At 1:05 on a Saturday morning, accompanied by strong detonations, a previously rare carbonaceous chondrite approached rural Mexico from the southsouthwest (approximately 215°) at a low-angle trajectory. Thousands of individual stones fell, creating the largest stone strewnfield recorded, measuring 50 km in length and at least 300 km² in area. A stone found in the village of Pueblito de Allende was taken to the newspaper office where news of the fall was disseminated rapidly. Material was quickly collected by both scientific and commercial interests, and within a few weeks research laboratories around the world were studying this new fall.
The largest recovered specimen was found eight months after the fall by Guadalupe Juarez as he was hunting a rabbit. The meteorite created a crater 32 inches by 52 inches and 13 inches deep, having a small rim on the north end. This specimen was estimated to have weighed ~110 kg before it was fragmented upon impact with the hard soil. The total recovered weight of the Allende fall was over 3 tons, a record for stone meteorites that is surpassed only by the Jilin, China fall in 1976 of ~4 tons.
X-ray diffraction techniques and Mössbauer spectroscopy have been used by Bland et al. (2004) to determine the modal mineralogy of Allende, and to quantify the compositional range of the olivine phases. In addition, the grain density can be readily estimated from the mode data, and therefore, in combination with the calculated bulk density, the porosity can be determined. Notably, from the limited studies performed thus far, the CV3 oxidized subgroup has a greater porosity than the reduced subgroup. The modal mineralogy (vol%) of Allende is as follows:
Fo100 ------------ 23.4
Fo80 -------------- 15.4
Fo60 -------------- 20.6
Fo50 -------------- 20.6
Fo25 ---------------- 3.9
Pentlandite ----------------- 8.0
Clinoenstatite (En90 ----- 6.6
Plagioclase (An100) ----- 1.2
Magnetite ------------------- 0.2
Fe-metal --------------------- 0.1
TOTAL ------------------ 100.0
grain density = 3.67 g per cubic cm
Allende and most other carbonaceous chondrites contain refractory inclusions rich in calcium and aluminum (CAIs) which, as determined through HfW systematics, probably formed ~4.5676 b.y. ago by nebular condensation at temperatures high enough to vaporize the Fe and Mg silicates. One theory places their formation early in nebular history when the heat source was the gravitational energy of the accreting stellar disk. Outward diffusion mechanisms allowed some CAIs to escape solar accretion and become stabilized in the outer zero-drag envelope of the newly formed Jovian gap. In addition, it was proposed that surface flares of Jupiter may have been one of the diverse mechanisms of chondrule formation. Allende chondrule formation is thought to have occurred ~4.5654 b.y. ago. It has been argued that multiple episodes of recycling ensued, during which time thermal, chemical, mechanical, and redox processing occurred, eventually resulting in both CAIs and chondrules being incorporated into a carbonaceous assemblage (Ruzicka et al., 2008). Other theories attribute the formation of the majority of chondrules to shock waves caused by gravitational instabilities.
An absolute age for Allende based on the PbPb chronometer, which represents the time of chondrule crystallization, was determined to be 4,565.45 (±0.45) m.y. (Connelly et al., 2007). This age indicates that chondrules in CV3 chondrites were formed by 1.66 (±0.48) m.y. after CAI formation. This history can be compared to that of other carbonaceous chondrite groups in which chondrule formation occurred as early as Allende chondrules to as late as 6 m.y. after CAIs (based on MnCr systematics; Trinquier et al., 2008). In another study of Allende chondrule formation ages conducted by Amelin and Krot (2007) and utilizing PbPb residueleachate isochrons, it was determined that the average age of Allende chondrules is 4,566.6 (±1.0) m.y.within error margins of the age associated with the formation of CAIs in CV3 chondrites (4,567.2 [±0.6] m.y.). An ultra-high-precision MnCr age anchored to D'Orbigny of 4567.91 (±0.76) m.y. was determined by Qing-Zhu Yin et al. (2009). These chondrules are the oldest measured from any meteorite, and the inference can be made that Allende chondrules formed virtually contemporaneously with CAIs over a span of time of at least 1.2 m.y., as resolved by the AlMg system (Amelin and Krot, 2007). This period also overlaps the time interval when basaltic crusts were being segregated and iron cores were being formed on other differentiated asteroids.
