JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, E00G09, doi:10.1029/2010JE003736, 2011Lunar mare deposits associated with the O...
E00G09                     WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN                                         ...
E00G09                      WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN                                        ...
E00G09                      WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN                                        ...
E00G09                       WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN                                       ...
E00G09                      WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN                                        ...
E00G09                      WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN                                        ...
E00G09                     WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN                                        E...
E00G09                           WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN                                   ...
E00G09                     WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN                                         ...
E00G09                     WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN                                         ...
E00G09                      WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN                                        ...
E00G09                      WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN                                        ...
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E00G09                     WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN                                      E00...
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
Lunar mare deposits_associated_with_orientale_basin
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Lunar mare deposits_associated_with_orientale_basin

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Lunar mare deposits_associated_with_orientale_basin

  1. 1. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, E00G09, doi:10.1029/2010JE003736, 2011Lunar mare deposits associated with the Orientale impact basin:New insights into mineralogy, history, mode of emplacement,and relation to Orientale Basin evolution from Moon MineralogyMapper (M3) data from Chandrayaan‐1Jennifer Whitten,1 James W. Head,1 Matthew Staid,2 Carle M. Pieters,1 John Mustard,1Roger Clark,3 Jeff Nettles,1 Rachel L. Klima,1,4 and Larry Taylor5Received 31 August 2010; revised 23 November 2010; accepted 30 December 2010; published 22 April 2011.[1] Moon Mineralogy Mapper (M3) image and spectral reflectance data are combined toanalyze mare basalt units in and adjacent to the Orientale multiring impact basin. Modelsare assessed for the relationships between basin formation and mare basalt emplacement.Mare basalt emplacement on the western nearside limb began prior to the Orientale event asevidenced by the presence of cryptomaria. The earliest post‐Orientale‐event mare basaltemplacement occurred in the center of the basin (Mare Orientale) and postdated theformation of the Orientale Basin by about 60–100 Ma. Over the next several hundred millionyears, basalt patches were emplaced first along the base of the Outer Rook ring (Lacus Veris)and then along the base of the Cordillera ring (Lacus Autumni), with some overlap inages. The latest basalt patches are as young as some of the youngest basalt deposits onthe lunar nearside. M3 data show several previously undetected mare patches on thesouthwestern margins of the basin interior. Regardless, the previously documented increasein mare abundance from the southwest toward the northeast is still prominent. We attributethis to crustal and lithospheric trends moving from the farside to the nearside, withcorrespondingly shallower density and thermal barriers to basaltic magma ascent anderuption toward the nearside. The wide range of model ages for Orientale mare deposits(3.70–1.66 Ga) mirrors the range of nearside mare ages, indicating that the small amount ofmare fill in Orientale is not due to early cessation of mare emplacement but rather to limitedvolumes of extrusion for each phase during the entire period of nearside mare basaltvolcanism. This suggests that nearside and farside source regions may be similar but thatother factors, such as thermal and crustal thickness barriers to magma ascent and eruption,may be determining the abundance of surface deposits on the limbs and farside. Thesequence, timing, and elevation of mare basalt deposits suggest that regional basin‐relatedstresses exerted control on their distribution. Our analysis clearly shows that Orientale servesas an excellent example of the early stages of the filling of impact basins with mare basalt.Citation: Whitten, J., J. W. Head, M. Staid, C. M. Pieters, J. Mustard, R. Clark, J. Nettles, R. L. Klima, and L. Taylor (2011),Lunar mare deposits associated with the Orientale impact basin: New insights into mineralogy, history, mode of emplacement, andrelation to Orientale Basin evolution from Moon Mineralogy Mapper (M3) data from Chandrayaan‐1, J. Geophys. Res., 116,E00G09, doi:10.1029/2010JE003736.1. Introduction [Head, 1976; Wilhelms, 1987; Yingst and Head, 1997, 1999] and larger more continuous deposits lying within and adja- [2] About 20% of the lunar surface is covered with mare cent to the large impact basins on the nearside of the Moondeposits, including mare patches on the farside and limbs [Head, 1976; Wilhelms, 1987; Head and Wilson, 1992] [see 1 Yingst and Head, 1997, Figure 1; Hiesinger and Head, 2006, Department of Geological Sciences, Brown University, Providence, Figure 1.21]. The generation, ascent, and eruption of mareRhode Island, USA. 2 Planetary Science Institute, Tucson, Arizona, USA. basalt magmas have been subjects of intense study and debate 3 U.S. Geological Survey, Denver, Colorado, USA. for many years. Petrologists have proposed and assessed 4 Now at Johns Hopkins University Applied Physics Laboratory, Laurel, models for the generation and petrogenetic evolution ofMaryland, USA. basaltic rocks returned from the Moon [e.g., Hess and 5 Planetary Geosciences Institute, University of Tennessee, Knoxville,Tennessee, USA. Parmentier, 1995, 2001; Shearer et al., 2006]. Geologists have described and classified the nature of the vents andCopyright 2011 by the American Geophysical Union. deposits and from this information inferred eruption condi-0148‐0227/11/2010JE003736 tions and durations [e.g., Head, 1976; Wilson and Head, E00G09 1 of 33
  2. 2. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G091980; Head and Wilson, 1992; Hiesinger and Head, 2006]. Lucey et al., 2006]. Geochronologists have provided aSpectroscopists have shown the mineralogical diversity of chronology of mare basalt emplacement and flux throughmare basalts and linked surface units to returned samples dating of returned samples [e.g., Nyquist and Shih, 1992] and[e.g., Pieters, 1978; Pieters et al., 1993; Staid et al., 1996; impact crater size–frequency distribution analyses of mare units [e.g., Stöffler et al., 2006; Hiesinger et al., 2000, 2003, 2011]. Volcanologists have modeled the processes of magma generation, ascent, and eruption and made predictions about the nature, volumes, and styles of eruptions [e.g., Wilson and Head, 1981; Head and Wilson, 1992]. Geophysicists have modeled the thermal history and structure of the lunar interior and provided a range of candidate scenarios for mare basalt generation [e.g., Wieczorek et al., 2006], as well as the rela- tionship of mare basalt emplacement and lithospheric thick- ness [e.g., Solomon and Head, 1979, 1980]. [3] Despite significant advances in all of these areas, several of the most fundamental questions in mare basalt petrogenesis and emplacement remain unresolved [e.g., Jolliff et al., 2006; Jolliff, 2008]. Mare basalts are known to occur preferentially on the lunar nearside, where both the crust [e.g., Ishihara et al., 2009] and lithosphere [Solomon and Head, 1980] are thinnest, and to be concentrated within and adjacent to impact basins, such as Imbrium, Serenitatis, Crisium and Humorum. The following are among the out- standing questions. [4] 1. What is the relationship of impact basin formation and mare basalt genesis and emplacement? For example, does the formation of an impact basin generate or assist in the formation of mare basalts [e.g., Ivanov and Melosh, 2003; Elkins‐Tanton et al., 2004; Elkins‐Tanton and Hager, 2005]? Does the presence of impact basins influence where mare basalts are likely to be extruded [e.g., Head and Wilson, 1992]? When mare basalts are emplaced in lunar basins, does the filling and loading influence further stages, styles, and locations of eruption and emplacement [e.g., Solomon and Head, 1979, 1980]? [5] 2. What is the influence of crustal thickness and litho- spheric thickness on the emplacement of mare basalts? Are mare basalts inhibited from ascending to the surface by buoyancy traps at the base of the crust or lithosphere [e.g., Head and Wilson, 1992]? Or are there other reasons for the differences in the nearside/farside distribution of mare basalts unrelated to these factors, such as fundamental differences in deep thermal structure and thus mare basalt genesis [e.g., Zhong et al., 2000]? [6] The Orientale multiring basin offers the opportunity to assess many of these questions. Located on the western limb of the lunar nearside (Figure 1), Orientale is the youngest multiring basin on the Moon [e.g., Wilhelms, 1987] and is located in the transitional region between thin nearside crust and thick farside crust [e.g., Ishihara et al., 2009]. It is sparsely filled with mare basalts and thus the relationships between mare basalts and a well‐preserved basin structure Figure 1. (a) Location map of Orientale Basin with four main rings and deposits labeled. (b) M 3 thermal image (2936 nm) mosaic of Orientale Basin. This wavelength band is effective at displaying topography. (c) Standard M3 color composite, highlighting mafic mineral absorptions (R, 1.0 mm integrated band depth (IBD); G, the 2.0 mm IBD; and B, reflectance at 1489 nm). Yellow regions indicate areas of strong 1.0 and 2.0 mm IBDs. 2 of 33
