Petrography, geochronology and source terrain characteristics of lunar meteorites Dhofar 925, 961 and Sayh al Uhaymir 449
Introduction
The Moon is a witness plate to Solar System processes and preserves a record of the geological evolution of small planetary bodies (NRC, 2007). Manned and unmanned missions to the Moon returned ∼382 kg of lunar rocks and soils (Vaniman et al., 1991). These were collected from within and around the nearside Procellarum KREEP Terrane (PKT) by the Apollo missions, and from equatorial latitudes on the eastern limb by the Luna missions. Therefore, interpretations of the Moon’s past have mostly been derived from a geographically restricted dataset on the lunar nearside. Lunar meteorites, which are sourced from potentially anywhere on the Moon’s surface, however, provide a better global representation of the geological and chronological history of the Moon (Korotev, 2005, Joy and Arai, 2013). To date, there have been ∼185 individual (named) lunar meteorites collected on Earth as hot and cold desert finds. These originated from perhaps as few as 40–50 source craters on the Moon (Basilevsky et al., 2010). Radiogenic isotope studies indicate that the majority of known lunar meteorites have been launched from the Moon in the last 10 Myr, and all have been launched in the last 20 Myr probably from small craters only a few kilometres or less in diameter (Warren, 1994, Head et al., 2002).
Remote sensing datasets provide information about the chemical and mineralogical diversity of the lunar surface (i.e., Lunar Prospector and the Kaguya gamma-ray spectrometer chemical data, Clementine, Chandrayaan-1 Moon Mineralogy Mapper, and the Kaguya Spectral Profiler spectral datasets). Although the spatial scales of these mapping efforts are often on the scale of hundreds of metres to tens of kilometre (depending on the method), many previous studies have used these datasets to test chemical and mineralogical similarities with lunar meteorite and infer potential source regions. For example, feldspathic lunar meteorites (i.e., samples with bulk rock FeO <7 wt%: Korotev et al., 2009) have been linked to origins in the highlands on the farside of the Moon (Palme et al., 1991, Korotev et al., 2003, Warren, 2005, Warren et al., 2005, Nyquist et al., 2006, Takeda et al., 2006, Arai et al., 2008, Yamaguchi et al., 2010, Joy et al., 2010a, Fritz, 2012). Basaltic meteorites (i.e., bulk rock FeO >17 wt%) have been linked to different mare basalt lava flow units predominantly on the nearside of the Moon (Joy et al., 2008, Fernandes et al., 2009, Arai et al., 2010, Robinson et al., 2012). Meteorites of intermediate-Fe composition (i.e., with bulk compositions between 7 and 17 FeO wt%) and with high concentrations of thorium (>2 ppm) and other incompatible trace elements (ITEs) have previously been linked with high-Th regoliths on the nearside of the Moon in the Procellarum KREEP Terrane (Gnos et al., 2004, Joy et al., 2011a), and tentatively with the South Pole-Aitken (SPA) impact basin on the farside of the Moon (Hill and Boynton, 2003, Korotev et al., 2007, Mercer et al., 2013).
Lunar meteorites Dhofar (Dho) 925 and 961 and Sayh al Uhaymir (SaU) 449 are breccias of intermediate-Fe composition (Demidova et al., 2005, Demidova et al., 2007, Korotev et al., 2009, Korotev, 2012). Henceforth, this group of stones will be collectively referred to here as the Dhofar group. They were collected in Oman and are thought to have originated in the same meteorite fall, and are also grouped with the Dho 960 stone (Demidova et al., 2005, Demidova et al., 2007, Korotev et al., 2010, Korotev, 2012). All stones are formally classified as impact melt breccias (Russell et al., 2004, Russell et al., 2005, Connolly et al., 2007). The meteorites have elevated concentrations of Th (1–3 ppm: Table 1) compared with many other intermediate-Fe brecciated lunar meteorites, indicating inclusion of an ITE-rich component. Previous studies of Dho 961 (e.g., Jolliff et al., 2007, Jolliff et al., 2008, Jolliff et al., 2009, Korotev et al., 2007, Korotev et al., 2009, Korotev et al., 2010, Zeigler et al., 2010a, Zeigler et al., 2010b, Zeigler et al., 2013) report that the bulk rock composition is not consistent with Apollo samples sourced from the Procellarum KREEP Terrane. Zeigler et al. (2013 and Refs. therein) argue that the meteorite may have originated from the South Pole-Aitken basin, which is the other notable Th-rich (i.e., ITE-rich) region of the Moon (Jolliff, 1998).