By a different approach to chondrule age determination, an absolute age of 4,568.3 (±0.7) m.y. was determined for CAI formation based on the HfW isochron of several angrites (Burkhardt et al., 2008). Comparing this age with the AlMg and UPb systematics of chondrules in certain carbonaceous chondrites, it can be inferred that CV chondrites such as Allende formed later than previously thought, ~2 m.y. after CAIs, while CR chondrites formed ~4 m.y. later than CAIs. This history indicates that formation of carbonaceous chondrites occurred even later than ordinary chondrules and is consistent with the lower degree of radiogenic heating experienced by carbonaceous chondrites compared to ordinary chondrites.
Still, in a study of U-isotopic variation among Allende CAIs (Brennecka et al., 2010), it was determined that correlations between 238U/235U values and the original Cm/U in the CAIs proved that these U-isotopic variations were caused by the decay of Curium-247, an element created during the r-process in supernovae. The calculated initial 247Cm/235U ratio infers that the interval between the production of the heavy isotope 247Cm and its delivery to the molecular cloud occurred 110140 million years before CAI formation. This decay of 247Cm would have resulted in higher 235U levels which affects the equations used in the PbPb dating method; ages for the earliest solar system events will need to be corrected by as much as 5 m.y. less than the ages determined by the previous 238U/235U initial ratio of 137.88.
In their study of the MnCr isochron in Allende and other carbonaceous chondrites, Scott and Sanders (2008) determined that fractionation of Mn from Cr occurred at the same time as CAI formation, prior to carbonaceous chondrule formation, as a result of the formation of the earliest planetesimals. They argue therefore, that carbonaceous chondrules could not have been formed by direct melting of pristine nebular dust because such a moderately volatile element fractionation would not have persisted within the nebula to be inherited much later by carbonaceous chondrules. They propose instead that chondrules were more likely formed from fine-grained splash or condensate material generated by collisions between very early-accreted planetesimals. In addition, fine-grained dust generated from such collisions may have been subsequently melted and mixed with both a volatile-rich and a refractory-rich nebular dust component to form some carbonaceous chondrules.
The matrix of Allende has been shown to be composed of primary nebula condensate material that formed after CAIs and chondrules in an oxidizing environment (Inoue et al., 2004). Following accretion, this material underwent an extended period of low temperature aqueous alteration~15 m.y. as revealed by IXe ages of dark inclusions (Hohenberg et al., 2004). Thereafter, this material experienced dehydration processes as evidenced by the fayalite-rich rims on forsterite grains, while FeNi-metal was oxidized to magnetite.
In their study of MnCr in Allende and other carbonaceous chondrites, Scott and Sanders (2008, 2009) determined that even though carbonaceous chondrite parent bodies were not formed until a few m.y. after the formation of CAIs, they do share a bulk rock MnCr isochron coincident with CAI formation of 4,568 (±1) m.y. They argue that carbonaceous chondrules could not have been formed by direct melting of pristine nebular dust because such a moderately volatile element fractionation would not have persisted within the nebula to be inherited at a much later time by carbonaceous chondrules. According to their current scenario, volatility fractionation occurred at this time, separating Mn and Cr grains into separate reservoirs: refractory Cr was rapidly concentrated into a first-generation of forsterite- and FeNi-metal-bearing planetesimals during the early, intensely hot stage of nebular condensation, while moderately volatile Mn persisted within the hot solar gas over a longer period of time, only condensing into planetesimals farther from the Sun as temperatures decreased. Both elements remained stored within these "precursor" planetesimals for 1.55 m.y.a time period during which steady bombardment shattered these objects and released fine-grained Cr- and Mn-rich ejecta which eventually became incorporated into chondrules. These chondrules were finally accreted to the various carbonaceous chondrite parent bodies.
