Possible mantle origin of olivine around lunar impact basins detected ...
LETTERS PUBLISHED ONLINE: 4 JULY 2010 | DOI: 10.1038/NGEO897
Possible mantle origin of olivine around lunar impact basins detected by SELENE Satoru Yamamoto1 *, Ryosuke Nakamura2 , Tsuneo Matsunaga1 , Yoshiko Ogawa3 , Yoshiaki Ishihara4 , Tomokatsu Morota5 , Naru Hirata3 , Makiko Ohtake5 , Takahiro Hiroi6 , Yasuhiro Yokota1 and Junichi Haruyama5 90
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The composition, structure and evolution of the Moon’s mantle is poorly constrained. The mineral olivine, one of the main constituents of Earth’s mantle, has been identified by Earth-based telescopic observations at two craters on the near side of the Moon, Aristarchus and Copernicus1–3 . Global reflectance spectra in five discrete spectral bands produced by the spacecraft Clementine4–6 suggested several possible olivine-bearing sites, but one of the candidate occurrences of olivine was later re-classified, on the basis of continuous reflectance spectra over the entire 1 µm band, as a mixture of plagioclase and pyroxene7 . Here we present a global survey of the lunar surface using the Spectral Profiler onboard the lunar explorer SELENE/Kaguya7,8 . We found many exposures of olivine on the Moon, located in concentric regions around the South Pole-Aitken, Imbrium and Moscoviense impact basins where the crust is relatively thin. We propose that these exposures of olivine can be attributed either to an excavation of the lunar mantle at the time of the impacts that formed the basins3 , or to magnesium-rich pluton in the Moon’s lower crust. On the basis of radiative transfer modelling4,8–10 , we suggest that at least some of the olivine detected near impact basins originates from upper mantle of the Moon. The lunar magma ocean (LMO) scenario proposes fractional crystallization of LMO-produced mafic cumulates that made up the mantle, and plagioclase floatation that made up the crust11,12 . Several models have been proposed that describe the compositional and structural evolution of a crystallizing magma ocean, but there are still uncertainties in the composition and structure. One of the reasons for the uncertainties is the lack of information on olivine exposure on the Moon, a plausible main material for the lunar mantle. Earth-based telescopic observations have reported only two nearside craters, Copernicus and Aristarchus, having olivine-rich spectral features1–3 . Although Earth-based observations produce continuous reflectance spectra, the observational points are sparse and limited to the lunar nearside. On the other hand, the UVVIS camera onboard the Clementine spacecraft (hereafter Clementine), which had five discrete bands, provided global data of the Moon4–6 . Olivine Hill in the South Pole-Aitken (SPA) basin and the central peaks of five craters were identified as possible olivine-bearing sites by Clementine5,6 . However, after a re-examination using data taken by the Spectral Profiler (SP) onboard the Japanese explorer Kaguya, one of the Clementine candidates, the Tsiolkovsky crater,
Figure 1 | Global distribution of olivine-rich points on the Moon. The background map is the total lunar crustal thickness (crustal materials and mare basalt fills) based on SELENE gravity and a topographic model produced by the Kaguya explorer13,28–30 . The red squares indicate olivine-rich points with multiple SP data points showing a clear olivine spectral signature. The small red crosses indicate single SP detections. Note that most of the olivine-rich points are distributed around impact basins. The SP successfully detected olivine at the Copernicus (C1) and Aristarchus (C6) craters, which were identified as olivine-bearing areas by Earth-based observation1–3 .
was classified as a mixture of plagioclase and pyroxene, rather than as pure olivine7 . This SP finding demonstrated the importance of obtaining continuous reflectance spectra over the visible and near-infrared range covering the entire 1 µm band, which can be used to as a diagnostic tool for olivine and other silicates in identifying olivine exposure sites on the Moon. The SP has obtained continuous spectral reflectance data for about seventy million points (a 0.2–0.5 km by 0.5 km footprint) on the Moon over the 0.5–2.6 µm wavelength range (λ) with a spectral resolution of 6–8 nm during its mission period from November 2007 to June 2009 (refs 7,8). Analysing all of the spectral data, we identified 245 olivine-rich points by picking up spectra having absorption band minima within the wavelength range of λ = 1.05 ± 0.03 µm after removing a linear tangential continuum. Most of the spectra for the selected points (hereafter referred to as olivine-rich points) show clear olivine bands with λ = 0.85, 1.05 and 1.25 µm as shown in Supplementary Figs S1–S3, although some of the spectra show less clear olivine bands, which may be due to the presence of minor amounts of high-Ca pyroxene or other geologic units in the SP field of view.
