Geology and composition of the Orientale Basin impact melt sheet

Authors


Abstract

[1] The Orientale Basin is one of the largest (930 km diameter) and youngest (~3.8 Ga) impact craters on the Moon. As the basin is only partly flooded by mare lava, its floor materials expose a major portion of the basin impact melt sheet, which some previous work has suggested might have undergone igneous differentiation. To test this idea, we remapped the geology of the Orientale Basin using images and topography from the Lunar Reconnaissance Orbiter, mineralogical information from the Chandrayaan-1 Moon Mineralogy Mapper, and elemental concentration maps from Clementine multispectral imaging and Lunar Prospector gamma ray data. The Maunder Formation (impact melt sheet of the basin) is uniform in chemical composition (equivalent to “anorthositic norite”) in at least the upper 2 km of the deposit. The deepest sampling of the basin melt sheet (maximum depths of ~3–5 km by the crater Maunder, 55 km in diameter) shows a variety of lithologies, but these rock types (anorthosite, anorthositic norite melt rocks, mare basalt, and gabbro) are not those predicted by the differentiation model. We conclude that no differentiation of the Orientale Basin melt sheet has occurred and that such a process is not evident from new remote sensing data for the Moon or in the Apollo lunar samples.

1 Introduction

[2] As Orientale is the youngest and best preserved multiring impact basin on the Moon, it has long served as a prototype basin and has been studied for clues to the processes and histories of older, more degraded features [e.g., Head, 1974; Moore et al., 1974; McCauley, 1977; Wilhelms, 1987]. The most recent geological map of the Orientale Basin was made using data from the Lunar Orbiter and Zond 8 missions [Scott et al., 1978]. Recent missions such as the Lunar Reconnaissance Orbiter have provided high-resolution images of the surface as well as compositional and physical information on basin deposits, allowing detailed study and new understanding of lunar processes and history to be developed.

[3] Large impact events produce copious volumes of shock melted, liquid rock, most of which lines the transient crater and ends up as the floor deposit of craters [Howard and Wilshire, 1975; Grieve et al., 1977; Cintala and Grieve, 1998]. It was recognized that the distinctive cracked and smooth deposits of the inner Orientale Basin (i.e., the Maunder Formation (abbreviated as “Fm”.) of McCauley [1977]) was probably an exposure of the impact melt sheet of the basin. New images and topographic data permit the full extent of the Maunder Fm. to be mapped and estimate of its thickness, volume, and composition to the made. Thus, we can fully characterize the properties of a body of impact melt associated with the Orientale impact, a major event in lunar history [Wilhelms, 1987].

[4] Some cratering studies have emphasized the very large volumes of impact melt generated during basin-forming events [e.g., Cintala and Grieve, 1998]. In addition, the idea that large bodies of impact melt may cool slowly and undergo igneous differentiation is currently popular; this notion was encouraged by the hypothesis that the Sudbury Igneous Complex, a large igneous body associated with the terrestrial Sudbury Basin, is a differentiated impact melt sheet [Grieve et al., 1991]. This idea has been thought to be applicable to the largest basins of the Moon, including Orientale [Vaughan et al., 2013]. An early study of the compositional variation of the Orientale melt sheet by Budney et al. [1998] found no evidence for differentiation, but this work was done before the new spacecraft data were obtained, and the question has assumed renewed importance and urgency in light of ongoing efforts to model the differentiation of the Orientale melt sheet [Vaughan et al., 2013] and those of other large basins, such as South Pole-Aitken [e.g., Vaughan and Head, 2013].

[5] Because the Orientale Basin is sparsely filled with mare basalt, the original basin floor is widely exposed and offers the opportunity to study a well-preserved melt sheet of a major impact feature. Using morphology to geologically classify the basin deposits [e.g., Head, 1974; Moore et al., 1974; McCauley, 1977], we can recognize and make a new geological map of the impact melt sheet from images and estimate its extent and thickness. Moreover, using this map, we can directly determine its elemental composition using Clementine spectral reflectance and Lunar Prospector gamma ray maps [e.g., Lucey et al., 2000; Gillis et al., 2004].

[6] The aim of this work is to map the geology of the Orientale Basin using images and data from the Lunar Reconnaissance Orbiter Camera (LROC), the Moon Mineralogy Mapper (M3) of Chandrayaan-1, Clementine and Lunar Prospector (LP) FeO and TiO2 maps, and the Lunar Orbiter Laser Altimeter (LOLA) topographic data and to determine the extent of differentiation of the basin impact melt sheet (if any). The updated geological map shows a more accurate representation of the distribution of units both inside and outside of the basin; it also provides a clearer insight into the distribution of the melt sheet, the geological context of impact melt inside the basin, and the nature of the basin-forming impact. Determining if the Orientale Basin melt sheet has differentiated can help us understand the behavior of large, shallow magma bodies on the Moon and how this behavior might differ from impact melt on Earth.

