DHOFAR 007

This coarse-grained, brecciated eucrite, with a total known weight of ~27 kg, was found broken into 37 fragments on the desert floor of Oman. The find site is a flat plain, covered with carbonate stones, interspersed with a quartz–carbonate sand. The area is overgrown by low shrubs. Based on structure, petrography, bulk chemistry, and REE patterns, this eucrite was initially classified as a cumulate by Dr. Marina Ivanova at the Vernadsky Institute of Geochemistry of the Russian Academy of Sciences, Moscow.

In contrast to noncumulate eucrites, cumulate eucrites have higher Mg contents (Mg# ~44–65), REE abundances much lower than chondritic, and are LREE-enriched with positive Eu anomalies (18–23 × CI). The source magma was probably derived from the mineral peridotite, a mixture of olivine, pigeonite, and plagioclase, and is the mineral forming the bulk of the Earth's upper mantle. Cumulate eucrites formed by fractional crystallization in a magma chamber about 4.5 b.y. ago. On the basis of augite lamellae widths and Ca zoning profiles, a burial depth for cumulate eucrites was calculated to be 7–8 km, with a cooling rate of 0.16–0.2°C/t.y.

Dhofar 007 and the similar EET 92023 both experienced anomalous thermal histories. The composition of Dhofar 007 consists of roughly equal proportions of anorthite, a Ca-rich plagioclase feldspar, and the clinopyroxenes pigeonite and augite, along with minor amounts of Fe-metal, troilite, chromite, and phosphates. Glassy melt veins pervade these components, reflecting an impact shock history. During studies of Dhofar 007 by Yamaguchi et al (2003), they identified various xenolithic inclusions, including a polymineralic impact melt clast, a Mg-rich orthopyroxene fragment, and a recrystallized plagioclase grain, demonstrating a polymict nature for this meteorite.

Another clast that was studied, which represents a large portion of this meteorite, is coarse-grained with a granular texture, composed primarily of equal amounts of pyroxene (pigeonite and augite) and plagioclase, along with minor silica and metallic phases. The derivation of the metallic phases is suggested by Yamaguchi et al. (2006) to have occurred through the injection of a metallic component during a high temperature impact shock event on the mesosiderite PB. They describe a scenario by which the eucrites were formed at a distance from the actual impact location of a large metallic projectile, while the mesosiderites were located in close proximity to the impact. Dhofar 007 is unusually enriched in siderophile elements, including Ni, Ir, Os, Au, Pd, and Co, similar to the abundances found in metal of mesosiderites. The platinum group elements also have ratios that are similar to those in metallic portions of mesosiderites.

Compared to cumulate eucrites, the cooling history of Dhofar 007 is anomalous. Following the shock heating/melting event in which FeNi-metal was injected, recrystallization occurred. Thereafter, excavation from depth caused very rapid cooling at high temperatures (850–1200°C) in pyroxenes, resulting in the formation of very thin augite exsolution lamellae. This cooling rate is on the order of 10,000 times higher than that of cumulate eucrites cooled at depth. Subsequent burial by an extensive ejecta blanket led to very slow cooling at lower temperatures (~700°C down to at least 300°C) as evidenced by the metallic phases, in a similar manner to that of mesosiderites. Later, moderate impact shock events produced brecciation and melt veins.

Based on these anomalies in siderophile content and cooling history, Yamaguchi et al. (2003, 2006) proposed that Dhofar 007 may possibly be a silicate fraction from a mesosiderite. However, despite these anomalies, the O-isotope composition, petrology, REE content, Mg#, FeO/MnO ratios, mineralogy, and textures are all consistent with a cumulate eucrite classification. The Antarctic polymict eucrite EET 92023 exhibits important similarities to the coarse-grained clasts in Dhofar 007, and the two may be genetically related.

Based on the calculated CRE age and Kr–Kr age of ~12–15 m.y., a terrestrial age for Dhofar 007 has been estimated to be 70 t.y. (Takeda et al., 2007). A previous estimate given by Miura and Nagao (2003) was 20 t.y. (Further information on the cumulate eucrites can be found on the NWA 1836 page. The photo above shows a 1.45 g partial end section of Dhofar 007 showing both fine- and coarse-grained clasts intruded by black shock veins. The photo below shows one of the fusion-crusted fragments as it was found lying on the desert plain.

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