1 Center
for Global Environmental Research, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan, 2 Information Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezato, Tsukuba, Ibaraki 305-8568, Japan, 3 ARC-Space/CAIST, The University of Aizu, Ikki-machi, Aizuwakamatsu, Fukushima 965-8580, Japan, 4 RISE project, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan, 5 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan, 6 Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA. *e-mail: [email protected].
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© 2010 Macmillan Publishers Limited. All rights reserved.
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NATURE GEOSCIENCE DOI: 10.1038/NGEO897
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Figure 2 | Local distribution of olivine-rich sites around various basins or maria. The background map is the surface topography obtained by the Kaguya mission28 . The larger rectangles and smaller circles indicate olivine-rich points with multiple and single SP detections, respectively. Photos A1, B2, C1, D1, E1 and F1 are close-up images of olivine-rich sites taken by the MI or the TC onboard Kaguya14,15 . On the close-up images, olivine-rich points are plotted as red rectangles with white 5 km scale bars. The accompanying plots show the continuum-removed reflectance spectra Rc at the location of the yellow square marked on each close-up image.
In Fig. 1 we plot 245 olivine-rich points on a lunar crustal thickness map obtained by Kaguya13 . Most of the olivine-rich points are grouped into several local sites. For example, SP detected 59 in the Copernicus crater and 4 in the Aristarchus crater. Taking into account the local geologic context, based on images obtained by Kaguya’s Multiband Imager (MI) or Terrain Camera (TC) during the SP observation14,15 , we found that most of the localities having multiple olivine-rich points are associated with small fresh, craters (Fig. 2). Therefore, we divided and assigned the 245 olivine-rich points to 34 olivine-rich sites (Supplementary Table S1). The representative spectra for the individual olivine-rich sites are shown in Supplementary Figs S1–S3. Figure 1 shows that most of the olivine-rich sites are located around impact basins: that is, (A) Mare Moscoviense, (B) Crisium, (C) Imbrium, (D) Humorum, the SPA basin ((E) Schrödinger and (M) Zeeman craters), (G) Nectaris, (H) Serenitatis, (I) Humboldtianum and (J) Australe. These basins are located on 534
thinner crusts with a thickness of about 30–50 km. Most of the olivine-rich sites are concentrated on the lunar nearside. Whereas there is no olivine-rich site in the Feldspathic Highlands Terrane16 , olivine-rich sites are found in the SPA and Moscoviense, on the far side, in locations where the crust is thin. Furthermore, in the vicinity of each basin, olivine-rich sites are distributed along the concentric region of the basin. For example, Fig. 2a shows that olivine-rich sites are distributed along the rim of Moscoviense, whereas there is no olivine-rich site in the central region of the mare or regions far from the outer ring. Another conspicuous example is Crisium, where the olivine-rich sites are limited to a narrow concentric region around the mare (Fig. 2b). Around Imbrium (Fig. 2c) we found olivine-rich sites in the Copernicus (C1), Eratosthenes (C3), Aristarchus (C6), the Montes Alpes (C2) and the terrace in the Sinus Iridum (C4 and C5). Their locations seem to correspond to the prominent rings of Imbrium17 . In the SPA (Fig. 2e), there are two craters with olivine-rich sites
NATURE GEOSCIENCE | VOL 3 | AUGUST 2010 | www.nature.com/naturegeoscience © 2010 Macmillan Publishers Limited. All rights reserved.