2 Methods

[7] The global image mosaics at 100 m/pixel of the LROC Wide Angle Camera (WAC) [Robinson et al., 2010] were used to create a new geological map of the Orientale Basin. An orthographic projection of this mosaic centered on Orientale (−19°, −93°) was used as a base map, and units were rendered in ArcGIS 10.0. The GLD100 global topographic map (derived from LOLA and WAC stereo images) [Scholten et al., 2012] was used to aid in the identification and mapping of various units and to estimate the thickness of interior deposits. In addition, Clementine FeO and TiO2 maps [Lucey et al., 2000] and LP iron maps [Gillis et al., 2004] were used to help distinguish areas of mare material (high FeO content) from texturally similar highlands plains material of the basin melt sheet. Clementine RGB false color composites [Pieters et al., 1994; Bussey and Spudis, 1997] enabled the distinction of major compositional units of the basin.

[8] Geological units were defined using characteristics such as position within the basin, surface texture, structure, and stratigraphic position. We adapted the unit names of the Orientale Group [McCauley, 1977], which defined a series of stratigraphic formations on the basis of their morphology and position. Stratigraphic formations were further subdivided into members [e.g., Wilhelms, 1987] on the basis of detailed surface textures and position. Although we are in the process of mapping the geology of the entire basin, for this effort, we focus only on basin interior deposits, particularly the inner basin Maunder Formation [McCauley, 1977], which has been convincingly interpreted by most workers as a remnant of the basin impact melt sheet [Head, 1974; Moore et al., 1974; McCauley, 1977; Wilhelms, 1987].

[9] The new geological map of the basin interior was used to create a stencil to extract compositional information of a given unit. The stencil and compositional image are registered in ArcGIS and statistics collected. Thus, we are able to estimate the chemical and mineralogical makeup of the surface of the impact melt sheet of the Orientale Basin. In addition to this information, we mapped the position and extent of the ejecta from over 300 superposed impact craters on the Maunder Fm. Using this stencil, we are able to collect information on the composition of the ejecta from these craters, which are derived from beneath the surface by varying amounts (depths up to 5 km). By this method, we use impact craters as natural drill holes to study possible compositional variations in vertical section.

[10] In addition to compositional data from Clementine and LP, the M3 obtained spectral data for the deposits of the Orientale Basin [Green et al., 2011], from which we can infer the dominant mineralogy [Pieters et al., 2008]. We have examined M3 data for several fresh craters on the Maunder Fm. and in particular, the deposits of the crater Maunder. At 55 km diameter, Maunder is the largest postbasin impact crater in the interior of Orientale (Figure 1) and hence a source of the deepest stratum (~3–5 km depth) of melt sheet lithologies. When information on the dominant mineralogy is complemented with estimates of chemical composition, we can infer the composition of mapped geological units in terms of known lunar rock types. As the computational models of differentiation make specific predictions about rock types as a function of depth in the melt sheet [Vaughan et al., 2013], we use this information to test those models.

Figure 1.

Orientale Basin interior with Maunder Fm. morphologies illustrated in the breakout images. M is the mare basalt fill of Mare Orientale, S is the smooth member, and R is the rough member of the Maunder Fm. The crater Maunder (55 km diameter; −14.6°, −93.8°) is illustrated by the arrow. Area of Montes Rook Fm. shown in Figure 4 is indicated by box.

3 Geology of the Basin Interior

[11] Orientale is only partly filled by mare basalt, so its original floor configuration can be clearly seen over most of the basin interior (Figure 1). We have remapped the geology of the basin interior utilizing the stratigraphic nomenclature of McCauley [1977], which has been largely unchanged except that some formations have been subdivided into members on the basis of surface texture. Several geological formations collectively make up the Orientale Group; in the interior of the basin, these formations are the Maunder, Montes Rook, and (of extremely limited extent here) Hevelius Formations. In addition to these formally named units, we also map two additional (informal) rock stratigraphic units: mare and massif material (Figure 2). The Orientale Basin interior displays a number of small melt ponds that were previously unmapped (and partly revealed by comparison with the Clementine FeO map) [Lucey et al., 2000].

Figure 2.

Geologic map of the inner Orientale Basin. The Maunder Fm. is largely restricted within the Outer Rook ring. Table 1 summarizes compositional data for these units.

