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 grains; these grains originated as an immiscible phase of a silicate melt during the formation of CAIs and chondrules.
Ningqiang contains a lower abundance of CAIs compared to CV3 chondrites, constituting ~2.0 vol% and 5.1 vol%, respectively. Several types of CAIs are present, including a rare anorthite-spinel-rich type which is thought to be both an alteration product of spinel-rich type A inclusions and the precursor material of type C inclusions; these CAIs indicate 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 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 such injection.
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 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 significantly smaller than that of CV chondrites, and is closest to that of the CK chondrites; however, the volume of chondrules is about two times greater than that in CK chondrites. 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.
Nevertheless, Ningqiang is more highly depleted in refractory lithophiles than either Allende-type CV or the more unequilibrated CK members, and is actually 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. Such a large difference in the refractory inclusion abundances between Ningqiang and Allende-type CV chondrites can be explained by a later formation for Ningqiang, giving the Allende-type CV chondrites the opportunity 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., which is much higher than most CV members. The meteorite has experienced only low shock effects.
In an effort to further resolve differences between the CV and CK chondrite groups, Yin and Sanborn (2019) analyzed Cr isotopes in a significant number and broad range of meteorites. Their study included samples from each of the three CV subgroups (oxA, oxB, Red), two anomalous CV3 (NWA 6047 and NWA 7891), a C3-ungrouped (Ningqiang), several CK members, and other potential CV-related meteorites (see top diagram below). It is demonstrated that the CV and CK meteorites are clearly resolved into two distinct isotopic reservoirs. In addition, it is shown in the top diagram below that the ε54Cr value for NWA 6047 puts it in a distinct location compared to other CV group meteorites, and therefore it may represent a separate carbonaceous chondrite parent body. Furthermore, despite the varied classification history of Ningqiang, it can now be assigned to the CK group. A coupled Δ17O vs. ε54Cr diagram plotting all of the meteorites in the study is shown in the bottom diagram below. Notably, anomalous CV3 NWA 7891 (Δ17O = 7.7 [±4.5] ) plots far below the range considered in the bottom diagram.
Cr Isotope Weighted Average For CV and CK Chondrites
click on photo for a magnified view
OCr Diagram For CV and CK Chondrites
CK: orange shades; CV: green shades; Achondrites: open
click on photo for a magnified view
Diagrams credit: Yin and Sanborn et al., 50th LPSC, #3023 (2019)
A unique dark inclusion (DI) was discovered in Ningqiang which represents some of the most pristine nebular material ever studied (Zolensky et al., 2003). It comprises two lithologies, both of which consist of µm-sized olivine and pyroxene crystals, but in only 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 for Ar-rich, heavy primordial noble gases that were produced in a plasma (Nakamura et al., 2003). In addition, an amorphous carbon phase in this DI contains both the dominant 'Q' noble gases (for 'quintessence') and exotic 'HL' noble gases (enriched in both heavy and light isotopes). This porous carbonaceous host phase for the Q-gases has been characterized by Amari et al. (2013) as nanoscale graphene platelets (see photo below). An in-depth investigation into the carbonaceous carrier of the Q-phase was conducted by Fisenko et al. (2018) utilizing the L4 chondrite Saratov. They contend that the carrier of the Q-gases is a nongraphitizing carbon phase present as curved, few-layer, graphene-like sheets which were likely formed in the protoplanetary nebula. The DI in the Ningqiang chondrite must have been incorporated 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, #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 L4 Saratov meteorite. Arrows indicate curled edges of graphene platelets.
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 does contain abundant grains of FeS and magnetite.
Trace amounts of the secondary alteration minerals sodalite and nepheline have been discovered in many components of Ningqiang. Considering the high abundance of sodalite observed in one DI relative to the expected abundance in the more porous matrix material, and understanding that the O-isotopic plot falls along the CCAM line, Wang and Hsu (2008, 2009) concluded that the sodalite and nepheline were formed in a nebular environment prior to parent body accretion. 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 thought likely that sodalite and nepheline formation occurred through an alkalihalogen metasomatic process involving anorthite prior to its accretion to the parent body. In further studies of Ningqiang, Matsumoto et al. (2014) concluded that the fine-grained nepheline and sodalite now present in the matrix was initially formed through NaFe metasomatism and replacment of precursor chondrules and CAIs in the early stages of parent body formation. Thereafter, disaggregation of the altered chondrules, CAIs, and host matrix occurred, and the nepheline/sodalite was transported in a fluid and incorporated into the matrix grains of the meteorite source rock prior to final lithification. Matsumoto et al. (2017) suggest that the low abundance of CAIs and the small size and irregular shape of chondrules in Ningqiang, relative to typical CV3 chondrites, can be attibuted to this late-stage process.
Presolar silicate and oxide grains as well as 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; however, 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 a significant probability exists for such an event to disrupt the presolar nebula instead of causing its gravitational collapse. 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 M⊙) is formed within ~25 parsecs, perhaps through rapid accretion or through 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 could 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 occurred 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, CK, and CO groups, usually as an anomalous member, or otherwise considered to be ungrouped (G. Kallemeyn, 27th LPSC, #1318 ). Other analyses indicating lower than usual refractory lithophile abundances led to the general conclusion that Ningqiang would be best classified as an ungrouped C3 chondrite (e.g., Wasson et al., 2013). More recent isotopic analyses conducted by Yin and Sanborn (2019) establish a CK group membership for Ningqiang. The photo shown above is a 1.2 g interior fragment of Ningqiang, while the photo below shows a prominent CAI exposed inside of a broken edge.