In Renazzo, Italy, at 8:30 P.M., a bright light was seen and three detonations were heard as several stones fell. Three stones were recovered, the largest weighing ~5 kg, having a total weight of 10 kg. This is a brecciated meteorite with a shock stage ranging from S1 to S3. Renazzo is a preserved fall with unique volatile element and N-isotopic compositions which distinguish it from the other carbonaceous chondrite groups. It serves as the type specimen of the Renazzo-type chondrite group and is thought to be the least altered, both thermally and aqueously, of the known meteorite groups. Renazzo is among the most primitive meteorites that we have for study.
In Renazzo, chondrules and chondrule fragments constitute ~50–60 vol% of the bulk composition. Multilayered, FeO-poor (type I), mostly porphyritic chondrules constitute the vast majority of the chondrules, while a very small population constituting <1 vol% is sulfide-rich, type-II (FeO-rich) chondrules and chondrule fragments. Rare relict type I grains have been identified within type II chondrules. These chondrules and other matrix components, including CAIs, AOAs, dark inclusions, FeNi-metal, and sulfides, were agglomerated in the solar nebula at approximately the same heliocentric distance as the CI chondrites.
Compared to the initial 26Al/27Al ratios calculated for chondrules from unequilibrated ordinary chondrites and CO3.0 chondrites, which are indicative of a formation 1–2 m.y. after CAIs, the chondrules in CR chondrites are consistent with a relatively late formation ~2.5–>4 m.y. after CAIs (Scott et al., 2007; Nagashima et al., 2008). Utilizing the Pb–Pb dating method, the formation age for CR chondrules was found to be 2.5 (±0.9) m.y. after CAIs (Amelin et al., 2002). Based on 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.
Initially, Ti-bearing perovskite condensates facilitated the condensation of forsterite. Lower temperatures and more highly reducing conditions prevailed as low-Ni, FeNi-metal grains and fine-grained pyroxene dust formed and adhered together. Thereafter, these coarse-grained aggregates were melted by a solar heat pulse and coalesced to form Ti-enriched, FeO-poor, porphyritic chondrules. Nebular-condensed C was dissolved in the metal and later exsolved in the newly formed chondrules (Kong et al., 1999). During this time period, mineral assemblages comprising troilite, phosphate, and FeNi-metal were formed in high-temperature gas–solid sulfidation processes in the nebula (Schrader et al., 2008).
Silica-rich igneous rims are present on most type I CR chondrules (although not evident in Renazzo or Al Rais due to their higher degree of aqueous alteration), which are presumed to have formed by direct condensation of silica onto chondrule surfaces from a cooling, fractionated nebular gas (Krot et al., 2003, 2004). Globular FeNi-metal forms rims on ~40% of CR chondrules, and occurs in the chondrule interior in some. These 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 resembles that of a shell rather than a ring. It is now considered more plausible that under high temperature conditions the molten metal had a lower surface tension with respect to the vacuum of space than it did to the molten silicates, and was induced to form along the outer surface (Wasson and Rubin, 2009). Upon cooling, the interface tension became more influential than the surface tension, causing the metal to take the form of small globules. Utilizing 3D microtomographic imaging of Renazzo chondrules, Ebel et al. (2009) found that some chondrules exhibit multiple concentric layering of silicate and metal, suggesting sequential accretion of independent metal and silicate components. These represent sequential generations of chondrules attributable to multiple local heating events within the same unique nebular source region. This accretionary period was followed by an interval of annealing.
Finer-grained matrix material was also formed in this same Ti-depleted nebular region, but only after substantial cooling had occurred. At this time, abundant water ices which had accreted 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, as evidenced by the incomplete recondensation of volatile elements previously evaporated during chondrule 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 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 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 O-isotope and bulk FeO compositions and have a heavier O-isotopic signature than that of type I chondrules. It is believed the gas 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 subject to radiogenic heating in the presence of water ices. This aqueous alteration produced abundant sulfide phases including pentlandite, magnetite, and tochilinite, the latter reflecting sulfer-rich aqueous alteration of FeNi-metal at temperatures of 50–200°C (Schrader et al., 2008). Consistent with its content of magnetite and the hydrated sulfide tochilinite, Renazzo reflects a greater degree of aqueous alteration than most CR group members.
Trace-element studies have identified primitive glass inclusions within olivines that sample the liquid–vapor barrier which 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 from 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 also been identified in Renazzo.
Refractory inclusions in Renazzo are small and scarce, just as in other CR chondrites. Pristine 16O-rich CAIs 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 any further equilibration with the cooling nebular vapor.
A small component of Al-rich (>10 wt% Al2O3) chondrules are present, thought to have formed by melting of spinel–anorthite–pyroxene CAI precursor material which 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 metallic FeNi in the form of taenite and kamacite (5–9 vol%), along with the sulfides pyrrhotite and pentlandite (1–4 vol%). Metal-bearing sulfides also contribute to the high metal content of the CR chondrites. Metal occurs in Renazzo in chondrule interiors, on chondrule rims, and as separate finer grains in the matrix. It is thought by some that during the melting event(s) in which chondrules were aggregated, 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 most metal grains on their rims. Because of the evaporation and migration of Fe to the rim metal, and 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.
Still, other components of CR metal are consistent with an origin through high-temperature silicate reduction and metal–silicate equilibrium processes, as evidenced by 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) proposed a model in which the FeNi-metal found in Renazzo and other chondrites was formed by the loss of S from an FeS melt during condensation from the solar nebula at high temperature (~1565°C) and high pressure (~1 atm), possibly as a result of the passage of a shock wave. They also determined that the FeNi-metal inclusions present within olivine grains, commonly known as "dusty olivine", is best modeled as having been formed through the reduction of FeO in the presence of kerogen.
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 which has not occurred in the more pristine ungrouped 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 their current designation of 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.
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. 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, and accordingly, they were accreted by the CR parent body during its the initial stage of formation. 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 the same 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. 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 in the inclusion of the largest known alkyl component in the insoluble organic material (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 unaltered CR chondrites QUE 99177 and MET 00426. 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), which 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.
It was found 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 of 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, has recently been classified as the first CR1 chondrite (Weisberg and Huber (2007). Although the chondrules have been completely replaced by phyllosilicates, magnetite, and sulfides, their textural integrity has been preserved and they are still discernible as type I. Similarly, 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.
Just as importantly, a meteorite previously classified as C3-ungrouped, Sah 00182, 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 CR2 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 has experienced very little thermal metamorphism. In addition, a few meteorites have alteration histories which 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 oxygen and nitrogen 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 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). Another study found that a chondritic source high in carbon and metal, similar in many respects to Renazzo, may have been the precursor material that melted to form the IAB complex iron meteorites. 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. The specimen of Renazzo pictured above is a 1.1 g partial end section with the fresh fusion-crusted side shown beneath.