Table 1. FeO and TiO2 Concentrations for Orientale Basin Geologic Units (Figure 2)
Geologic unitFeO wt %TiO2 wt %
Maria11.0 ± 3.32.3 ± 1.4
Maunder Fm. (smooth member)4.5 ± 1.90.6 ± 0.3
Maunder Fm. (rough member)4.4 ± 2.00.6 ± 0.3
Montes Rook Fm. (knobby member)4.6 ± 1.10.5 ± 0.1
Montes Rook Fm. (smooth member)5.1 ± 1.10.5 ± 0.1
Massifs (average)4.1 ± 1.60.5 ± 0.2
Massifs (anorthosite, average of eight)0.6 ± 0.50.3 ± 0.8

[12] The mare basalts of Orientale have been mapped and described in detail by previous workers [Head et al., 1993; Gillis, 1998; Whitten et al., 2011]. The basalts thinly cover the innermost center of the basin and occur as small patches at the base of and concentric to major rings. Submare Maunder Fm. crops out in central Mare Orientale as small kipukas (Figure 2) and as rim material of the crater Hohmann (16 km diameter; −17.9°, −94.1°), indicating that the basalts here must thin to a featheredge. The nearly pure basaltic composition of ejecta from the crater Il'in (13 km diameter; −17.8°, −97.5°) indicates that the mare basalts here must be on the order of at least 1 km thick. Thus, the inner basin floor is highly irregular in relief, displaying a wide range of basalt thicknesses. The basalts of Mare Orientale appear to be moderate in titanium content (~2.3 wt %, Table 1, but see also Whitten et al. [2011]), which is relatively low Ti compared to the Apollo samples but higher than the Ti content of other typical farside maria [Gillis, 1998].

[13] The other informal rock unit is made up of the massifs (mountains) of the inner basin rings (Figure 2). Massifs typically make up the two intermediate rings (i.e., the Inner and Outer Montes Rook). The other two basin rings (inner shelf ring and Montes Cordillera; see below) are more scarp like. Massifs can be as large as 20–30 km across, are typically equant in plan, cluster in some locales and occur as isolated peaks in others. Parts (but not all) of the Inner Rook ring are composed of massifs made up of pure anorthosite [Spudis et al., 1984; Bussey and Spudis, 1997; Hawke et al., 2003; Cheek et al., 2013]. In some cases, these anorthosites are shocked to levels of at least 20 GPa [Spudis et al., 1984], but less than 30 GPa, as evidenced by the presence of the 1250 nm plagioclase absorption feature [Cheek et al., 2013].

[14] The Orientale Basin interior displays at least four distinct concentric rings (Figure 3) [Hartmann and Wood, 1971; Moore et al., 1974; Wilhelms, 1987]. The innermost ring (320 km diameter) is expressed as a simple scarp covered by the Maunder Fm., with up to 3 km of relief between the upper shelf and the flat, mare-covered floor of the basin. The next larger ring (480 km diameter) is the Inner Montes Rook, an irregular but broadly circular arrangement of equant massifs and massif clusters. Some of these massifs are composed of nearly pure anorthosite [Spudis et al., 1984; Hawke et al., 2003; Cheek et al., 2013]. The next larger ring is the Outer Montes Rook (620 km diameter), again made up of massive, blocky mountains, roughly arranged in a circular pattern. The outer ring (and main topographic rim of the basin) is the Montes Cordillera (930 km diameter), which has the morphology of a gigantic scarp. In most cases, the rings demark the limit of exposure of units of the inner Orientale Group (e.g., the Montes Rook Fm. is largely confined between the Cordillera and Outer Rook rings) but exceptions occur locally (Figure 2).

Figure 3.

Ring configuration of the inner basin. Three prominent rings occur within the basin topographic rim and the fourth makes up that rim. GLD100 topographic image.

[15] The Maunder Formation (Figure 1) has been split into two members—a smooth (plains-forming) member and a rough member (Figure 2). The Maunder smooth member is relatively flat and appears to occur within topographic lows. The Maunder rough member has a relatively large range of surface relief and appears to be draped over the preexisting topography; it is extensively fractured in some areas (termed “corrugated facies” by Head [1974]). These cracks may have been created as the melt sheet settled and cooled over the rugged parts of the basin floor. On the basis of their position with respect to the basin rim and their resemblance to the floor deposits of fresh complex craters such as Tycho [e.g., Howard and Wilshire, 1975], both members of the Maunder Fm. are interpreted as the remnant of the impact melt sheet of the basin [McCauley, 1977; Wilhelms, 1987]. The smooth plains member likely represents small local areas where the melt has ponded and filled in hollows and small valleys in the floor.

