A fresh, black, carbonaceous chondrite weighing just 90 g was found in the desert of Oman. Dhofar 225 has textural characteristics similar to typical CM chondrites, but differs from members of that group in mineralogy, bulk composition, and O-isotopic composition (Ivanova et al., 2010). The chromium oxide content of Dhofar 225 indicates a petrologic type below 3.0. A likely paired 16.6 g stone designated Dhofar 2046 was found nearby by Pierre-Marie Pelé in 2014 (see photo). Notably, a 6 g stone designated Dhofar 2016 and classified at The Natural History Museum in Berlin (A. Greshake) as CM2 was recovered in 2010 within ~1 km of Dhofar 225.
Dhofar 225 has an O-isotopic composition that is enriched in heavy oxygen (18O, 17O), and has a plot very close to the C2-ungrouped Tagish Lake (oxygen isotope plot) and to the CM/CI-like, thermally metamorphosed/dehydrated Antarctic meteorite group termed 'CY' by Y. Ikeda (1992 [Consortium Summary]) which consists of Belgica 7904 (C2-ung), Y-82162 (C1/2-ung), Y-86029 (CI1), Y-86720 (C2-ung), Y-86789 (C2-ung, likely paired with Y-86720), and Y-980115 (CI1). See the MetBull oxygen isotope plot and diagrams below.
Diagram credit: King and Russell, 50th LPSC, #1386 (2019)
See also open accessarticle by King et al. in Chemie der ErdeGeochemistry, vol. 79 (2019)
Diagram credit: Greenwood et al., GCA, vol. 277, p. 381 (2020, open access link)
'Linking asteroids and meteorites to the primordial planetesimal population'
Other possibly related dehydrated CM-like meteorites include WIS 91600 (CM2.2 [or CI]), EET 96010 (CM2), and PCA 02012 (CM2). In addition, Nakamura (2006) identified two regolith breccias containing solar-wind-implanted noble gases which belong to this dehydrated group, Y-793321 (CM2) and A-881458 (CM2), while M.M.M. Meier (2014) found that the meteorite Diepenveen (CM2-an) also contains similar trapped solar gases. Dhofar 1988, which was found by M. Cimala in 2011 and initially classified as an ungrouped C2 chondrite, has a similar O-isotopic composition (oxygen isotope plot; photo courtesy of Marcin Cimala). This meteorite was the subject of an in-depth study by Suttle et al. (2021) who suggest a reclassification to CY. In addition, the ungrouped C chondrite Dhofar 2066 (oxygen isotope plot; photo courtesy of Lukasz Smula) also has a heavy oxygen isotope composition consistent with the CY group, as do both Y-86737 and Y-980134 which are classified as CI1 (King and Russell, 2019). Notably, Dhofar 225 has many features and an oxygen isotopic composition that are similar to the anomalous CM chondrite Dhofar 735 (oxygen isotope plot; photo courtesy of Thomas Witzke), which along with Belgica 7904, Y-86720, and PCA 02012, have experienced the highest temperatures (~900°C) over a brief time interval (PCA 02012 estimated at tens of hours; Nakato et al., 2013) compared to other members of the CY group.
The case for a distinct CY group as proposed by Y. Ikeda (1992) is strengthened by a mineralogical comparison conducted by King and Russell (2019). The significantly higher modal sulfide content in the CY-group chondrites Y-980115 and Y-82162 compared to that of average CM and CI chondrites is difficult to reconcile with an attribution to hydration/dehydration processes, but is instead more consistent with a difference in primary mineralogy. Similarly, in a geochemical comparison, Russell et al. (2019) demonstrated that the matrix compositions of CY1 chondrites Y-82162 and Y-980115 were unlike those of both CI1 and CM1, but instead consist of two different components: a unique Mg-rich serpentine and a CI-like serpentine/saponite (see diagrams below).
Diagram credit: King and Russell, 50th LPSC, #1386 (2019)
click on image for a magnified view
Diagram credit: Russell et al., 82nd MetSoc, #6402 (2019)
Differences exist between Dhofar 225 and Dhofar 735 on one hand, and the Belgica-like grouplet on the other. In contrast to the FeNi-metal grains present among Belgica-like meteorites, those in Dhofar 225 and Dhofar 735 are not enriched in Cr and P (Ivanova et al., 2010). Moreover, the bulk chemistry between the Dhofar and Belgica metamorphosed meteorites are different. Similar to the Belgica grouplet, but unlike typical CM chondrites, Dhofar 225 exhibits considerable but incomplete dehydration of matrix phyllosilicates (<2 wt% water), Fe and S depletions, and contains tiny grains of tetrataenite within the matrixall features consistent with a higher thermal metamorphism than that experienced by typical CM group members. However, sharp zoning profiles of olivine in the chondrule-like objects of Dhofar 225 severely constrain the maximum temperature of metamorphism. In particular, zoning of olivine grains observed in Dhofar 735 and Belgica 7904 indicates a short heating duration that negates the theory of heating by decay of radioactive elements (Nakato et al., 2011). Instead, impact heating is considered to be most consistent with the heterogeneous heating observed among the CY chondrites (King et al., 2019). Aqueously altered carbonaceous chondrites that have experienced thermal metamorphism have been classified according to their degree of heating and corresponding phyllosilicate dehydration. Estimates of dehydration temperatures are shown below (Nakamura, 2005):
Chondrules in Dhofar 225 are sparse (24 vol%), and similar in size (0.3 mm) to those of CM chondrites. Olivine is forsteritic and commonly occurs as aggregates up to 0.6 mm in size, and as chondrule-like objects. The matrix constitutes 70 vol% and is primarily composed of phyllosilicates (serpentine) along with minor sulfides, phosphides, phosphates, FeNi-metal, and chromite, and only rare CAIs (2 vol%). A previously unknown mineral phase, Ca,Fe-oxysulfide, was identified in the matrix, possibly an oxidation product of a primary sulfide phase (Ivanova et al., 2010). Tochilinite, characteristically abundant in CM chondrites, has been mostly thermally decomposed to troilite and oxides in both Dhofar 225 and Dhofar 735 as well as in the Belgica-like grouplet. The similarly thermally unstable P-rich oxysulfides only occur in very low abundances (Ivanova et al., 2005). Other rare minerals identified include eskolaite and Cr-barringerite.