NATURE GEOSCIENCE DOI: 10.1038/NGEO897 1.00
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Figure 3 | Colour-composite image maps of olivine-rich sites D1 and E1 taken by the MI. (See ref. 14 for more detail on the MI images.) Blue, green and red are assigned to reflectances of 900, 1,050 and 1,250 nm, respectively. The continuum-removed reflectance spectra Rc and reflectance factor9 (REFF) at the six locations marked A–F in the images are also plotted. The locations of A and E show olivine-rich spectra. All reflectance spectra are given as the average of a 500 m × 500 m area to remove spatial variation. Saturated data areas are masked (black filled areas).
(Schrödinger and Zeeman). They are located near the edge of the SPA, whereas there is no olivine-rich site in its central region. Although the number of olivine-rich sites was limited, the same distribution pattern was observed at other basins (Fig. 2d–i). At each olivine-rich site, most of the olivine exposure was detected at several consecutive SP footprints. This indicates that the olivine-rich exposures extend over several footprint sizes spanning several kilometres. They are found on crater walls (for example, B2, D1 and E1) and on continuous ejecta (for example, A1 and F1). Figure 3 shows the MI images for sites D1 and E1, where the olivine-rich spectra appear in the landslide features on the crater wall. At site E1, there is also an area that has a clear plagioclase spectrum showing a strong 1.25 µm band on the crater wall (marked ‘F’). On the other hand, spectral features for areas outside olivine (or plagioclase) exposures are too unclear to allow correct interpretation of their mineral compositions. This is because most of the lunar surface is covered with mixtures of various minerals. Space weathering also obscures spectral features. The olivine-rich exposures, however, are found in fresh areas such as landslide features on crater walls or recently formed craters (for example, F1). Figure 1 does not includes the Olivine Hill, Langrenus, Keeler, Crookes and Tsiolkovsky craters, which were suggested as olivinebearing areas by Clementine5,6 . This is because the Clementine analysis was based on discrete spectral data with a limited wavelength coverage of λ ≤ 1 µm, whereas the SP has continuous spectral data with λ = 0.5–1.6 µm (ref. 7). However, the Theophilus suggested by Clementine is identified as an olivine-rich site by the SP. Figure 2g shows that this crater is located in the concentric region around Nectaris. In summary, olivine exposures on the Moon are limited to concentric regions around the impact basins that have thinner crusts. On a local scale, they are found mainly on small, fresh crater walls or continuous ejecta. What mechanism produced this distribution? We propose that basin formation is responsible for the observed distribution of the olivine exposures. Each basin formation could have blasted away the upper crust, excavating and redistributing deep-seated olivine-rich material to the rim. Whereas the central region of the basin would be covered with basaltic lava that erupted later in the cases of nearside basins and Moscoviense, the rim region would not. For the SPA the impact resulted in the production of a large amount of melted material, which puddled on the floor of the excavated cavity as a melt sheet. Local differentiation occurred in these melt layers, forming an orthopyroxene layer that overlies the olivine-rich layer18 . Indeed, a recent SP survey8 revealed the existence of an extensive layer of differentiated orthopyroxene in the central region of the SPA; the central peaks of the Finsen, Antoniadi, Bhabha and Lyman craters show clear orthopyroxene spectra. Thus,
the deep-seated olivine-rich layers in the central region of the SPA would be hidden by the differentiated impact melt. Although olivine in the rim regions would have been covered with ejecta from the surroundings, later impacts could have excavated the olivine, exposing it to the surface. As a result, olivine-rich sites are observed only at fresh craters in the concentric regions around large basins. Although most main thin-crust basins (for example, Moscoviense, Crisium, Humboldtianum) have olivine-rich sites, some basins (for example, Mare Smythii) do not. This may be due to the incomplete coverage of our survey. Some basins located in thin-crust regions may have olivine exposures in their concentric regions that the SP survey did not discover. Where did the olivine-rich