  3. 3. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09can be readily assessed. Furthermore, the remote sensing Orbiter (LRO), provides a precise global lunar topographiccharacteristics of the mare basalts have been previously model and geodetic grid by pulsing a laser instrument duringstudied in a variety of analyses [e.g., Head, 1974; Scott et al., orbit [Zuber et al., 2010; Smith et al., 2010]. A 64 pixel per1977; Greeley et al., 1993; Kadel et al., 1993; Yingst and degree (∼473 m/pixel) global gridded data set was used forHead, 1997; Bussey and Spudis, 1997, 2000; Staid, 2000]. basin profiles and mare pond elevation analyses in this study. [7] Recent acquisition of high spatial and spectral resolu- In addition, LOLA profile data were used to determine thetion spectrometer data for the Orientale region of the Moon widths and depths of the identified sinuous rilles.by the Moon Mineralogy Mapper (M3) instrument on the 2.2.2. Impact Crater Size‐Frequency DistributionsChandrayaan‐1 mission permits further characterization of [12] Crater counting was conducted using a M3 mosaic ofmare basalt deposits in this region, and enables an analysis Orientale Basin at ∼140 m/pixel [Boardman et al., 2011], asof their mineralogy, ages, affinities, and their relationships well as the CraterStats program [Michael and Neukum, 2010].to Orientale impact basin deposits, topography and evo- The chronology and production functions of Neukum et al.lution. In this analysis, we utilize a range of M3 and related [2001] were used to calculate the ages from analysis of thedata (e.g., Lunar Orbiting Laser Altimeter, LOLA) to address new M3 data; crater ages were obtained by plotting unbinnedthese outstanding questions concerning (1) the relation of data. Specific count regions of similarly sized area weremare basalt origin and emplacement to impact basins and defined for both the Maunder Formation (melt sheet deposit)(2) crustal/lithospheric thickness differences. and the Hevelius Formation (basin ejecta deposit) to obtain [8] The purpose of this paper is to investigate the char- model ages for the basin‐forming event. Each count area foracteristics of volcanic features and deposits in the Orientale the Maunder Formation was between 1,000 and 3,000 km2Basin using the latest available data, including (1) the ages of and count areas for the Hevelius Formation ranged fromthe various volcanic deposits, the melt sheet, and the basin 190,000 to 250,000 km2. For dating the basin‐forming eventevent itself; (2) the areal extent and volume of the identified only primary craters larger than 5 km were counted, whereasmare deposits; (3) the spectral characteristic and composi- for the mare ponds primary craters greater than ∼0.75 km intional implications of Mare Orientale and other volcanic diameter were counted. Only if the crater population abovedeposits; (4) their altimetry, including both elevation and 0.75 km was too small to give statistically significant ageslocal or regional tilts; and (5) the volcanic features associated were craters from 0.5 km in diameter considered. We foundwith the basin, including sinuous rilles, domes, and vents. no obvious flow fronts and no distinct differences in mareCombining all of this new information can assist in further composition in the individual mare ponds in Lacus Veris anddefining the relationship between large impact basins and Lacus Autumni; thus, we assume that individual mare pondsvolcanism on the lunar surface in a nearside/farside transi- are the result of single eruptive events [Yingst and Head,tion region. 1997, 1999]. There is some indication of subtle differ- [9] We begin with a description of the new M3 data and ences within individual mare pond composition, which couldother data and techniques in order to provide background on either be due to lateral and vertical mixing of feldspathichow we have utilized these data to bring new insight into the material with mare soils or actual mineralogical differencesproblems outlined above. [Kadel, 1993; Staid, 2000]. Further investigation is needed to assess these trends in mixing and apparent mineralogical2. The Moon Mineralogy Mapper (M3) differences.Experiment, Related Data, and Analysis 2.2.3. Spectral Properties and Background on Lunar Near‐Infrared Reflectance Spectroscopy2.1. Background: The M3 Instrument [13] The mineralogic composition of the lunar surface has [10] Within the last several years, petabytes of new data been investigated for decades using the techniques of visible,have been returned to Earth from instruments on several near‐infrared (VNIR) spectroscopy, owing to the abundancelunar orbiters (e.g., Kaguya, Chang’ E, Chandrayaan‐1, of diagnostic mineral absorptions in this region of the elec-Lunar Reconnaissance Orbiter). Among these instruments tromagnetic spectrum [e.g., McCord et al., 1981; Pietersis the Moon Mineralogy Mapper (M3), a NASA imaging et al., 1996; Ohtake et al., 2009; Isaacson et al., 2011].spectrometer onboard the Indian Chandrayaan‐1 spacecraft. These diagnostic absorptions in the VNIR are the result ofM3 was designed to operate in two modes, a global mode and molecular electronic transitions that are specific to crystal-a targeted mode. The data used in this paper were obtained in lographic sites in mineral crystals. The characteristics of theseglobal mode, which collected reflectance spectra in 85 bands, crystallographic sites vary between different minerals andranging from 400 to 3000 nm with a spatial resolution of mineral compositions [Burns, 1993]. Thus, the shape and140 m/pixel from a 100 km orbit [e.g., Green et al., 2007; location of mineral absorptions are indicative of composition.Pieters et al., 2009]. Data from the M3 instrument were used The two most common mafic minerals on planetary surfacesin crater counting, measurement of the area and volume of old are olivine and pyroxene, both having diagnostic absorptionsand newly identified mare ponds, determination of the length that vary with composition. Pyroxenes have two diagnosticand width of sinuous rilles, and the definition of the spectral absorptions near 1 and 2 mm [e.g., Adams, 1974; Cloutis andcomposition of Mare Orientale and related deposits. Gaffey, 1991], and olivines have a combination absorption feature near 1 mm, composed of three separate overlapping2.2. Methods mineral absorptions [e.g., Burns, 1970, 1974; Sunshine and2.2.1. Altimetry Pieters, 1998]. [11] The Lunar Orbiting Laser Altimeter (LOLA), one of [14] Oftentimes the continuum slope of spectra arethe instruments carried by the NASA Lunar Reconnaissance described as either “blue” or “red,” referring to the slope of 3 of 33
  4. 4. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09the visible part of the electromagnetic spectrum. Blue light is basic approach was used in place of the modified Gaussianthe shortest wavelength and red light is manifested at longer model (MGM), which can derive much more specific com-wavelengths. Thus, when a spectrum is described as “blue” positional information by deconvolving individual absorp-that means the shorter wavelength region of the electromag- tion bands [e.g., Sunshine et al., 1990; Pieters et al., 1996;netic spectrum is dominating and the spectrum has a flat Klima et al., 2007], in order to reach initial conclusions aboutslope; “red” refers to positive continuum slopes controlled by the composition of basalts in Mare Orientale.the longer wavelength regions of the electromagnetic spec-trum. This continuum slope is controlled by composition of a 3. Review of Models of Mare Basalt Genesisgeologic material, whether it is mixed with another spectrally Related to Impact Basin Formation/Evolutiondistinct geologic material, and the maturity or degree of space and Global Crustal Thickness Variationsweathering experienced by the surface and the composition ofthe material [e.g., Staid, 2000; Staid and Pieters, 2000]. [18] Early in the modern study of the Moon, basic models [15] Approximately 120 M3 spectra from Mare Orientale for the formation of mare basalts were postulated thatwere collected to characterize basalt compositions. Optical involved internal radiogenic heat sources and sequentialperiod 1B (OP1B) [Boardman et al., 2011] data strips from partial melting of a layered mantle [e.g., Runcorn, 1974;two regions of Mare Orientale, previously identified as Solomon, 1975]. Subsequent petrological evidence suggesteduncontaminated with feldspathic basin