An alternate theory of CAI origin places the formation at a later period, when the accretion phase was over and the Sun was in its T Tauri phase. The solar wind swept the volatile-rich gas from the outer layers of the nebular disk leaving behind only refractory-rich dust. Shock-wave heating then evaporated the Fe and Mg silicates, leaving the dust enriched in Al. This dust finally coalesced and was melted to form the CAIs that are found in most carbonaceous meteorites. More recently, studies have shown that some CAIs were accreted rapidly into larger bodies, heated by the decay of 26Al, and thermally metamorphosed. Thereafter, these bodies were disrupted and the CAIs were returned to the nebula to be remelted and recycled into later-forming carbonaceous chondrite bodies. Studies have demonstrated that multiple melting events did occur in some CAIs, and that in some, 16O-poor melilite has crystallized within the short time interval of 0.4 m.y. after the initial crystallization of 16O-rich melilite (Ito et al., 2006).
According to Krot et al. (2009), the depletion mechanism of 16O is thought to have occurred in the outer region of the protoplanetary disk due to isotopic self-shielding during UV photolysis of CO. This process preferentially dissociated isotopically heavy molecules from CO including 17O and 18O, which were subsequently incorporated into water ice. Thereafter, this 16O-depleted water ice/dust mixture migrated to the inner disk region and evaporated, leaving this region depleted in 16O. While CAIs were formed in the inner disk during a low-water ice, high 16O CO interval, most chondrules were formed in an ~4 m.y. period after the 16O-depletion occurred.
Layered rims surrounding some CAIs, called WarkLovering rims or accretionary rims, were possibly formed by a flash heating event in a more oxidizing environment of the solar nebula. This heating event was measured in fractions of a second and resulted in a loss of volatiles with enrichment of the refractory component, along with the subsequent diffusion of O and Mg. During the flash heating event, temperatures at the rim are inferred to have approached 3000°C, steeply decreasing to temperatures of ~1700°C just 1 mm below the rim. Subsequent chemical and isotopic exchange, corresponding to the grain size and porosity of specific minerals, most likely occurred in situ on the parent body.
Through investigations of the systematics of radiogenic PbPb in Allende CAIs, values for ThU were calculated (Yin et al., 2008). Using these values, a formula was employed to determine the age of the interstellar dust clouds from which the condensation of the CAIs occurred:
Tgalaxy = 21.8*[log(U/Th)0log(1/κ)]; where Tgalaxy is expressed in billion years, and (U/Th)0 is the production ratio, and κ is the time integrated model 232Th/238U ratios (Cowan et al., 1999)
According to Cowan et al. (1999), actinide chronometers have been used to determine Galactic ages by (1) predicting 232Th/238U and 235U/238U ratios in r-process calculations; (2) applying them in Galactic evolution models, which include assumptions about the histories of star formation rates and r-process production; and finally (3) comparing these ratios with meteoritic data which provide the Th/U and U/U ratios at the time of formation of the solar system. Age ranges of 10.010.9 b.y. and 13.614.2 b.y. were determined utilizing two separate Allende CAIs. These ranges are consistent with the age of the galaxy as determined through astronomical observations.
The CV3 group has been subdivided into four subgroups (McSween, 1977; Weisberg et al., 1997; Wasson et al., 2013):
Reduced subgroup: e.g., Arch, Efremovka, Leoville, Vigarano, and QUE 93429
Oxidized-Allende subgroup: e.g., Allende, Axtell, Tibooburra, and ALH 84028
Oxidized-Bali subgroup: e.g., Bali, Grosnaja, Kaba, and Mokoia
The four recognized subgroups reflect varying degrees of aqueous/oxidative alteration, which has been found to be correlated with the amount of ice-bearing matrix that was initially accreted (Ebel et al., 2009), as well as varying degrees of metamorphism, reflecting impact-generated crushing, thermal alteration, and recrystallization processes (Wasson et al., 2013).
