New Geochemical Constraints on Pairing of the Dhofar 961 Clan of Lunar Meteorites

Compositionally and petrologically, Dhofar 961, a glassy-matrix regolith breccia, is unique among lunar meteorites (Fig. 1) and distinct from any Apollo sample. In a recent paper [1], we make two observations. (1) If the ~65 known lunar meteorites represent even as few as only 30 source craters on the Moon, then there is still an 80% chance that at least one is from the South PoleAitken basin. (2) Among lunar meteorites, Dhofar 961 is the most likely SPA candidate on the basis of its mineralogy and unusual composition [1,2]. Dhofar stones 925 (49 g), 960 (35 g), and 961 (22 g) were reportedly found together at a point distant from other lunar meteorite stones and have been regarded as mutually paired on the basis of field location, texture, clast population, and mineral chemistry [3–5]. Our compositional data for 9 small samples (239 mg total mass) of Dhofar 961 scatter greatly ([1], Fig. 2). Nevertheless, our 2 samples of Dhofar 925 are substantially more feldspathic and poorer in incompatible elements than all but one of the Dhofar 961 samples. This difference led us to question whether the two stones were actually paired [1]. To confuse the issue further, Dhofar 925 is compositionally indistinguishable from and texturally similar to SaU 449, reportedly found 212 km away. We have subsequently obtained new bulk compositional data on a small sample (16 mg) from a different slice of Dhofar 961 as well as data for 6 samples (138 mg total mass) of previously unanalyzed Dhofar 960. The Dhofar 960 samples mostly overlap the fields of Dhofar 925 and SaU 449 (Figs. 1,2). The new sample of Dhofar 961 lies outside the compositional range of the first 9 samples but is clearly more similar to the other Dhofar 961 samples than it is to samples of the other 3 stones (Fig. 2). In a companion abstract we report on the petrography of the four stones [6]. On the basis of that work and the new compositional data we conclude that Dhofar 925, 960, and 961 are all stones from a common fall and together they represent a regolith consisting of a variety lithologies of widely varying composition, a regolith that is heterogeneous at millimeter to centimeter scale. The differences in composition between Dhofar 961 and the other stones occurs because Dhofar 961 contains a high (~50%) proportion of an IMB (impact-melt breccia) that is both mafic and moderately rich in incompatible elements. This component is uncommon in Dhofar 925, Dhofar 960, and SaU 449. The large degree of scatter among the Dhofar 961 samples (Fig. 2) is caused by the large compositional diversity and coarse grain size of the clastic components – basalt, magnesian granulites, anorthositic granulites, and at least two kinds of impact-melt breccia [6]. The mafic IMB component of Dhofar 961 is only moderately rich in incompatible elements compared to otherwise similar breccias in the Apollo collection and does not contain a high-Sm KREEP norite component ([1], Fig. 2b). This characteristic argues that Dhofar 961 is not from the Procellarum KREEP Terrane [1].