material [Staid, 2000], a wide range of source depths for mare basalts. The docu-were utilized for data collection. To avoid complications re- mentation of increasing complexities in the ages and petro-sulting from highland contamination, spectra were collected genesis of mare basalts led to the proposal of several majorfrom craters < ∼1 km in diameter that showed an immature new ideas for their generation (see summary by Shearer et al.signature in a 0.989 mm/0.750 mm ratio parameter map. [2006]). Models for the generation, ascent and eruption ofCraters have an excavation depth that is approximately 1/10th mare basalts (as observed in the distribution of mare deposits)of the diameter [Melosh, 1989]; thus, collecting spectra from can be classified into five types, as follows.craters < ∼1 km in diameter ensures that only the surfacedeposit is being sampled. The 0.989 mm band, located at 3.1. Model 1: Nearside/Farside Mare Basaltapproximately the center of one of the main absorptions for Asymmetry Due to Crustal Thickness Differencesmafic minerals, and the 0.750 mm band, located outside of the [19] One of the more fundamental characteristics of theabsorption on the continuum part of a spectrum, provide a Moon is the nearside‐farside asymmetry in the distribution ofgood indication of the strength of the 1 mm absorption in mare basalts [e.g., Head, 1976]. Early models accounted formafic minerals. The closer the value of the ratio is to 1, the this by calling on differences in nearside‐farside crustalweaker the mineral absorption. thickness [e.g., Solomon, 1975; Head and Wilson, 1992]: for [16] It was necessary to take spectral measurements of the globally heterogeneous mare basalt source regions at depth,more freshly exposed areas to avoid any effects of maturity dikes rising to a constant level were much more likely to eruptor space weathering, such as decreased depth of mineral onto the surface in the thinner nearside crust than dikes risingabsorption bands, increased red slope of the spectra, and a in a thicker farside crust. The general paucity of farside maredecrease the overall albedo of the surface [e.g., Pieters et al., basalts and the concentration of most farside basalts in the2000; Anand et al., 2004; Noble et al., 2007]. Calibration of relatively deep South Pole–Aitken basin seemed consistentthe data is explained in detail by R. O. Green et al. (The Moon with this hypothesis.Mineralogy Mapper (M3) imaging spectrometer for lunar [20] Global altimetry obtained by the Clementine missionscience: Instrument, calibration, and on‐orbit measurement [e.g., Zuber et al., 1994; Neumann et al., 1996] revealed,performance, submitted to Journal of Geophysical Research, however, that the depth of the South Pole–Aitken basin was2011). Separate from general corrections, a mild spectral comparable to mare basalt elevations in basins on the near-correction was applied to the data in order to suppress residual side, and yet the SPA basin was not as extensively flooded byartifacts in M3 data. A detailed explanation of this correction mare basalts as the nearside basins [e.g., Head et al., 1993;is provided by Isaacson et al. [2011]. Greeley et al., 1993; Yingst and Head, 1997, 1999; Pieters [17] In this analysis, only the 1.0 mm absorption band was et al., 2001]. These observations cast doubt on the assump-investigated due to factors, such as thermal effects, becoming tion of global mare basalt source symmetry and the role ofincreasingly important to the 2.0 mm absorption, which is crustal thickness in explaining nearside‐farside mare basaltin the process of being resolved in the ongoing calibration of asymmetry, and pointed out the necessity of adding addi-the new M3 data. Features of the 1 mm absorption that were tional factors to models of magma ascent and eruption [e.g.,studied include the band center and the band strength. It has Wieczorek et al., 2001]. This led to the development of sev-been shown in numerous studies that absorptions centered eral alternative models for the emplacement of basalts in thearound 1.0 mm and 2.0 mm shift to longer wavelengths as South Pole–Aitken basin and for the Moon as a whole, raisingFe2+ and Ca2+ substitute for Mg2+ in pyroxenes [e.g., Adams, the questions (1) were there fundamental differences in the1974; Cloutis and Gaffey, 1991; Pieters et al., 1996; Klima nearside‐farside source regions established early in lunaret al., 2007]. Measurements of the position of the 1 mm history [e.g., Zhong et al., 2000; Parmentier et al., 2002];absorption band center were made using the methods of (2) did the Procellarum KREEP Terrain (PKT) play a fun-Cloutis and Gaffey [1991]. In these calculations, it was damental role in mare basalt generation [e.g., Wieczorek andassumed that the absorptions were the result of a single Phillips, 2000]; (3) was there a basic difference in theelectronic transition absorption, where spectroscopic min- nearside‐farside thermal gradient that influenced both basineral absorptions are actually composed of several over- relaxation and mare basalt generation, ascent and eruptionlapping electronic transition absorption bands. This more [e.g., Solomon and Head, 1980]; and (4) could the formation 4 of 33
  5. 5. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09Table 1. Summary of the Various Models of Mare Basalt GenesisModel Summary Predictions 1 Nearside/farside asymmetry due to crustal thickness differences 1a) A close correlation exists between crustal thickness and distribution of mare basalts. 1b) No correlation exists between these two variables, instead differences in the nature and distribution are due to nearside/ farside mantle source region differences. 2 Pressure release melting and associated secondary convection Large quantities in situ pressure release melting are produced instantaneously and located in the basin center. Smaller quantities of basalts are produced as a result of adiabatic melting induced by convection and persist for extended periods of time after impact (i.e., 350 Ma). The lowest‐titanium content mare basalts erupt last. 3 Enhanced KREEP layer induces volcanism in the Western The presence of KREEP and possible derivatives (i.e., "red spot" nearside Procellarum KREEP Terrain (PKT) volcanism such as the Gruithuisen Domes) might control the distribution of mare basalts. 4 Large‐scale overturn causes eruption of mare basalts High‐titanium basalts erupt earliest, followed by a suite of other types of basalts. Model is independent of basin formation. 5 Mare basalt emplacement is related to global thermal evolution Early mare volcanism is located in the center of the basin, and the and basin evolution youngest deposits can be found on the outside rim of the basin. This model is not necessarily related to basin‐forming impacts.of a basin the size of SPA have induced sufficient convection and picritic magmas and predict an order of eruption, with theto have stripped away a subsurface KREEP layer, and thus most primitive, lowest‐titanium magmas last.to have inhibited the formation of mare basalts below the [24] In a more recent treatment of impact‐induced con-basin [e.g., Arkani‐Hamed and Pentecost, 2001]? vection, Ghods and Arkani‐Hamed [2007] used a suite of [21] Model 1a (correlation with crustal/lithospheric thick- numerical models to show that this mechanism might be ableness) thus predicts a close correlation between these factors, to account for the formation of mare basalts, the range ofand model 1b (no correlation) predicts not only no correla- depths of their source regions, the observed delay betweention, but differences in the nature and distribution of mare impact basin formation and initiation of basaltic volcanism,basalts in space and time due to nearside‐farside mantle and the long duration of emplacement of mare basalts.source region differences (Table 1). These models treat basins of different sizes (e.g., ranging from Orientale, through Imbrium, to South Pole–Aitken),3.2. Model 2: Impact Basin Pressure‐Release Melting make different predictions about the record of mare basaltand Associated Secondary Convection emplacement for each, and can be readily tested against the [22] Pressure release melting is known to be an important geological record of mare basalt volcanism.mechanism for basalt generation, but the lunar pressure [25] Models for pressure release melting predict initiallygradient, combined with the composition of the crust and the large volumes of mare basalts in the basin center, followed byapparent depth of origin of mare basalts, has led to this mech- smaller amounts emplaced over several hundred millionanism generally not being favored for mare basalt formation. years, and the lowest‐Ti content lavas last (Table 1). Key testsRecently, Elkins‐Tanton et al. [2004] have reassessed the thus involve the timing, duration, volumes, styles and themagmatic effects of large basin formation, and introduced a comparison of the record in different sized basins.two‐stage model for melt creation beneath lunar basins trig-gered by basin formation itself. In the initial stage, crater 3.3. Model 3: Enhanced Sub‐Procellarum KREEPexcavation depressurizes and uplifts underlying mantle Layermaterial so that it melts in situ instantaneously, forming large [26] Although clearly outside the Orientale Basin region,quantities of melt below the basin (in addition to impact more than 60% of the mare