material originate? This is an important question for increasing our understanding of the structure and evolution of the Moon. Here we propose two possible scenarios. The first scenario is that the olivine-rich exposures originated in the upper lunar mantle. The basins with olivine-rich sites are located only in regions where the crust is relatively thin (Fig. 1). For example, if the general impact cratering theory19 is applied to the mare Crisium (∼1,000 km diameter), the depth of the excavation is >∼ 100 km. The original crust thickness at Crisium could have been thinner than the maximum thickness of the current feldspathic crust, which is about 100 km (Fig. 1). Thus, basin formation impacts could plausibly have penetrated to the crust–mantle boundary. The second scenario is that the olivine-rich exposure originates from the mafic-rich lower crust. In other words, the basin formations excavated the Mg-rich pluton intruding into the lunar lower crust20–22 . Note that some of the olivine-rich sites are associated with plagioclase; the Schrödinger and Aristarchus craters were reported as the purest-anorthosite-bearing regions14 . In addition, site E1 in the Schrödinger crater (Fig. 3) has areas that exhibit the 1.25 µm plagioclase absorption band adjacent to areas showing olivine-rich spectra. This may suggest that the basin formations excavated intrusions with spatially inhomogeneous plagioclase/olivine ratios in the lower crust, although there is the possibility that the excavation of the crust–mantle boundary resulted in mixtures of mantle olivine and anorthosite during the excavation process. If this scenario is true, the spatial distribution of olivine exposures (Fig. 1) gives important insights into constraints on early lunar basaltic magmatism and crustal growth after the crystallization of the LMO (refs 23,24). Which of the above scenarios is more plausible? If the olivinerich exposure originates from the upper mantle, the composition should be similar to dunite rather than troctolite in the lower lunar crust. To confirm whether this is the case, we examined the spectral data for some of the olivine-rich sites using radiative transfer modelling based on an intimate mixture model4,8–10 (see Supplementary Information). Supplementary Fig. S4 shows that the
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NATURE GEOSCIENCE DOI: 10.1038/NGEO897
LETTERS representative spectra are more consistent with a dunite-dominant model than with troctolite. This is mainly because most of the spectral data for the olivine-rich sites in Supplementary Fig. S4 have lower absolute reflectance (
Possible mantle origin of olivine around lunar impact basins detected by SELENE Satoru Yamamoto1 *, Ryosuke Nakamura2 , Tsuneo Matsunaga1 , Yoshiko Ogawa3 , Yoshiaki Ishihara4 , Tomokatsu Morota5 , Naru Hirata3 , Makiko Ohtake5 , Takahiro Hiroi6 , Yasuhiro Yokota1 and Junichi Haruyama5 90
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The composition, structure and evolution of the Moon’s mantle is poorly constrained. The mineral olivine, one of the main constituents of Earth’s mantle, has been identified by Earth-based telescopic observations at two craters on the near side of the Moon, Aristarchus and Copernicus1–3 . Global reflectance spectra in five discrete spectral bands produced by the spacecraft Clementine4–6 suggested several possible olivine-bearing sites, but one of the candidate occurrences of olivine was later re-classified, on the basis of continuous reflectance spectra over the entire 1 µm band, as a mixture of plagioclase and pyroxene7 . Here we present a global survey of the lunar surface using the Spectral Profiler onboard the lunar explorer SELENE/Kaguya7,8 . We found many exposures of olivine on the Moon, located in concentric regions around the South Pole-Aitken, Imbrium and Moscoviense impact basins where the crust is relatively thin. We propose that these exposures of olivine can be attributed either to an excavation of the lunar mantle at the time of the impacts that formed the basins3 , or to magnesium-rich pluton in the Moon’s lower crust. On the basis of radiative transfer modelling4,8–10 , we suggest that at least some of the olivine detected near impact basins originates from upper mantle of the Moon. The lunar magma ocean (LMO) scenario proposes fractional crystallization of LMO-produced mafic cumulates that made up the mantle, and plagioclase floatation that made up the crust11,12 . Several models have