Mineralogy of the last lunar basalts - Brown University27,888 STAID AND PIETERS: MINERALOGY OF THE...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. Ell, PAGES 27,887-27,900, NOVEMBER 25, 2001

Mineralogy of the last lunar basalts: Results from Clementine

Matthew I. Staid

U.S. Geological Survey, Flagstaff, Arizona, USA

Carl• M. Pieters

Department of Geological Sciences, Brown University, Providence, Rhode Island, USA

Abstract. The last major phase of lunar volcanism produced extensive high-titanium mare deposits on the western nearside which remain unsampled by landing missions. The visible and near-infrared reflectance properties of these basalts are examined using Clementine multispectral images to better constrain their mineralogy. A much stronger 1 }am ferrous absorption was observed for the western high-titanium basalts than within earlier mafia, suggesting that these last major mare eruptions also may have been the most iron-rich. These western basalts also have a distinctly long-wavelength, 1 }am ferrous absorption which was found to be similar for both surface soils and materials excavated from depth, supporting the interpretation of abundant olivine within these deposits. Spectral variation along flows within the Imbrium basin also suggests variations in ilmenite content along previously mapped lava flows as well as increasing olivine content within subsequent eruptions.

1. Introduction

The lunar maria encompass a wide range of basaltic compositions. The Apollo and Luna missions sampled a subset of these basalts, which erupted between 3.9 and 3.1 billion years ago [Lofgren et al., 1981 ]. However, the last major phases of lunar volcanism produced extensive deposits on the western nearside which remain unsampled [Boyce, 1976; Soderblorn et al., 1977; Whitford-Stark and Head, 1980]. Crater degradation studies have dated several western flows as younger than 3.0 Ga [e.g., Boyce, 1976 and Schaber, 1973a], and other studies suggest that some of the western high titanium basalts may have erupted a billion years or more after those in the sample collection [Schultz et al., 1976; Schultz and Spudis, 1983; Young, 1977; Hiesinger and Head, 1999]. Telescopic spectral measurements and Apollo gamma-ray data demonstrate that most of the western basalts erupted during the Era- tosthenian period (-•3.2-1.1 Ga) are rich in titanium [Pieters, 1978; Davis, 1980; Wilhelms, 1987]. However, unlike the older high-titanium basalts sampled by Apollo 11 and Apollo 17, spectra of the western high-titanium flows also exhibit a unique combination of a strong 1 gm feature and a relatively weak or attenuated 2 gm absorption [Pieters et al., 1980]. Orbital data also suggest that the western mafia are more radioactive than eastern deposits [Soderblorn et al., 1977; Ad- ams et al., 1981; Lawrence et al., 1998], perhaps providing a source of heat for late stage melting [Taylor, 1982; Wilhelrns, 1987]. These properties indicate fundamental compositional differences resulting from major changes in the nature the Moon's volcanism over time. Though lunar volcanism was

Copyright 2001 by the American Geophysical Union

Paper number 2000JE001387. 0148-0227/01/2000JE001387509.00

regionally complex during the peak period of basalt emplacement, the Moon's last eruptions of mare basalt produced unique compositions that are not observed elsewhere on the lunar surface.

In the current study the visible and near-infrared spectral properties of the high-titanium western basalts are compared to other nearside deposits to provide new constraints on their mineralogy. The reflectance properties of crystalline crater materials within the western basalts are examined using the high spatial resolution data from the Clementine ultraviolet- visible (UVVIS) camera. These observations provide spectral information about relatively unweathered basalt regoliths for comparison with surface soils. Clementine UVVIS data are also used to examine spectral variations along previously mapped flows in Mare Imbrium [Schaber et al., 1970; Schaber, 1973a, 1973b], thus providing information about how the mineralogy of the Imbrium flows may have evolved during emplacement and over subsequent eruptions. To- gether, this information can be considered within a petrologic framework to better constrain the mineralogy of the last extensive eruptions of lunar basalts.

2. Previous Work

Experimental studies of mare basalt compositions suggest that the lunar basalts were produced by partial melting of mafic sources within the lunar mantle at depths of < 550 km [Lofgren et al., 1981; Longhi, 1992; Wieczorek and Phillips, 2000]. These sources are generally thought to be the cumu- late rocks produced during the early differentiation of the Moon. A useful chemical property for separating the returned mare basalts into compositional groups is TiO2 content. High-titanium basalts in the sample collection include Apollo 11 low- and high-K samples as well as Apollo 17 basalts. Low-titanium basalts include the Apollo 12 olivine, pigeonite,

27,887

27,888 STAID AND PIETERS: MINERALOGY OF THE LAST LUNAR BASALTS

and ilmenite basalts as well as Apollo 15 olivine and pigeonite basalts. Very low titanium basalts appear in the samples returned by Luna 24 and in small amounts of the Apollo 17 samples. A summary of these sample groups with respect to TiO2 content is presented by Papike et al. [1998]. Varia- tions within many subgroups can be related to low-pressure fractionation of olivine or clinopyroxene in the low-titanium basalts or Fe-Ti oxides in the high-titanium basalts. However, in general, it is not possible to relate the high-, low-, and very low titanium basalts through low-pressure fractionation schemes [Lofgren et al., 1981; Longhi, 1992], indicating different sources for the three groups.

