CM-anomalous, ungrouped
(thermally metamorphosed/dehydrated)
standby for dhofar 225 photo
Found January 15, 2001
18° 21.6' N., 54° 11.3' E.

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.

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 and to the CM/CI-like, thermally metamorphosed/dehydrated Antarctic meteorite grouplet of Belgica 7904 [3.1], WIS 91600, Y-86720, PCA 02012, and possibly Y-82162 and Y-86789 (see diagram below). In addition, Nakamura (2006) identified two regolith breccias containing solar-wind-implanted noble gases which belong to this group, Y-793321 and A-881458, while M.M.M. Meier (2014) found that the meteorite Diepenveen also contains similar trapped solar gases. Notably, Dhofar 225 has many features in common with the anomalous CM chondrite Dhofar 735, which along with Belgica 7904 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 belonging to this dehydrated group.

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 matrix—all 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). 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):

  Dehydration Temperature
Stage I <300°C
Stage II 300–500°C
Stage III 500–750°C
Stage IV >750°C

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), with minor sulfides, phosphides, phosphates, FeNi-metal, and chromite, with 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 might have affected Dhofar 225 resulting in its extant tochilinite.

While this group of metamorphosed carbonaceous chondrites could have been derived from normal CM chondrites, in accord with their many common characterisics, 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 two Dhofar meteorites and the Belgica meteorites attest to the fact that their source material was not exactly the same. It was accepted that these metamorphosed meteorites could not have been derived from typical CM2 material through dehydration processes, but rather were formed in a similar O-reservoir (Ivanova et al., 2010).

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 hydration–dehydration 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 extant water ice, which was then utilized in the hydration of silicate rock—then followed burial, metamorphism, and dehydration of this rock. They propose that this hydration–dehydration cycle may have occurred multiple times to produce the isotopic and geochemical differences observed among these meteorites.

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 a varying temperatures and heating durations. They demonstrated that heating at a high temperature of 900°C for 1–96 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 FeNi–metal+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.

Current studies suggest that both cometary dust and meteorites should be produced from the disruption of Jupiter-family comets which 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, a near-Earth asteroid is favored as the common source object. One possible candidate is the binary asteroid 1998 ST27, which appears to match the required spectrographic characterisics 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 meteorites; i.e., the heat source for their metamorphism may be associated with their perihelion close to the Sun. The specimen of Dhofar 225 shown above is a 0.69 g partial end section.