed in the CR chondrite formation region. From a weighted mean of 21 CR chondrules, Schrader et al. (2017) obtained a preferred Al–Mg age of 3.75 (±0.24) m.y. after CAIs. A high-precision Hf–W isotopic analysis of Renazzo and three other CR chondrites was conducted by Budde et al. (2018) in order to constrain the timing of chondrule formation, accretion, and metal–silicate segregation for the CR parent body. They determined a Hf–W age of 3.6 (±0.6) m.y., which is in agreement with that determined by the Pb–Pb and Al–Mg chronometers. In addition, they ascertained a weighted mean age based on all three chronometers of 3.73 (±0.21) m.y. after CAIs.

Utilizing Mn–Cr systematics, Trinquier et al. (2008) calculated an equilibrium age for CR2 chondrites of between 1.1 and 6.2 m.y. after CAIs. Because the CR chondrules were accreted into the developing parent body after most of the radiogenic 26Al had decayed, the degree of thermal metamorphism was limited. Among other petrographic features, the high abundance of Cr2O3 that remains in the metal phase attests to this low degree of thermal metamorphism (Wasson and Rubin, 2010; Schrader et al., 2015 [see diagram above]). Jilly-Rehak et al. (2016) conducted new 53Mn–53Cr analyses of secondary carbonates—calcite in the matrices of Renazzo and GRO 95577 and of dolomite in a Renazzo dark inclusion. They determined aqueous alteration ages anchored to the D’Orbigny angrite of 4.5634 (+0.0028/–0.0074) b.y. for Renazzo calcite, and 4.5554 (+0.0014/–0.0021) b.y. for GRO 95577 calcite; these ages correspond to 4.6 (+7.4/–2.8) m.y. and 12.6 (+2.1/–1.4) m.y. after CV CAIs (4.56794 [±0.00031] b.y.; Bouvier et al., 2011), respectively. The prolonged period of aqueous alteration revealed by this study indicates that impact shock was necessarily a significant heating mechanism on the CR parent body following the cessation of radiogenic heating from 26Al decay.

A basic scenario for the early petrogenesis of Renazzo has been described. Initially, Ti-bearing perovskite condensates facilitated the condensation of forsterite. Lower temperatures and more highly reducing conditions prevailed as small low-Ni, FeNi-metal grains and fine-grained pyroxene dust combined to form aggregates. Thereafter, these precursor coarse-grained aggregates were melted by a solar heat pulse and then cooled from peak temperatures at rates of 0.9–96.7 K/hour. These conditions are more consistent with nebula shock wave-induced heating rather than through lightning or the x-wind (Chaumard et al., 2015). These melted aggregates ultimately coalesced to form the Ti-enriched, FeO-poor, porphyritic type-I chondrules composing Renazzo. Nebular-condensed C was dissolved in the metal and later exsolved in the newly formed chondrules (Kong et al., 1999). Contemporaneously, mineral assemblages comprising FeNi-metal, sulfides (pentlandite and pyrrhotite), and phosphate were formed in the aftermath of high-temperature gas–solid sulfidation processes in the solar nebula (Schrader et al., 2008, 2015).

Igneous rims composed of silica-rich pyroxene are present on most CR type-I chondrules (although not evident in Renazzo or Al Rais due to their higher degree of aqueous alteration). These rims are presumed to have formed by direct condensation of silica onto chondrule surfaces from a cooling, fractionated nebular gas (Krot et al., 2003, 2004). FeNi-metal occurs along the rims of a significant portion (~40%) of type-I chondrules in CR chondrites, and also occurs both in chondrule interiors and as isolated grains in interchondrule matrix material. The amoeboid-shaped metal grains on chondrule rims were once considered to have formed by migration of FeNi-metal grains from chondrule interiors through centrifugal forces associated with rapid spinning of these chondrules (Grossman and Wasson, 1985; Kong and Palme, 1999); however, the distribution of the metal bears more resemblence to a discontinuous shell than a ring. Noting the minor effects centrifugal forces may have, it was argued by Wasson and Rubin (2009) that under high temperature conditions, with the molten metal having lower surface tension with respect to the vacuum of space than to the molten silicates, metal was induced to form along the outer surface. Upon cooling, the interface tension became more influential than the surface tension, causing the metal to take the form of small globules. The formation of metal in CR chondrites was examined in-depth by Jacquet et al. (2013, 2014), who assessed the likelihood of the four competing theories: 1) direct condensation from the nebula, 2) silicate reduction processes, 3) evaporation/recondensation, and 4) desulfurization of FeS. A brief synopsis of their scenario is presented below (∗). A fifth mechanism has been proposed by Chaumard et al. (2014) to describe the formation of an isolated, igneously-zoned, mm-sized matrix metal grain in Renazzo—fractional crystallization of a molten droplet with subsequent recondensation of volatile siderophile elements on the exterior margin which diffused inwards during cooling.