Remote observations of the lunar surface have greatly expanded the study of mare basalt types beyond those represented in the returned samples. Apollo X-ray and gamma-ray data have provided compositional information for limited areas of the Moon at low spatial resolutions [e.g., Davis, 1980] and the recent Prospector mission has expanded such measurements to global data sets at 30 +/-15 km/pixel resolution [e.g., Lawrence et al., 1998, 2000; Elphic et al., 2000; Feldman et al., 2000a, 2000b; Maurice et al., 2000]. Earth- based telescopic studies in the 0.4-2.4 lam range also have provided information about the compositional diversity of lunar basalts. Specifically, telescopic studies have classified unsampled regions into a number of "basalt types" based on continuum shape, albedo, and near-infrared absorption features in mature soils [Pieters and McCord, 1976; Johnson et al., 1977; Matson et al., 1977; Pieters, 1978, 1993]. In the Pieters [1978] nomenclature the first letter characterizes the ultraviolet/visible ratio related to titanium content (H, high; h, medium high; m, medium; L, low), the second letter to albedo (D, dark; I, intermediate; B, bright), and the last two letters to the strength of ferrous absorptions at 1 and 2 lam bands (S, strong; G, general; or W, weak for the 1 lam band and P, prominent; A, attenuated; or-, undetermined for the 2 •tm absorption). Through the combined efforts of sample and remote studies it has become clear that there is no simple correlation of iron and titanium content with age for the lunar basalts. The complex spatial and stratigraphic distribution of lunar basalt types requires more complex models of mare basalt genesis than the effects of partial melting zones moving to greater depth through time [Lofgren et al., 1981]. Instead, lunar volcanism appears to have been multiphased with basin- scale magmatism evolving independently of neighboring regions [Pieters, 1993].

The Apollo 11 and 17 missions collected high-titanium basalts from the ancient mare deposits on the eastem nearside of the Moon. Radioactive isotope dates for these samples range from 3.55 to 3.85 Ga and are older than most other mare basalt samples (summary given by Wetherill et al. [ 1981 ]). The distribution of the Apollo 11 type high-titanium basalts was mapped telescopically as the HDWA spectral class by Pieters [1978] and occur primarily within Mare Tranquillitatis. Johnson et al. [1991a] and Melendrez et al. [1994] further refined the characterization of the high- titanium soils on the basis of detailed mapping of the UV/VIS

Mare Tranquillitatis can be subdivided into three distinct compositional and stratigraphic units. The oldest of the three consist of high-titanium basalts (Th) associated with the brightest and most heavily cratered mare regions within cen- tral Tranquillitatis. Except for small topographically high regions, most of these basalts were buried by later flows. Above the Th basalts, a higher-titanium unit (Tvh-B) extends across the entire basin and appears to be the source of high- titanium, low-potassium basalts sampled at both Apollo 11 and Apollo 17 [Snyder et al., 1992, 1994; Staid et al., 1996]. The youngest and highest-titanium basalts in Tranquillitatis (Tvh-A) Occur in the western portion of Tranquillitatis and are thought to be related to the higher-potassium, high-titanium Apollo 11 Group A basalts [Jerde et al., 1994; Staid et al., 1996].

The dark and relatively blue (high UV/VIS ratio) volcanic flows on the western nearside are believed to have TiO2 abundances which are similar to the older Apollo 11 and 17 high-titanium basalts on the basis of Apollo gamma-ray data [Davis, 1980] and their ultraviolet-to-visible spectral properties [Charette et al., 1974; Pieters, 1978; Johnson et al., 1991a; Giguere et al., 2000]. These western high titanium flows cover older mare units within the Imbrium basin and

the eastern portion of Oceanus Procellarum. Within the Im- brium basin the young high-titanium flows flood small and large Eratosthenian craters and cover older Irabrian mare units, providing a clear context for their stratigraphic age [Wilhelms, 1987]. The Eratosthenian flows in the Imbrium basin are particularly well mapped, and three extensive flow phases extend north and northeast for great distances [Schaber, 1973a, 1973b]. In northern Oceanus Procellarum, high-titanium flows also are relatively young, and some have been dated as only twice the age of the crater Copernicus, or 1.6-2.0 b.y. old [Young, 1977]. More recent crater counts [Hiesinger and Head, 1999; Hiesinger et al., 2000] date a small area of the hDSA spectral type in northern Oceanus Procellarum (the Roris Basalt, Sharpe Formation) as young as 1.3 Ga. The distribution of these Eratosthenian high-titanium deposits as mapped telescopically by Pieters [1978] is shown in Figure 1.

Though the high-titanium deposits in Mare Imbrium and Oceanus Procellarum have ultraviolet to visible reflectance

properties which are similar to the older Apollo 11 type basalts, Pieters [1978] classified the western basalts into separate spectral types (HDSA and hDSA rather than the Apollo 11 HDWA class) on the basis of differences in mafic band shape and strength at longer wavelengths. The hDSA and HDSA basalts occur together in the Procellarum region and are similar enough spectrally to be considered two subgroups of the same basalt type that vary in titanium content [Pieters, 1978]. Because these two western deposits exhibit albedo and UV/VIS ratio properties in the same range as the Apollo 11 basalts, ilmenite abundance and its effect on absorption strength are assumed to be comparable. Therefore the relative strength of optical absorption bands is most likely to be con- trolled by the relative abundance of pyroxene, glass, and oli-

spectral properties of the Tranquillitatis region, identifying at vine in a similar manner for both high-titanium soils [Pieters least two compositionally distinct mare units with respect to et al., 1980]. This effect is demonstrated in continuum- wt% TiO2. Staid and Pieters [1996] continued the classifica- removed telescopic spectra of mare soils from several eastem tion of Tranquillitatis basalts through an analysis of Galileo and western maria as reproduced in Figure 2 from Pieters et Solid-State Imaging (SSI) multispectral data and Clementine al. [1980]. The pyroxene contributions to the spectra of high images. As a result, the high- to very high titanium basalts in titanium mare soils of Apollo 11 and Flamsteed 1 are thought

STAID AND PIETERS: MINERALOGY OF THE LAST LUNAR BASALTS 27,889

UNIT TYPE DESIGNATION AREA

HDSA "• FLAMSTEED (RING) hDSA •-• IMBRIUM (BLUE)

