At 7:00 in the evening, this unique carbonaceous chondrite fell in Ningqiang County, Shaanxi Province, The People's Republic of China. Four stones were recovered weighing 0.35, 0.38, 0.78, and 3.1 kg for a total of 4.61 kg. The curator for the majority of the material is the Zijin Shan Observatory, Academia Sinica, in Nanjing, People's Republic of China.
Ningqiang is an unequilibrated carbonaceous chondrite that petrographically and texturally resembles the oxidized CV3 chondrites (such as Allende) in its large, well-defined chondrules, its high abundance of dark inclusions and fine-grained, fayalitic olivine matrix (50.5 vol%), its high magnetite/FeNi-metal ratio, and in containing awaruite (Ni>65 wt%) as its principal metal phase. A metallic phase in Ningqiang containing ~39 wt% Co (wairauite?), similar to a Co-rich phase found in certain LL and R chondrites, is associated with troilite and pentlandite. Ningqiang also contains opaque assemblages within CAIs, chondrules, and matrix which are typically found in CV3 chondrites (Wang et al., 2006). These assemblages are low-temperature aqueous alteration products (peak temperatures for the Ningqiang parent body were no higher than 300°C; Hsu et al., 2011) of pre-existing metal grainsgrains which originated as an immiscible phase of a silicate melt during the formation of CAIs and chondrules.
Ningqiang has a lower abundance of CAIs than CV3 chondrites, ~2.0 vol% compared to 5.1 vol%. Several types of CAIs are present, including a rare anorthite-spinel-rich type, which is thought to be an alteration product of spinel-rich type A inclusions and precursor material of type C inclusions; these indicate that a link exists between type A and C inclusions (Wang and Hsu, 2009). In addition, these two inclusion types were the likely precursor material for the formation of Al-rich chondrules, which are consistent with a derivation from low refractory material. Hsu et al. (2011) contrasted the absence of hibonite in CV (and former CK) chondrites with the ~7% of CAIs in Ningqiang that are hibonite-bearing. They hypothesize that the component of hibonite-bearing CAIs which is 26Al-free/poor formed early in the inner solar system prior to the injection of 26Al into the solar nebula from a nearby stellar source, while that which is 26Al-rich formed after injection.
Most of the CAIs in Ningqiang contain Na-rich nepheline aggregates replacing melilite, which is thought to have occurred by a hydrothermal process (Sugita and Tomeoka, 2008). Moreover, the Ningqiang matrix has a higher Na content than CV3 matrices, and is composed of two components derived from distinct parent body reservoirs: the first component consists of sub-µm-sized magnesian olivine with included nepheline (also derived from the reservoir from which the nepheline-containing CAIs originated), while the second component consists of larger than µm-sized ferroan olivine, only rarely associated with nepheline, but containing abundant grains of FeS and magnetite.
Ningqiang has a similar ratio of volatile to moderately volatile elements (e.g., Zn/Mn) and a similar induced TL sensitivity to that of Allende-type CV group members. However, it is more enriched in volatiles, carbon, FeNi-metal, and magnetite compared to the Allende-type CV group. Ningqiang has a bulk composition close to that of the equilibrated CV chondrites (formerly CK chondrites) for most elements, with other elements having abundances closer to those of the unequilibrated CV group members. Although the O-isotope ratios of Ningqiang are enriched in 16O relative to Allende, the ratios are more consistent with the CO group, suggesting that a close relationship exists between them. Interestingly, Ningqiang contains phosphoran olivine, an extrememly rare meteorite phase found only in main-group pallasites (and three specific terrestrial sources).
As a further comparison, Ningqiang has an average chondrule size closest to that of the former CK chondrites, but the volume of chondrules is about two times larger. Ningqiang contains only one-tenth the number of coarsely rimmed chondrules than do members of the CV group, possibly due to an inefficient, low-temperature rim attachment. An unusually high abundance of silicate inclusions known as aggregational chondrules are present. These chondrules probably formed in the solar nebula at an early stage of melting, and were sintered together in a low-temperature environment. Utilizing grain and bulk density measurements, Macke et al. (2011) have determined the porosity of Ningqiang to be 23.6%, with certain properties showing consistencies with the oxidized CV groups.
Still, Ningqiang is more highly depleted in refractory lithophiles than either Allende-type CV or the more unequilibrated (former CK) members, and is most similar to the CO chondrites in this respect. This depletion is probably due to the lower abundance and smaller size of the refractory inclusions. This large difference between refractory inclusion abundances can be explained by a later formation for Ningqiangthe Allende-type CV chondrites were the first to agglomerate a large portion of the coarser refractories that settled out early into the nebular midplane. Ningqiang also has a unique cosmic-ray exposure age of ~42 m.y., much higher than most CV members. The meteorite has experienced only low shock effects.