Utilizing 3-D microtomographic imaging of Renazzo chondrules, Ebel et al. (2009) and Ebel and Downen (2011) discovered that some chondrules exhibit multiple discrete, concentric, metallic layers alternating with silicate layers, suggesting sequential accretion of independent metal and silicate components. These represent sequential generations of chondrule formation attributable to multiple local heating events within the same unique nebular source region. These accretionary periods were followed by intervals of rapid annealing, which occurred in a cooling disk environment under reducing conditions. During this period, impacts produced a petrofabric in the form of chondrule flattening in this matrix-rich (31 vol%) meteorite, resulting in a chondrule axial ratio of ~1.3 (Kallemeyn et al., 1994).

The finer-grained matrix material was also formed in this same Ti-depleted nebular region, but only after substantial cooling had occurred. During this time, abundant water ices that had accreted along with the matrix material promoted the formation of phyllosilicates. All of these various components constituting the CR parent body agglomerated in a geological instant, at a time ~2.5 m.y. after CAI formation. It was demonstrated that if the bulk chemical compositions of both the highly variable population of Renazzo chondrules and the matrix materials are calculated together, they preserve the solar elemental abundance ratios. This complementarity indicates that the accretion process of CR chondrites probably occurred as a closed system within a unique chondritic region of the protoplanetary disk (Ebel et al., 2009).

The much rarer FeO-rich (type-II) chondrules present in CR chondrites lack accretionary rims, exhibit mostly broken surfaces, and some contain relict grains with Fa values and O-isotopic ratios indicative of recycling from an earlier generation of type-I chondrules (Connolly et al., 2003, 2008). They exhibit a wide range of bulk FeO and O-isotopic compositions and have a heavier O-isotopic signature than that of type-I chondrules. It is considered that the gas may have evolved from reducing to highly oxidizing during the interval between type-I and type-II chondrule formation. In addition, the O-isotopic signature evolved from 16O-rich to 16O-poor; this enrichment of heavy oxygen isotopes was due to the evaporation of water ice, which also created a more highly oxidizing environment (Connolly and Huss, 2010). The O-isotopic composition of type-II chondrules overlaps that of ordinary chondrites, suggesting a complex formation history in an oxidizing and sulfidizing environment. After an episode of fragmentation, which was possibly associated with the accretion of the CR parent body, the type-II chondrules were subjected to impact heating and aqueous alteration from previously accreted water ices. Consistent with its content of magnetite, Renazzo reflects a greater degree of aqueous alteration than many CR group members. Aside from its brecciated structure, the extent of aqueous alteration is responsible for a porosity that ranges from 3.7% to 18.2% (Macke et al., 2011).

Results of trace-element studies have identified primitive glass inclusions within olivines that sample the liquid–vapor barrier that lead to olivine formation. These inclusion glasses formed contemporaneously with the host olivine in a dust- and oxygen-enriched region of the condensing nebula. These primary glass inclusions are found to be either Al-rich and derived from unfractionated nebular condensates, Al-poor and derived through fractionation and removal of refractory components from the nebula vapor, or Na-rich and derived from Al-rich parental glass after a metasomatic (solid–vapor) exchange of Ca for Na in the nebula (Varela et al., 2001). Presolar grains containing anomalous Xe isotopes (Xe-HL) have been identified in Renazzo. Xe-HL is a mixture of Xe-H (enriched in heavy xenon isotopes) and Xe-L (enriched in light xenon isotopes) (Bekaert et al., 2018 and references therein). Rare presolar silicate grains representing ~18 ppm, plus rare presolar silicon carbide grains representing ~55 ppm, have been identified in fine-grained accretionary chondrule rims (Leitner et al., 2012).

Refractory inclusions in Renazzo are small (generally <1 mm and most <0.5 mm) and scarce, just as in other CR chondrites. They include pristine 16O-rich CAIs that were formed over a period of ~100,000 to 400,000 years in a similar nebular reservoir as those in CV chondrites (Makide et al., 2009). CAIs constitute <1 vol% of CR chondrites and have primarily melilite-rich compositions, while others are grossite- or hibonite-rich, or more rarely, anorthite-rich. Fine-grained aggregates of nebular gas-solid condensation, known as amoeboid olivine aggregates (AOAs), are minor constituents in Renazzo. These AOAs preserve some of the most primitive relicts of early nebular condensation similar to those present in CAIs, including refractory minerals such as perovskite and spinel, and Mn-rich forsterite; primary FeNi-metal blebs also occur in some (Weisberg et al., 2008). Evidence indicates that AOAs formed during a period intermediate between the final stages of Wark-Lovering rim formation on type-A CAIs and the onset of chondrule formation. The occurrence of CAI–chondrule compound objects attests to subsequent remelting of some CAIs with chondrules in an evolved, 16O-depleted solar nebula. Both AOAs and CAIs have similar 26Mg excesses derived from initial 26Al values, and they share an 16O-rich composition likely derived from the same nebular gas reservoir (Weisberg et al., 2004; 2007). Following their formation, AOAs did not experience further equilibration with the cooling nebular vapor.