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Figure 1. Telescopic classification of basalt types after Pieters [ 1978] relevant to this study. The first letter characterizes the ultraviolet/visible ratio related to ti.tanium content (H, high; h, medium high; m, medium), the second letter to albedo (D, dark; !, intermediate), and the last two letters to the strength of the I and 2 lam bands (S, strong, W, weak; P, prominent; A, attenuated). Modified from Pieters [1978] and reprinted with permission from Elsevier Science.

to be equivalent because the two basalt types share a similar, but weak, 2 lam band [Pieters et al., 1980]. However, the 1 lam band depth for the younger HDSA basalts is approximately twice as strong as that of the older Tranquillitatis basalts for regions of optically mature soils [Pieters, 1978]. Pieters et al. [1980] also observed that the deep I lam mafic band of the western HDSA and hDSA soils are centered near

I lam and hav.e a distinctly broad and asymmetric shape. On the basis of such measurements, Pieters et al. [1980] con- cluded that the unusual strength and shape of the I lam absorption in the western high-titanium basalts results from an additional ferrous absorption from olivine and/or iron-beating glass. Because olivine and glasses are less absorbing than

ß

pyroxene and require a large abundance to be detect.ed, the olivine- or glass-to-pyroxene absorption would have to be > 1 [Pieters et aJ., 1980]. Either abundant olivine or Fe-rich glass could produce the strong, long-wavelength 1 lam band, but a glassy iron-rich surface throughout the western deposits could

rich mare soils [Pieters and McCord, 1976]. Compositional interpretations Of these measurements rely on the assumption that as regional basalt surfaces weather to soil their ge o - chemical identity is maintained even if much of the original mineralogical character is lost [Pieters, 1977]. Because space-we•athering processes weaken diagnostic absorption features, it is difficult to characterize the detailed mineralogy of the parent basalt from spectra of mature soils. Composi- tional interpretations based on regional spectra of mature mare surfaces are also-problematic because all lunar soils may also contain a significant fraction of foreign nonmare materials [Laul and Papike, 1980; Haskin and Warren, 1991; Jolliff et al., 2000]. However, laboratory reflectance studies of lunar soil and rock powders demonstrate that crystalline lunar materials exhibit very diagnostic absorption features related to sample mineralogy [e.g. Adams and McCord, 1970 and McCord and Adams, 1973]. Observations of relatively crystalline and unweathered b,asaltic regoliths such as those ex-

only form by rapid cooling along the exterior surfaces of posed at young impact craters are thought to be the best ana- flows. There is no photogeologic evidence for a regional volcanic glass component contributing to the strong, long- wavelength I lam band; instead, the distinct mafic absorption is observed uniformly throughout the western hDSA and HDSA ba•salts in Oceanus Procellarum and Mare Imbrium

[e.g., Pieters and McCord, 1976]. Previous telescopic classification of unsampled basalts has

been based on spectral measurements of mature agglutinate-

1ogue to sample rock powders. As a result, mare craters are believed to be important sites for the remote assessment of the mineralogy of unsampled volcanics because their ejecta blankets expose less weathered materials [e.g. McCord et al., 1981]. Telescopic observations of fresh mare craters [e.g. Pieters, 1977] demonstrate distinct mineral absorption features which can be interpreted using returned lunar samples. However, telescopic spectral measurements of craters smaller


Eastern Mare Soils

' '•' Apollo 11

Mare SeremtaUs miSP

0.8 1.2 1.6 2.0 2.4

Western Qh Titanium Mare Soils • Flaresteed

' HDSA

0.8 1.2 1.6 2.0 2.4

Wavelength (pm) Figure 2. Near-infrared telescopic spectra of mature mare soils within the four basalt types discussed [from Pieters et al., 1980]. A straight line continuum has been removed from these spectra to enhance the visibility of spectral differences between mare units.

than -5 km are very difficult to obtain because of the practical resolution limits of telescopes on Earth [Pieters, 1993]. Be- cause the depth of material excavated by craters scales as a function of diameter [Melosh, 1989], a 5 km crater has sampled at least the upper 500 m of the mare surface. As a result, telescopic measurements have not allowed studies of craters small enough to guarantee that individual basalt types are measured without the mixing of underlying matedhals from different geochemical units [Pieters, 1993]. The western high-titanium flows are particularly thin, and many craters resolvable with telescopic data are believed to have excavated earlier low-titanium units [Pieters, 1977; Pieters et al., 1980].

In the spring of 1994 the Clementine satellite acquired high spatial resolution multispectral images (-100-200 m/pixel) from the ultraviolet to the near-infrared of more than 90% of

the lunar surface [Nozette et al., 1994; Eliason et al., 1999]. Clementine's UVVIS camera contained five filters centered at

415, 750, 900, 950, and 1000 nm. Unlike previous Earth- based and Galileo imagery, Clementine resolved the spectral properties of immature crater deposits small enough to sample individual volcanic flows [Staid and Pieters, 1996, 2000]. A

strategy has been developed to reevaluate lunar basalt types using Clementine images of such fresh mare craters and their associated soils [Staid and Pieters, 1998, 2000]. This approach was previously applied to the compositional study of early eastern mare units whose compositions are better known and sampled than the late stage western basalts [Staid and Piet,ers, 2000].