In contrast to the low-Ni, low-Co content of the metal within chondrules of Dhofar 225, the composition of the matrix metal is high-Ni, high-Co taenite and tetrataenite. The Fe/Si matrix ratio of Dhofar 225 is consistent with that of the CM chondrite group. The absence of Cr and P in the metal of Dhofar 225 is similar to that in the metamorphosed meteorites Belgica 7904 and Y-86720. Although the matrix of Dhofar 225 is compositionally similar to CI chondrites, especially Y-82162, as well as to the metamorphosed-CM chondrite Y-86720, only Dhofar 225 has retained moderate abundances of tochilinite-cronstedtite intergrowths (TCI; formerly PCP or "poorly characterized phases"). This specific mineralogy suggests that the grouplet experienced a period of variable aqueous alteration followed by a low level heating/dehydration phase, probably caused by impacts (Choe et al., 2010). A later episode of aqueous alteration may have affected Dhofar 225 resulting in its extant tochilinite. Notably, in an analysis of Dhofar 225 conducted by Lentfort et al. (2020, Supporting Information), it was determined that the bright areas thought to be TCIs are more likely areas composed of sulfidic or metallic components.
While this group of metamorphosed carbonaceous chondrites could in theory have been derived from normal CM chondrites in accord with their many common characteristics, some researchers consider it more likely that they originated from one or more separate parent bodies. This scenario can explain the significant difference in O-isotopic compositions between the metamorphosed Dhofar meteorites (and the Belgica-like grouplet) and typical CM chondrites (Choe et al., 2010). Furthermore, geochemical variations that exist between the Dhofar meteorites and the Belgica meteorites attest to the fact that their source material was not exactly the same. It was experimentally demonstrated by Ivanova et al. (2010) that these thermally metamorphosed meteorites could not be derived from typical CM2 material through heating/dehydration processes, but rather were formed in a similar oxygen reservoir.
Continued research by Ivanova et al. (2012, 2013) has demonstrated that the differences observed in the O-isotopic composition between the metamorphosed carbonaceous chondrites of the Dhofar and Belgica-like groupings and typical CM chondrites are consistent with multiple cycles of hydrationdehydration on a common parent body. Following aqueous alteration of silicates involving a source of water enriched in 18O, the resulting phyllosilicate (primarily serpentine) was also enriched in 18O by ~10%. Moreover, subsequent dehydration processes led to a further enrichment in 18O by ~7%. They reasoned that a low degree of heating at some distance from an impact crater would result in melting of existing water ice, which was then utilized in the hydration of silicate rockthen followed burial, metamorphism, and dehydration of this rock. They propose that this hydrationdehydration cycle might have occurred multiple times to produce the isotopic and geochemical differences observed among these meteorites.
Subsequent heating experiments were conducted by Nakato et al. (2014, 2016) in which samples of the C2-ungrouped Tagish Lake, a meteorite that shares many characteristics with metamorphosed carbonaceous chondrites, were exposed to varying temperatures and heating durations. They demonstrated that heating at a high temperature of 900°C for 196 hours caused progressive reduction and dehydration, resulting in mineralogical and textural changes similar to those observed in the Belgica group of thermally metamorphosed carbonaceous chondrites; e.g., fibrous textured phyllosilicates, reduction of magnetite to form FeNimetal+troilite assemblages. In addition, both Tagish Lake and the Belgica group meteorites have Si-rich matrix compositions compared to typical low-temperature CM chondrites. The degree of change in the O-isotopic composition of these heated samples is yet to be established.