Here we report the composition, mineralogy and chronology of the Dhofar group of meteorites to investigate their geological history, and test the hypothesis that the samples represent South Pole-Aitken basin material. A launch locality in SPA would be significant, as geological samples from this massive impact basin are expected to hold the answer to several key lunar science questions (NRC, 2007, Jolliff et al., 2010) including: (i) the age of the basin, which is believed to be the largest and one of the oldest impact basins on the Moon (Wilhelms et al., 1987, Spudis, 1993). Defining its age will help to constrain the early lunar bombardment record, which may help to anchor the early Earth-Moon impact flux chronology (NRC, 2007, Norman, 2009); (ii) determine the extent and nature of products of the Moon’s differentiation by studying igneous rock samples from the lunar farside (e.g., Arai et al., 2008, Ohtake et al., 2012, Gross et al., 2014); (iii) characterise products of impact melt sheet differentiation (e.g., Vaughan et al., 2012, Vaughan and Head, 2014, Hurwitz and Kring, 2014); (iv) determine the composition and timing of farside mare volcanism to shed light on the magmatic history of the Moon (e.g., Hagerty et al., 2011); (v) directly sample lunar mantle material, which may have been excavated during the SPA basin-forming event (Pieters et al., 1997, Yamamoto et al., 2010, Potter et al., 2012), helping to characterise the stratification of the mantle and address models of lunar differentiation and evolution (Elardo et al., 2011, Elkins-Tanton et al., 2011).
Section snippets
Samples and method
We obtained three authenticated meteorite chips (EA1.1 to EA1.3) of Dho 925 (0.136 g), Dho 961 (0.331 g) and SaU 449 (0.764 g). Two 100 μm thick sections (named Dho 925,1, Dho 925,2 and Dho 961,1 and Dho 961,2) and a 30 μm thin section (named Dho 925,3 and Dho 961,3) were prepared from each of the Dho 925 and 961 stones using Buehler Epo-Thin resin. The SaU 449 sample was split into two chips, and the larger portion (0.535 g) was prepared as two 100 μm thick sections (named SaU 449,2 and SaU 449,3)
Dhofar 925
The Dho 925,1 section is ∼7 × 6 mm and is a clast-bearing dark-grey glassy impact melt breccia (Fig. 1a and EA1.1). Clasts range from small (<10 μm) mineral and glass fragments up to 2 mm lithics, including magnesian granulites, basalts (quenched variolitic and ophitic texture), crystalline impact melt breccias and clast-bearing glassy melt breccias, lithic breccias, and rare Si-K-feldspar assemblages. The sample is cross-cut with fractures that are filled with terrestrially deposited minerals
Chronology results
Pb–Pb and U–Pb isotope data was collected from 38 phosphate grains (apatite and merrillite) in Dho 961,1 (Table 2, EA1.10 and EA1.11). Some of these were measured in discrete grains with no to little petrographic context (i.e., they occur as mineral fragments in the Dho 961,1 matrix: EA1.11), and others were in crystalline impact melt breccias (EA1.10 and EA1.11) and an equilibrated granulite (EA1.11). The U–Pb data (Table 2), both uncorrected for initial Pb, and when corrected using the Stacey
Remote sensing data and potential source regions
It may be possible to constrain the source region of the meteorites using remote sensing geochemical datasets (see approach of Jolliff et al., 2009, Corrigan et al., 2009). We searched the Lunar Prospector gamma-ray spectrometer datasets using a method similar to Joy et al. (2010a, 2011a) and Mercer et al. (2013), assuming that meteorites were derived from a compositionally homogeneous terrane exposed on the scale of individual pixels. We used the bulk FeO and Th and TiO2 composition (Table 1)
Are the meteorites from the South Pole-Aitken basin?
It has been argued (see Zeigler et al., 2013 and Refs. therein) that the Dhofar group originated from a moderately Fe- and Th-rich region of the Moon that is not within the Procellarum KREEP Terrane. An alternative source region has been suggested to be the South Pole-Aitken basin (Jolliff et al., 2007, Jolliff et al., 2008, Jolliff et al., 2009, Korotev et al., 2007, Korotev et al., 2009, Korotev et al., 2010, Zeigler et al., 2010a, Zeigler et al., 2010b, Zeigler et al., 2013). We weigh up the
Conclusions
The Dho 925, 961 and SaU 449 samples are lithologically diverse (Fig. 1). The group represent products of impact cratering event(s), which affected several types of target rocks and mixed them together as an impact melt breccia. Collectively, the breccias contain fragments of at least 4 different mare basalt textural types from very low-Ti and low-Ti lavas; two main impact melt breccia types including crystalline and clast-bearing glassy, which both have several different sub-varieties; two
Acknowledgements
This research was funded by NASA Lunar Science Institute contract NNA09DB33A, David A. Kring PI. KHJ acknowledges funding from the Leverhulme Trust, UK (grant 2011-569). We thank David Mann for sample preparation. We acknowledge the resources of Dr. Randy Korotev’s Lunar Meteorite List, and NASA’s Apollo and Lunar Meteorite sample compendium. We thank Drs. Axel Wittmann, Tomoko Arai and Romain Tartèse for thoughtful reviews, and Dr. Marc Norman for his AE handling and suggestions which greatly
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