been proposed that describe the compositional and structural evolution of a crystallizing magma ocean, but there are still uncertainties in the composition and structure. One of the reasons for the uncertainties is the lack of information on olivine exposure on the Moon, a plausible main material for the lunar mantle. Earth-based telescopic observations have reported only two nearside craters, Copernicus and Aristarchus, having olivine-rich spectral features1–3 . Although Earth-based observations produce continuous reflectance spectra, the observational points are sparse and limited to the lunar nearside. On the other hand, the UVVIS camera onboard the Clementine spacecraft (hereafter Clementine), which had five discrete bands, provided global data of the Moon4–6 . Olivine Hill in the South Pole-Aitken (SPA) basin and the central peaks of five craters were identified as possible olivine-bearing sites by Clementine5,6 . However, after a re-examination using data taken by the Spectral Profiler (SP) onboard the Japanese explorer Kaguya, one of the Clementine candidates, the Tsiolkovsky crater,
Figure 1 | Global distribution of olivine-rich points on the Moon. The background map is the total lunar crustal thickness (crustal materials and mare basalt fills) based on SELENE gravity and a topographic model produced by the Kaguya explorer13,28–30 . The red squares indicate olivine-rich points with multiple SP data points showing a clear olivine spectral signature. The small red crosses indicate single SP detections. Note that most of the olivine-rich points are distributed around impact basins. The SP successfully detected olivine at the Copernicus (C1) and Aristarchus (C6) craters, which were identified as olivine-bearing areas by Earth-based observation1–3 .
was classified as a mixture of plagioclase and pyroxene, rather than as pure olivine7 . This SP finding demonstrated the importance of obtaining continuous reflectance spectra over the visible and near-infrared range covering the entire 1 µm band, which can be used to as a diagnostic tool for olivine and other silicates in identifying olivine exposure sites on the Moon. The SP has obtained continuous spectral reflectance data for about seventy million points (a 0.2–0.5 km by 0.5 km footprint) on the Moon over the 0.5–2.6 µm wavelength range (λ) with a spectral resolution of 6–8 nm during its mission period from November 2007 to June 2009 (refs 7,8). Analysing all of the spectral data, we identified 245 olivine-rich points by picking up spectra having absorption band minima within the wavelength range of λ = 1.05 ± 0.03 µm after removing a linear tangential continuum. Most of the spectra for the selected points (hereafter referred to as olivine-rich points) show clear olivine bands with λ = 0.85, 1.05 and 1.25 µm as shown in Supplementary Figs S1–S3, although some of the spectra show less clear olivine bands, which may be due to the presence of minor amounts of high-Ca pyroxene or other geologic units in the SP field of view.
1 Center
for Global Environmental Research, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan, 2 Information Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezato, Tsukuba, Ibaraki 305-8568, Japan, 3 ARC-Space/CAIST, The University of Aizu, Ikki-machi, Aizuwakamatsu, Fukushima 965-8580, Japan, 4 RISE project, National Astronomical Observatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate 023-0861, Japan, 5 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan, 6 Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA. *e-mail: [email protected].
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© 2010 Macmillan Publishers Limited. All rights reserved.
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NATURE GEOSCIENCE DOI: 10.1038/NGEO897
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Figure 2 | Local distribution of olivine-rich sites around various basins or maria. The background map is the surface topography obtained by the Kaguya mission28 . The larger rectangles and smaller circles indicate olivine-rich points with multiple and single SP detections, respectively. Photos A1, B2, C1, D1, E1 and F1 are close-up images of olivine-rich sites taken by the MI or the TC onboard Kaguya14,15 . On the close-up images, olivine-rich points are plotted as red rectangles with white 5 km scale bars. The accompanying plots show the continuum-removed reflectance spectra Rc at the location of the yellow square marked on each close-up image.