A unique dark inclusion (DI) was discovered in Ningqiang that represents some of the most pristine nebular material ever studied (Zolensky et al., 2003). It comprises two lithologiesboth lithologies consist of µm-sized olivine and pyroxene crystals, but only in one are the silicates rimmed by amorphous to microcrystalline material. These rims are thought to have formed through irradiation by bipolar outflows or FU-orionis flares from the nascent Sun, and in fact, this rim material is the carrier of Ar-rich, heavy primordial noble gases which were produced in a plasma (Nakamura et al., 2003). In addition, this DI contains exotic noble gases (HL) along with the dominant "Q" gases (for quintessence, which includes He, Ne, Ar, Kr, and Xe) that are present in an amorphous phase of carbon. The porous carbonaceous host phase of Q noble gases has been characterized by Amari et al. (2013) as nanoscale graphene platelets (see photo below). The Q gases are thought to have been implanted through ion irradiation (Matsuda et al., 2010). The DI must have been incorporated into the Ningqiang chondrite after parent body aqueous alteration processes were complete, and subsequent to partial annealing, since noble gases would have been quickly lost from host minerals through such oxidizing alteration processes (Yamamoto et al., 2006).
Photo and caption: Sachiko Amari et al., The Astrophysical Journal, Vol 778, Nr 1 (2013)
High-resolution aberration-corrected scanning transmission electron microscopy (STEM) image shows planar carbon ring structures
inside graphene platelets in Q from acid-resistant residue of the Saratov meteorite. Arrows indicate curled edges of graphene platelets.
Trace amounts of the secondary alteration minerals sodalite and nepheline have been discovered in many components of Ningqiang. The fact that a high abundance of sodalite is observed in one DI compared to the amount expected to be present in the more porous matrix material, and the fact that the O-isotopic plot falls along the CCAM line, led to the conclusion that the sodalite and nepheline were formed in a nebular environment prior to parent body accretion (Wang and Hsu, 2008, 2009). Nakashima et al. (2008) presented further isotopic evidence indicating that nebular alteration processes were responsible for the formation of these secondary mineral phases in Ningqiang. It is considered that sodalite and nepheline formation occurred through an alkalihalogen metasomatic process with anorthite prior to its accretion to the parent body. In further studies of Ningqiang, Matsumoto et al. (2014) concluded that the nepheline and sodalite now present in the matrix was initially formed through metasomatism of chondrules and CAIs in the early stages of parent body formation, and was subsequently incorporated into the matrix component of the meteorite source rock prior to its final lithification.
Presolar silicate and oxide grains, along with grains exhibiting O-isotopic anomalies, have been identified in Ningqiang matrix areas, with the abundance in one area reaching 230 ppm (Zhao et al., 2011). Most of these grains have 17O enrichments and likely formed around low- to intermediate-mass red giant branch stars and asymptotic giant branch stars. A small number of the grains probably formed in supernovae. These presolar grains are all enriched in Fe through a secondary Fealkalihalogen metasomatic process.
A ChineseAmerican team of scientists from the Chinese Academy of Sciences and Arizona State University have identified two short-lived radionuclides in Ningqiang60Fe and 36Clboth of which likely formed inside an earlier generation of massive stars, perhaps attaining 3060× the mass of the Sun. Rapidly expanding UV radiation from such a massive star could have produced a shock wave that triggered the formation of low-mass stars like the Sun. The final life stage of such a massive star is a supernova, which would have enriched our protoplanetary disk with the short-lived radionuclides that we observe. However, it was recognized that the probability of disrupting the presolar nebula rather than causing its gravitational collapse is significant. An alternative 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 solar masses) is formed within ~25 parsecs, perhaps through rapid accretion or stellar mergers. This massive star is conjectured to have undergone core collapse and transition into a supernova within a short interval of ~35 m.y., injecting short-lived nuclides into the existing protoplanetary disk(s). This scenario is also consistent with the finding that the earliest CAIs contain no 26Al.
Bizzarro et al. (2007) found that the differentiated meteorites do not contain 60Fe, but that later-formed chondrites do, and that all of the meteorite types do contain 26Al. They believe the radiometric evidence indicates that 26Al was infused into the local nebula very early through stellar winds from a nearby massive star. Sahijpal and Gupta (2009) suggest that this massive star belonged to a common stellar cluster and was located ~3.5 parsecs from the protosun. Injection of radionuclides by this massive star into the presolar nebula might have occurred during a Wolf-Rayet stage or during a core collapse supernova ~1 m.y. after the onset of planetesimal agglomeration, after which 26Al became homogeneously mixed throughout the nebula. The stellar winds from the massive star could even be primarily responsible for the initial collapse of the protosolar disk. Only after the supernova explosion was the 60Fe released from the massive star's interior and injected into the dust of newly accreting chondritic parent bodies. For additional information on the studies of Bizzarro et al. (2007) read the PSRD article by G. Jeffrey Taylor: "The Sun's Crowded Delivery Room", July 2007.
Ningqiang has had a variable history with its classification, at one time or another being associated with the CV, former CK (now equilibrtaed CV), and CO groups, usually as an anomalous member, or as ungrouped (G. Kallemeyn, LPS, vol. 27, 03/1996). More recent analyses indicating lower than usual refractory lithophile abundances led to the conclusion that Ningqiang is best classified as an ungrouped C3 chondrite (Wasson et al., 2013). Studies of Ningqiang and other ungrouped carbonaceous chondrites such as Acfer 094 (probably CO-related; Simon and Grossman, 2015), Mulga West, and Adelaide will continue to broaden our database and improve our understanding of early solar system history. The photo shown above is a 1.2 g interior fragment of Ningqiang. The photo below shows a prominent CAI exposed inside of a broken edge.