The small component of Al-rich (>10 wt% Al2O3) chondrules are thought to have formed by melting of spinel–anorthite–pyroxene CAI precursor material that was mixed with type-I precursor material (Krot et al., 2006). In contrast to the 16O-poor type-I and -II ferromagnesian chondrules, a significant percentage of Al-rich chondrules exhibit O-isotopic heterogeneity due to inclusion of 16O-rich relict CAI material, and to isotopic exchange processes with an evolving nebular gas.

The CR group contains unusually high abundances of FeNi-metal in the form of taenite and kamacite (5–9 vol%), and the metal-bearing sulfides pyrrhotite and pentlandite (1–4 vol%) contribute to this high metal content. FeNi-metal occurs in Renazzo in chondrule interiors, on chondrule rims, and as separate finer grains in the matrix. It has been suggested (Connolly et al., 2001; Zanda et al., 2002) that during the heating event(s) in which chondrules were forming, FeO and other volatiles present in the precursor condensates were evaporated and then recondensed onto the chondrule rims, later diffusing inward. The chondrules that were melted to the highest degree, corresponding to those with the most circular shapes, developed the highest abundance of metal grains on their rims. Because of the evaporation and migration of Fe to the rim metal—the most stable arrangement (factoring in properties such as surface tension and temperature)—and then its incomplete rediffusion back into the interior metal, the metal in the chondrule interiors became enriched in Ni, P, and other siderophile components. Trace element data and Ni–Co correlations support this scenario, although they indicate that certain components of chondrule metal, especially the core grains, did originate through direct, high temperature nebular condensation processes (Schönbeck and Palme, 2003; Ebel et al., 2009). The subsequent introduction of the chondrules to an oxidizing environment may also be responsible for the removal of Fe from the core.

In contrast to the argument of Wood (1963) and others for a direct condensation model, Wasson and Rubin (2010) proposed that a nebular fractional crystallization process, occurring at ~1750K and proceeded by a slow cooling rate, was responsible for producing the observed Ni gradients (core to rim decrease) and granoblastic textures of chondrules. They found that this nebular process, rather than either reduction of FeO or evaporation/recondensation, was more consistent with creating the positive correlation between Co and Ni in surface metal, as well as the negative correlation between olivine Fa content and Ni. The subsequent evaporation of S from the surficial FeS component of the chondrule rim would then increase the interface tension, ultimately leading to surface energy forces influencing the formation of coarse metal globules rather than a potential homogeneous metal film.

Still, other components of CR metal are consistent with an origin through high-temperature silicate reduction and metal–silicate equilibrium processes, as evidenced through results of Fe-isotopic mass fractionation studies. Experimental results by Cohen et al. (2006) demonstrate that type-I chondrule metal is consistent with formation by such a reduction process, and constrains the associated chondrule formation time to ~1 hour. Based on the results of evaporation experiments in a low-pressure furnace, Cohen and Hewins (2004) advanced a model in which the FeNi-metal found in Renazzo and other chondrites was a product of desulfurization of sulfides through volatilation, possibly as FeS liquid was condensing from the solar nebula at high temperature (~1565°C) and high pressure (~1 atm); the heat for this process may have been generated by the passage of a shock wave. They also argued that the FeNi-metal inclusions present within olivine grains, commonly known as ‘dusty olivine’, are best modeled as having been formed through the reduction of FeO in the presence of carbon (kerogen).

∗ In an effort to establish a definitive history of the FeNi-metal in CR chondrites, Jacquet et al. (2013) conducted trace element analyses of numerous metal grains from nine CR chondrites. These metal grains were present in three configurations—as chondrule ‘interior grains’, as chondrule surface, rim, or ‘margin grains’, and as chondrite matrix or ‘isolated grains’. After examining the geochemical relationships among these different metal grains, a formation scenario for CR type-I chondrules was developed which is consistent with the totality of their findings—some aspects of their scenario are contrary to portions of previous propositions outlined above:

  1. Precursor grains were produced under conditions of incomplete condensation, inheriting a depleted volatile element abundance, and began to aggregate into chondrules.
  2. As small interior metal grains were heated and melted, they coalesced to form rounded grains.
  3. An extended period (>1 day) of high temperatures resulted in equilibration between the interior metal grains and olivine, accompanied by evaporative loss of some Fe from those chondrules which were completely melted (leaving them Ni-enriched); an open-system redox processing is favored by the evidence.
  4. Continued accretion and heating of fine-grained material onto these early-formed chondrules produced the igneous rims; this period reflects either a single prolonged accretional/heating event, or possibly involves multiple heating events.
  5. As the cooling rate began to increase, a layer of pyroxene formed at the chondrule periphery, possibly by interaction with a Si-enriched gas, and these newly accreted margin grains coalesced and experienced a lower-degree of melting to form zoned amoeboid-shaped grains.
  6. Continued accretion of some fine-grained material onto chondrules proceeded.
  7. If a single localized heating episode was involved, whereas these stages occurred concurrently, then this may be indicative of these chondrules forming in a high-temperature region with a high dust density, followed by a sudden onset of cooling upon exiting this region; the temperature curve fitting this scenario is inferred to be inconsistent with that of the often cited shock wave model of chondrule formation.