basalts by area occur within themelt in the cavity). This model thus predicts huge quantities boundaries of the Procellarum KREEP Terrain (PKT), whichof in situ pressure release melt (98–100% of the melt created) makes up only ∼16% of the surface of the Moon. Thisproduced instantaneously and available to be extruded into observation was one of the major factors that led Wieczorekthe impact basin as lunar mare basalts. and Phillips [2000] to propose that there was a cause and [23] In the second stage, the cratered region rises isostati- effect in terms of the occurrence of KREEP and the genera-cally, warping isotherms upward and inducing convection, at tion of mare basalts. They postulated that the enhancement ofwhich time adiabatic melting can occur. This second stage is heat‐producing elements implied by the elevated KREEPminiscule in terms of melt produced (1–2% of the total) but levels significantly influenced the thermal evolution of thecan last for a longer period of time, up to ∼350 Ma. In the region, causing the underlying mantle to partially melt overElkins‐Tanton et al. [2004] model, mafic mantle melts can be much of lunar history to generate the observed basaltic vol-generated from depths of 150–560 km, depending on mantle canic sequence. The thermal model of Wieczorek and Phillipspotential temperature. Assuming that 10% of the melt gen- [2000] predicts that melting occurs only directly below theerated erupts, Elkins‐Tanton et al. [2004, Figure 5] find that PKT. Partial melting of the mantle begins immediately afterthe volumes of magma reported for basins are similar to their the model is started at 4.5 Ga and continues to a lesser degreepredictions. Elkins‐Tanton et al. [2004] also model the origin to the present. Melting initiates immediately beneath theand emplacement of high‐alumina, high‐TiO2, KREEP‐rich, KREEP basalt layer and becomes deeper with time, with the 5 of 33
  6. 6. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09maximum depth of melting being ∼600 km, and the KREEP and geometry of fill and the resulting distribution of loading‐layer is kept above its solidus for most of lunar history. induced stresses (influenced by global thermal evolution and [27] Does this hypothesis account for the origin of mare the net state of stress in the lithosphere) controls the locationbasalts in terms of the timing, duration, areal distribution, of eruptive vents and the style of emplacement [e.g., Solomonvolumes, and changes in depth with time? If not, is it related and Head, 1979, 1980; McGovern and Litherland, 2010].to any other volcanism within the PKT region, such as the Using linear tectonic rilles and wrinkle ridges, Solomon and“red spot” volcanic domes and related deposits? Hess and Head [1980] established that there are two stress systems,Parmentier [2001] pointed out several difficulties with this one local and another global, that affect the volcanism thatmodel in terms of (1) petrogenetic evolution, (2) geophysical occurred in the large lunar basins. The local stresses are theevidence against the long‐term duration of a near‐liquid result of lithospheric loading by the basalt fill of the basinsKREEP layer, and (3) the fact that such a layer might form an and global stresses originate from the thermal evolution of theimpenetrable barrier to the eruption of mare basalts (e.g., how Moon. The onset of mare volcanism in each basin is favoredcan denser mare basalt liquids penetrate through a >30 km by the presence of the local extensional stresses in the litho-thick layer of less dense, KREEP‐rich liquid?). This model sphere. Loading by central mare fill induces flexure andpredicts (Table 1) that the presence of KREEP and possible favors migration of vents to the margins of the basin.derivatives, such as “red spots,” might control the distribution [31] This hypothesis leads to the prediction (Table 1) thatof mare basalts, and thus we examine the new data for such early mare volcanism will be focused in the basin center andevidence. that the youngest mare volcanism will have occurred on the edges of the basin. Once the global compressive stress, from3.4. Model 4: Large‐Scale Overturn of Initial Unstable global cooling of the Moon, overrides the local extensionalStratification stresses, ascent and eruption of mare basalts becomes more [28] In model 4, crystallization of the lunar magma ocean difficult, and ultimately terminates.(LMO) forms a chemically stratified lunar interior with ananorthositic crust separated from the primitive lunar interior 3.6. Tests of the Modelsby magma ocean cumulates. Dense, ilmenite‐rich cumulates [32] Although these five models treat different aspects ofwith high concentrations of incompatible radioactive ele- the origin, evolution and emplacement of mare basalts, eachments are the last magma ocean cumulates to form, and makes different predictions (Table 1) and can be tested withunderlying olivine‐orthopyroxene cumulates are also strati- the further characterization of mare basalts in the Orientalefied with later crystallized, denser, more Fe‐rich composi- Basin. Our analysis of mare basalts in Orientale thus has thetions at the top. These layers are gravitationally unstable. following elements: (1) When and where were OrientaleRayleigh‐Taylor instabilities cause the dense cumulates to deposits first emplaced? (2) When and where did the terminalsink toward the center of the Moon and to form a dense core stages of volcanism in Orientale basin occur (what is their[Hess and Parmentier, 1995]. Subsequently, the ilmenite‐ age, what is their mode of emplacement)? (3) What was therich cumulate core undergoes radioactive heating and this flux of mare basalt volcanism? What is the volume of mareheats the overlying mantle, causing melting. The source basalt deposits as a function of time, when did it peak, wereregion for high‐TiO2 basalts is thus envisioned to be a mixed there multiple peaks, and what was the shape of the declinezone above the core‐mantle boundary containing variable toward the present? (4) How do the Orientale mare depositsamounts of ilmenite and KREEP and involves deep, high‐ compare to the global distribution of mare basalts as a func-pressure melting, delayed for a period of time subsequent to tion of time? (5) How does the mineralogy of the OrientaleLMO formation and overturn. Thermal plumes rise into mare deposits compare to the global distribution of marechemically stratified surroundings of the mantle (chemically basalt types? (6) How do eruption styles compare to theless dense but colder) above the core and cause mixing and global distribution of mare basalt vent types and impliedhomogenization. The resulting lower thermal boundary layer eruption conditions (e.g., J. W. Head and L. Wilson, Lunarmay be partially to wholly molten depending on mineralogy volcanic vent types, landforms and deposits: A synthesis andand the range of input parameters. assessment of modes of emplacement, manuscript in prepa- [29] Melting at the top of the mixed layer to produce mare ration, 2011)? (7) How do the individual and total volumes ofbasalt magmas must occur at low enough pressure for melt Orientale mare basalts help constrain the total volume ofbuoyancy and at high enough pressure to satisfy the depth mantle melting that has occurred (using estimates of extrusionindicated by phase equilibria. The onset time of mare volca- to intrusion ratios, and depths of origin, to provide an order ofnism is constrained by bulk core radioactivity, and TiO2‐rich magnitude assessment of the total melting that occurred,mare basalt liquids must be positively buoyant enough to updating earlier estimates)? (8) How does the distribution ofform dikes rather than sink. This model predicts (Table 1) pyroclastics in Orientale compare to the nature and globalearly high‐Ti mare basalts followed by a suite of other types distribution of mare pyroclastic deposits [e.g., Gaddis et al.,and is completely independent of impact basin formation. 2003], vent characteristics, eruption conditions, and associ- ated mare basalt types? (9) How do the model ages and model3.5. Model 5: Nature and Location of Source Regions age ranges of Orientale mare basalt emplacement relate to theand Emplacement Styles Related to Global Thermal general thermal evolution of the Moon and to the loading andEvolution and Basin Evolution but Independent of Basin subsidence observed in other basins [e.g., Solomon and Head,Formation 1980]? (10) Is there any evidence in Orientale for the type of [30] In model 5, mare basalt emplacement is not necessarily “red spot” extrusive domes and related deposits [e.g., Chevrelrelated to the formation of the impact basin in which the et al., 1999; Wagner et al., 2002, 2010; Wilson and Head,basalts reside. Rather, the shape of the basin, the sequence 2003] that may relate to suspected KREEP basalt extrusions 6 of 33
