DHOFAR 011

A single 150 g stone was found on the surface of the desert in Oman. It was classified at the Vernadsky Institute (M. Ivanova) as an LL7, S3, W3 meteorite. Fission track thermochronometry indicates that type 7 chondrites cooled more slowly at greater depths than did those of lower petrologic types (Trieloff et al., 2003). Consequently, type 7 chondrites experienced a longer period of thermal metamorphism within this interior layer, and now exhibit extensively recrystallized textures that are transitional to an achondrite classification. Type 7 ordinary chondrites were originally defined by Dodd et al. (1975) according to specific petrographic characteristics. They listed three metamorphic criteria to distinguish between petrologic types 6 and 7:

1. the presence of poorly defined chondrules in type 6, but only relict chondrules in type 7
2. low-Ca pyroxenes contain no more than 1.0 wt% CaO (1.0 wt% = ~1.9 mol% Wo) in type 6, but more than 1.0 wt% in type 7; conversely, the CaO content of high-Ca pyroxenes decreases from type 6 to type 7
3. feldspar grains gradually coarsen to reach a size of at least 0.1 mm in type 7

In Dhofar 011, chondrules are virtually absent within the coarse-grained, thoroughly recrystallized matrix. Furthermore, CaO in the low-Ca pyroxenes comprise 1.14 wt%. With reference to Dodd (1981), the classification of Dhofar 011 is consistent with that of type 7.

In the intervening years since Dodd et al. proposed their classification parameters, additional type 7 chondrites have been found and studied. As a result of more recent studies, it was proposed by Wittke and Bunch (pers. comm., 2004) that a type 7 category should not comprise meteorites containing any relict chondrules, but rather, should represent a metamorphic extreme in which no sign of chondrules remains. This would lump those meteorites containing "poorly defined" chondrules and "relict" chondrules into the type 6 category.

In further contrast to Dodd et al., Wittke and Bunch (2004) suggest that the relative size of all of the silicates, rather than only the feldspar grains, would provide a better gauge of a petrographic type 7 since silicates attain an equigranular texture only under the highest metamorphism. They have also discovered that simple twinning of plagioclase occurs only in type 7, and suggest that this could be utilized as an additional parameter. Beyond that, it was revealed that modal metal contents decrease significantly during late metamorphic stages; i.e., low-Ni metal, as well as pyroxenes, are consumed to produce olivine, resulting in only small amounts of Ni-rich metal along with lower amounts of orthopyroxene and clinopyroxene compared to those amounts present in lower metamorphic grades.

Research has been published which identifies specific characteristics that distinguish type 7 chondrites from primitive achondrites. The following characteristics are typically observed in primitive achondrites (Ford et al., 2004):

1. an equigranular (igneous) texture with no extensive segregation
2. experienced temperatures to levels necessary for FeNi-metal, FeS, and silicate partial melting (~1200°C, perhaps by shock melting)
3. migration of free metal from olivine fayalite and chromite as a result of reduction processes (i.e., by reaction with graphite), resulting in Mg-rich olivine and chromite and low-Ni metal
4. Cr acting as a chalcophile element during reduction leading to its incorporation into troilite
5. close to chondritic bulk composition

For those meteorites that experienced metamorphic temperatures high enough for metal–sulfide melting to occur, which most commonly occurs as a result of impact events, an igneous texture would be produced (Mittlefehldt and Lindstrom, 2001). In these cases the use of the Van Schmus–Wood classification scheme is no longer valid, and these meteorites may be called primitive achondrites or even impact melts. Dhofar 011 does not show any evidence of having lost a low-melting fraction of metal–sulfide, and it exhibits an unfractionated chondritic composition. Those meteorites which have undergone more extensive thermal processing and have lost their original geochemical and isotopic features (e.g., members of the HED suite) would be called achondrites.

This meteorite may be paired with the 414 g LL7 Dhofar 014. The specimen of Dhofar 011 shown above is a 1.29 g partial slice. Pictured below is the main mass of Dhofar 011 as found in the rock-covered desert.

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