3. Analysis of Clementine UVVIS Images Clementinc UVVIS data of the western high-titanium ba-

salts are examined for large regions of Oceanus Proccllarum (HDSA) and Marc Imbrium (hDSA). The spectral properties of these western regions were compared to the sampled Apollo 11 basalts in Marc Tranquillitatis (HDWA), which have similar albedos and ultraviolet-to-visible spectral proper- tics and are thus inferred to be compositionally similar with respect to TiO: content (as ilmcnitc). For reference, the west- crn basalts also were compared to the low-titanium basalts in Marc Scrcnitatis (mISP). The iron-rich composition of these Serenitatis basalts has been well established by a wide variety of orbital and telescopic remote-sensing data [e.g., Pieters, 1978; Davis, 1980; Lucey et al., 1998; Staid and Pieters, 2000]. According to remotely sensed parameters and assum- ing that surface rcgolith compositions faithfully represent basalt compositions, the basalts within Scrcnitatis and Tran- quillitatis are among_the most iron-rich basalts on the eastern nearside [Lucey et al., 1998; Lawrence et al., 1998; Staid and Pieters, 2000]. The general location of each study region and their associated spectral classifications [Pieters, 1978] are provided in Figure 3a.

The Clementinc UVVIS images require extensive calibration and processing to produce multispectral image mosaics [Pieters et al., 1994]. A U.S. Geological Survey (USGS) five-band calibrated mosaic with a spatial resolution of 0.5 km/pixcl [Eliason et al., 1999] was used to compare the spectral properties of each study area within several useful parameters. Full spatial resolution mosaics (100 m) also wcrc prepared by the authors to examine the reflectance properties of small marc craters within each region. The following steps were used in calibrating each frame to derive bidirectional reflectance and are discussed in detail by C. M. Pictors ct al., Clementinc UVVIS data calibration and processing, available at http://www.planctary.brown.cdu/clcmentinc/calibration.html and McEwen et al. [1998]: (1) an offset correction based on the camera's electronic setting, (2) correction for dark current as a function of exposure time, temperature, and CCD row position, (3) a column-dependent electronic shutter offset correction, (4) a fiat field correction for spatial nonuniformity, (5) photometric corrections to a standard viewing geometry (i = 30 ø, c = 0ø), and (6) spectral calibration to bidirectional reflectance using the Apollo 16 site as a lunar standard. Mul- tispectral image cubes from selected mare regions were then mosaicked into 10 ø latitude regions for extraction of spectra.

Because much of the optical differences between basalt types are revealed by variations in continuum slope, albedo, and ferrous (1 pm) band strength [Pieters, 1978], relatively simple spectral parameters can be used to compare the general properties of different basalt types. Visible continuum slope can be approximated by the Clementine UV/VIS ratio 0.41/0.75 pm, albedo can be represented by 0.75 pm reflectance, and 1 pm band strength can be approximated by a


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Figure 3. (a) The nearside of the Moon as seen in a Clementine 0.75 gm USGS global mosaic showing the locations of the four mare study regions. (b) A Clementine 0.41/0.75 lam ratio mosaic of the lunar nearside sensitive to titanium content within mare soils.


1.0/0.75 lam ratio. Mineralogical and compositional information is inferred from spectral differences between mare units over a range of maturity states. This approach is described in detail by Staid and Pieters [2000] and has been adopted here to evaluate the spectral differences between eastern and western mare units. To allow direct comparisons between regions, scatterplots of these useful spectral parameters were con- structed by sampling 100,000 evenly spaced pixels from each mare region within areas indicated by white boxes in Figure 3. Scatterplots comparing the mare study areas are shown in Plates 1 a and lb. The intensity of each pixel within the plots displays the relative abundance of data points containing a given value in the parameter space. Because mature soils dominate the surfaces exposed, the density distribution of each data cloud has been presented after a root stretch to enhance the visibility of the less abundant immature materials [Staid and Pieters, 2000]. For several mare units the areas around the largest craters (e.g., > 10 km) were eliminated to avoid the inclusion of underlying materials in the analysis.

Exposure to space weathering weakens absorption features over time, and the freshest and least weathered materials within a given unit exhibit the strongest absorption bands. The scatterplot of the 1.0/0.75 lam ratio and 0.75 lam reflectance shown in Plate 1 b allows trends related to maturity to be evaluated as revealed by band strength at 1 lam [Fischer and Pieter& 1996; Lucey et al., 1995, 1998; Staid and Pieters, 2000]. For a given basalt type the most immature materials display the maximum albedo and strongest 1 lam band and occur in the lowermost limit of the vector passing through the data cloud. The spectral properties of immature matehals and mature soils can be compared to determine how each unit alters optically as it weathers from relatively crystalline regolith to homogeneous soils. Materials that represent the most immature materials within the study area of each mare unit were selected by identifying pixels that corresponded to the lower right limit of each mare unit's 1.0/0.75 lain ratio versus 0.75 lam scatterplot cloud. Spectra of these materials are compared in Figure 4a.

In order to evaluate how the spectral properties of relatively crystalline mare materials vary spatially and with depth, five- color spectra also were collected for hundreds of fresh craters of various sizes for each mare region. The following steps were used to isolate immature mare craters for each study area and group them on the basis of size (discussed in detail by Staid and Pieters [2000]). In the first step, Clementine images from each mare region were calibrated and mosaicked as discussed in the previous section. All images were registered to USGS Clementine 0.75 lain base maps and resampled to 100 m spatial resolution. In the second step a ratio of the 0.75/1.0 lain Clementine filters was used to identify materials exhibiting the strongest 1 lam absorptions relative to surrounding soils. Because absolute band strength varies from one basalt type to another, the 6% of materials with the strongest 1 lain ratio in a given unit were used to identify immature materials in order to select a large number of craters from each unit in an unbiased manner. This threshold was

identified independently for each mare unit and was used to isolate materials associated with individual fresh craters while

minimizing the inclusion of overlapping ejecta blankets of adjacent craters. In the third step a pattern recognition pro- cedure is used to identify and bin fresh craters on the basis of their estimated diameter. This processing step identifies

groups of contiguous pixels by size and shape and sorts them into size bins on the basis of their area. For each unit, results contain statistics from hundreds of craters within the smaller

size bins (<0.5, 0.5-1, 1-2 kin) and tens of craters or less in the largest bins (2-3, 3-5, 5+ km). Previous mare soil unit designations of Pieters [1978] are used for comparison. The spectra for each unit as a function of crater size are provided in Figure 4b.