standby for cr chondrule photo
Sketch of the formation scenario for CR type-I chondrules
Image credit: Jacquet et al., MAPS, vol. 48, #10, p. 1995 (2013)
‘Trace element geochemistry of CR chondrite metal’ (http://dx.doi.org/10.1111/maps.12212) In their studies of primitive chondrites, Kimura et al. (2008) found that the FeNi-metal phases serve as one of the most sensitive indicators of the onset of thermal metamorphism. Their work reveals that primary martensite decomposes to fine-grained plessite during low degrees of metamorphism as observed in the LL chondrite Semarkona, but that this has not occurred in the more pristine ungrouped (probably CO-related; Simon and Grossman, 2015) carbonaceous chondrite Acfer 094. Furthermore, they found that metal in and around Semarkona chondrules does not show a solar ratio of Co/Ni like that in Acfer 094, and that low temperature aqueous alteration has occurred in Semarkona as well. In addition, Kimura et al. (2008) included the carbonaceous chondrites of groups CR, CH, CB, and CM as probable 3.00 subtype specimens, notwithstanding the fact that some are currently designated as subtype 2 due to aqueous alteration features. In light of this petrologic typing paradox, they proposed that a separate scale be adopted to describe aqueous alteration which is distinct from the scale currently used for thermal metamorphism.

A preliminary aqueous alteration scale analogous to that employed for CM chondrites has been constructed for CR chondrites by Harju et al. (2011, 2014, 2015). This scale has a range from petrologic type 2.0, which is equivalent to highly altered type 1, to petrologic type 3.0, representing virtually no alteration; it was found that ~70% of CR chondrites are type 2.8. As alteration increases petrologic changes occur:

  1. chondrule igneous rim material (and later, mesostasis) is altered to phyllosilicates
  2. matrix S concentration decreases
  3. clear isotropic glass becomes altered (e.g., leaching, zoning, hydration)
  4. FeNi-metal grains undergo conversion to magnetite, along with increasing abundances of oxide phases and Fe-carbonate
  5. Δ17O values tend to increase from ~ –2.6‰ in type 2.8 to ~ –0.4‰ in type 2.0, likely due to exchange with 16O-poor fluids
  6. abundances of amino acids and O-anomalous presolar grains tend to decrease

According to this scale, Renazzo has been designated petrologic subtype 2.4. Hydrothermal alteration phases are abundant in Renazzo, including serpentine, smectite, and certain chlorite group minerals, which suggest an alteration temperature no higher than ~50–150°C; other CR chondrites have compositions more consistent with alteration temperatures as high as ~300°C. This isotopically heavy fluid (D-enrichment higher than that found in comets) has transformed the composition of certain CR samples to a greater degree than others—the CR2.3 chondrite Al Rais represents an isotopically heavy endmember, while the CR2.8 QUE 99177 represents an isotopically light endmember. The variability of D-enrichment within samples has been attributed to parent body processes rather than to accretion of an ice mixture (Bonal et al., 2013). Studies by Abreu and Stanek (2012) demonstrate that the CR chondrites which experienced the highest degrees of aqueous alteration now have the lowest Fe and the highest S content, while those that experienced the highest shock stages accompanied by volatile loss now have the highest Fe and the lowest S contents. The water/rock ratio on the CR parent body was calculated to be as high as 1.157 (Schrader et al., 2010).

Abundant xenolithic dark clasts are present in CR chondrites (~8 vol%), but oxygen isotopic data indicate their precursor is different from the CR parent body; they were likely accreted by the CR parent body during its early formative stage. Oxygen isotopic compositions and mineralogical characteristics of these dark clasts show similarities to type 3 carbonaceous chondrites that were aqueously altered prior to their incorporation into the CR host. Important similarities exist between the CR group and the CI group, suggesting that they both formed in a similar nebular region, but with the CR chondrites undergoing reduction during metamorphism and hydrothermal alteration.