  7. 7. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09[e.g., Spudis et al., 1988] or to upland and Cayley plains that underlying Orientale Basin: the Hevelius Formation, themight represent aluminous basalts or cryptomaria [e.g., Montes Rook Formation, the Maunder Formation and relatedAntonenko et al., 1995; Antonenko, 1999]? plains material [Scott et al., 1977]. The Hevelius Formation consists of the radial basin ejecta blanket located outside the4. The Orientale Basin: Background and Setting Cordillera mountain ring. Interior to that deposit is the Montes Rook Formation, between the Cordillera and Outer [33] The Orientale multiring impact basin, located on the Rook rings. It consists of hummocky knobs situated in awestern limb of the Moon (19°S and 93°W), is the youngest rough textured matrix. The outer facies of the Maunder For-lunar basin, dated to the Upper Imbrium period of lunar mation has a highly fractured, undulating and corrugatedhistory [e.g., Wilhelms, 1987; Kadel, 1993] (Figure 1). surface and is interpreted to be composed of melt sheetOrientale is particularly interesting because, unlike most material. Last, the inner facies of the Maunder Formationlunar basins, its interior has not been completely filled with consists of higher‐albedo plains material that has a smoothmare deposits, permitting investigations into the volcanic surface texture, embays topography and is broken by distinctprocesses involved in the evolution and emplacement of mare linear fracture patterns; this facies is interpreted to be morebasalts [e.g., Head, 1974; Greeley, 1976; Spudis et al., 1984; pure impact melt lacking the abundance of admixed clastsHead and Wilson, 1992; Bussey and Spudis, 1997, 2000; that result in draping and cracking of the outer part of theYingst and Head, 1997]. The preservation of the Orientale Maunder Formation [Head, 1974; Moore et al., 1974; Spudisring topography distinctly separates its various mare deposits et al., 1984].and allows for a closer examination of their ages, compo-sitions and modes of emplacement. The nature, ages, andlocation of volcanic vents and deposits within multiringed 5. Distribution, Areas, and Volumes of Marebasins provide evidence for the role the basins play in the Basalts and Related Volcanic Featuresgeneration of volcanism and provide important information [37] Using M3 data, we first examined the Orientale regionon the thermal evolution of the lunar interior. for any evidence of previously undetected mare patches. The [34] There are four main rings associated with Orientale increased spatial and spectral resolution of M3 has allowedBasin (Figure 1a) [Head, 1974; Moore et al., 1974; Scott for the identification of several additional mare deposits to theet al., 1977; Wilhelms, 1987; Spudis, 1993; Head et al., west and south of the basin interior (Figure 2), although the1993; Head, 2010]. The ∼930 km Cordillera Mountain ring major distribution of mare from the southwest to the north-is an inward facing mountain scarp that defines the basin. The east of the basin remains unchanged. Orientale Basin itself issecond largest ring, the Outer Rook ring, is an interconnected ∼930 km in diameter and covers an area of ∼700,000 km2network of massifs spanning ∼620 km in diameter. The next [Head, 1974]. Within the basin, the largest of the mareinnermost ring is the Inner Rook ring, ∼480 km in diameter. It deposits is Mare Orientale, which covers an area ofis composed of many isolated massifs resembling central 52,700 km2 (Table 2). Determination of the volume ofpeaks in complex craters and together, these resemble a ring Mare Orientale requires information on the thickness of theof peaks as seen in peak ring basins. The most interior ring is a deposit. Previous estimates of the thickness of Mare Orientalecentral depression ∼320 km in diameter that has been inter- were less than ∼1–2 km [Head, 1974; Solomon and Head,preted to have formed by ∼3 km of thermal subsidence [Bratt 1980], and perhaps up to ∼1 km thick [Greeley, 1976; Scottet al., 1985a]. The preservation of this ring topography et al., 1977].separates the various mare deposits and allows for a closer [38] We used M3 spectral reflectance data to assess post-examination of their ages, compositions, duration of volca- mare impact craters that penetrate into and possibly throughnism and modes of emplacement. the mare deposit, and stratigraphic relationships, to assess the [35] The three large previously defined mare deposits in range of mare thickness and to derive an average thicknessOrientale Basin include (Figure 1) Mare Orientale, Lacus estimate (Figure 3a). Craters that impacted into the mare andVeris, and Lacus Autumni. Mare Orientale is located in the showed mare signatures in their interior and ejecta providedcenter of the basin, almost entirely encompassed by the rim of minimum thickness estimates, while impacts into mare thatthe central depression, and is the largest of the mare deposits. excavated nonmare highlands material provided maximumIncluded in the defined volcanic units of Mare Orientale is the estimates of the thickness of fill. These craters were chosenpolygonal mare deposit directly to the southwest of the center. based on their location in the mare deposits, although, theNext in areal extent is Lacus Veris, located between the Inner appearance of the crater and ejecta spectra were important asRook and Outer Rook rings. This mare deposit is composed well. It was necessary for the geologic materials to still haveof five large ponds along with several smaller scattered ponds an identifiable spectrum in order to differentiate between theall oriented in an arcuate belt, extending from ∼NNW to E. basaltic mare and more feldspathic highland signatures. MareSimilar in broad location within the basin interior is Lacus spectroscopic signatures contain mineral absorptions at bothAutumni, positioned between the Outer Rook and Cordillera 1 and 2 mm, owing to the presence of olivine and pyroxene inmountains between ∼ENE to E. Only three relatively shallow the mare basalt, and have lower albedos (lower reflectanceponds comprise Lacus Autumni. The deposits within each set values), as compared to the highland material. Spectral sig-of rings become progressively smaller in area with distance natures of highland material lack absorption features, butfrom the center of the basin. A large mafic ring occurs in the have higher albedo values. This is due to the fact that theSSW part of the Orientale interior, centered on the Outer highlands are thought to be composed largely of anorthosite,Rook ring (Figure 1c). which is usually spectrally neutral on the Moon. Anorthosite [36] In addition to the volcanic deposits located between has an absorption around 1.2 mm if Fe2+ is included in thethe basin rings, there are four main deposits related to the mineral structure [e.g., Bell and Mao, 1973; Adams and 7 of 33
  8. 8. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09 Figure 2. Areal distribution of mare ponds, sinuous rilles, and domes in Orientale Basin. Ponds are repre- sented by gray regions with black numbers, sinuous rilles are represented by red curvy lines with red letter labels, and domes are outlined in yellow.Goullaud, 1978] and if the rock has not been subjected to diameter relationships defined by Pike [1977], Il’in excavatessufficiently high shock pressures to erase evidence of this between ∼0.8 km [Stöffler et al., 1975] and ∼1.3 km [Melosh,signature [e.g., Adams, 1979; Johnson and Hörz, 2003]. 1989] into mare material (Figures 3a and 3b). The crater Il’in, [39] Crater impacts throughout lunar history have promp- however, is superposed on a mare wrinkle ridge (Figure 3b),ted lateral and vertical mixing of geologic materials over the and on the basis of the interpreted three‐dimensional struc-entire surface of the Moon. Mixing of feldspathic highland ture of these contractional features, it is possible that thematerial and mafic mare material can influence estimates mare basalts in which the arch and ridges formed have beenof the thickness of mare deposits as well as the composition thickened by the folding and thrusting thought to accompanyof mare basalt. Highland material could contaminate mare their formation [e.g., Lucchitta, 1976; Sharpton and Head,material and change the shape of the spectra, including the 1988].strength of the absorptions, and the location of absorption [41] The crater Hohmann, ∼100 km east of Il’in, has aband centers [e.g., Staid, 2000]. Several researchers have nonmare feldspathic signature on its rim and is partially filledinvestigated the mixing relationships along mare‐highland with mare material. Hohmann is ∼17 km in diameter, andboundaries, investigating the importance of lateral and ver- based on the relationships defined by Stöffler et al. [1975] andtical transport of materials [e.g., Fischer and Pieters, 1995; Melosh [1989] is estimated to have sampled between ∼1.1 kmMustard and Head, 1996; Mustard et al., 1998; Li and and ∼1.7 km into the crust (Figures 3a and 3c). HohmannMustard, 2000]. Mustard and Head [1996] investigated the crater rim is composed entirely of highland material, makingeffects of three contact geometries and found lateral mixing this crater the maximum limit for the depth of Mare Orientale.to be the dominant transport mechanism when considering [42] Interspersed throughout Mare Orientale are severalboundaries with highland massifs bordering mare deposits. smaller craters with diameters on the order of 4–5 km exca- [40] The ∼13 km diameter crater Il’in, in northwest Mare vating feldspathic basin material. Diameters of this sizeOrientale, lacks a highland feldspathic signature in its impact indicate sampling depths of < ∼300 m (Figure 3a). In addi-ejecta and interior [see also Staid, 2000]. Based on the depth/ tion, mare deposits along the margins of Mare Orientale show 8 of 33