4. Late-Stage Mare Basalts 4.1. Titanium Content

The UV/VIS ratio has been used extensively to estimate titanium content in mature mare soils [e.g., Charette et al., 1974; Pieters, 1978; Johnson et al., 1991a, 1991b]. The boundaries of the spectrally blue (high UV/VIS ratio) Im- brium and Procellarum basalts can be seen clearly in a Clementine UV/VIS ratio image presented in Figure 3b. The UV/VIS ratio coupled with the 0.75 lam parameter has been applied more recently to estimate titanium content across many lunar materials [Lucey et al., 1998; Blewett et al., 1997; Giguere et al., 2000; Lucey et al., 2000]. A quantitative comparison of these UV/VIS ratio values and 0.75 •tm albedo for each study region is presented in the scatterplot in Plate l a. High-titanium basalts plot in the upper left portion of Plate l a because of their low albedo and high UV/VIS ratio values.

All three high-titanium basalts (Tranquillitatis HDWA, Procellarum HDSA, and Imbrium hDSA) are observed to be darker and bluer than the low-titanium Serenitatis basalt

(mlSP), consistent with an elevated TiO2 abundance. The Tranquillitatis HDWA basalts are the darkest and bluest basalts, whereas the Procellarum HDSA and Imbrium hDSA basalts are seen to be progressively less dark and blue. Clementine UV/VIS ratio values for the Procellarum HDSA

unit are similar to but slightly lower than HDWA Apollo 11 basalts. These values are consistent with previous evaluation of the western high-titanium basalts using telescopic [Pieters, 1978; Johnson et al., 1991a] and Apollo gamma-ray data [Davis, 1980] which suggest only a minor difference in TiO2 contents between these mare deposits. Applying the modified Charette empirical approach of Johnson et al. [1991b] to the Clementine UV/VIS ratio values of mare soils within each

region provides estimates of wt% TiO2. Estimates are 4-5% for the hDSA basalts and >8% for both the HDSA and

HDWA basalts compared with an estimate of_<2 wt% TiO2 within the basaltic deposits of Mare Serenitatis. Application of the more recent Ti-mapping approach described by Blewett et al. [1997] and Lucey et al. [1998] results in estimates of approximately 8-10 wt% TiO2 for the hDSA basalts, 11-15 wt% for the HDWA basalts, and approximately 11-14 wt% for the HDSA basalts. Analysis of neutron spectrometer data collected by Lunar Prospector suggests that the Clementine Ti-mapping approach [Blewett et al., 1997; Lucey et al., 1998] may be overestimating TiO2 content for these high- titanium areas by as much as 5 wt% [Elphic et al., 2000].

4.2. Mafic Mineralogy

Though the eastern and western high-titanium basalts exhibit similar albedo and UV/VIS spectral properties, they differ significantly in their 1 lam absorption properties [Piet-


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Figure 4. (a) Five-color Clementinc spectra of the least weathered materials identified in each mare study region. (b) Clementinc spectra for each mare region obtained by averaging all craters for each size bin.

ers et al., 1980]. The scatterplot in Plate lb captures the 1 Hm absorption strength and albedo of large areas of each basalt type over a range of optical maturities. Materials whose soil surfaces are less mature are slightly brighter and display a stronger ferrous band [Lucey et al., 1998; Staid and Pieters, 2000]. For each basalt type this relation results in a roughly parallel range of values for these spectral parameters, forming distinct "weathering clouds" of data [Staid and Pieters, 2000].

The western HDSA and hDSA basalts show a much lower

1.0/0.75 Hm ratio (stronger mafic absorption) than the Tran- quillitatis basalts for both mature soils and immature crater materials (Plate lb). Despite a higher abundance of Fe-Ti oxides (which subdue absorption features) compared to the Serenitatis mISP basalts, the western HDSA and hDSA mare units also exhibit a stronger mafic ratio than the iron-rich Serenitatis basalts (Plate 1). These combined properties indi-

cate an exceptionally high abundance of mafic minerals and suggest that the Eratosthenian deposits within Oceanus Pro- cellarum may be the most iron-rich basalts extruded on the surface of the Moon. Applying the Fe-mapping approach of Lucey et al. [1998] produces estimates of around 19-20 wt% FeO for the high-titanium basalts in both western Procellarum and Mare Tranquillitatis. However, as shown in Plate I b, the stronger ferrous absorption within the mature soils and immature regoliths of western high-titanium deposits relative to eastern basalts is not well resolved along a rotational axis in the band strength versus albedo parameter space. As a result, FeO abundance appears to be underestimated by the rotational Fe-mapping techniques for the western high-titanium deposits. The strong mafic band exhibited by the western high- titanium basalts indicates that they have a substantially higher iron content than representative basalts sampled within Tran- quillitatis (Plate lb), which have approximately-19-20% FeO [e.g., Taylor et al., 1991]. This relationship leads to an estimate of FeO contents which are >20 wt% for the western

high-titanium 'basalts. These estimates of iron abundance are consistent with the most recent Prospector gamma-ray results, which also indicate that the western high-titanium basalts are more iron rich than those in Mare Tranquillitatis [Lawrence et al., 2001 ].

Previous telescopic studies demonstrate that the strong ferrous band in the mature soils of the western high-titanium deposits has a longer-wavelength center than is observed for other maria [Pieters et al., 1980]. To investigate this property of the western basalts, regions that represent the most immature materials within each mare area were identified by selecting pixels that correspond to the lower right limit of each mare unit's 1.0/0.75 Hm versus 0.75 Hm scatterplot cloud in Plate lb. These spectra, shown in Figure 4a, allow comparisons of the strong ferrous absorption for the most crystalline materials within each basalt type. The shape of the 1 Hm feature is much flatter and centered at a longer wavelength in the spectra of the western Procellarum basalts compared to the eastern Serenitatis and Tranquillitatis basalts. Spectra of immature materials from the HDSA unit exhibit the strongest downturn in the ferrous band at 1 Hm of all the mare units in Figure 4a. These observations of immature crater regoliths are consistent with telescopic measurements of mature soils within the western maria, which suggests a longerJwavelength I Hm absorption resulting from the presence of olivine or Fe- bcaring glass [Pieters et al., 1980].