[16] Moving outward from the basin center, the next unit is the Montes Rook Formation (Figure 2). This unit has also been split into two members—a rough, knobby member (“domical” facies of Head [1974]; “knobby terrain” of Moore et al. [1974]) and a smoother, plains-like member. The knobby member appears to contain large, equant, bulbous hummocks of material giving these areas a blocky and uneven appearance. The Montes Rook Fm. is similar in appearance to the Alpes Fm. of the nearside Imbrium Basin [Wilhelms, 1987], but unlike the Alpes, the Montes Rook Fm. is largely restricted to the Orientale Basin interior, with minor extensions beyond the Cordillera Basin rim crest. Flow lobes within the Montes Rook Fm. have been observed against some of the massifs or against the Cordillera Ring in some areas (Figure 4), suggesting some type of fluid movement of material during late stages of emplacement. The smooth member is mainly concentrated in the southwestern quadrant of the basin interior.

Figure 4.

Flow lobe within the Montes Rook Fm. (arrow) suggesting that impact melt makes up at least part of this unit. Montes Rook Fm. laps up onto Cordillera scarp here in southwestern corner of Orientale Basin. Field of view is about 200 km across; LROC WAC mosaic.

[17] The origin of the knobby surface texture displayed by the Montes Rook Fm. is unknown. Head [1974] suggested that seismic shaking during megaterrace formation of the Cordillera ring induced a “domical” morphology to the unit. Moore et al. [1974] equated the Montes Rook Fm. with the Maunder Fm. and suggested that both units were largely composed of impact melt created during the basin-forming event. McCauley [1977] proposed that the knobs are the surface manifestation of coherent, ejecta megablocks quarried from deep stratigraphic horizons. None of these suggestions explains the widespread occurrence of this surface texture across much of the lunar nearside [Spudis et al., 2011]. The idea that the Montes Rook Fm. is composed of impact melt at least in part is supported by the presence of the flow lobes in the southwestern corner of the basin interior (Figure 4).

[18] The basin exterior deposits are collectively named the Hevelius Formation [McCauley, 1977; Wilhelms, 1987]. This unit displays a variety of morphologies, including radially lineated, swirl-like, transverse (to the basin rim), and hummocky. At ranges beyond about one half to one basin radii, the continuous Hevelius Fm. transitions into a set of discontinuous units, including smooth, Cayley-like [Wilhelms, 1987] plains and abundant secondary impact craters, including singles, open clusters, and sequential chains oriented radial to the basin center. As our focus in this study is on the basin impact melt sheet, we will defer detailed discussion of the morphology and distribution of the Hevelius Fm. for a later contribution. The vast bulk of the Hevelius Fm. occurs beyond the Cordillera rim, but we have identified a few small occurrences of this material inside the basin rim (Figure 2).

[19] Vaughan et al. [2013] used LOLA topography and thermal model considerations to estimate the thickness of the Orientale Basin melt sheet (i.e., the Maunder Fm.). They estimated an initial thickness of about 15 km near the basin center. As an alternative, we examined the topographic relief concentrically within the Inner Montes Rook ring (Figure 5), a feature that is partly embayed by the melt sheet. If the exposed western section of the Inner Montes Rook is representative of the currently buried eastern segments (which seems likely), then the maximum amount of relief within this exposed portion of the ring represents the likely maximum thickness of the melt sheet at this radial position. The observed maximum relief within the exposed Inner Montes Rook ring is about 6.3 km.

Figure 5.

(left) The Inner Montes Rook ring (480 km diameter) and the topographic relief along its circumference (from A clockwise to B). (right) Relief is up to 6.5 km from highest to lowest segments; as Maunder Fm. partly covers this ring, this value may be taken as an approximate limit to the thickness of the basin melt sheet.

[20] As the Maunder Fm. sometimes completely covers the Inner Montes Rook ring (near the southeastern sections; Figure 1) and sometimes laps up against it, we suggest that the thickness of the melt sheet is no greater than about 6 km. The thickness of the melt sheet in the basin center cannot be directly determined but is not likely to be significantly greater than this estimate plus the elevation difference between the Maunder “shelf” and basin center, about 2 km [Vaughan et al., 2013]. Thus, we suggest that the Maunder Fm. is about 500–600 km in diameter and has an average thickness of ~6 km or less, possibly thickening locally to 8–9 km in the center of the basin. A melt sheet of this extent and thickness would have a volume of ~8.5 × 105 km3, a value roughly comparable to the independent estimates of one million cubic kilometers by Potter et al. [2013] and Vaughan et al. [2013]. This estimate may represent a minimum value for the total amount of melt generated in the Orientale Basin-forming impact, as significant amounts of impact melt would be ejected as crater size increases [Cintala and Grieve, 1998].