  9. 9. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09Table 2. Areas, Volumes, and Elevations of Mare Ponds in Orientale Basin Identifiera Area (km2) Y‐H Areab (km2) Minimum Volume (km3) Y‐H Minimum Volumeb (km3) Elevation (km)Mare Orientale 52200 ‐ 10440 ‐ −2.741 25 ‐ 1 ‐ 2.372 230 ‐ 15 ‐ −0.1553 935 ‐ 65 ‐ −0.4274 80 ‐ 10 ‐ −0.1705 160 ‐ 10 ‐ 0.4096 160 ‐ 15 ‐ −0.3757 1620 ‐ 235 ‐ −2.378 75 ‐ 2 ‐ −0.6539 1580 1740 300 870 −1.0410 145 165 10 20 −1.0611 1505 1640 135 820 −1.0812 2200 2560 240 1280 −0.70713 720 ‐ 50 ‐ −2.4014 8890 11510 1385 5755 −0.52615 55 ‐ 3 ‐ −0.42516 140 ‐ 5 ‐ −0.77317 515 845c 25 150 −0.98118 165 ‐ 25 ‐ −1.1319 100 145 3 20 −0.6520 105 ‐ 5 ‐ −0.1921 815 905 65 160 −0.36922 2060d 2240 115 390 −0.23323 985 945 110 120 −0.14924 510d 730 25 130 −1.58 a Identifier refers to the numbering system introduced in Figure 2. b Areas and volumes calculated by Yingst and Head [1997] (Y‐H). c Pond is defined differently; Yingst and Head [1997] pond is a combination of our ponds 16 and 17. d Sections of these ponds were missing in our mosaic. Therefore, other data sets (i.e., Clementine albedo images) were used to fill in the gaps.evidence for lateral mixing, where highlands ejecta has been suggest a typical thickness of ∼200 m (Figure 3a), and kipukathrown in from the adjacent basin deposits, and vertical distribution data (Figure 3e) are consistent with this. Apply-mixing, caused by material being excavated from below thin ing this as an average thickness to the area of Mare Orientalemare deposits and incorporated into the regolith. For instance, provides an estimated volume of ∼10,440 km3 (Table 2).a good example of predominantly lateral mixing is observed [44] Volumes and areas of terrestrial continental floodin the rectangular mare deposit to the southwest of Mare basalts have been calculated by many researchers [e.g.,Orientale. It is mantled with feldspathic soils while most of Richards et al., 1989; Camp et al., 2003; Reidel, 2005; Jaythe craters show a mafic signature. However, the larger of the and Widdowson, 2008]. For instance, estimated volumescraters (∼2 km in diameter) have a faint feldspathic signature for the Deccan Traps are between 0.5 and 2 × 106 km3 overon their floor, similar to the soil signature in the polygon. In an area of 1 × 106 km2. In comparison to the Deccan Traps,comparison, small craters on the order of several hundred all of the flows that make up Columbia River Basalts havemeters in diameter have only a mafic signature. This suggests both a smaller calculated volume (∼2 × 105 km3) [Jay andthat locally this mare unit is very thin, less than 40 m thick Widdowson, 2008] and areal extent (>2 × 105 km2) [Reidel,(Figure 3d). 2005]. Mare Orientale has an area of ∼52,700 km2 and a [43] Furthermore, there are numerous kipukas (islands of volume of ∼10,400 km3, much less in extent and volume thanbasin deposits surrounded by, and protruding through, the the Columbia River basalts.maria) whose spectral properties are consistent with the [45] Lacus Veris (Figure 1) is the next largest mare deposit,Maunder Formation protruding through Mare Orientale with each of its five ponds varying considerably in area. The(Figures 1c and 3e). The abundance of these preexisting largest of the Lacus Veris ponds is ∼8,890 km2 and thetopographic features in the mare, combined with the shallow smallest pond is ∼145 km2. Using the largest craters to con-slopes from mare margins suggested by the decrease in strain the thickness of these deposits [Yingst and Head, 1997]kipuka density (Figure 3e), support the interpretation that gives minimal volume estimates ranging from ∼10 km3 toMare Orientale is shallow in these specific areas. For exam- ∼1,385 km3 (Table 2). The largest pond in Lacus Verisple, there is a line of preexisting basin topography through the appears to vary in thickness throughout its extent in a mannercenter of the basin protruding through the surface of the mare similar to Mare Orientale. In the northern part of the pond adeposit (Figures 3a and 3e). Thus, it appears that the center ∼7 km diameter crater excavates into mafic material, butof the basin, in a north‐south direction, is much shallower approximately 30 km southeast a ∼5 km diameter craterthan adjacent mare to its east and west. The juxtaposition of situated between kipukas excavates feldspathic materialall these features demonstrates that the basin topography (Figure 3f).underlying Mare Orientale varies significantly, especially [46] Lacus Autumni is composed of several large pondsfrom east to west. On the basis of these observations and data, as well. The largest of its three ponds covers an area ofour best estimates for the thickness and volumes of Mare ∼2,060 km2 and the smallest is ∼815 km2. These mare pondsOrientale are as follows: small crater depth of sampling data are some of the shallowest in the basin, as evidenced by the 9 of 33
  10. 10. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09 Figure 3. M3 thermal images (1489 nm) showing the variable thickness in Mare Orientale and Lacus Veris. (a) Map of Mare Orientale with thickness labeled, as inferred from crater excavation depths [Stöffler et al., 1975]. Craters highlighted in yellow represent those excavating mare material and those in white represent craters ejecting feldspathic basin material. (b) Il’in crater (∼13 km in diameter) excavates only mafic material. (c) Hohmann crater (∼17 km in diameter) flooded with mare, has a feldspathic rim. (d) Southwest polygon. (e) Kipukas protruding from mare surface in the center of the basin. (f) Close‐ up of the north central region in Lacus Veris with distinct example of thickness variation. Shares the same color scheme as Figure 3a.abundance of kipukas. Based on current crater investigations, small craters. The new total calculated area and volume forponds in Lacus Autumni have volumes between ∼65 km3 and ponds 1–24 from this study (19,350 km2 and 2,400 km3) is∼115 km3 (Table 2). smaller than the previous estimates of Yingst and Head [47] Yingst and Head [1997] previously investigated the [1997] (23,400 km2 and 9,700 km3). The total (minimum)areas and volumes of Lacus Veris and Lacus Autumni to volume of mare in the Orientale Basin is ∼46,000 km3,understand better the mode and rates of emplacement for throughout an area of ∼700,000 km2. The smaller areas andthese deposits. Both volume and area values were recal- volumes are very likely due to higher‐resolution data used inculated in the current analysis because of the availability defining the margins of the deposits, the spectral data thatof higher‐resolution image, compositional and topographic helped define the mare nature of smooth mare areas and thedata, allowing for more precise identification of ponds and 10 of 33
  11. 11. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09 Figure 4. Pyroclastic ring in the southwest of Orientale Basin. Elongate vent is located within the Outer Rook ring. (a) Thermal M3 image (2936 nm). (b) Standard M3 color composite (R, 1 mm IBD; G, 2 mm IBD; B, 1489 nm reflectance).high spatial and spectral resolution data that permitted much (Figure 2). Despite their small size, these deposits identifiedbetter estimates of the thickness of the mare basalts. with M3 data show that volcanism in Orientale was active [48] Yingst and Head [1997] found that South Pole– throughout the entire basin area and not simply confined toAitken contains ∼153,000 km3 of mare over its areal extent the eastern part of Orientale.(∼4,000,000 km2). Comparing the basin areas and mare areal [51] A large, 154 km diameter dark annular ring was firstextent of Orientale and South Pole–Aitken shows that, pro- discovered and documented in Soviet Zond 8 images [Lipsky,portionally, Orientale Basin has more mare covering its 1975], and is located in the south‐southwestern part ofinterior than South Pole–Aitken, when taking Mare Orientale Orientale Basin centered on the Outer Rook ring (Figure 4).into account. In contrast to these two incompletely filled Earlier interpretations concluded that the dark mantle ringbasins, most other lunar nearside basins have volumes deposit (DMRD) consisted of a large number of individualbetween 200,000 km3 (e.g., Tranquillitatis and Fecunditatis) vents producing an annulus of dark mantle pyroclasticto 1,000,000 km3 (e.g., Crisium, Serenitatis) [Bratt et al., deposits; the mantle was interpreted to have erupted from1985b], 4 to 20 times more mare than Orientale. vents marking the location of a 175 km diameter pre‐ [49] We identified several ponds (1, 4, 5, 6, 8) using M3 Orientale Basin crater, thus explaining its circular naturedata (Figure 1). Pond 4 (see identification scheme in Figure 2) [Schultz and Spudis, 1978]. This interpretation had a signif-is ∼80 km2 and lies just inside massifs on the western side of icant influence on attempts to locate the Orientale Basinthe Inner Rook ring. Ponds 2 and 3, ∼230 km2 and ∼935 km2, transient cavity rim crest, since the presence, and thus pres-have been noted previously in Zond and Galileo data [Scott ervation, of such a large crater would tend to place the tran-et al., 1977; Kadel, 1993; Head et al., 1993] and both are sient cavity rim crest inside its location, at or inside thelocated adjacent to the Outer Rook ring, in a similar position Inner Rook ring [Schultz and Spudis, 1978].to the ponds of Lacus Veris. Ponds 1, 5, and 6, having areas of [52] Clementine data revealed the presence of a 7.5 km∼25 km2, ∼160 km2, and ∼160 km2, lie along the scarp of the wide by 16 km long elongate depression located at the centerCordillera mountain ring and mirror the occurrence of the of the DMRD [Weitz et al., 1998; Head et al., 2002].ponds of Lacus Autumni. Placing volumetric constraints on This elongate depression, similar to features seen in associ-these identified ponds is more difficult because they have ation with other dark mantle deposits (e.g., Sulpicius Gallusvery few impacts owing to their small size. Table 2 shows [Lucchitta and Schmidt, 1974]), had no obvious adjacentminimum estimates of these pond volumes derived from deposits of dark mantle or mare, but on the basis of its locationcraters with mafic signatures. in the approximate center of the DMRD, Weitz et al. [1998] [50] Other small deposits have been identified around and and Head et al. [2002] investigated the possibility that itbetween previously known mare deposits. Pond 20, covering could be a source crater for an eruption producing the DMRD.