Interstellar-sourced organic compounds enriched in deuterium (D) and heavy nitrogen (15N) are present in Renazzo and other CR chondrites, which is considered to reflect accretion within the cold outer region of the protoplanetary disk (Budde et al., 2006 and references therein). These isotopes are associated with carbon (C) contents of between 1.2 and 2.7 wt%. These organic macromolecules are composed of mostly 1–4 ring (up to 15 rings have been identified) aromatic C compounds comprising both oxidized (hydrous alteration) and reduced (anhydrous alteration) species. Minimal aqueous processing in Renazzo and other CR carbonaceous chondrites is indicated by the low degree of hydroxylation of toluene to phenols, and by the failure to liberate 15N-rich organic species, as well as by the presence in the insoluble organic material of an inclusion with the largest known alkyl component (Cody and Alexander, 2005). In addition, the organic material maturity parameter, PAI 1, indicates only mild aqueous alteration was involved (Pearson et al., 2006). The low degree of sulfur-based thiophenes is indicative of a low degree of thermal metamorphism. In addition, through organic material maturity parameters (such as MNR), it is demonstrated that Renazzo has experienced the least thermal processing among the CR chondrite members.

Floss and Stadermann (2009) identified high abundances of both nanocrystalline and amorphous, O-anomalous, ferromagnesian presolar silicate grains in the mostly unaltered CR2.8 chondrites QUE 99177 and MET 00426 (hydrated to serpentine in the latter; Le Guillou et al., 2013). These presolar grains are thought to have originated in oxygen-rich, low-mass red giant and asymptotic giant branch (AGB) stars. The silicate/oxide ratios of these presolar grains are higher than those found in the most primitive meteorites analyzed (Acfer 094 and ALHA77307), and are similar to those found in interplanetary dust particles (IDPs); this attests to the pristine nature of these CR chondrites, and may indicate the actual protosolar cloud abundances of presolar silicate and oxide grains. These presolar grains also contain primitive C of interstellar origin and have a high amino acid content. It appears likely that the high degree of aqueous alteration experienced by other CR chondrites is the reason they lack similar presolar silicate grains. In the CR2.8 GRV 021710, an abundance of 174 (±30) ppm O-anomalous grains, 203 (±36) ppm C-anomalous grains, 165 (±29) ppm presolar silicate grains, 9 (±6) ppm presolar oxide grains, 135 (±35) ppm SiC grains, and 50 (±12) ppm carbonaceous grains were calculated to be present (Zhao et al., 2011).

It was discovered that some Antarctic CR chondrites contain the highest amino acid abundances measured in any meteorite group. These are extraterrestrial amino acids as evidenced by their enrichment of 13C, and they have C-isotope values similar to those present in CM2 chondrites, possibly indicating a common presolar precursor. It is thought that final synthesis of certain amino acids, such as the α-amino acids alanine, glycine, isovaline, and α-aminoisobutyric acid, took place on the parent asteroid through aqueous alteration of existing presolar carbonyl precursors (Martins et al., 2007). Renazzo has a relatively low abundance of amino acids, probably due to degradation through higher degrees of aqueous alteration.

Using Raman spectroscopy, researchers have demonstrated that the degree of structural order of the organic matter in Renazzo and other unequilibrated ordinary chondrites is correlated with their petrologic type (Quirico et al., 2003). They also found that the abundances of D and 15N were also correlated. From these data they determined that Renazzo contains the most pristine organic matter among all chondrites, and that LL3.01 Semarkona, which was identified as being slightly less primitive than Renazzo, is by far the most pristine among the ordinary chondrite groups. In addition, a study of FeNi-metal and sulfide composition and texture by Kimura et al. (2006) has revealed that the anomalous carbonaceous chondrite Acfer 094 experienced even less metamorphism than Semarkona, and is consistent with the lowest petrologic type assignment of 3.00.

One CR chondrite with a very high degree of hydration, GRO 95577, was classified by Weisberg and Huber (2007) as the first CR1 chondrite (CR2.0 after Harju et al., 2011). Although the chondrules have been completely replaced by phyllosilicate layers, magnetite, and various sulfides, their textural integrity has been preserved and they are still discernible as type-I. Similarly, all of the metal has been replaced by magnetite, and there is a prevalence of sulfides and carbonates where matrix and dark inclusions had previously been located. Alteration processes involving hydrothermal fluids probably occurred at temperatures of ~300°C.

Also of note is a meteorite previously classified as C3-ungrouped, Sah 00182, which shares many similarities with the CR chondrite group (Weisberg, 2001). In addition to having similar chondrule textures, most chondrules in both Sah 00182 and other CR members are Mg-rich type-I. Metal within chondrules has a similar Ni content (4.7–6.6 wt%) and Co/Ni ratio. Although Sah 00182 lacks hydrous phyllosilicates consistent with a petrologic grade of 3, it is nevertheless petrographically similar to the CR chondrites given their degree of aqueous alteration. However, the O-isotopic ratios of Sah 00182 plot within the field of the CV chondrites, and therefore Sah 00182 remains an ungrouped carbonaceous chondrite which has retained a pristine nebular signature within its components (chondrules, FeNi-metal, CAIs, AOAs), and it has experienced very little thermal metamorphism. In addition, a few meteorites have alteration histories that may be consistent with a CR3 classification, including Acfer 324 (Sipiera and Cole, 2004).