In Fig. 1 we plot 245 olivine-rich points on a lunar crustal thickness map obtained by Kaguya13 . Most of the olivine-rich points are grouped into several local sites. For example, SP detected 59 in the Copernicus crater and 4 in the Aristarchus crater. Taking into account the local geologic context, based on images obtained by Kaguya’s Multiband Imager (MI) or Terrain Camera (TC) during the SP observation14,15 , we found that most of the localities having multiple olivine-rich points are associated with small fresh, craters (Fig. 2). Therefore, we divided and assigned the 245 olivine-rich points to 34 olivine-rich sites (Supplementary Table S1). The representative spectra for the individual olivine-rich sites are shown in Supplementary Figs S1–S3. Figure 1 shows that most of the olivine-rich sites are located around impact basins: that is, (A) Mare Moscoviense, (B) Crisium, (C) Imbrium, (D) Humorum, the SPA basin ((E) Schrödinger and (M) Zeeman craters), (G) Nectaris, (H) Serenitatis, (I) Humboldtianum and (J) Australe. These basins are located on 534
thinner crusts with a thickness of about 30–50 km. Most of the olivine-rich sites are concentrated on the lunar nearside. Whereas there is no olivine-rich site in the Feldspathic Highlands Terrane16 , olivine-rich sites are found in the SPA and Moscoviense, on the far side, in locations where the crust is thin. Furthermore, in the vicinity of each basin, olivine-rich sites are distributed along the concentric region of the basin. For example, Fig. 2a shows that olivine-rich sites are distributed along the rim of Moscoviense, whereas there is no olivine-rich site in the central region of the mare or regions far from the outer ring. Another conspicuous example is Crisium, where the olivine-rich sites are limited to a narrow concentric region around the mare (Fig. 2b). Around Imbrium (Fig. 2c) we found olivine-rich sites in the Copernicus (C1), Eratosthenes (C3), Aristarchus (C6), the Montes Alpes (C2) and the terrace in the Sinus Iridum (C4 and C5). Their locations seem to correspond to the prominent rings of Imbrium17 . In the SPA (Fig. 2e), there are two craters with olivine-rich sites
NATURE GEOSCIENCE | VOL 3 | AUGUST 2010 | www.nature.com/naturegeoscience © 2010 Macmillan Publishers Limited. All rights reserved.
NATURE GEOSCIENCE DOI: 10.1038/NGEO897 1.00
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Figure 3 | Colour-composite image maps of olivine-rich sites D1 and E1 taken by the MI. (See ref. 14 for more detail on the MI images.) Blue, green and red are assigned to reflectances of 900, 1,050 and 1,250 nm, respectively. The continuum-removed reflectance spectra Rc and reflectance factor9 (REFF) at the six locations marked A–F in the images are also plotted. The locations of A and E show olivine-rich spectra. All reflectance spectra are given as the average of a 500 m × 500 m area to remove spatial variation. Saturated data areas are masked (black filled areas).
(Schrödinger and Zeeman). They are located near the edge of the SPA, whereas there is no olivine-rich site in its central region. Although the number of olivine-rich sites was limited, the same distribution pattern was observed at other basins (Fig. 2d–i). At each olivine-rich site, most of the olivine exposure was detected at several consecutive SP footprints. This indicates that the olivine-rich exposures extend over several footprint sizes spanning several kilometres. They are found on crater walls (for example, B2, D1 and E1) and on continuous ejecta (for example, A1 and F1). Figure 3 shows the MI images for sites D1 and E1, where the olivine-rich spectra appear in the landslide features on the crater wall. At site E1, there is also an area that has a clear plagioclase spectrum showing a strong 1.25 µm band on the crater wall (marked ‘F’). On the other hand, spectral features for areas outside olivine (or plagioclase) exposures are too unclear to allow correct interpretation of their mineral compositions. This is because most of the lunar surface is covered with mixtures of various minerals. Space weathering also obscures spectral features. The olivine-rich exposures, however, are found in fresh areas such as landslide features on crater walls or recently formed craters (for example, F1). Figure 1 does not includes the Olivine Hill, Langrenus, Keeler, Crookes and Tsiolkovsky craters, which were suggested as olivinebearing areas by Clementine5,6 . This is because the Clementine analysis was based on discrete spectral data with a limited wavelength coverage of λ ≤ 1 µm, whereas the SP has continuous spectral data with λ = 0.5–1.6 µm (ref. 7). However, the Theophilus suggested by Clementine is identified as an olivine-rich