∼105 km2, has been identified to the east of the Outer Rook They proposed that the dark ring is the manifestation of aring. Unlike the other deposits identified by M3, this mare pyroclastic eruption originating at a fissure vent (the elongatepond appears to mantle the preexisting topography instead of depression) and forming an Ionian‐like eruption plume. Headponding in a topographic low. Many other small deposits et al. [2002] outlined a scenario in which the event producinghave been identified between the ponds in Lacus Autumni the eruption began with a dike rapidly emplaced from sub-and spread around the eastern base of the Outer Rook ring crustal depths to within ∼3–4 km of the surface. The dike 11 of 33
  12. 12. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09stabilized and degassed over ∼1.7 years to form an upper there is no reason to believe that they lie outside the range offoam layer, which then penetrated to the surface to cause an model ages for the mare deposits.eruption, lasting ∼1–2 weeks. This eruption produced an [56] The total duration of volcanic activity in Orientalealmost 40 km high symmetrical spray of pyroclasts into the spans 0.85–1.50 Ga (see section 7), but despite this lengthylunar vacuum at velocities of ∼350 to ∼420 m/s; as a result of duration, the ponds have relatively low volumes [Yingst andthis eruption, the pyroclastic material accumulated in a Head, 1997]. In comparison to other large impact basins onsymmetrical ring around the vent to produce the DMRD. the Moon, Orientale contains a very small total volume ofHead et al. [2002] attributed the paucity of pyroclastic rings mare material, ∼46,000 km3. Bratt et al. [1985b] calculatedof this type on the Moon to the low probability of a dike total volumes for basins on the nearside of the Moon, whichstalling at just the right depth (∼3–4 km) to create these include the volume of the topographic depression and mareeruption conditions. fill. For comparison, these volumes are taken as the maximum [53] In general, lunar volcanic glasses can be identified in volume of mare. Serenitatis and Nectaris, two basins similarspectra by broad absorptions at 1.0 and 2.0 mm. The amount in size to Orientale, have total basin volumes of 1 × 106 km3of titanium in the glasses can be determined by an iron‐ and 7 × 10 5 km 3 , respectively. According to Bratt et al.titanium charge transfer absorption feature in the visible [1985b], Orientale has a total basin volume of 7 × 105 km3,region of the electromagnetic spectrum. The higher the Ti meaning that there is ∼6.55 × 105 km3 that was not filledcontent in the glasses, the stronger the absorption edge in the with mare. Consequently the mare load in the center ofvisible [Bell et al., 1976]. Black beads that represent the Orientale Basin is not as massive as the initial central loadscrystallized equivalents of the orange glasses have a strong, that are likely to have characterized the deeper basin interiorbroad absorption centered at ∼0.6 mm that is due to the in other lunar basins. For example, Solomon and Head [1980]presence of ilmenite. estimated the thickness of Mare Orientale to be 1–2 km in [54] Galileo images provided the first multispectral modeling the mare load and its influence on flexure. Theimages of the DMRD region and the dark halo deposit itself; mantle plug that was uplifted into the excavated cavity whenPieters et al. [1993] showed that the ring deposit is some- it collapsed [Wieczorek and Phillips, 2000], and its super-what brighter than nearside mantling deposits, has a very isostatic state may thus be of more significance in terms ofweak 1 mm absorption band, and has an ultraviolet‐visible a cause of the local extensional stress along the edge of the(UV/VIS) ratio that is relatively high but lower than that seen basin.in nearside deposits of black spheres. They concluded thatthese characteristics could be consistent with contamination 6. Craters Kopff and Maunderof the deposit by highland material (as suggested by Greeleyet al. [1993]). They also proposed an alternative interpre- [57] Two large craters in the interior of Orientale, Maundertation, that the actual spectral properties of the pyroclastics and Kopff (Figure 5), have attracted attention for years duethemselves dominate the measured spectral properties. In the their similar sizes but contrasting morphology. A first‐orderfirst case the deposits could be interpreted as similar to examination of the 55 km diameter Maunder crater indicatesilmenite‐rich dark mantling material such as the black beads that it is a classic example of an impact crater, having a centralsampled at Apollo 17. In the second case, however, the weak peak, a flat floor, terraced walls, a raised rim and continuous1 mm band could indicate that the deposits are not homoge- ejecta deposit, and a system of radial rays [El‐Baz, 1974;neous glass but are in a crystallized form. The low UV/VIS Pike, 1980]. In contrast, the 42 km diameter Kopff craterratio, relative to black beads seen elsewhere, could be due to has no central peak, no wall terraces, an unusual rim shape,the lower abundance of ilmenite, the opaque component that and unusual smooth crater ejecta deposits and secondaries.causes darkening. In the latter case, the Orientale ring deposit Several authors hypothesized that Kopff crater was volcanicmight have affinities with the local medium‐Ti deposits in origin based on its subdued morphology [McCauley, 1968;identified in the Orientale region [Greeley et al., 1993]. Guest and Greeley, 1977], with one hypothesis favoring an [55] The Clementine UV/VIS spectral data for the explosion caldera [Pai et al., 1978]; other researchers favoredOrientale DMRD show a slight absorption at 0.9–1.0 mm, an unusual impact event, for example, into partially moltensimilar to that seen in Aristarchus DMD spectra [Weitz et al., material [Wilhelms and McCauley, 1971; Guest and Greeley,1998]. On the basis of these characteristics, the beads that 1977; Spudis et al., 1984]. Still others hypothesized thatcompose the Orientale DMRD were interpreted to be domi- Kopff was a volcanically altered impact crater [Schultz, 1976;nated by glasses, rather than the crystallized beads typical of Wilhelms, 1987].the Taurus‐Littrow DMD deposit. The implications are that [58] The rim crest of Kopff is very irregular, with multiplethe DMRD beads cooled rapidly after eruption at the surface, undulations along its circumference. The largest crater rim‐preventing crystals from forming. Low optical density in the to‐floor relief is ∼1.7 km. This is less than half of the cratereruption plume, due to either a high gas content or a low mass rim crest‐to‐floor measurement of Maunder, ∼4 km deep. Theeruption rate, is required to enhance rapid cooling and explain depth of Kopff is too shallow to be a typical fresh impactthe dominance of glasses in the deposit. This implies rela- crater and its floor diameter of ∼36 km is too large [Pike,tively rapid cooling times for the eruption products, consis- 1980] (Figures 6a and 6b). Both these characteristics, intent with the Head et al. [2002] scenario of a dike stalling and addition to the observed floor flooding, indicate that the craterbuildup of volatile foams prior to eruption. Although analysis has undergone at least some volcanic modification. Mea-is not yet complete, M3 data suggest that the pyroclastic surements of morphologic features that would not be affecteddeposits have been weathered and are glass‐rich only at by volcanic modification, such as rim height and width,small fresh craters. Although the age of these deposits is not indicate that Kopff has characteristics consistent with anknown due to the difficulty of dating friable mantle deposits, impact crater (Figures 6c and 6d). Its rim height value is on 12 of 33