4 Composition of the Maunder Formation

[21] The new geological mapping (Figure 1) described above can be used to determine the chemical composition of the Orientale Basin melt sheet. Using the map of the extent of the Maunder Fm., a stencil was created to mask out data of non-Maunder Fm. units. The remaining pixels represent the samples of the Maunder Fm., yielding its surface composition and variations (Figures 6 and 7). Both FeO and TiO2 concentration maps were prepared from the Clementine spectral reflectance images using the algorithms of Lucey et al. [2000] and Gillis et al. [2004]. Results of this analysis are given in Table 1, where mean iron and titanium values are given for all inner basin geological units. The iron content of the surface of the Maunder Fm. is very uniform (Figure 7), at around 4.5 wt % FeO and 0.6 wt % TiO2. Low-Ca pyroxene dominates the spectra observed in M3 data in fresh exposures of the Maunder Fm., but as pyroxene is a spectrally dominant mineral, its influence on a reflectance spectrum is not proportional to its petrologic modal abundance. These characteristics lead to the conclusion that the surface of the Maunder Fm. is made up of “anorthositic norite” (Al2O3 ~28 wt %, estimated from average FeO values) [see Spudis et al., 2002; Zellner et al., 2002], a rock type common to the upper crustal composition of the lunar highlands. A chemical and mineralogical analog to this composition in the Apollo collections are the samples 68415 and 68416, impact melts collected from Station 8 at the Apollo 16 landing site [Meyer, 2010].

Figure 6.

Geologic stencil, WAC mosaic, and Clementine FeO image used to derive the elemental composition of the Maunder Formation.

Figure 7.

Iron concentrations of the Maunder Fm. surface derived from geologic map stencil over the Clementine FeO image. Curve is well-behaved, narrow peak with mean FeO of 4.4 ± 2.0 wt %, indicating a very uniform composition. See Table 1 for details.

[22] While the surface composition of the Maunder Fm. is very uniform (Figure 7), is this composition representative of the unit as a whole? Because of its great age (older than 3.8 Ga), the Orientale Basin has a large number of impact craters superposed on its units. We mapped the ejecta deposits of over 300 small (0.5–18 km diameter) impact craters on the Maunder Fm. This map was used to create another stencil so that we could assess the variation in iron and titanium composition of the melt sheet with depth. Results are shown in Figure 8. These plots show that the composition of the Maunder Fm. is remarkably uniform and homogeneous down to depths of almost 2 km. Virtually all compositions fall within one standard deviation of the mean value of the surface composition except for a population at the smallest diameters, which seem to be contaminated with a fraction of more mafic members. As these points show both enhanced iron and titanium, they are probably caused by the inclusion of near-surface mare basalt flows and feeder dikes related to the emplacement of the lavas of Mare Orientale. The fact that these mafic anomalies are restricted to the smallest crater diameters suggests that we were not totally successful in restricting our geological mapping solely to the Maunder Fm. at the smallest scales, probably caused by the moderate resolution (~100 m/pixel) of the images used for mapping.

Figure 8.

Composition of the ejecta deposits of craters on Maunder Fm. Solid horizontal lines are the mean values of the surface; shaded areas are one standard deviation. With increasing crater size, deeper stratigraphic horizons are excavated. Yet the largest craters (excavating the deepest) show no significant differences in composition with the surface of the melt sheet, suggesting that there are no changes in composition to depths of about 2 km.

[23] The largest craters sampled were approximately 18 km across and excavated material from ~1.5–2 km depth within the melt sheet [Croft, 1980, 1985]. As there is no apparent correlation between crater size (and hence depth of excavation) and elemental composition, we conclude that the upper ~2 km of the Orientale Basin impact melt sheet is chemically homogeneous and identical to the surface composition. A recent study that modeled the crystallization of the Orientale melt body produced a hypothetical melt sheet stratigraphy of ~1 km of “anorthosite debris” overlying a layer of “norite” that extends to depths of about 4 km [Vaughan et al., 2013]. Such composition and structure are not observed in our data. We find evidence for spatially isolated outcrops of “pure anorthosite” (i.e., FeO content <1 wt %) only within some isolated massifs of the Inner Rook basin ring and in a few small impact craters near this ring. But the Maunder Fm. itself is not anorthosite but anorthositic norite (FeO ~4.5 wt %), and this composition manifests itself to depths of at least 2 km.

5 Composition of the Deposits of Maunder Crater

[24] The crater Maunder (55 km diameter; −14.6°, −93.8°) is the largest postbasin crater whose target consisted of inner basin deposits, in this case, the Maunder Fm. and part of the inner basin (scarp) ring (Figures 9 and 10). As such, the composition of its ejecta, rim deposits, melt sheet, and central peak can provide critical information about the composition and nature of the Orientale melt sheet at depth. Thus, we undertook a special study of this crater and its geology to better understand the nature of the interior of the basin melt sheet.