It was shown that the oxidized-Allende subgroup contains twice the amount of matrix and half the amount of chondrules as the reduced subgroup. As a consequence, the oxidized-Allende subgroup has experienced significant metasomatic alteration of primary minerals producing nepheline, sodalite, andradite, sulfides, magnetite, and other secondary phases (Amelin and Krot, 2007). The oxidized-Bali subgroup exhibits a still higher degree of aqueous alteration than the oxidized-Allende subgroup, which can be observed in the replacement of the primary mesostasis and FeNi-metal by phyllosilicates, magnetite, sulfides, and other secondary phases. The reduced subgroup exhibits a much lower degree of alteration than the oxidized subgroups (for more mineralogical relationships, see Appendix I, Carbonaceous Chondrites).
A recent study was undertaken by Bonal et al. (2004, 2006) to refine the subtypes of several CV3 chondrites. They utilized several methods to obtain their data, including Raman spectrometry of organic material, a petrologic study of Fe zoning in olivine phenocrysts, presolar grain abundance, and a noble gas study. These methods are in contrast to that of TL sensitivity data of feldspar which is typically used to determine subtypes of ordinary chondrites and which was previously applied to the CV3 chondrites. They suggest that TL sensitivity data is not applicable to aqueously altered carbonaceous chondrites because of loss of feldspars through dissolution, leading to an underestimate of the petrologic subtypes. They have redefined the petrologic subtypes of the common CV3 members as follows:
These differences in petrologic subtype are explained by Greenwood et al. (2009) in their study of CV and CK chondrite relationships. They assert that there is a decoupling between the silicate and organic components with respect to measurements involving thermal metamorphism.
Allende contains other inclusions such as diamonds and silicon carbide grains that have unusual isotopic and rare-gas element compositions. Recent investigations have discovered other forms of "poorly-graphitized carbon" having three-dimensional, closed structures, including carbon spheres (fullerenes), graphene sheets, and various other pyrocarbon phases (Vis et al., 2002). It was determined that an amorphous phase of carbon experienced implantation through ion irradiation of planetary noble gases (the "plasma model"), which now constitutes the carrier phase Q (Matsuda et al., 2010). These Q-gases, specifically, He, Ne, Ar, Kr, and Xe, are released through oxidation processes resulting in a rearrangement of the carbon structure.
Coarse-grained lithic clasts composed of forsterite grains with annealed, granoblastic textures, were identified within type-I chondrules of some CV chondrites by Libourel and Krot (2006). Due to the lack of O-isotopic equilibration between the olivine clasts and the host pyroxene, it was conjectured that the clasts may represent relict material of early-accreted, differentiated and metamorphosed planetesimals that formed prior to chondrule formation (Krot et al., 2009).
There is evidence in the form of unvaporized interstellar material as well as in short-lived radionuclides, especially in the presence of 60Fe that is produced only through stellar nucleosynthesis, that one or more nearby type-Ia and/or type-II supernovae and/or massive asymptotic giant branch (AGB) stars seeded our stellar nursery during its earliest stages, but after the first solids had already formed (Huss et al., 2007). Another investigation of short-lived radionuclide origins conducted by Huss et al. (2009) suggests that intermediate-mass AGB stars, supernovae of type-II having >20 Msun, and supernovae of type-Ib or -Ic having >~33 Msun could each have been possible progenitors which injected short-lived radionuclides into the pre-collapse stage of the molecular cloud. They also found that r-process elements likely came from type-II supernovae having ≤11 Msun and 1225 Msun. Based on an alternative model, Trigo-Rodríguez et al. (2009) assert that a nearby massive AGB star of ~6.5 Msun, which had experienced the third dredge-up and hot bottom burning, injected a volume of 0.01 Msun of short-lived radionuclides from its envelope into the protosolar cloud. This injection, the volume of which was subsequently diluted by a factor of 300, occurred 0.53 m.y. prior to the formation of the first nebular solids.