In order to obtain more detailed information about how the

spectral properties of the western basalts vary with depth, spectra from large numbers of small mare craters were examined as a function of their diameter. The average reflectance properties of small fresh craters within the high-titanium western basalts (HDSA and hDSA) are compared to similar craters in Tranquillitatis (HDWA) and Serenitatis (mISP) in Figure 4b. Optically immature craters in the western mare units again display a consistently stronger mafic band than craters of similar size in the eastern high-titanium basalts. The crater spectra also demonstrate that the absorption feature for the young western basalts occurs at a longer wavelength than it does for the eastern mare units. A strong and long- wavelength mafic absorption is found to be pervasive throughout the fresh materials in the western maria that have been excavated by hundreds of craters from different size groups and depths within the maria. The presence of this


absorption throughout the soils and crater deposits of western high-titanium basalts confirms that this optical property is inherent to the mineralogy of the basalts. This observation thus excludes a glassy cooling surface occurring within the uppermost surface or at discrete depths as a source for the 1 lam feature and instead supports the alternate interpretation of abundant olivine within these iron-rich basalts (olivine/pyroxene > 1).

5. Compositional Variation along the Imbrium Flows

Apollo 15 and Apollo 17 orbital photographs acquired at low-Sun illumination provide excellent data for the morpho- logical study of lava flows in southwestern Mare Imbrium. Schaber [1973a, 1973b] mapped the Imbrium flows in detail, identifying three distinct eruptive phases within the western high-titanium basalts. These flows and associated maps provide an excellent opportunity to evaluate the composition of the western high-titanium basalts in relation to their proposed source vent. The boundaries of each flow relative to their

proposed source near Euler [5 are reproduced on the basis of Schaber's [1973b] map in Figure 5a. Eratosthenian units based on Wilhelms and McCauley [ 1971] are shaded in this figure.

In order to detect compositional changes along and between flow phases, several spectral parameters were investi- gated, including UV/VIS ratio, albedo, and mafic band strength and shape. On the basis of petrologic analyses of lunar samples, high-titanium lunar basalts show a trend in which TiO2 content decreases with increasing fractionation after Fe-Ti oxide saturation is achieved, reflecting the extraction of Fe-Ti oxide minerals from the melt [Lofgren et al., 1981 ]. An important implication of this observation is that if high-titanium lavas flow long distances, fractionation can produce significant Ti gradients along flow lines [Lofgren et al., 1981]. Spectral profiles obtained along the Imbrium flows provide an opportunity to investigate evidence for such fractionation along the very long Imbrium flow phases which extend 1200, 600, and 400 km for phases I through III [Schaber, 1973b]. If the late high-titanium basalts in the Im- brium basin experienced fractionation, sample studies suggest that Fe-Ti oxides such as ilmenite would fractionate first dur-

ing its early stages of cooling [Lofgren et al., 1981]. For a long flow, TiO2 content would be expected to decrease along the flow line as Fe and Ti opaques crystalized and were removed from the melt.

Lunar sample analyses also suggest that most sampled mare basalts have experienced some olivine removal en route to the surface [Lofgren et al., 1981]. When present, olivine is commonly an early crystallizing phase that is an important contributor to the fractionation process. Compositional subgroups of several Apollo samples may be related through the evolution of magma as a result of fractionation. For exam- ple, the textural and chemical variations within subgroups of the high-titanium Apollo 17 basalts have been attributed to moderate amounts of near-surface crystal fractionation dominated by the removal of olivine, armalcolite/ilmenite, and chrome spinel [Rhodes et al., 1976].

If the late stage western basalts are rich in olivine, then variations in the content of olivine might occur between eruptive phases or along flows as well. Different degrees of frac-

tionation may have occurred during ascent of the magma to the surface, resulting in a clear petrologic sequence from early to late phases within these Imbrium eruptions. Removal of magnesium-rich olivine could produce a more iron-rich resid- ual liquid, but the effects of olivine fractionation would com- pete with the removal of Fe into opaque phases. As a result, variations in albedo and band strength may result from changes in FeO or opaque content along the flows as they were emplaced.

Schaber's [1973b] map of the Imbrium flows was coregis- tered to USGS mosaics of Clementine images with 0.5 km/pixel spatial resolution [Eliason et al., 1999]. The boundaries of the western high titanium basalts in Imbrium and Procellarum can be clearly seen in the Clementine 0.41/0.75 lam ratio image in Figure 5b. Difference images sensitive to the shape of the 1 lam mafic band are also shown for the Imbrium deposits in Figures 5c (1.0-0.90 lam ) and 5d (1.0-0.95 lam). Both parameters measure the slope of the spectrum around 1 [tm, identifying materials with long- wavelength ferrous absorptions. Lower parameter values (dark in Figures 5c and 5d) clearly differentiate the long- wavelength absorption unique to the western high-titanium basalts. On the basis of these band difference images, both surface soils and crater deposits within the youngest high titanium basalts display this characteristic long-wavelength property. However, older low-titanium deposits within Im- brium have a consistently higher difference value resulting from the shorter-wavelength pyroxene absorption more com- mon in the eastern basalts. High titanium basalts in the eastem portion of Figure 5b mapped as pre-Eratosthenian [Wil- helms and McCauley, 1971 ] also lack the long-wavelength absorption observed in younger basalts (Figures 5c and 5d).

