At 9:30 P.M. in the farming community of Vigarano Pieve, Italy, stones were seen and heard to fall. Two stones of 11.5 kg and 4.5 kg were foundthe larger stone was recovered immediately, while the smaller stone was found a month later. This is the type specimen for the CV class, which has now been subdivided into four subgroups based on secondary mineralogy (McSween, 1977; Weisberg et al., 1997): reduced, oxidized Allende, and oxidized Bali. The CV-oxidized and CV-reduced subgroups are separated on the basis of metal abundances and the Ni content of sulfide (Howard et al., 2010). The previously used discriminator, magnetite abundance, has been shown to overlap between oxidized and reduced subgroups. The oxidized-Bali subgroup has a higher degree of aqueous alteration than oxidized-Allende (for more mineralogical relationships, see Appendix I, Carbonaceous Chondrites). Other members of the reduced subgroup to which Vigarano belongs include Arch, Efremovka, and Leoville.
A recent study was undertaken by Bonal et al. (2004, 2006) to refine the subtypes of several CV3 chondrites. They employed 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 are 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:
The differences that exist between these methods of subtype determination 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.
The reduced subgroup members are the least altered of the CV-group members. The oxidized subgroups have a range of characteristics which attest to a higher degree of thermal metamorphism than that experienced by the reduced subgroup. In a comparison between the inclusions in olivine grains in both reduced and oxidized CV3 subgroups, it was concluded by Abreu and Brearley (2011) that matrix olivines in the oxidized subgroup (Allende-like) could not have been derived through thermal processing of reduced (Vigarano-like) material. Still, it is generally presumed that all of the oxidized groups did derive from previously reduced material.
Vigarano is a regolith breccia, shocked to stage S12, containing solar-wind gases and both reduced and oxidized components, including oxB clasts and chondrules, and CAIs of both oxB and oxA types. Fayalite grains have been identified in oxB clasts which were formed prior to brecciation, during early aqueous alteration processes (Jogo et al., 2006). Application of MnCr dating techniques to the fayalite grains indicate a formation time of 4.561 (±.001) b.y. ago, identical to that of other oxB members.
Clasts have been identified in Vigarano which represent layered regolithic ponds composed of multiple layers (beds) that were developed through a seismically-driven, gravitational sorting/settling mechanism (Zolensky et al., 2013). Each bed is similarly composed of a band of iron-rich silicate having both a gradual (top) and an abrupt (bottom) transition into the other fine-grained mineral components making up the layer.
Phyllosilicate-rich clasts and rims on many chondrules provide evidence of extensive aqueous alteration occurring on the Vigarano parent body. Calcium carbonate of terrestrial origin occurs in veins throughout the meteorite, which was precipitated by aqueously-induced oxidation of metal and a consequent increase in pH. This secondary product is present in greater abundance in the smaller, later recovered stones (Abreu and Brearley, 2005). Notably, minor occurrences of pre-terrestrial carbonate have also been identified, which was formed through the extended process of augite replacement by calcite. Metal and sulfide phases are present in relatively low concentrations (e.g., 50100 ppm troilite; A. Brearley, 2007).
The three recognized CV3 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). Consistent with this, Fagan and Aoki (2015) reason that both the deformation of chondrules and the reduced modal abundance of matrix that is observed in the reduced subgroup compared to the oxidized subgroup is the result of an early impact event that reduced the porosity and voided the accreted ices, thus inhibiting fluid-assisted aqueous alteration. The shock pressure necessary to cause the observed deformation of chondrules was estimated to have been >20 GPa (Almeida et al., 2015). The matrix component in the reduced subgroup is significantly less than in the oxidized subgroup, only half as abundant in Vigarano than in Allende, and the former has a composition that is phyllosilicate-poor. Matrix material in Vigarano is present in two distinct forms: 1) a porous matrix that contains primordial noble gases; and 2) a compact matrix that contains both primordial noble gases and low abundances of solar noble gases (Noguchi et al., 2003). Both the Vigarano matrix and the fine-grained rims surrounding chondrules and CAIs are compositionally very similar (Hammond et al., 2007). Moreover, this fine-grained material has a complementary composition to the chondrules, suggesting that all of these components formed within a common nebula region. It was inferred that the shock wave model of formation is most consistent with these observations.
The original O-isotopic composition of Vigarano is thought to have been 16O-rich, later becoming 16O-poor through exchange with nebular gases. Dark inclusions in Vigarano have undergone hydration and dehydration processes, leading to a mass-dependent fractionation manifest in a heavy-isotope (δ18O) enrichment (Clayton and Mayeda, 1999). Despite the reduced nature of Vigarano, this heavy-isotope enrichment is thought to have been produced in an oxidizing, low-temperature aqueous environment. An unusual fine-grained dark inclusion found in Vigarano shows characteristics of having been formed through sedimentary processes in a fluvial environment, which occurred prior to its incorporation in the Vigarano host.
All members of the CV class contain high-temperature oxide and silicate minerals of calciumaluminumtitanium composition (CAIs). Relict presolar SiC grains are present within Vigarano CAIs. Based on the presence in Vigarano of material from all CV subgroups showing different degrees of aqueous alteration and thermal metamorphism, it can be inferred that all CV material originated on a single, heterogeneously altered parent body mixed by regolith gardening.
Evidence for a large differentiated CV planetary body has been accumulating over time (see the 'The Breakup of Antaeus'). This hypothesis is also attested by the fact that CV chondrites acquired a strong unidirectional natural remanent magnetization ~9 m.y. after CAI formation, reflecting the existence of an internal core dynamo (e.g., Weiss et al., 2010; Elkins-Tanton et al., 2011; Carporzen et al., 2010, 2011; Gattacceca et al., 2013, 2016). Employing multiple investigation techniques, Shah et al. (2017 [EPSL open access article]) investigated the paleointensity of 19 Vigarano chondrules and found values of 1.1150 µT. The observed magnetic remanence is considered to have been acquired during brecciation events that occurred ~7 m.y. after initial parent body accretion, with impact shock pressures reaching 1020 GPa. Therefore, they reason that the original paleofield would have been ~40 µT, which is too high to be attributable to the solar wind field, but is in the range of that expected for a planetary core dynamo. As research continues, further evidence could advance the hypothesis for the catastrophic disruption of a large planetary-sized body. The specimen of Vigarano shown above is a 0.8 g partial slice, and the bottom image is an excellent petrographic thin section micrograph of Vigarano, shown courtesy of Peter Marmet.
click on image for a magnified view
Photo courtesy of Peter Marmet