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, with the largest weighing ~5 kg. It was noted by Brian Mason in 1962 that only ~1.1 kg was accounted for in museums (Abreu et al., 2020). 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 abundances in FeO-rich olivine were obtained by Schrader et al. (2015) for a large CR chondrite sampling. The data indicate that EET 96259 could represent the least thermally altered, most pristine sample of the CR parent body presently known (subtype 3.00; see top diagram below). Improvements were made to the data analysis methods for the sample set of Schrader et al. (2015), and it was determined by Davidson et al. (2019) that MIL 090657 is among the most pristine CR chondrites studied (2.7 based on bulk isotopic composition, presolar grain abundance, and Cr content of ferroan olivine; see bottom diagram below). Other highly pristine CR chondrites with a very low degree of thermal alteration include 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 further down the page.
Metamorphic Sequence Based on Cr2O3 Content of Ferroan Olivine
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'
Revised Metamorphic Sequence From Improved Comparative Data
Diagram credit: Davidson et al., GCA, vol. 267, pp. 240256 (2019)
'Re-examining thermal metamorphism of the Renazzo-like (CR) carbonaceous chondrites:
Insights from pristine Miller Range 090657 and shock-heated Graves Nunataks 06100'
The matrix component in Renazzo, which includes igneous fragments, isolated mineral grains, CAIs, AOAs, dark inclusions, and metal grains, accounts for ~3540 vol% of the bulk composition (Bayron et al., 2014). The abundance of chondrules and chondrule fragments in Renazzo is ~5060 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, and possibly beyond Saturn (van Kooten et al., 2020).
Schematic Illustration of the Metal-Rich CC Reservoir
Diagram credit: Van Kooten et al., MAPS, vol. 55, #3, p. 584 (2020 open accesslink)
'The role of Bells in the continuous accretion between the CM and CR chondrite reservoirs'
Based on the initial 26Al/27Al ratios calculated for chondrules from unequilibrated ordinary chondrites and CO3.0 chondrites, they are considered to have formed 12 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 PbPb 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 PbPb chronometer was also applied to the CR chondrite NWA 6043, and it revealed a wide range of agesfrom 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 AlMg 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 AlMg age of 3.75 (±0.24) m.y. after CAIs. A high-precision HfW 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 metalsilicate segregation for the CR parent body. They determined a HfW age of 3.6 (±0.6) m.y., which is in agreement with that determined by the PbPb and AlMg chronometers. In addition, they ascertained a weighted mean age based on all three chronometers of 3.73 (±0.21) m.y. after CAIs.
Utilizing MnCr 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 53Mn53Cr analyses of secondary carbonatescalcite 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 0.996.7 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 gassolid 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 resemblance 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 (see a brief synopsis of their scenario in the paragraph below). A fifth mechanism has been proposed by Chaumard et al. (2014) to describe the formation of an isolated, igneous-zoned, mm-sized matrix metal grain in Renazzofractional 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). On the other hand, recent studies have addressed the chondrulematrix complementarity issue and have concluded that advocacy for such a genetic relationship is unnecessary to explain the elemental ratios observed in these two components (e.g., Patzer et al., 2020).
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 liquidvapor 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 (solidvapor) exchange of Ca for Na in the nebula (Varela et al., 2001). Presolar grains (nanodiamonds) carrying primordial noble gases with anomalous Xe isotopes (Xe-HL) have been identified in Renazzo. The 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 CAIchondrule 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 spinelanorthitepyroxene 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 (59 vol%), and the metal-bearing sulfides pyrrhotite and pentlandite (14 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 metalthe 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 NiCo 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 metalsilicate 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 volatilazation, 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 submicron-sized kamacite inclusions embedded in forsterite grains, commonly known as "dusty olivine", are best modeled as having been formed through the reduction of FeO in the presence of carbon (kerogen) prior to parent body accretion. Chondrules containing dusty olivine metal can carry a thermoremanent magnetization (TRM) that provides a high-fidelity record of the nebular magnetic field (Weiss et al., 2021; see Fig. 5). 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 configurationsas 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 findingssome aspects of their scenario are contrary to portions of previous propositions outlined above:
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014)
Huyskens et al. (2019) derived and compiled chronological data from multiple dating systems for four potentially 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, it has been proposed by Agee et al. (2020) that each of these four meteorite pairing groups along with certain other meteorites are members of a new group named 'ténéréites'.
17O vs. ε54Cr and ε50Ti for CR Carbonaceous Achondrites
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.