Dho 908

Lunar Feldspathic Breccia
standby for dhofar 908 photo
Found January 2003
19° 19.9′ N., 54° 47.0′ E. Nine individual stones, having a total combined weight of 245.46 g and exhibiting heterogeneous compositions, were recovered by a German expedition searching in Wadi Quitbit within the Dhofar dense collection area of Oman. The expedition was searching within the strewnfield in which the lunar pairing group of Dhofar 303, 305, 306, 307, 309, 310, 311, 730, 731, 489 (found 24 km away), 908, 909, 911 (comprising nine separate stones), 950, and 1085 was recovered. Terrestrial weathering has produced significant staining from hematite (R. Korotev).

The finder of the individual Dhofar 908 stone, Norbert Classen, adopted the term ‘Rosetta Stone’ to describe this 81 g lunaite due to its having three distinct lithologies that link the diverse finds Dhofar 302, 303, 305, 306, 307, 309, 310, 311, 730, 731, and 489 together; most of these other stones represent only one of the three lithologies. Importantly, Dhofar 908 established a clear pairing relationship among all of these separate finds (see photo below).

Following the analysis at the Institut für Planteologie in Münster, these meteorites were classified as lunar feldspathic breccias, specifically, impact-melt breccias. Interestingly, the lunar feldspathic fragmental breccia that comprises the individual stones Dhofar 081, 280, 910, and 1224, was found in the western half of this same strewnfield, which encompasses an area of 1.4 × 1.2 km—an astounding case of overlapping lunar strewnfields.

Cosmogenic nuclide studies of Dhofar 908 based on 10Be and 26Al have enabled the determination of the excavation depth on the Moon (>6 m), the Moon–Earth transit time (4 ±1 t.y.), and the terrestrial age (~300 t.y.) (Nishiizumi and Caffee, 2006). Sm–Nd data yield a crystallization age of 4.31 (±0.07) b.y., and might reflect derivation from magnesian troctolitic-anorthosite precursor material from plutons that intruded into the early ferroan crust after solidification of the lunar magma ocean (Nyquist et al., 2010). Gross et al. (2012) have presented evidence found in most feldspathic highlands meteorites that a global lunar magma ocean did not form, and they lack features of such a scenario; i.e., they contain no ferroan anorthosites, KREEP, or Mg-suite rocks. Instead, anorthosite intrusions rise continuously in diapirs, resulting in compositional diversity among the crustal regions.

An Ar–Ar age for the mostly troctolitic matrix material of Dhofar 908 was found to be 4,256 (±20) m.y., taking into consideration evidence for solar wind Ar trapped during residence on the lunar surface. This age may reflect material derived from an old basin within early-formed anorthositic crust on the lunar farside in which low Th and FeO exist (Karouji et al., 2010). The crustal asymmetries that exist between the farside and nearside can be explained by the tilted convection model. A potential ejection site for the Dhofar pairing group is considered to be the Derichlet-Jackson basin.

Studies of Dhofar 489, a member of the pairing group, revealed the presence of unique magnesian anorthosite clasts and a spinel troctolite clast. Its bulk analysis is highly depleted in Th (proxy for ITEs) and FeO (Takeda et al., 2007). In addition, studies of the paired stone Dhofar 309 revealed clasts of anorthosite and troctolite composition, considered to be metamorphosed and annealed crystalline rocks associated with an impact-melt pool. A reddish-orange clast found in these samples has a crystalline texture and contains plagioclase crystals and rounded olivine grains; it is thought to be an impact-melt clast derived from spinel troctolite, but which includes a pyroxene component derived from norite.

Studies of the paired Dhofar 307 led to the discovery of magnesian anorthositic granulite clasts, originally derived from impact melts, some of which contain large olivine fragments embedded in a glassy plagioclase matrix (Takeda et al., 2008, 2010). These olivine fragments may represent ejected mantle rock from the massive impact that created the largest impact crater in the Solar System, the South Pole–Aitken basin, located on the lunar farside. These magnesian anorthositic granulite clasts may be ancient (up to 4.3 b.y.), and are thought to represent basaltic plutons which were emplaced into plagioclase-rich crust following solidification of the lunar magma ocean. The bulk compositions of the anorthositic granulite clasts (FeO ~4.5 wt%; Al2O3 ~28 wt%; Th <1 ppm) are similar to those calculated for the Feldspathic Highlands Terrane (FHT) on the lunar farside. The magnesian anorthosite clasts represent a distinct geochemical lunar component that is widespread across the lunar surface (Treiman et al., 2010). These clasts contain too much magnesium to be related to the Ferroan Anorthosite Suite (FAN) components, and are too feldspathic and lacking in KREEP to be related to the Mg-Suite components, both of which are typical contaminates in lunar breccias recovered from the nearside of the Moon.

Since Dhofar 908 and its pairing group represent a quickly cooled impact-melt breccia that could have formed at a significant depth beneath a regolith, which could potentially have been contaminated with incompatible elements, its low-Th, low-FeO signature taken by itself is not an adequate determinant for either a nearside or a farside origin for this type of lunar meteorite (R. Korotev). However, when these analyses are considered together, along with the possible discovery of a norite component, it can be inferred that the origin of this pairing group is most likely the lunar farside—possibly from the area of the South Pole–Aitken basin, or perhaps the FHT at the northern portion of the lunar farside.

An alternative formation scenario has been proposed by Takeda et al. (2008, 2010) in which a large impact into magnesian anorthosites, likely on the northern farside, excavated a basin at least 80 km in diameter and produced an extensive melt sheet. Deep-seated lithologies present in the meteorite were excavated as well. Rapid cooling and subsequent impact gardening within this basin led to the final consolidation of this brecciated rock. A large basin containing many craters in which the Dhofar 908 breccia may have formed is the Dirichlet-Jackson basin, located in the low-Th region of the farside. The mineralogy, chemistry, and petrology of the various members of this pairing group indicate that they were all derived from a common precursor lithology, one having a spinel troctolite composition consistent with a location at a significant depth (>5 m) within the crust.

The specimen pictured above is a 0.143 g very thin partial slice from the main mass of Dhofar 908. The photo below shows the main mass of Dhofar 908, the first stone to be found, weighing 81.43 g. Dhofar 908 was exported by permit from the Ministry of Commerce and Industry, Sultanate of Oman. The bottom photo is a beautiful 0.61 g thin slice from the Dhofar 908 main mass showing three separate lithologies (3.1 cm in longest dimension). standby for dhofar 908 photo
Photo courtesy of Norbert Classen
dhofar 908
click on photo for a magnified view

Three Lithologies:
left: IMB clast-poor lithology
top: mature regolith w/ dark matrix
bottom: IMB clast-rich lithology
Photo courtesy of Stephan Kambach

For additional information on the magnesium-rich granulites, read the PSRD article by Linda Martel—‘Unraveling the Origin of the Lunar Highlands Crust’, Sept. 2010.

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