Profiles of Clementine UVVIS data 1.5 km in width were

extracted along the center of each eruptive phase mapped by Schaber [1973a, 1973b] as illustrated in Figure 5a. The extracted spectral data were then smoothed by averaging values in 10 km increments along each profile to map large-scale spectral changes along each flow phase and reduce high- frequency variations resulting from small craters and rays. The Clementine profile along the youngest (phase III) flow extends from just north of the proposed source near Euler [5 to the end along the longest arm of these flows -•400 km to the northeast. The phase II Clementine profile begins northwest of Lambert (Figure 5a) -•270 km from Euler [5 and extends to the end of the flows for another 330 km as mapped by Schaber [1973b]. The first several hundred kilometers of phase II are not clearly differentiated from phase I or are heavily contaminated by nonmare materials near Lambert. As a result, this portion of the phase II Imbrium flows is not in- cluded in the Clementine profile. The youngest flows (phases II and III [Schaber, 1973a, 1973b]) were mapped on the basis of distinct flow scarps in the Apollo photographs, whereas the earlier flows of phase I were mapped on the basis of color and albedo owing to a lack of obvious flow scarps within these flows (G. G. Schaber, personal communication, 2000). On the basis of the Clementine ratio images in Figure's 5b and 5c, it is not clear whether the area east of Le Verrier repre- sents the flow path of this phase (as originally mapped) or an eastern extension of the high-titanium flows farther to the west. As a result, the Clementine profile along phase I begins north of Le Verrier and extends for-•400 km to the western

limit of these lavas as mapped by Schaber [1973b].

27,896 STAID AND PIETERS' MINERALOGY OF THE LAST LUNAR BASALTS

Figure 5. The Mare Imbrium region. (a) Map of high-titanium flows in Mare Imbrium: phase I maps the earliest basalts, whereas phase III flows were the last to be extruded (modified and reprojected from $chaber [ 1973b]). Dashed lines along flows show where Clementine images were sampled to obtain profiles (Figure 6) of spectral parameters along each flow. Shaded areas are assigned an Eratosthenian age by Wilhelrns and McCauley [1971]. (b) A Clementine 0.41/0.75 lum UV/VIS ratio image, and (c) 1.0 lum- 0.90 lum and (d) 1.0 lum- 0.95 lum band difference images of the Mare Imbrium region.

No consistent pattern of change was observed along the flow phases with respect to either albedo or band strength. Instead, the largest variations in brightness and ferrous absorption strength within each flow phase are dominated by variations in optical maturity and mixing associated with impact craters rather than changes in the composition of the emplaced basalt. On average, mafic band strength was stronger in the two youngest flow phases when compared to the earliest phase and older Imbrium basalts (Table 1). The average albedo also increased within later eruptions, though all are significantly darker than neighboring low-titanium deposits (Table 1).

The UV/VIS ratio and mafic band shape parameter display more prominent differences across each profile and are plotted for each phase of Imbrium volcanism as well as older Imbrium basalts in Figures 6a and 6b. All three phases show a similar peak value in the TiO2-sensitive UV/VIS ratio along their flows (Figure 6a). The youngest and shortest flows (phase III) display a relatively constant UV/VIS ratio along their entire length from a source near Euler [5 to the end of the flows. Phase II, the second longest flow phase, exhibits similar values along the beginning of the profile (the middle of the flows with respect to a source near Euler B), then a gradual decrease in the last third of these flows. The oldest and long-

STAID AND PIETERS' MINERALOGY OF THE LAST LUNAR BASALTS

Table 1. Spectral Comparison of Average Mare Surfaces Within Imbriurn, Procellamrn, and Tranquillitatis

27,897

Imbrium hDSA High Ti Procellarum Imbrium Tranquillitatis Average Standard Phase I Phase II Phase III HDSA Low Ti High Ti Deviation of Regions

750 nm albedo, % Reflectance 9.77 750/1000 nm ratio 0.96

415/750 nm ratio 0.61

1000-900 nm, % Reflectance 0.21

10.04 10.00 9.09 11.55 9.08 0.411 0.99 0.98 0.97 0.96 0.92 0.012 0.62 0.63 0.66 0.55 0.67 0.008 0.05 0.06 0.03 0.39 0.31 0.044

est flows (phase I) show the largest decrease in UVVIS ratio values, which occurs along the last two thirds of the profile, corresponding to approximately the last third of the distance from Euler 13. Lower TiO2 values far from the source are consistent with the fractionation of ilmenite from these Ti-

rich basalts. Alternately, each flow phase may represent mul- tiple flow events with flows far from the original source shar- ing a similar change in composition. Mixing of underlying low-titanium materials also may influence UV/VIS ratio values within thin flows near the end of phase I but does not explain the broad decrease observed over large distances within the two longest flow phases. Regardless of its source, the decrease in UV/VIS ratios along each flow phase is larger than differences between phases. The highest average ratio values (and, correspondingly, the most TiO2 rich basalts) occur in the youngest phase, whereas average values for the older phases decrease slightly (•able 1).

A second set of profiles map variation in the 1 iam band shape along each flow. These profiles are plotted for the Im- brium flows in Figure 6b and track changes in the relative strength of the ferrous absorption at the wavelengths measured by the Clementine 0.90 and 1.0 iam filters (1.0 pm - 0.90 iam). Both the 1.0-0.90 [tm and the 1.0-0.95 iam band difference parameters were found to be useful for distinguishing differences in mafic band shape among mare basalts. The 1.0-0.90 lain difference presented in Figures 5c and 6 b is more sensitive to maturity variations than the 1.0-0.95 iam

difference (e.g., Figure 5d) but has a higher overall signal to noise and is less sensitive to orbit to orbit variations in the

Clementine data. Such a difference image has been used by Pieters et al. [2001] as a spectral parameter called "tilt" to map rock types with long-wavelength ferrous absorptions. Materials dominated by a longer-wavelength absorption display smaller values for this parameter, whereas a shorter- wavelength absorption would produce higher values. Differ- ences in the amount of olivine within subsequent eruptions would lead to variations in this parameter between phases. Fractionation of olivine within a long flow (i.e., decreasing in

,

abundance with distance) could also result in an increase in this spectral parameter as a function of distance.