Figure 9.

Maunder (55 km diameter; −14.6°, −93.8°), an Eratosthenian complex crater whose impact target was the Maunder Fm. overlying the inner basin shelf ring. The LROC WAC image shows morphology, the M3 composite image (red = band depth at 950 nm, green = band depth at 1050 nm, and blue = band depth at 1250 nm) displays mineralogy, and the Clementine iron map shows FeO weight concentrations (colors as in Figure 6). In the M3 composite image, the yellow/orange colors indicate pyroxene-rich lithologies, blue is crystalline plagioclase, and white (small fresh craters, mostly on the SW side) is olivine-rich. Note the bilateral symmetry in ejecta, with mafic ejecta in the SW and NE quadrants and feldspathic ejecta in the NW and SE sectors.

Figure 10.

Geologic map of Maunder Crater. Units are recognized by morphology and position; ejecta deposits have been subdivided according to FeO content. Mineralogy is estimated from extracted M3 spectra of fresh small (~100 m) craters superposed on a given unit. See Table 2 for unit compositions and interpretations.

Table 2. Average FeO and TiO2 Contents, Mineralogy (lpx = Low-Ca Pyroxene, cpx = Clinopyroxene, ol = Olivine, and plg = Plagioclase), and Interpreted Lithology of Maunder Crater Depositsa
UnitFeO (wt %)TiO2 (wt %)Dominant MineralInterpretation
  1. aSee geologic map of Figure 10 for location of units.
Central peak9.7 ± 1.60.6 ± 0.2lpx, cpxnorite, gabbro
Crater floor9.3 ± 1.40.8 ± 0.1mafic glassbasaltic impact melt
Wall materials4.5 ± 1.60.6 ± 0.1lpx, plgnorite, anorthosite, basin melt sheet
Feldspathic ejecta4.7 ± 1.50.6 ± 0.2plgbasin melt sheet rocks, feldspathic massif materials
Mafic ejecta8.3 ± 2.21.2 ± 0.4ol, mafic glassolivine vitrophyre mare basalt plus basin melt sheet debris
Distal ejecta6.0 ± 2.30.9 ± 0.4cpx, olbasin melt sheet debris on basalts of Mare Orientale

[25] Maunder is a complex crater, showing a flat floor and central peak, and is of Late Eratosthenian age [Scott et al., 1978; Wilhelms, 1987]. On the basis of its diameter and morphology, the excavation cavity of the crater was on the order of 30–35 km diameter and maximum depth of excavation would be a bit more than one tenth that number (3–4 km) [Croft, 1980, 1985]. Central peaks of complex craters represent slightly deeper stratigraphic horizons, with the Maunder central peak coming from roughly 5–5.5 km depth [Cintala and Grieve, 1998]. Thus, Maunder would have both excavated and brought up through structural uplift the deepest stratigraphic horizons of the melt sheet. If the hypothesized mafic cumulates exist, we might expect them to make up part of the Maunder deposits.

[26] The spectral parameters used to generate the M3 color composite in Figure 9 (middle image) are the depths of the absorption bands at 950 nm (red), 1050 nm (green), and 1250 nm (blue). The position of the mafic absorption band is indicative of mineralogy and mineral composition, and these parameters were selected to exploit that fact. Low-Ca pyroxene has a maximum absorption near 950 nm. The wavelength of maximum absorption increases with increasing Ca to near 1050 nm. Therefore, orange and yellow regions in Figure 9 (middle) show locations of low- to high-Ca pyroxene-rich lithologies. Crystalline anorthosite (>90% plagioclase) containing minor amounts of FeO (at least ~0.5 wt %) has a diagnostic absorption centered near 1250 nm. Bright blue areas in Figure 9 (middle) show the locations of anorthosite exposures. Three closely spaced absorptions in olivine overlap to appear as one broad absorption band centered near 1100 nm [Burns, 1993; Sunshine and Pieters, 1998] but still absorb a significant amount light at 950 and 1250 nm. Therefore, white and pale yellow in Figure 9 (middle) represent olivine-rich material.

[27] A geological map of Maunder is shown in Figure 10. We used M3 data to examine the spectral properties of Maunder units [Whitten et al., 2011] and extracted point spectra for small, fresh craters superposed on Maunder units (Figures 10 and 11, color symbols and spectra). From Clementine images, the iron and titanium concentrations were determined for each mapped unit and used in conjunction with mineral information to interpret the lithological makeup of Maunder materials (Table 2). It is important to note that M3 data alone do not uniquely determine lithology, although they can often be used to infer the composition of the spectrally dominant mineral [e.g., Burns, 1993; King and Ridley, 1987; Sunshine and Pieters, 1998; Isaacson et al., 2011; Klima et al., 2011]. Likewise, geochemical information alone cannot uniquely determine what rock types are present. However, both sources of information together can distinguish between contrasting (and drastically different) alternative lithologic interpretations. For example, an olivine-bearing unit containing FeO ~10 wt % could be dunite. However, if that unit also contains 1.2 wt % TiO2, it is more likely to be made up of olivine-rich mare basalts mixed with highland materials. While the latter is not a unique determination, it is more geologically reasonable than a dunite with an anomalously high abundance of titanium.