Although recognizing the existence of significant petrological differences among them, Weisberg et al. (1995) proposed to include the CB, CH, and CR chondrite groups within a clan hierarchy based on similarities in mineralogical, bulk chemical, and O- and N-isotopic characteristics. In a contrary opinion, Makide et al. (2009) argue that the CR chondrite group should not be included with the CB and CH groups because of its dissimilarities in chondrule texture, metal abundance, matrix abundance, CAI composition and isotopic systematics, and component history (pristine nebula vs. impact vapor plume).

The ungrouped C3 chondrite LEW 85332 (Rubin and Kallemeyn, 1990) is considered to be the closest match to the likely precursor material of the CR clan with regard to its isotopic and chemical properties (Clayton and Mayeda, 1999). Notably, it has been proposed by Alexander and Bowden (2018, #6063) that LEW 85332 and the CR2 chondrite MIL 090001 represent a duo from a unique parent body. From comparisons made between the reflectance spectrum of asteroid 2 Pallas and that of the carbonaceous chondrite groups, it was shown that a close similarity exists to Renazzo. Compared to all other meteorite groups, both CR chondrites and IIC irons have significant δ183W excesses, have elevated δ15N, and share similar Mo isotope systematics, and therefore a genetic link is inferred (Kruijer et al., 2017; Budde et al., 2018).

It has been proposed by many investigators that a large (~400 km diameter) differentiated CR parent body formed in the early history of the solar system and subsequently experienced a collisional disruption. For more information pertaining to this scenario, see the LPSC abstract ‘Primitive’ and igneous achondrites related to the large and differentiated CR parent body by Bunch et al. (2005), and the MetSoc abstract Tafassasset and Primitive Achondrites: Records of Planetary Differentiation by Nehru et al. (2014). The diagram below includes numerous meteorites that plot within the CR chondrite field and potentially represent daughter components of a single large disaggregated body. standby for o-isotopic diagram
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014) Huyskens et al. (2019) derived and compiled chronological data from multiple dating systems for four different achondrite parent bodies that accreted in the CR reservoir, comprising the pairing groups of NWA 011/2976/4587, NWA 6704/6693/10132, Tafassasset/NWA 3100, and NWA 6962/7680. They determined that each of these parent bodies accreted and differentiated early in Solar System history and over a relatively short timespan ~4.5637 to 4.5624 b.y. ago. Each of these CR-like objects have Cr- and Ti-isotopic compositions that when coupled to the O-isotopic compositions plot in distinct locations (see diagrams below). Notably, the CR2 chondrite Renazzo plots nearest to NWA 6962/7680 in O–Cr space, but no comparable Ti isotope data is yet available. 17O vs. ε54Cr and ε50Ti for CR Carbonaceous Achondrites
standby for ox vs. cr and ti diagrams
click on photo for a magnified view

Diagrams credit: Huyskens et al., 50th LPSC, #2736 (2019)
The brecciated nature of Renazzo is also manifest as heterogeneity in its porosity, which ranges from 3.7% to 18.2% (Macke et al., 2011). The specimen of Renazzo pictured above is a 1.1 g partial end section, showing both the interior with armored chondrules and the exterior with fresh fusion crust.


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NWA 7317

CR6
(possibly NWA 011-related) standby for nwa 2646 photo
click on photo for a magnified view Purchased June 2012
no coordinates recorded

A single meteorite weighing 1,096 g was purchased in Morocco by T. Jakubowski and M. Cimala of PolandMet. The stone was analyzed at the University of Washington in Seattle (A. Irving) and NWA 7317 was classified as a recrystallized, texturally evolved CR6 chondrite paired to NWA 2994 [4,756 g], NWA 3250 [916 g], NWA 6901 [1,197 g], and NWA 6921 [1,749 g] (T. Bunch and J. Wittke, NAU; A. Irving, UWS). At least one other stone presumed to belong with this pairing group remains to be studied. This pairing group constitutes one of only a few rare CR6 chondrites to be classified. See a photo of two whole stones exhibited in the ‘Encyclopedia of Meteorites’ by Marcin Cimala. As with NWA 3100, NWA 7317 and pairings contain evidence of relict barred chondrules in thin section, logically disqualifying it as a ‘primitive’ achondrite, and not meeting the requisite advanced thermal metamorphism to be termed a type 7 or metachondrite.

