Elsevier

Geochimica et Cosmochimica Acta

Volume 144, 1 November 2014, Pages 299-325
Geochimica et Cosmochimica Acta

Petrography, geochronology and source terrain characteristics of lunar meteorites Dhofar 925, 961 and Sayh al Uhaymir 449

https://doi.org/10.1016/j.gca.2014.08.013Get rights and content

Abstract

Dhofar (Dho) 925, 961 and Sayh al Uhaymir (SaU) 449 are brecciated lunar meteorites consisting of mineral fragments and clasts from a range of precursor lithologies including magnesian anorthositic gabbronorite granulites; crystalline impact melt breccias; clast-bearing glassy impact melt breccias; lithic (fragmental) breccias; mare basalts; and evolved (silica-rich) rocks. On the similarity of clast type and mineral chemistry the samples are likely grouped, and were part of the same parent meteorite. Phosphate Pb–Pb ages in impact melt breccias and matrix grains demonstrate that Dho 961 records geological events spanning ∼500 Ma between 4.35 and 3.89 Ga. These Pb–Pb ages are similar to the ages of ‘ancient’ intrusive magmatic samples and impact basin melt products collected on the lunar nearside by the Apollo missions. However, the samples’ bulk rock composition is chemically distinct from these types of samples, and it has been suggested that they may have originated from the farside South Pole-Aitken impact basin (i.e., Jolliff et al., 2008). We test this hypothesis, and conclude that although it is possible that the samples may be from the South Pole-Aitken basin, there are other regions on the Moon that may have also sourced these complex breccias.

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

References (132)

  • J. Gross et al.

    Lunar feldspathic meteorites: constraints on the geology of the lunar farside highlands, and the origin of the lunar crust

    Earth Planet. Sci. Lett.

    (2014)
  • J.A. Hudgins et al.

    Mineralogy, geochemistry, and 40Ar–39Ar geochronology of lunar granulitic breccia Northwest Africa 3163 and paired stones: comparisons with Apollo samples

    Geochim. Cosmochim. Acta

    (2011)
  • K.H. Joy et al.

    The petrogenesis of miller range 05035: a new lunar gabbroic meteorite

    Geochim. Cosmochim. Acta

    (2008)
  • K.H. Joy et al.

    Petrogenesis and chronology of lunar meteorite Northwest Africa 4472

    Geochim. Cosmochim. Acta

    (2011)
  • R.L. Korotev

    Compositional variation in the Apollo 16 impact-melt breccias and inferences for the geology and bombardment history of the Central Highlands of the Moon

    Geochim. Cosmochim. Acta

    (1994)
  • R.L. Korotev

    Lunar geochemistry as told by lunar meteorites

    Chem. Erde

    (2005)
  • R.L. Korotev et al.

    Feldspathic lunar meteorites and their implications for compositional remote sensing of the lunar surface and the composition of the lunar crust

    Geochim. Cosmochim. Acta

    (2003)
  • D. Liu et al.

    Comparative zircon U–Pb geochronology of impact melt breccias from Apollo 12 and lunar meteorite SaU 169, and the age of the Imbrium impact

    Earth Planet. Sci. Lett.

    (2012)
  • J. Longhi et al.

    The pattern of Ni and Co abundances in lunar olivines

    Geochim. Cosmochim. Acta

    (2010)
  • F.M. McCubbin et al.

    Fluorine and chlorine abundances in lunar apatite: implications for heterogeneous distributions of magmatic volatiles in the lunar interior

    Geochim. Cosmochim. Acta

    (2011)
  • A. Morbidelli et al.

    A sawtooth-like timeline for the first billion years of lunar bombardment

    Earth Planet. Sci. Lett.

    (2012)
  • A.A. Nemchin et al.

    SIMS U–Pb study of zircon from Apollo 14 and 17 breccias: implications for the evolution of lunar KREEP

    Geochim. Cosmochim. Acta

    (2008)
  • M.D. Norman et al.

    A 4.2 billion year old impact basin on the Moon: U–Pb dating of zirconolite and apatite in lunar melt rock 67955

    Earth Planet. Sci. Lett.

    (2014)
  • L. Nyquist et al.

    Feldspathic clasts in Yamato-86032: remnants of the lunar crust with implications for its formation and impact history

    Geochim. Cosmochim. Acta

    (2006)
  • H. Palme et al.

    Lunar highland meteorites and the composition of the lunar crust

    Geochim. Cosmochim. Acta

    (1991)
  • J.J. Papike et al.