From a study of O-isotopic anomalies of the Sun, Lee et al. (2008) recognized that the Sun must have formed within a stellar cluster coincident with a massive star. According to Ouellette et al. (2007, 2009), the mechanism of collapse for the protosolar molecular cloud is consistent with that of a shock wave from a supernova(e) (~1525 Msun) having a speed of 530 km/sec. They envision a prior Wolf-Rayet phase in which many short-lived radionuclides are ejected by the star and homogenized into the protoplanetary disk, including 26Al and 41Ca. Only later, during the actual supernova(e) explosion did the initial injection of Fe-enriched dust grains occur through a magnetic hydrodynamic mechanism, followed thereafter by intermediate-mass elements, and finally, unburned C+O; this scenario is consistent with the lack of 60Fe in some very early-accreted (~0.50.7 m.y. after CAIs) planetesimals. Carbonaceous chondrites would have accreted later than most other bodies, after the late-ejection of C+O by the supernova(e), within a region having low-temperature conditions (Ustinov, 2006).
While the late injection model may be correct, a related scenario was investigated by Nittler (2007) in an effort to understand the origin of a specific group of presolar grains having unusual isotopic ratios. He argued that during the explosion of a Type II supernova, a jet of 16O-rich material from the inner-zone was mixed with material from the outer-zone to form these unusual presolar grains. This supernova may have also injected the short-lived radionuclides into an already collapsed solar disk. He submitted that the late injection of 26Al into the solar disk during a possible Wolf-Rayet phase, as proposed by Ouellette et al. (2007) (see above), would result in the loss of the star's entire envelope before the burning and mixing of the various zones required to produce the studied grains could be accomplished. However, Boss (1995) determined that a slowed shock front (<50 km/sec; Foster and Boss, 1996) from a supernova located at a distance of several parsecs from the protoplanetary disk could both trigger the collapse and inject radionuclides from different shells to account for the short-lived radionuclide abundances observed in Allende. The supernova could have been much closer, <1 parsec, had the solar disk already been concentrated.
Another model was presented by Sahijpal and Gupta (2007) in which low-mass star formation occurs first as a result of local density fluctuations, and thereafter, a massive star (>40 Msun) is formed within ~25 parsecs, perhaps through stellar mergers. This massive star underwent core collapse to become a supernova within a short interval of ~35 m.y., injecting short-lived nuclides into the assemblage of protoplanetary disks. More detailed solar formation scenarios by Sahijpal and Gupta (2009) and others can be found on the Ningqiang page.
Meteorite studies in recent years have revealed the existence of several primitive achondrites that are texturally-evolved chondrites, some of which have O-isotopic compositions that plot in the CV chondrite field. The name metachondrite has been applied to these meteorites (Irving et al., 2005). In addition, O-isotopic compositions of members of other meteorite groups
also plot within the CV chondrite field, supporting the inference of a large differentiated CV parent body, possibly comprising the NWA 176 and Bocaiuva silicated irons, the Eagle Station pallasites, the NWA 1839 and NWA 3133 metachondrites, the more highly metamorphosed CV46 chondrites (formerly the CK groupGreenwood et al., 2009; Wasson et al., 2013), and an Allende-like, low metamorphosed chondritic regolith (Irving et al., 2004).
In support of such a differentiated asteroid model, the detection of a strong natural remanent magnetization has been confirmed in Allende (Weiss et al., 2009). This paleomagnetization has been determined to be the result of an internally generated dynamo, and excludes any external sources such as the T Tauri Sun. Further studies by Elkins-Tanton and Weiss (2009) led them to conclude that following its early, rapid accretion, an internally-generated convecting magma ocean and metallic iron core was formed on the CV parent body, a planetesimal which was probably at least 160 km in diameter. Metasomatism occurred over millions of years, leading to the acquisition of a remanent magnetization by minerals in the unmelted, insulating, still-chondritic shell. Spectroscopic studies of asteroids have led investigators to propose the existence of a possible link between the differentiated CV planetesimal and the diverse Eos asteroid family, including members that may be achondritic (Weiss et al., 2010).
Allende has a 21Ne-based CRE age of 5.2 m.y. It was shocked to stage S1 (1.7 GPa), perhaps as part of a breccia lens. The specimen of Allende shown above is a 10.2 g partially fusion-crusted individual stone, along with a 7.2 g partial slice exhibiting abundant chondrules and CAIs within a dark matrix. The 10.2 g individual stone was the first meteorite acquired for the Weir Meteorite Collection in 1983, purchased from Robert Meteorite Man Haag, pictured below holding his meteorwrong Rachel.