Figure 11.

Representative spectra of the principal geological units of Maunder Crater; numbers and colors correspond to units shown in mineral key at top right of Figure 10.

[28] From the mapping and compositional data presented here (Figures 10 and 11 and Table 2), we suggest the following interpretation of the geology of Maunder Crater. The impact occurred on Maunder Fm. (basin melt sheet), which here overlies the scarp of the innermost basin ring. Two principal lithologies make up the deep subsurface (>2 km depth) of the melt target here: a large norite pluton or massif striking NW-SE and intrusive bodies of mare basalt (or gabbro), likely of the same composition as the surface flows of Mare Orientale and possibly making up feeder dikes for those lavas. These bodies are overlain by basin impact melt, which has a composition of anorthositic norite, as described above. These rock types were excavated during the formation of Maunder Crater, creating the heterogeneous and mixed ejecta blanket that surrounds the feature. The central peak is made up mostly of norite and lesser amounts of gabbro, suggesting a complex petrologic structure at depth. Floor material (impact melt sheet of Maunder Crater) is a “highland basalt” (FeO ~10 wt %) displaying spectra indicative of a mafic glass. It is probably similar to the basaltic impact melts ubiquitous in the Apollo collections; although its KREEP content is unknown, it is probably low. There is no evidence in any of the crater deposits for ultramafic rock types such as pyroxenite and dunite; if such products exist, they must occur at depths greater than 5–6 km. The crater exterior ejecta displays two principal compositions: a mafic, olivine-bearing unit that probably consists of a mixture of fragments of the basin melt sheet (feldspathic) and mare basalt and another more feldspathic unit, largely composed of basin melt sheet fragments with minor blocks of anorthosite (Figures 9 and 10). A reconstruction of the inferred subsurface stratigraphy of the Maunder Fm. and comparison to the configuration proposed in Vaughan et al. [2013] are shown in Figure 12.

Figure 12.

Comparison of the reconstructed stratigraphy of the Maunder Fm. of Orientale Basin in the case of simple equilibrium crystallization in the large “melt sea” of Vaughan et al. [2013] and inferred subsurface structure derived from the mapping and analysis of this study.

6 Discussion

[29] Our studies of the Maunder Fm. of the Orientale Basin have shown that although partly covered by mare basalt, the impact melt sheet is exposed and widespread in the center of the basin, with an estimated maximum thickness of ~6 km. It has a remarkably uniform surface composition (Figure 7) and this homogeneity continues to depths of at least 2 km (Figure 8). Its bulk composition is significantly more feldspathic than norite, corresponding to an anorthositic norite (Al2O3 ~ 28 wt %). Our one data point for deeper stratigraphic levels is provided by the deposits of Maunder Crater, which show a complex association of norite, gabbro, mare basalt, and feldspathic basin melt rocks.

[30] The idea that large bodies of impact melt might differentiate in place has gained some currency in lunar science [e.g., Cintala and Grieve, 1998; Morrison, 1998; Vaughan et al., 2013]. This supposition is based on analogy to the terrestrial Sudbury Igneous Complex, which has been hypothesized to represent a differentiated impact melt sheet [Grieve et al., 1991]. Large melt sheets supposedly contain lesser amounts of cold clasts relative to the impact melt sheets of smaller craters and hence would retain more heat for long periods of time [Cintala and Grieve, 1998]. Previous studies of the melt sheets at other terrestrial craters [Grieve et al., 1977; Floran et al., 1978] and lunar samples [Ryder and Wood, 1977] led to a different understanding—that impact melts mix energetically during crater formation and thoroughly homogenize diverse target lithologies into a very uniform, blended melt composition. (A corollary of this idea is that chemically defined groups of impact melts must have been derived from distinct impact events; see Spudis [1993, p. 172–180].) Rapid cooling is precipitated by the physical incorporation of abundant unmelted clasts as the crater grows, which are intimately mixed into the melt during cratering flow [Simonds et al., 1976]. While the impact melt remains mobile enough to settle, drape topography, pond, and partly flow for short distances, it cools relatively quickly, resulting in the production of fine-grained, clast-laden, chemically uniform melt rocks [Simonds et al., 1976; Spudis and Ryder, 1981]. Which (if either) of these two scenarios are correct and under what conditions?