    Ion microprobe investigation of plagioclase and orthopyroxene from lunar Mg-suite norites: implications for calculating parental melt REE concentrations and for assessing postcrystallization REE redistribution

    Geochim. Cosmochim. Acta

    (1996)
  • J.J. Papike et al.

    Evolution of the lunar crust: SIMS study of plagioclase from ferroan anorthosites

    Geochim. Cosmochim. Acta

    (1997)
  • J.J. Papike et al.

    Silicate mineralogy of martian meteorites

    Geochim. Cosmochim. Acta

    (2009)
  • R.W.K. Potter et al.

    Constraining the size of the South Pole-Aitken basin impact

    Icarus

    (2012)
  • G. Ryder et al.

    The complex stratigraphy of the highland crust in the Serenitatis region of the Moon inferred from mineral fragment chemistry

    Geochim. Cosmochim. Acta

    (1997)
  • D.W. Schnare et al.

    A laser-ablation ICP-MS study of Apollo 15 low-titanium olivine-normative and quartz-normative mare basalts

    Geochim. Cosmochim. Acta

    (2008)
  • C.K. Shearer et al.

    Early crustal building processes on the moon: models for the petrogenesis of the magnesian suite

    Geochim. Cosmochim. Acta

    (2005)
  • C.K. Shearer et al.

    Origin of sulfide replacement textures in lunar breccias. Implications for vapor element transport in the lunar crust

    Geochim. Cosmochim. Acta

    (2012)
  • J.W. Shervais et al.

    Ion and electron microprobe study of troctolites, norite, and anorthosites from Apollo 14: evidence for urKREEP assimilation during petrogenesis of Apollo 14 Mg-suite rocks

    Geochim. Cosmochim. Acta

    (1998)
  • T. Arai et al.

    Four lunar meteorites: crystallization trends of pyroxenes and spinels

    Meteorit. Planet. Sci.

    (1996)
  • T. Arai et al.

    A new model of lunar crust: asymmetry in crustal composition and evolution

    Earth Planets Space

    (2008)
  • J. Berlin et al.

    Fe-Mn systematics of type IIA chondrules in unequilibrated CO, CR, and ordinary chondrites

    Meteorit. Planet. Sci.

    (2011)
  • M.G. Bersch et al.

    Mineral compositions in pristine lunar highland rocks and the diversity of highland magmatism

    Geophys. Res. Lett.

    (1991)
  • D. Bogard

    Impact ages of meteorites: a synthesis

    Meteoritics

    (1995)
  • H.C. Connolly et al.

    The meteoritical bulletin, No. 93, 2008 March

    Meteorit. Planet. Sci.

    (2007)
  • Corrigan C. M., Dombard A. J., Spudis P. D., Bussey D. B. J. and McCoy T. J. (2009) Candidate Source Regions for the...
  • Demidova S. I., Nazarov M. A., Kurat G., Brandstätter F. and Ntaflos T. (2005) New lunar meteorites from Oman: Dhofar...
  • S.I. Demidova et al.

    Chemical composition of lunar meteorites and the lunar crust

    Petrology

    (2007)
  • V.A. Fernandes et al.

    40Ar–39Ar age determinations of lunar basalt meteorites Asuka 881757, Yamato 793169, Miller Range 05035, LaPaz Icefield 02205, Northwest Africa 479, and basaltic breccia Elephant Moraine 96008

    Meteorit. Planet. Sci.

    (2009)
  • V.A. Fernandes et al.

    The bombardment history of the Moon as recorded by 40Ar–39Ar chronology

    Meteorit. Planet. Sci.

    (2013)
  • Friel J. J., and Goldstein J. I. (1977) The relationship between lunar metal particles and phosphate minerals. Lunar...
  • Garrick-Bethell I., Fernandes V. A., Weiss B. P. and Shuster D. L. (2008) 4.2 billion year old ages from Apollo 16, 17,...
  • E. Gnos et al.

    Pinpointing the source of a lunar meteorite: implications for the evolution of the Moon

    Science

    (2004)
  • J.J. Hagerty et al.

    Thorium abundances of basalt ponds in South Pole-Aitken basin: insights into the composition and evolution of the far side lunar mantle

    J. Geophys. Res.

    (2011)
  • S.E. Haggerty

    Luna 20: mineral chemistry of spinel, pleonaste, chromite, ulvöspinel, ilmenite and rutile

    Geochim. Cosmochim. Acta

    (1973)
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