Petrography and Geochemistry of Dhofar 733 – an Unusually Sodic, Feldspathic Lunar

Introduction: Dhofar 733 is a 98-g lunar meteorite collected in the Dhofar region of Oman in 2002 [1]. In the initial description [1], Dhofar 733 is described as an anorthositic granulitic breccia, one that is probably not paired with any other lunar meteorite on the basis of its unique texture and isolation from other lunar meteorites (the next closest lunar meteorite was found 22 km away). The brownish gray stone does not have a fusion crust and is “moderately weathered.” Terrestrial alteration minerals include gypsum, smectite, and Fe hydroxides [1]. Methods: We obtained from a meteorite dealer a 32mg sample of Dhofar 733 (0.1× the mass we would have preferred, but we have not been able to obtain more). We divided the sample into two splits for INAA (instrumental neutron activation analysis, Table 1). The compositions of both splits are similar (e.g., 3.28 and 3.25% FeO). After INAA, we fused one split into a glass bead and determined major-element concentrations by EPMA (electron probe microanalysis, Table 1). We prepared a polished thick section of the other split and determined mineral compositions via EPMA (Table 2). Plagioclase was analyzed with a 10–20-μm beam whereas mafic minerals were analyzed with a 1μm beam. Results: With 29.5% Al2O3, Dhofar 733 is one of the most feldspathic of all lunar meteorites [3,4]. Our sample of Dhofar 733 contains a plagioclase matrix (87% normatively, 84% modally) with interstitial granoblastic grains of olivine (10 vol% both by norm and mode). Poikiloblastic low and high-Ca pyroxene (2.5% norm (vol%), 1.5% modal) and minor amounts (0.15%) of the accessory minerals armalcolite, chromite, rutile, ilmenite, and Ca phosphate (Fig. 1). Olivine textures are commonly anhedral and equant, with boundaries forming smooth curves approaching 120° triple junctions, consistent with the classification of feldspathic granulitic impactites [4–7] (Fig. 1). The plagioclase has a relatively sodic (for the Moon) average composition of An93, with a small amount of compositional zoning observed (An91–96). The olivine (Fo70–77) and orthopyroxene (Wo4En78) are both magnesian and relatively well equilibrated. Alteration comprises some 4 vol.% of the section Compositionally, Dhofar 733 is unusual among feldspathic lunar meteorites, as well as Apollo samples, in being magnesian (bulk molar Mg/[Mg + Fe] = 74%) and having high (again, by lunar highlands standards) concentrations of Na and Eu (Fig. 2). Some samples from Apollo 16 (mainly from North Ray Crater) are similarly sodic, but have ferroan-anorthositic suite affinities [8]. The whole-rock Na2O concentration of Dhofar 733 requires plagioclase with a mean composition of An94 (Fig. 2), similar to the average plagioclase composition obtained via EPMA, An93. Thus, the plagioclase in Dhofar 733 is about twice as sodic, on average, as that in most feldspathic lunar meteorites (An96–97) and the Eu concentration is also about twice as great (Fig. 2). Discussion: Dhofar 733 must have formed at depths well below the regolith in order to account for the low concentrations of siderophile and incompatible elements [3], the high degree of equilibration witnessed among mineral phases, and the granulitic textures. These textures are characteristic of post-formation high-temperature metamorphism [4–7]. It has been noted by [4] that homogeneity of mineral compositions within feldspathic granulitic impactites is typically on the scale of a few Table 1. Bulk composition of Dhofar 733. SiO2 44.3 Sc 3.89 Eu 1.63 TiO2 0.27 Co 9.9 Tb 0.109 Al2O3 29.5 Ni 35 Yb 0.360 Cr2O3 0.05 Sr 410 Lu 0.050 FeO 3.27 Zr 10 Hf 0.34 MnO 0.05 Ba 90 Ta 0.060 MgO 5.37 La 1.58 Ir 1.3 CaO 16.2 Ce 3.66 Au 5.0 Na2O 0.70 Nd 2.3 Th 0.104 K2O 0.04 Sm 0.580 U 0.14 P2O5 0.02 sum 99.77 Oxides in wt%, others in μg/g, except Au and Ir in ng/g.