DHOFAR 225

A fresh, black, carbonaceous chondrite weighing just 90 g was found in the desert of Oman. Dhofar 225 has certain textural, mineralogical, and bulk chemical similarities to the CM chondrites, but differs from that group in important ways. Dhofar 225 has an O-isotopic composition that is enriched in heavy oxygen and plots close to the C2-ungrouped Tagish Lake and to the CI/CM-like, thermally metamorphosed/dehydrated Antarctic meteorite grouplet of Belgica 7904, WIS 91600, Y-86720, and possibly Y-82162 and Y-86789 (see diagram below). Notably, Dhofar 225 has many features in common with the anomalous CM chondrite Dhofar 735.

Like the Belgica grouplet, Dhofar 225 exhibits considerable dehydration of matrix phyllosilicates, Fe and S depletions, and 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 severely constrain the maximum temperature of metamorphism. 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, and are 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 is primarily composed of phyllosilicates, with minor sulfides, phosphides, phosphates, FeNi-metal, chromite, and eskolaite, with only rare CAIs. A previously unknown mineral phase, Ca,Fe-oxysulfide, was identified in the matrix. Tochilinite, characteristically abundant in CM chondrites, has been mostly thermally decomposed to troilite and oxides, and 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, 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 Meta-CM chondrite Y-86720, although only Dhofar 225 has retained moderate abundances of PCP and tochilinite with intergrown phyllosilicates. 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.

To date, seven CM chondrites and two CI chondrites found in Antarctica have been described as thermally metamorphosed (Zolensky et al., 2005). To this group has been added two metamorphosed CM chondrites from Oman—Dhofar 225 and 735. This metamorphosed group may have formed from normal CM chondrites since they have many characterisics in common, or they may have originated from a separate carbonaceous chondrite parent body; the latter scenario best explains the significant difference in O-isotopic compositions (Choe et al., 2010). In consideration of the young CRE age of all of these 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. The specimen of Dhofar 225 shown above is a 0.69 g partial end section.

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