Dho 011

LL7 or Meta-LL
standby for dhofar 011 photo
Found December 5, 1999
18° 20.7′ N., 54° 11.9′ E. 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. An identical 414 g stone designated Dhofar 014 was found nearby and is considered to be paired. 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 definition would lump those meteorites containing ‘poorly defined’ chondrules and ‘relict’ chondrules, such as Dhofar 011/014 and DaG 1022, into the type 6 category.

In further contrast to Dodd et al., Wittke and Bunch (2004) suggest that the relative size of all 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. In addition, Tomkins (2014) determined other petrogrphic criteria to mark the chondrite–achondrite transition. They found that beyond petrologic type 6, skeletal plagioclase forms an interconnected network between olivine and pyroxene grains, and as a low degree (<5%) of partial melting occurs, plagioclase exhibits wetting textures in contact with olivine and pyroxene grains. This was in fact shown to be the case in the H7 chondrite Watson 012 studied by Tait et al. (2004). In addition, low-Ca exsolutions were observed in many clinopyroxene grains in meteorites beyond petrologic type 6, and relict chondrules were very limited in number—only 0.25 per cm² in Watson 012.

Among the several type-7 LL chondrites studied by Friedrich et al. (2014), both Uden and EET 92013 exhibit some evidence for very low degrees (incipient) of partial melting and occasional mobilization of both FeNi–FeS and plagioclase, as well as evidence of rare vestigial chondrules; a relict barred chondrule was observed in their LL7 Y-82067. In a study of the bulk elemental abundances of several H7 and LL7 chondrites, Yoshioka et al. (2014) observed only slight volatile element depletions (Se), and abundance patterns that exhibit both a negative Eu anomaly and an HREE enrichment through REE fractionation, features likely resulting from plagioclase loss. In their research on the metamorphic transition to type 7, Tait et al. (2014) studied the H7 chondrite Watson 012 employing various techniques including X-ray computed tomography. They observed evidence of incipient silicate partial melting in the form of an interconnected plagioclase network composed of coarse-grained skeletal crystals, often associated with large globular metal–troilite grains encapsulated at triple junction node expansions; in addition, some orthopyroxene and clinopyroxene grains have crystallized from this melt phase. They argue that the partial melt was produced early in Solar System history (10–15 m.y. old) through a combination of radiogenic heating and impact-shock heating (>S4), resulting in a combined peak temperature of ~1156°C. This equilibrium-controlled partial melting occurred at a depth of ~15–20 km, and thereafter, a period of slow cooling and annealing ensued. Tait et al. (2014) have proposed a revision to the Van Schmus and Wood (1967) classification system to include criteria for petrologic type 7, incorporating updates from Dodd (1981), Sears and Dodd (1988), Brearley and Jones (1998), and Tait et al. (2014), which can be viewed here.

For those chondritic meteorites that experienced metamorphic temperatures high enough for significant metal–sulfide melting to occur, which commonly occurred as a result of impact events (after the decay of most radiogenic 26Al), an igneous texture would be produced (Mittlefehldt and Lindstrom, 2001). In these cases the use of the Van Schmus–Wood classification system is no longer valid, and these meteorites could be characterized as impact melts or even primitive achondrites. Dhofar 011 does not show any evidence of having lost a low-melting fraction of metal–sulfide, and it exhibits an unfractionated chondritic composition consistent with subtype 7. Finally, 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 termed achondrites.

Research has been published that 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 of an already hot parental source)
  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

The Dhofar 011 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.

standby for dhofar 011 in situ photo

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