It was experimentally demonstrated by Lindgren et al. (2020) that the slope of 0.48, which represents the increase in heavy oxygen isotopes from heating of CM chondrite ALH 83100 to 800°C, does not intersect the CY compositional field (see diagram below). Therefore, they concluded that CY chondrites cannot be derived from CM2 chondrites through any thermal process.
click on diagram for a magnified view
Diagram credit: Lindgren et al., GCA, vol. 289, p. 77 (2020, open accesslink)
'Signatures of the post-hydration heating of highly aqueously altered CM carbonaceous chondrites and implications for interpreting asteroid sample returns'
Employing position-sensitive-detector X-ray diffraction (PSD-XRD), King et al. (2015) determined the modal mineralogy of a suite of CI and CY chondrites (see diagrams below). The total phyllosilicate abundances, ~83 vol% (8184 vol%) for CI and ~79 vol% (including potentially back-transformed olivine) for CY, indicates that both parent bodies experienced similar extensive aqueous alteration to near completion. However, there is a significant difference in sulfide abundances between the two groups, ~6 vol% for CI and 19 vol% for CY, which reflects a difference in primary S content for the respective protoliths. King et al. (2015) also found a difference in the modal abundance of magnetite between the two groups, ~79 vol% for CI and 2 vol% for CY. Based on this data they inferred that the two groups likely originated on separate parent bodies.
X-Ray Diffraction Patterns for CI and CY Chondrites
Diagrams credit: King et al., GCA, vol. 165, pp. 148160 (2015, open accesslink
'Modal mineralogy of CI and CI-like chondrites by X-ray diffraction'
Employing micro-Computer Tomography along with high-precision triple oxygen isotope analysis, Suttle et al. (2020) investigated a number of the largest (>500 µm), least melted, and least weathered micrometeorites from the Transantarctic Mountains (TAM) collection. They found that two intensely hydrated, 16O-poor micrometeorites,
TAM50-25 and TAM19B-7, share a common oxygen isotope trend line with CO, CM, and CY chondrites, possibly reflecting differences in accumulated isotopically-heavy water ice (see diagram below). Although the two micrometeorites have significantly lower sulfide abundances as well as smaller chondrules compared to the CY
chondrites, they maintain that the micrometeorites could have originated from the CY parent body.
click on diagram for a magnified view
Diagram credit: Suttle et al., EPSL, vol. 546 (2020)
'Isotopic and textural analysis of giant unmelted micrometeoritesidentification of new material from intensely altered 16O-poor water-rich asteroids'
Current studies suggest that both cometary dust and meteorites should be produced from the disruption of Jupiter-family comets that originate in the Kuiper belt. Studies have shown that Antarctic micrometeorites have a similar carbonaceous chondrite:ordinary chondrite ratio (~7:1) as the composition of zodiacal dust (M.M.M. Meier, 2014). Based on observational evidence and current modeling, it is thought that comets should be dark in color and have a low density and strength, a high porosity, a solar ratio of elements, an elevated ratio of C, H, O, and N, a high interstellar grain content, anhydrous and highly unequilibrated silicates, few to no chondrules, and a low cosmic-ray exposure age (<10 m.y.). Both the CI and CM groups of meteorites exhibit characteristics which are consistent with the above descriptions.
Orbital data obtained from several carbonaceous chondrites (e.g., CI Orgueil [eyewitness plotting]; CMs Maribo and Sutter's Mill [instrument recording]) are a good match to the orbits expected from the disruption of Jupiter-family comets, but are unlike the orbits of ordinary chondrites and most other asteroidal objects (M.M.M. Meier, 2014). Both the orbital eccentricity and semimajor axis for Maribo is nearly identical to those of Comet Encke and the associated Taurid swarm of objects (Haack et al., 2011). On the other hand, a CRE age study of CM chondrites conducted by Meier et al. (2016) shows a possible relationship exists to the asteroid breakup event ~8.3 m.y. ago that formed the Ch/C/Cg-type members of the Veritas family. In addition to the large abundance of 3He-enriched interplanetary dust discovered in 8.2 m.y.-old deep-sea drill cores, ~1/6 of all CM meteorites have 21Ne-based CRE ages that are consistent with derivation from this catastrophic breakup, while others with significantly younger CRE ages could represent secondary collisions among the Veritas fragments.
In consideration of the young CRE age of all of the Belgica group meteorites (≤1.3 m.y.), a near-Earth asteroid is favored as the common source object (King et al., 2019). One possible candidate is the binary asteroid 1998 ST27, which appears to match the required spectrographic characteristics of these meteorites. Moreover, its binary nature is consistent with the likelihood for disruption and injection of material into an Earth approaching orbit. Other source asteroids, such as Phaethon, Icarus, and 2008 FF5 are considered by Ivanova et al. (2013) as potential sources for these meteoritesthe heat source for their metamorphism may be associated with their perihelion close to the Sun.
It is noteworthy that the C-type asteroid (162173) Ryugu has some spectral similarity to experimentally-heated hydrous carbonaceous chondrites, and may be analogous to spectra of the CY group (Matsuoka et al., 2018; King and Russell, 2019; King et al., 2019). Preliminary analyses of the Ryugu sample material reported by Yada et al. (2021) shows that it contains no chondrules or CAIs and is most similar to CI chondrites; however, Ryugu material is darker (but similar to Tagish Lake), more porous, and more fragile. In addition, Ryugu material has a lower bulk density than either CI chondrites or Tagish Lake. The specimen of Dhofar 225 shown above is a 0.69 g partial end section.