  13. 13. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09 [60] In order to evaluate processes of modification for Kopff, Maunder crater was “flooded” to an elevation of approximately −2.6 km from its rim crest (comparable to the depth of the floor of Kopff below the rim crest) to evaluate its morphology (Figure 7). Comparing the still uncovered por- tion of Maunder with the morphology of Kopff could help to determine if the flooding scenario is possible. Assuming an initial depth of 3.5 km for the floor of Kopff, this flooded elevation is similar to the amount of flooding Kopff is assumed to have experienced. At this flood level (∼2.2 km), Maunder is filled with a volume of ∼1900 km3. Using the various morphometric impact crater measurements for Kopff [Pike, 1980] then yields a volume of ∼1700 km3. This cal- culated volume is significantly different from that determined from compositional information. The ∼3 km diameter crater on the east of Kopff floor that excavated nonmare material may have impacted into an uplifted crater floor, or possibly a buried slump terrace. [61] The floor of Kopff is very asymmetric due to a small depression in its center and a slight bulge on its eastern side, superposed by a floor fracture. The uneven nature of the mare floor indicates that after flooding, some type of volcanic activity continued to occur and eventually warped the surface. The bulge and fracturing of the floor could result from a small mare intrusion forming beneath the crater floor, creating a floor‐fractured crater [Schultz, 1976]. [62] In addition to the unusual morphology of Kopff, crater count statistics provide additional evidence concerning its origin. The Maunder Formation has been dated at ∼3.64 Ga on average, and Kopff formed at ∼3.63 Ga, based on model ages derived from its smooth ejecta deposit (Table 3). The Kopff impact occurred soon after the emplacement of the melt sheet. If the central uplifted area of the Orientale Basin and the overlying melt sheet had not thermally equilibrated after ∼10 Ma, perhaps the thermal state of the substrate influenced the formation and early modification of the morphology of Kopff [e.g., Spudis et al., 1984]. Dates on the volcanic flooding of the interior of Kopf are ∼3.36 Ga, significantly later than the formation of the crater itself. This favors vol- canic modification of an unusual crater type.Figure 5. (a) M3 thermal image (2936 nm) of Maunder andKopff craters. (b) Standard M3 color composite (R, 1 mm 7. Ages of Mare BasaltsIBD; G, 2 mm IBD; B, 1489 nm reflectance). [63] Crater production and chronology functions [e.g.,the lower end of collected measurements, but this could be the Neukum, 1983; Ivanov et al., 1999, 2001; Hartmann et al., 2000; Neukum et al., 2001] have been developed that per-result of impact into partially molten material. In comparison, mit the assignment of ages from crater size‐frequency dis-the rim height of Maunder is slightly above the trend, likely to tributions for the surfaces of the terrestrial planets. Craterbe the result of its location on preexisting topography along counting is very useful for dating lunar surface units due tothe edge of the inner depression. the high degree of preservation of impact craters. This dating [59] An impact crater ∼3 km in diameter located on the method can be employed to determine the timing of largeeastern edge of the mare‐covered floor of Kopff has exca- basin‐forming impacts, ages of different volcanic units orvated anorthositic material (<2 wt % FeO) [Bussey and flows, and the duration of volcanism. Important considera-Spudis, 2000], confirmed here with M3 data (Figure 5b). In tions when dating volcanic surfaces include the effects ofthe standard M3 color composite (Figure 5b) the rim of Kopffhas a distinctly feldspathic signature, suggesting that it is resurfacing events, secondary craters and endogenic craters [e.g., Hiesinger et al., 2000; Greeley and Gault, 1971;composed largely of premare basin material. The depth of Oberbeck and Morrison, 1974], all of which can produceexcavation of the ∼3 km superposed crater is ∼0.2 km, con- ages that differ from the ages of the actual surface units.straining the mare deposit volume to be <145 km3, using [64] Crater count analyses have been conducted on Maretechniques described by Pike [1977], Stöffler et al. [1975], Orientale, Lacus Veris and Lacus Autumni by previousand Yingst and Head [1997]. If Kopff was a typical impact researchers [e.g., Greeley et al., 1993; Kadel, 1993; Morotacrater, its depth should be on the order of ∼3.5 km (Figure 6a). et al., 2010]. Initially, Hartmann and Yale [1968] interpreted 13 of 33
  14. 14. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09 Figure 6. Graphs modified from Pike [1980] to include Maunder and Kopff crater characteristics. All values are measured in kilometers. (a) Crater depth versus rim crest diameter. (b) Floor diameter versus rim crest diameter. (c) Rim height versus rim crest diameter. (d) Rim width versus rim crest diameter.early crater counts to mean that Lacus Veris and Autumni and Spudis, 2000] and the identified mare ponds in Lacusoccurred soon after the Orientale event, followed later by Veris and Lacus Autumni of sufficient size to produce reliableinfilling of Mare Orientale. Using Lunar Orbiter photo- model ages (Table 3). Previous estimates of the model agegraphs, Greeley et al. [1993] determined that the oldest mare of the Orientale Basin event are ∼3.8 Ga [Baldwin, 1974;was emplaced in south central Mare Orientale at ∼3.70 Ga, Nunes et al., 1974; Schaeffer and Husain, 1974; Neukum,followed by the emplacement of maria in western and 1983; Baldwin, 1987; Wilhelms, 1987; Kadel, 1993].southeastern Mare Orientale at ∼3.45 Ga. Moving further Updated crater age equations produce a model age offrom the center of the basin, Greeley et al. [1993] showed ∼3.68 Ga for the Orientale impact event (Figure 8). The olderthat Lacus Veris had an average model age of ∼3.50 Ga, and Neukum [1983] production and chronology functions yield aLacus Autumni is the youngest deposit with a model age of model age of ∼3.8 Ga, using the counted regions from this∼2.85 Ga. Even younger model ages have been reported for study. This difference in calculated ages arises from an updatecentral Lacus Veris, at ∼2.59 and ∼2.29 Ga [Kadel, 1993]. of Neukum production and chronology equations. NewBased on these findings, mare volcanism in Orientale was counts on impact craters in and around Orientale Basin per-estimated to have lasted between 0.85 and 1.50 Ga [Greeley mitted a reestimation of the size‐frequency distribution curveet al., 1993], beginning ∼100 Ma after basin formation. for craters between 1 km and 20 km [Neukum et al., 2001]. [65] Using high‐resolution M3 data at 140 m/pixel, as Earlier basin model age estimates made using the Hartmanncompared to the Lunar Orbiter ∼3.5–7.6 km/pixel resolution production function could differ as a result of the inherentused by Greeley et al. [1993], crater counts were conducted differences between the two approaches for deriving a produc-on the ejecta deposit of Orientale Basin, parts of Mare tion function. Neukum uses an 11th‐order polynomial, whereasOrientale, the Maunder Formation [see Head, 1974; Bussey Hartmann uses a log incremental equation to approximate 14 of 33
  15. 15. E00G09 WHITTEN ET AL.: VOLCANISM IN THE LUNAR ORIENTALE BASIN E00G09 Figure 7. (a) M3 thermal image (2936 nm) of Maunder crater in its current state. (b) M3 thermal image of Kopff crater. (c) M3 thermal image of Maunder “flooded” to the same proportion that Kopff is suspected to be flooded based on crater morphology relationships defined by Pike [1980]. (d) Graph of the volume of mare versus the elevation of the mare surface at 200 m steps of flooding in Maunder.the size‐frequency distribution curve. These two functions craters > 0.75 km on the Maunder Formation in an attempt todiffer most at diameter bins between 2 and 20 km [Neukum count the most preserved craters on each deposit. Our cal-et al., 2001, Figure 8]. culated crater retention ages are almost the same, ∼3.68 Ga [66] Our data suggest that the melt sheet material of the for the ejecta and ∼3.64 for the melt sheet; this suggestsMaunder Formation solidified shortly after the basin forming our efforts were successful. Mare Orientale was emplacedevent, ∼3.64 Ga. In theory, the melt sheet and basin ejecta shortly after the Maunder Formation ∼3.58 Ga (Figure 8 andages should be the same, since the melt is produced instan- Table 3). This delay of ∼60–100 Ma between basin formationtaneously with the impact, and comes to rest during the short‐ and volcanism in Mare Orientale, and later extended vol-term modification stage, the terminal stage of the cratering canism in Lacus Veris and Lacus Autumni argues againstevent. The discrepancy between ages is likely to be due to Orientale impact pressure release melting (Table 1) being adifferences in the degree of crater preservation, or possibly significance factor in mare basalt production [e.g., Elkins‐due to differences in material properties [e.g., van der Bogert Tanton et al., 2004], at least for the last volcanic depositset al., 2010]. Melt sheet material tends to drape topography, that reset the age of these surfaces.creating smooth surfaces that allow for the preservation of [67] Impact craters were counted, size‐frequency distribu-smaller craters (∼<10 km). Ejecta blankets tend to have tions were compiled and ages were calculated for the fiverougher textures that may lead to poorer preservation of largest ponds in Lacus Veris; this yielded an age range ofsmaller craters. Thus, the melt sheet age is typically the more ∼3.20 to ∼3.69 Ga (Figures 9a–9f and Table 3). These modelreliable of the two for the age of the impact event. However, ages are relatively consistent with those calculated fromwe counted craters > 5 km on the Hevelius Formation and previous studies, but these are differences in grouping and 15 of 33

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