All three high-titanium flow phases in Imbrium display lower 1.0-0.90 iam values (flatter bands) than the surrounding low-titanium basalts and older high-titanium basalts within Tranquillitatis (Table 1). Though the absolute magnitudes of these (1.0-0.90 iam) differences are small (<0.4% reflectance),' they represent a significant change in band shape when comRared to the narrow range of valui:s for this parameter across •ifferent mare soil compositions on the Moon. The high-titaniu m deposits within western Procellamm and the youngest Imbrium flows exhibit the flattest bhnds (with difference values around 0%), which distinguish them from all other nearside basalts. Most other mare regions on the nearside, including pre-Eratosthenian high-titanium basalts, have difference values between 0.3 and 0.4% with very little vari-

0.64 .......................... * ................ i ..................... * .............. ' ....................

• 't,½•:.; •: :• Phase I! 0.60

0.58-

Imbrium

0.56 low Ti ß . •-. .•, •

O. 54 U t ••• ................... 0 ! O0 200 300 400 500

a}

'"" '• Phase l

Limit of flows

Distance along Flows (krn)

l;• !:&• )mbrium •i ?!, i ';• IowTi

0.3, a ;.?"%,,, Phase

Phase !1

o.•

o

-o.•

b)

•:•" •:"::•:':Phase III

Limit of flows

f t L

100 200 300 400 500

Distance along Flows (km)

Figure 6. Profiles, of spectral parameters along the Imbrium flows in Figure 5. (a) The Clementine 0.41/0.75 iam UV/VIS ratio and (b) the band shape from the 1.0-0.90 gm filter difference are presented for each high- titanium Imbrium flow phase and neighboring low-titanium basalts.


ability within or between units of different composition. Within Mare Imbrium, the earliest Eratosthenian flows (phase I, Figures 5a and 6b) exhibit band differences that are intermediate between pre-Eratosthenian basalts and later Imbrium flows (0.2%). The youngest basalts in Mare Imbrium (phases II and III) have longer-wavelength band shapes and difference values similar to those observed throughout the western Pro- cellarum high-titanium deposits (<0.1%, Table 1). These observations are consistent with subsequent eruptions of the hDSA basalts being more olivine-rich. A decrease in long- wavelength absorbing components (higher difference values) along flows would suggest the removal of olivine crystals from an olivine-rich melt during emplacement. Although there is no clear change in shape along each profile (Figure 6b), a small increase in difference values near the end of profile II may indicate a small drop in olivine content far from flow sources. However, the larger differences between profiles suggest that the most significant changes in olivine content occurred prior to emplacement as an increase in olivine abundance within later eruptions.

6. Summary and Conclusions

The examination of mare materials over a range of optical maturities provides new information about the mineralogy of late stage lunar volcanism. The reflectance properties of relatively crystalline mare crater materials exhibit more pro- nounced mineralogic absorptions than soils whose spectral properties have been weakened and altered by space- weathering processes [e.g., McCord et al., 1981; Pieters, 1977]. The spectral properties of the western high-titanium basalts derived from Clementine imagery confirm the presence of a strong and long-wavelength 1 [tm ferrous band. The band observed for optically immature craters within these western basalts is much stronger than the absorption feature observed for similar materials in the high-titanium Tranquilli- tatis basalts. Despite the presence of abundant opaques, which subdue band strength, the mafic band in the western high-titanium hDSA and HDSA basalts is also observed to be stronger than the band observed in high-Fe (but low-titanium) Serenitatis mare materials.

The optical properties of the young western basalts indicate that they contain >20 wt% FeO and that their mineralogy may be more iron-rich than any other large mare deposits on the lunar surface. The unique long-wavelength shape of the ferrous band within these basalts is found to be pervasive both in surface soils [Pieters, 1978] and materials excavated from depth, supporting the presence of abundant olivine as a significant source for this absorption feature. Spectral properties observed along mapped flows in Mare Imbrium [Schaber, 1973a, 1973b] provide evidence for fractionation of ilmenite during the emplacement of these basalts. Subsequent phases emplaced in Mare Imbrium display band characteristics consistent with an increased abundance of olivine within later

eruptions. A separate study of Clementine data comparing many

nearside mid farside mare regions indicates that the iron- and olivine-rich compositions of basalts discussed here are unique to the western nearside of the Moon [Staid, 2000]. Initial results from the recent Lunar Prospector mission [Lawrence et al., 1998] are also consistent with a very iron rich composition for these late western basalts as well as the unusual com-

positional properties of the western nearside as a whole. The unique petrology of these last major phases of lunar volcanism (Fe-rich composition and ilmenite and olivine-rich mineralogy) along with new elemental information about the Im- brium and Procellarum regions provide new insight into the nature of the lunar interior and the evolution of mare volcanism over time.

Acknowledgments. NASA funding to the authors for this research through the Solar System Planetary Geology and Geophysics and Lunar Data Analysis programs is gratefully acknowledged. We thank Brad Jolliff, Jeffrey Johnson, and Lisa Gaddis for detailed and helpful reviews of the original manuscript.

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M. I. Staid, U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, AZ 86001, USA. (mstaid•usgs.gov)

(Received September 15, 2000; revised April 11,2001; accepted June 5,2001 .)

Mineralogy of the last lunar basalts - Brown University27,888 STAID AND PIETERS: MINERALOGY OF THE...

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