Polymict breccia, cumulate
(or mesosiderite inclusion, or ungrouped achondrite)
18° 20.1' N., 54° 10.9' E.
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 quartzcarbonate sand. The area is overgrown by low shrubs. Based on structure, petrography, bulk chemistry, and REE patterns, this eucrite was 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# ~4465), REE abundances much lower than chondritic, and are LREE-enriched with positive Eu anomalies (1823 × 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 78 km, with a cooling rate of 0.160.2°C/t.y.
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 parent body. 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 more complex. 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 (8501200°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 might possibly be a silicate fraction from a mesosiderite.
Despite these anomalies, the petrology, REE content, Mg#, FeO/MnO ratios, mineralogy, and textures are all consistent with a cumulate eucrite classification. However, utilizing an oxygen three-isotope diagram, Greenwood et al. (2017) demonstrated that Dhofar 007 plots far away from the eucrite fractionation line (see diagram below). Whether this reflects impactor contamination on the eucrite parent body (4 Vesta) or a separate parent body is still an open question. Interestingly, a Cr-isotopic analysis was subsequently conducted by Pang et al. (2020) for both Dhofar 007 and Dhofar 778 at the University of Science and Technology of China, and the ε54Cr values of 0.46 (±0.09) and 0.51 (±0.07), respectively, support the O-isotopic evidence for a possible common parent body distinct from the HEDs and other anomalous eucrites. It was also recognized that the Δ17O value for the anomalous eucrite-like meteorite NWA 12338 (0.18 [±0.01]) is very similar to that for both Dhofar 007 (0.173 [±0.010]) and Dhofar 778 (0.160 [±0.054]), and it was suggested that the three meteorites might be genetically related; Cr isotopes still need to be determined for NWA 12338 (Pang et al., 2020).
click on image for a magnified view
Diagram credit: Greenwood et al., Chemie der ErdeGeochemistry, vol. 77, p. 25 (2017)
'Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies'
(open access: http://dx.doi.org/10.1016/j.chemer.2016.09.005)
The unbrecciated eucrite-like achondrite EET 92023 exhibits important petrological and O-isotopic similarities to the coarse-grained clasts in Dhofar 007, and it has been considered by some that the two might be genetically related. In their study of EET 92023, Yamaguchi et al. (2017) determined that its source rock experienced a multistage thermal history including a very early impact by a IAB or IVA iron projectile, but evidence indicates that its anomalous O-isotopic signature is intrinsic and not the result of impactor contamination. However, it has not yet been determined if the similar O-isotopic signature of Dhofar 007 is intrinsic or is perhaps due to impactor contamination.
In addition to Dhofar 007 and EET 92023, a number of anomalous eucritic meteorites are known including Ibitira, Pasamonte, NWA 1240, PCA 82502/91007, Bunburra Rockhole, A-881394, and Emmaville, each of which are resolved from typical eucrites and the HED parent body both isotopically and compositionally. It is notable that EET 92023, Bunburra Rockhole, and A-881394 have close O-isotope values, which suggests the possibility of a genetic relationship (Barrett et al., 2017, O-isotopic diagram 1; Mittlefehldt et al., 2018, O-isotopic diagram 2).
Another useful tool to help resolve potential genetic relationships among meteorites is the Fe/Mn ratio. While Fe and Mn do experience nebular fractionations, they are not readily fractionated during parent body igneous processing, and therefore different Fe/Mn values are inherent in different parent objects. Mittlefehldt et al. (2017) utilized a number of eucrites and anomalous eucrite meteorites, including A-881394, EET 92023, Ibitira, and Emmaville, to compare the Fe/Mn and Fe/Mg ratios for low-Ca pyroxenes. Contrary to the results from O-isotopic analyses, these four meteorites plot in separate locations on an Fe/Mn vs. Fe/Mg coupled diagram, which suggests that they derive from separate parent bodies (see diagram below). Moreover, although Bunburra Rockhole and A-881394 have overlapping oxygen and chromium isotope compositions, new in-depth analyses of Bunburra Rockhole conducted by Benedix et al. (2017, and references therein) have revealed that these two meteorites have very different textures and mineral chemistries; e.g., Bunburra Rockhole has plagioclase with An8790, while A-881394 has plagioclase with An98. Based on their results, they consider it likely that these two meteorites derive from separate parent bodies. Further details about the anomalous eucrites can be found on the Pasamonte page.
Diagram adapted from Mittlefehldt et al., 47th LPSC, #1240 (2016)
Based on the calculated CRE age and KrKr age of ~1215 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 about 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.