Dho 081

Lunar Feldspathic Breccia
(fragmental breccia or immature regolith breccia)
standby for dhofar 081 photo
Found November 29, 1999
19° 19.32′ N., 54° 50.85′ E. This fresh, fusion-crusted lunar meteorite, weighing just 174 g, was found in the Dhofar region of Oman. Although this meteorite has a terrestrial age of 40 (±20) t.y. (Nishiizumi et al., 2004), terrestrial weathering effects are very slight, as evidenced by µm-sized carbonate and sulfate minerals. Although Dhofar 081 is classified as a feldspathic fragmental highland breccia, its relatively high 20Ne content might be diagnostic of an immature regolith breccia (Warren et al., 2005). It is composed of various clasts of older impact-melt breccias and plutonic rocks. Individual clast types include crystalline fragments of gabbroic anorthosites, granulites, highland basalts, various impact-melt breccias, and high-An plagioclase feldspars. The matrix minerals consist mostly of olivine and augite, with minor FeNi-metal, ilmenite, spinel, and rare devitrified brown glass, but no evidence of mafic regolith, KREEP, or mare basalt components was identified. Some clasts have plagioclase and mafic mineral compositions that plot within the gap separating ferroan anorthosite suite rocks (FAN) and high-magnesium suite rocks (HMS). The identification of somewhat higher abundances of solar He and Ne is consistent with a regolith setting (Korotev, 2012 and references therein).

A second stone recovered nearby weighing 251.2 g was investigated for possible pairing with Dhofar 081. Dhofar 280, has a similar chemistry and mineralogy to that of Dhofar 081, but it exhibits some important differences. Only Dhofar 280 contains regolith clasts incorporating glass spherules and FeNi-metal grains. Dhofar 280 is also distinct in containing three new Fe-silicide phases. In light of these differences, further studies were conducted to better constrain the probability of their pairing. Nazarov et al., 2003, and Demidova et al., 2003, found that Dhofar 081 and 280 form a unique group among the large number of recently recovered lunar meteorites from the Dhofar region of Oman. Dhofar 081 and 280 have similar and distinctive compositions, including a low abundance of Sm and Sc, a high abundance of feldspar, and the lowest Mg# among lunaites, and they share similar degrees of weathering. An Ar systematics study conducted by Korochantseva et al. (2015) revealed that the two meteorites had very different irradiation histories on the Moon—~400 m.y. for Dhofar 280 and 500–600 m.y. for Dhofar 081. They also determined that Dhofar 280 was enriched in KREEP compared to Dhofar 081, and for the two meteorites to have experienced a common ejection event, they must have formed in separate locations in close proximity. Two additional stones, designated Dhofar 910 and Dhofar 1224, were subsequently recovered from this strewnfield and may be additional pairings. A meteorite from northwest Africa, NWA 2998, was found to have an identical highly-feldspathic composition, although with a different texture to that of the Dhofar 081 pairing group, and may be source-crater paired (Korotev et al., 2008).

Based on mineralogy, chemistry, petrography, and find location, the Oman lunar meteorites that have been analyzed so far can be tentatively resolved into the following groups, listed in order of location from north to south (expedition team member, pers. comm.; Korotev et al., 2008):

  1. Dhofar 081, 280, 910, and 1224 are likely paired.
  2. Dhofar 490 and 1084, which were found about 76 km south of the Dhofar 081/280/910/1224 strewnfield, are not compositionally distinguishable and the two are likely paired.
  3. Dhofar 302, 303, 305, 306, 307, 309, 310, 311, 730, 731, 908, 909, 911, 950, 1085, and 489 are all likely paired, sharing a strewn field (~1.4 × 1.2 km) with the Dhofar 081 pairing group.
  4. Dhofar 025, 301, 304, and 308 are likely paired; they were found within a small area of ~0.5 km² (and approximately 100 km southwest of the Dhofar 081 pairing group).
  5. Dhofar 287 is the only mare basalt found; it was recovered <0.5 km from some members of the Dhofar 025 group.
  6. Dhofar 026 and Dhofars 457–468 are likely paired; they constitute a strewnfield ~30 km long oriented west to east, and located ~20 km south (to ~50 km southwest) of the Dhofar 025 group.

Further studies of these finds will help to determine exactly how many independent falls are represented among the many individual Dhofar lunaites. A concise and up-to-date table of all known lunar meteorites can be found on Norbert Classen’s An Up-to-date List of Lunar Meteorites. Multiple shock features are present throughout Dhofar 081, including recrystallization, fracturing, mosaicism, and in situ melting. These features indicate that shock pressures as high as ~15–20 GPa were experienced prior to the final ejection event. One or more shock events also produced melt veins with vesicles and the lithification of previously unconsolidated material. Strong zoning of olivine grains is indicative of rapid cooling within an impact ejecta blanket. Noble gas studies suggest that Dhofar 081/280/910/1224 is not paired with other lunar meteorites from the Dhofar region, but it may be launch paired with the Antarctic lunaite, MAC 88104/5, based on their similar noble gas records. The CRE age of Dhofar 081, 680 (±140) m.y., actually represents the duration of its exposure at depth—calculated to be ~100 cm within the lunar regolith based on Kr and Xe (Lorenzetti et al., 2005), or 111–128 cm deep based on Be and Al (Nishiizumi and Caffee, 2006). Consistent with a launch from shallow depth, Dhofar 081 had a short Moon–Earth transit time of ~5,000 years (Nishiizumi, 2003; Nishiizumi and Caffee, 2010), and the stones resided on Earth for another ~12,000 years.

Feldspathic fragmental breccias similar in composition to Dhofar 081 were the most common rock type excavated at the North Ray Crater, Central Highlands, landing site of Apollo 16. However, even though Dhofar 081 has a very low abundance of mafic components and a highly anorthositic composition similar to Apollo 16 breccias, the absence of KREEP and high magnesium suite constituents makes it more likely that it originated from a ferroan, anorthosite-rich, Feldspathic Highlands Terrane, probably on the far side of the Moon. The specimen of Dhofar 081 pictured above is a 0.143 g very thin partial slice.

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