This meteorite has been very weakly shocked (S2) and has experienced very minor terrestrial weathering (W0/1). The oxygen isotopic composition for NWA 7317 was determined at the Carnegie Insitution in Washington DC (D. Rumble, III; CR chondrite comparison plot), while that of NWA 3250 CR chondrite comparison plot) was previously determined at the Open University, UK (I. Franchi and R. Greenwood; Δ17O = –1.72). Including the plot from NWA 3100 (University of Western Ontario; T. Larson and F. Longstaffe), they all fall within the field for CR chondrites. The anomalous achondrites Tafassasset and LEW 88763 are also geochemically and isotopically related to the CR parent body, but they have experienced a higher degree of thermal metamorphism and recrystallization. Since relict chondrules have now been reported in the texurally-evolved Tafassasset (as well as in LEW 88763), it would perhaps be more appropriately termed CR6 as well.

A cooperative study was undertaken of a number of previously ungrouped achondrites, primitive achondrites, and silicated irons which have O-isotopic compositions that plot along the CR oxygen isotope trend line (Bunch et al., 2005—Northern Arizona University, University of Washington, and University of Western Ontario). From the meteorites that were studied, including NWA 3100, NWA 801, Tafassasset, NWA 011 pairing group, LEW 88763, Sombrerete, and NWA 468, it was determined that some or all of them may have originated in the core, mantle, crust, and chondritic regolith of a large, at least partially differentiated CR-type parent body that was subsequently collisionally disaggregated. Compared to all other meteorite groups, both CR chondrites and IIC irons have significant δ183W excesses, have elevated δ15N, and share similar Mo isotope systematics, and therefore a genetic link is inferred (Kruijer et al. (2017; Budde et al., 2018). standby for CR trend line diagram
click on photo for a magnified view

Diagram credit: Bunch et al., 36th LPSC, #2308 (2005) Continued research on this front has been ongoing (e.g., Bunch et al., 2005; Floss et al., 2005, [MAPS vol. 40, #3]; Irving et al., 2014 [#2465]; Sanborn et al., 2014 [#2032]). As provided in the Sanborn et al. (2014) abstract, a coupled Δ17O vs. ε54Cr diagram is one of the best diagnostic tools for determining genetic relationships among meteorites. Moreover, Sanborn et al. (2015) demonstrated that ε54Cr values are not affected by aqueous alteration. The diagrams below include the NWA 7317 pairings NWA 6901, 6921, and NWA 2994, and it is apparent that they plot within the CR chondrite field. standby for o-isotopic diagram
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014)

<!– standby for 17o-54cr diagram
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014) –> 17O vs. ε54Cr and ε50Ti for CR Carbonaceous Achondrites
standby for o-cr diagram
click on image for a magnified view

Diagrams credit: Sanborn et al., GCA, vol. 245, pp. 577–596 (2019)
‘Carbonaceous Achondrites Northwest Africa 6704/6693: Milestones for Early Solar System Chronology and Genealogy’
(https://doi.org/10.1016/j.gca.2018.10.004)
However, results of a study of the paired meteorite NWA 6901 conducted by J. Zipfel (2014, #5346) led to a different conclusion. He determined that despite having similar oxygen and chromium isotopic values, this meteorite has a major element composition that is inconsistent with a derivation from a CR-like source. He suggests that the infiltration of a trace element-rich melt phase similar to that of the phosphates present in the ungrouped achondrite NWA 011 (possibly CR-related) could explain the trace element abundance pattern of NWA 6901. These two meteorites also plot near each other on a coupled Δ17O vs. ε54Cr diagram (see above), and therefore he proposes that these meteorites might be related.

The specimen of NWA 7317 shown above and in the top photo below is a 3.07 g partial slice. The excellent photos at the bottom show the main mass and a petrographic thin section micrograph of NWA 7317. standby for nwa 2646 photo
click on photo for a magnified view

standby for nwa 7317 main mass photo
Photo courtesy of Tomasz Jakubowski

standby for nwa 7317 ts photo
click on photo for a magnified view
Photo courtesy of Peter Marmet


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NWA 5131

CR7 or Meta-CR
(LL7 in MetBull 97)
standby for nwa 5131 photo
Found 2007
no coordinates recorded A single stone weighing 533 g was found in Northwest Africa and purchased by F. Kuntz. A sample was submitted for analysis and classification (Wittke and Bunch, NAU; Irving, UWS) and NWA 5131 was found to have a mineralogical composition similar to that of LL chondrites. However, following an O-isotopic analysis (Rumble III, Carnegie Institution; #5222), it could be demonstrated that the oxygen three-isotope plot falls within the CR chondrite field.

The meteorite is a highly metamorphosed assemblage with strong similarities to the CR6 (or CR7, or metachondrite) Tafassasset. Fine-grained portions exhibit 120° triple junctions, while other areas consist of mineral phases described as having a poikiloblastic texture, defining possible relict chondrules. These recrystallized chondrule relicts would be consistent with a porphyritic, metal-bearing, olivine–pyroxene chondrule type.