site by the SP. Figure 2g shows that this crater is located in the concentric region around Nectaris. In summary, olivine exposures on the Moon are limited to concentric regions around the impact basins that have thinner crusts. On a local scale, they are found mainly on small, fresh crater walls or continuous ejecta. What mechanism produced this distribution? We propose that basin formation is responsible for the observed distribution of the olivine exposures. Each basin formation could have blasted away the upper crust, excavating and redistributing deep-seated olivine-rich material to the rim. Whereas the central region of the basin would be covered with basaltic lava that erupted later in the cases of nearside basins and Moscoviense, the rim region would not. For the SPA the impact resulted in the production of a large amount of melted material, which puddled on the floor of the excavated cavity as a melt sheet. Local differentiation occurred in these melt layers, forming an orthopyroxene layer that overlies the olivine-rich layer18 . Indeed, a recent SP survey8 revealed the existence of an extensive layer of differentiated orthopyroxene in the central region of the SPA; the central peaks of the Finsen, Antoniadi, Bhabha and Lyman craters show clear orthopyroxene spectra. Thus,
the deep-seated olivine-rich layers in the central region of the SPA would be hidden by the differentiated impact melt. Although olivine in the rim regions would have been covered with ejecta from the surroundings, later impacts could have excavated the olivine, exposing it to the surface. As a result, olivine-rich sites are observed only at fresh craters in the concentric regions around large basins. Although most main thin-crust basins (for example, Moscoviense, Crisium, Humboldtianum) have olivine-rich sites, some basins (for example, Mare Smythii) do not. This may be due to the incomplete coverage of our survey. Some basins located in thin-crust regions may have olivine exposures in their concentric regions that the SP survey did not discover. Where did the olivine-rich material originate? This is an important question for increasing our understanding of the structure and evolution of the Moon. Here we propose two possible scenarios. The first scenario is that the olivine-rich exposures originated in the upper lunar mantle. The basins with olivine-rich sites are located only in regions where the crust is relatively thin (Fig. 1). For example, if the general impact cratering theory19 is applied to the mare Crisium (∼1,000 km diameter), the depth of the excavation is >∼ 100 km. The original crust thickness at Crisium could have been thinner than the maximum thickness of the current feldspathic crust, which is about 100 km (Fig. 1). Thus, basin formation impacts could plausibly have penetrated to the crust–mantle boundary. The second scenario is that the olivine-rich exposure originates from the mafic-rich lower crust. In other words, the basin formations excavated the Mg-rich pluton intruding into the lunar lower crust20–22 . Note that some of the olivine-rich sites are associated with plagioclase; the Schrödinger and Aristarchus craters were reported as the purest-anorthosite-bearing regions14 . In addition, site E1 in the Schrödinger crater (Fig. 3) has areas that exhibit the 1.25 µm plagioclase absorption band adjacent to areas showing olivine-rich spectra. This may suggest that the basin formations excavated intrusions with spatially inhomogeneous plagioclase/olivine ratios in the lower crust, although there is the possibility that the excavation of the crust–mantle boundary resulted in mixtures of mantle olivine and anorthosite during the excavation process. If this scenario is true, the spatial distribution of olivine exposures (Fig. 1) gives important insights into constraints on early lunar basaltic magmatism and crustal growth after the crystallization of the LMO (refs 23,24). Which of the above scenarios is more plausible? If the olivinerich exposure originates from the upper mantle, the composition should be similar to dunite rather than troctolite in the lower lunar crust. To confirm whether this is the case, we examined the spectral data for some of the olivine-rich sites using radiative transfer modelling based on an intimate mixture model4,8–10 (see Supplementary Information). Supplementary Fig. S4 shows that the
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NATURE GEOSCIENCE DOI: 10.1038/NGEO897
LETTERS representative spectra are more consistent with a dunite-dominant model than with troctolite. This is mainly because most of the spectral data for the olivine-rich sites in Supplementary Fig. S4 have lower absolute reflectance (