[31] A recent model of the differentiation of an Orientale Basin-sized melt body produced an igneous body having a stratigraphy of norite on top (8 km thick) overlying pyroxenite (4 km) and basal dunite (2 km) [Vaughan et al., 2013]. Although the upper surface of the Orientale melt sheet is anorthositic norite (significantly more feldspathic than norite), there is little evidence for the presence of the other predicted mafic compositions at depth (Table 2). Given the estimated initial thickness of the melt sheet—which ranges from 6 km (this study) up to 15 km [Vaughan et al., 2013]—at least some level of differentiation would have occurred by analogy with the largest impact melt sheets on Earth (e.g., the Sudbury Igneous Complex; Grieve et al. [1991]). The results of this study indicate no differentiation of the Orientale melt sheet along the lines predicted by the Vaughan et al. [2013] modeling. Although some lithologic diversity appears to be present in the subsurface beneath the crater Maunder, it is not the diversity predicted by the differentiation model but is consistent with a complex and heterogeneous basin floor overlain by a homogeneous melt sheet.

[32] In a more philosophical vein, this work calls into question the whole idea of the igneous differentiation of lunar impact melt sheets. At nearly 1000 km diameter, Orientale is one of the largest impact basins on the Moon [Wilhelms, 1987]. As such, it would be expected, at least to some degree, to manifest the features and phenomena characteristic of the biggest impacts. The lack of evidence for differentiation of its impact melt sheet calls into question assumptions that the impact melt produced by the largest lunar impact crater, the South Pole-Aitken Basin, underwent differentiation [e.g., Morrison, 1998; Vaughan et al., 2013]. A lack of such differentiation is supported by some interpretations of remote sensing data [e.g., Pieters et al., 2001; Nakamura et al., 2009].

[33] To look at it from another perspective, if the melt sheets in lunar basins undergo widespread differentiation, what is the origin of the ubiquitous fine-grained basaltic impact melts in the Apollo collection? These rocks occur at every landing site and in many lunar meteorites, contain clasts of at least middle (if not deep) crustal origin (i.e., nonsurficial material), all date from the time of heavy bombardment (circa 3.9 Ga), have distinctive meteoritic siderophile signatures, and are found proximate to the latest and largest impact basins of the lunar near side [e.g., Spudis, 1993]. In bulk composition, they are more mafic than at least the upper few kilometers of the lunar crust, so they cannot be derived from the melt sheets of typical highland craters. These facts seem to point to an origin in a large (basin-forming) impact for most of these rocks, even if the basin to which each rock belongs is questioned [cf. Spudis and Ryder, 1981; Haskin, 1998]. As the plutonic rocks in the lunar samples appear to be of endogenous origin (most have crystallization ages (~4.5 to 4.1 Ga) prior to the ages youngest basins and extremely low meteoritic siderophile element concentrations), we have no obvious candidates for melt sheet differentiates in the lunar samples. If the basaltic melt rocks are ejecta, they would say nothing about the possibility of melt sheet differentiation, except that the starting compositions of such complexes might be quite different from the ones currently assumed by existing modeling [e.g., Vaughan et al., 2013].

[34] Although the idea of differentiation of impact melt sheets is intriguing, at present there is no direct evidence that such a process occurs on the Moon. It would require multiple sample returns from a complex area, such as the floor of South Pole-Aitken Basin, to establish this process as an important contributor to lunar surface evolution. Melt sheet differentiation is inconsistent with the geology and compositions of the Orientale Basin and with the characteristics of the lunar sample collection.

7 Conclusions

[35] An updated geological map of the Orientale Basin shows the relations of basin units and has been used to study the variation in composition of the basin's impact melt sheet. We find that the Maunder Fm. (basin melt sheet) varies from 1–6 km thick and has a near-uniform composition of anorthositic norite (FeO ~4.5 wt %). This composition does not change over the upper 2 km of the unit. Deeper horizons of the Maunder Fm. in at least one location show a greater diversity of composition but include lithologies neither expected from nor consistent with impact melt sheet differentiation. Results of this study confirm predictions of the previous model of rapidly quenched impact melt of uniform chemical composition. Study of the new data returned by the fleet of exploration missions over the last few years have greatly increased our ability to address these and other complex questions in lunar science.

Acknowledgments

[36] The work of DJPM is partly supported by the LPI Summer Intern Program. We thank A. Fagan, N. Petro, and B. Sharpton for review and comment on an earlier version of this paper. This is Lunar and Planetary Institute contribution 1764. This work is partly supported by NASA Lunar Science Institute contract NNA09DB33A (PI David A. Kring).

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