A more advanced stage of metamorphism than that exhibited by the CR6 chondrites NWA 7317 (and pairings) and NWA 3100 has been invoked to explain the recrystallized poikiloblastic texture in NWA 5131, and therefore the term metachondrite might be most appropriate for this meteorite (Wittke et al., 2011). It has also been argued that the similarity in O-isotopic compositions that is observed among the non-metamorphosed CR chondrites, the metamorphosed CR6 chondrites, and NWA 5131, compared with the igneous achondrite NWA 011 (and pairings), is consistent with their derivation from a common, large parent body, one which experienced internal partial melting while retaining a chondritic regolith.

Northwest Africa 5131 is a recrystallized meteorite that is petrographically consistent with a low-degree partial melt of Renazzo-like precursor material which has retained its metal component. The rock subsequently experienced equilibration processes through an extended period of thermal metamorphism. The specimen of NWA 5131 shown above is a 2.73 g partial slice.


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NWA 3100

CR6
(Primitive achondrite in MetBull 89)
standby for nwa 3100 photo
Purchased June 2003
no coordinates recorded

A single meteorite weighing 136 g was purchased in Rissani, Morocco. The stone was analyzed at Northern Arizona University (T. Bunch and J. Wittke) and was initially thought to represent a very fine-grained, completely recrystallized L7 chondrite (Bunch et al., 2005). Two very small relict chondrules were identified in the thin sections studied. This meteorite has been shocked to stage S1 and terrestrially weathered to grade W2.

Despite its similarities to an L7 chondite, certain elemental ratios such as Fe/Mn and Ca/Na were inconsistent with those known from any ordinary chondrite group, and therefore an O-isotopic analysis was conducted. Based on the values from the completed analysis, conducted at the University of Western Ontario (T. Larson and F. Longstaffe), it was demonstrated that the O-isotope plot of NWA 3100 falls along the trend line of the CR chondrites. This suggests that NWA 3100 is likely genetically related to the CR carbonaceous chondrite group. Studies of the REE pattern for NWA 3100 also demonstrate a similarity to the CR-related, FeO-rich achondrite LEW 88763 (Bunch et al., 2008 and reference therein); however, new analyses of LEW 88763 by Day et al. (2015) led them to propose its reclassification as an anomalous achondrite, with a possible relationship to the ungrouped achondrite NWA 6704 pairing group.

A cooperative study was undertaken of a number of previously ungrouped achondrites, primitive achondrites, and silicated irons which have O-isotopic compositions that plot along the CR oxygen isotope trend line (Bunch et al., 2005—Northern Arizona University, University of Washington, and University of Western Ontario). From the meteorites that were studied, including NWA 3100, NWA 801, Tafassasset, NWA 011 pairing group, LEW 88763, Sombrerete, and NWA 468, it was determined that some or all of them may have originated in the core, mantle, crust, and chondritic regolith of a large, at least partially differentiated CR-type parent body that was subsequently collisionally disaggregated. Compared to all other meteorite groups, both CR chondrites and IIC irons have significant δ183W excesses, have elevated δ15N, and share similar Mo isotope systematics, and therefore a genetic link is inferred (Kruijer et al. (2017; Budde et al., 2018). standby for CR trend line diagram
click on image for a magnified view

Diagram credit: Bunch et al., 36th LPSC, #2308 (2005) Northwest Africa 3100 is a recrystallized, texturally evolved chondrite with an elevated Fe/Mn ratio and Ca-rich plagioclase (features possibly reflecting metasomatism), and an O-isotopic composition that plots within the CR chondrite field. These features may be most appropriately associated with the newly proposed group of carbonaceous metachondrites (Irving et al., 2005). The more highly fractionated, CR-related chondrite NWA 2994 and the equilibrated CR-an (or CR7) Tafassasset, both meteorites with O-isotopic ratios that plot with the CR chondrite group, may each represent lithologies on the CR parent body that experienced a higher degree and/or a longer duration of thermal metamorphism (Bunch et al., 2008) as well as metasomatism.

Continued research on this topic has been ongoing (e.g., Bunch et al., 2005; Floss et al., 2005, [MAPS vol. 40, #3]; Irving et al., 2014 [#2465]; Sanborn et al., 2014 [#2032]). As provided in the Sanborn et al. (2014) abstract, a Δ17O vs. ε54Cr diagram is one of the best diagnostic tools for determining genetic relationships among meteorites (see diagrams below). Moreover, Sanborn et al. (2015) demonstrated that ε54Cr values are not affected by aqueous alteration. The specimen of NWA 3100 shown above is a 3.1 g partial slice. standby for o-isotopic diagram
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014)

<!– standby for 17o-54cr diagram
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014) –> 17O vs. ε54Cr and ε50Ti for CR Carbonaceous Achondrites
standby for o-cr diagram
click on image for a magnified view

Diagrams credit: Sanborn et al., GCA, vol. 245, pp. 577–596 (2019)
‘Carbonaceous Achondrites Northwest Africa 6704/6693: Milestones for Early Solar System Chronology and Genealogy’
(https://doi.org/10.1016/j.gca.2018.10.004)