Stratigraphy and Evolution of Basalts in Mare Humorum and Southeastern Procellarum

Meteoritics & Planetary Science 41, Nr 3, 479–488 (2006) Abstract available online at http://meteoritics.org

Stratigraphy and evolution of basalts in Mare Humorum and southeastern Procellarum

Terence HACKWILL1, John GUEST1, and Paul SPUDIS2

1Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK 2Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland 20723-6099, USA *Corresponding author. E-mail: [email protected] (Received 18 January 2005; revision accepted 08 October 2005)

Abstract–We have studied the mare basalts of Mare Humorum and southeastern Procellarum (30°W– 50°W, 0°–40°S). One hundred and nine basaltic units have been identified from differences in their FeO wt% and TiO2 wt% content, and variations in crater densities. Crater counting and reference to isotopically dated Apollo samples have provided an age for 33 major units. Some evidence for three distinct periods of volcanic activity has been found. We found that the large unit in the middle of Mare Humorum is the oldest in the basin. This supports the suggestion that the oldest central unit sank causing the lithosphere to bend and create dykes through which lava flowed to produce the outer units. No evidence of a trend in FeO wt% and TiO2 wt% content against time is found within Mare Humorum. There appears to be no lateral trend of basalts in terms of FeO and TiO2 wt% over the entire area with time. An increase in FeO content with time is found in the 33 major units and there is some evidence for an increase in TiO2 in the same units. A correlation between FeO wt% and TiO2 wt% content is evident when all 109 units are compared. A notable feature of this correlation is a sharp increase in gradient of TiO2 wt% content when the FeO wt% content rises above about 17%.

INTRODUCTION Spectrometers (ISIS). These display the area by FeO content, TiO2 content, as a color composite and in “false” (or Ultraviolet and visible wavelength images from the “stretched”) color. The color composite image was prepared Clementine mission can be used in conjunction with from images taken through the 450 nm (blue), 750 nm (green) algorithms devised by various workers (Lucey et al. 1995; and 950 nm (red) filters fitted to the Clementine camera. Blewett et al. 1997; Lucey et al. 2000) to display the extent of Variations in iron and titanium content produce subtle lunar basaltic lava units and determine their FeO and TiO2 changes in color of the lunar surface. The “false” color image content. This paper describes how we have used this has been created from the three colors that produced the color information to map individual lava flows in the Mare composite image with 415/750 nm controlling the blue, 750/ Humorum and the southeastern Procellarum region of the 950 nm controlling the green, and 750/415 nm controlling the Moon. Additionally, each unit has been relatively dated by red. These ratios suppress albedo differences, allowing crater counting, and absolutely dated by using a graph compositional variation to be more easily recognized, and prepared from Apollo returned samples by Schultz and exaggerate real color differences. High-resolution Lunar Spudis (1983). Inferences about the regularity, spatial Orbiter IV images were used for crater counting because their direction of eruptions and possible trends of iron and titanium resolution is higher than that of Clementine images (down to content with time in Mare Humorum and SE Procellarum are 60 m compared with 200 m). then made. Bugiolacchi et al. (2006) have completed a similar There were five main stages to the project: assessment of Mare Nubium and Mare Cognitum (0°, 30°W, • Delineate the individual basaltic units; 0°, 30°S). • Assess the relative ages of the individual flows by crater counting; METHOD • Derive ages from crater counts for individual lava flows; • Assess any spatial trends of eruptions over time; Four images were prepared by the Lunar and Planetary • Look for any trends in iron and titanium content of the Institute using Integrated Software for Imagers and basalt over time.

479 © The Meteoritical Society, 2006. Printed in USA. 480 T. Hackwill et al.

Delineate the Individual Basaltic Units their circumference was inside the boundary. Obvious secondary craters were ignored. Individual basaltic units were identified from their FeO The 500-meter size was chosen as a suitable minimum and TiO2 content as it was expected that each flow would size because it is small enough to give a meaningful number have had its own melting history producing a unique of craters and allows easy calibration against Apollo 12 and chemistry (Taylor et al. 1991). Many of the units are obvious 15 sample ages using the data in Basaltic volcanism on the on the iron, titanium, and false color maps and could easily terrestrial planets (BVTP), Table 8.8.1 (Basaltic Volcanism be drawn in on the false color image. Units that were less Study Project 1981). A diameter of 500 m approaches the certain were also added because any unit found to be part of minimum resolution of some Lunar Orbiter IV images. It is an adjacent unit could be reunited with it later. For each unit, also large enough to avoid problems that occur if very small 12 FeO and 12 TiO2 wt% measurements were taken and craters are counted. The main problems for crater diameters averaged. Lucey et al. (1995), Blewett et al. (1997), and <300 m, quoted by Schultz et al. (1976), are: i) many of these Lucey et al. (2000) demonstrate the robustness of their craters are asymmetric, making measurement difficult, ii) algorithms for determining FeO and TiO2 wt%, although small primary craters easily become subdued and can be shortcomings are suggested by some researchers (e.g., Staid confused with secondary craters, which are often created with and Pieters 2000; Clark and McFadden [2000]). The TiO2 a subdued morphology because of the slower impact speed showed much greater variation between units than the FeO than for primaries, iii) the erosion rates for small craters and so was used to compare each unit with all its adjacent appears to be different to that of larger ones and so small ones units to see if there was a significant difference (1 standard disappear sooner; this will distort number of craters being deviation). Where a significant difference between unit X and used for dating (Young 1976). unit Y only occurred in one direction (i.e., unit X’s average It was hoped that it would be possible to work out relative TiO2 value fell outside unit Y’s average ± one standard ages for all the units on the map, but after counting it became deviation, but not the other way round) a further 12 samples apparent that small units did not have a sufficient number of were taken. The potential units were considered to be a single craters with diameters >500 m to determine whether one was unit if a significant difference in both directions was still not significantly different from others if they had nearly the same seen. Apparent adjacent units having no significant number of craters. It was decided to only use units greater difference in either direction were also considered to be a than 500 km2, and there were 33 such units. They are single unit. summarized in Table 1.

Assess the Relative Ages of the Individual Flows by Crater Derive Actual Ages for Individual Lava Flows Counting Returned samples can be radiometrically aged Crater densities can be used to assess relative ages of (Papanastassiou and Wasserburg 1971; Tera et al. 1974; geological units on the Moon (Neukum et al. 1975; Neukum Basaltic Volcanism Study Project 1981). Schultz and Spudis 1977). Lunar Orbiter IV high resolution frames were used to (1983) took the ages of radiometrically dated Apollo 12 and crater count each unit in preference to Clementine images 15 samples, and plotted them against the crater counts for the because of their higher resolution (sometimes down to 60 m units from which they were obtained. They also plotted some for Lunar Orbiter compared with 200 m/pixel for inferred ages against crater counts. Their crater counts used Clementine). Lunar Orbiter IV frames of the area were craters with diameters greater than 500 m. It is reproduced as scanned. The scale of the Lunar Orbiter IV images was Fig. 1. Apollo 11 and 17 crater counts have been added to obtained from NSSDC 71-13 (Anderson and Miller [1971]) allow pre-Apollo 15 dating. These were obtained from BVTP, and used to determine that each pixel represents 39.8 m × which was used as the source of data for consistency with 39.8 m. Sample areas of the scanned images were compared other work by Paul Spudis. While we recognize that the in detail with the original ones and were found to be perfectly abrupt change in direction in the trend line at the Apollo 15 adequate for showing detail as small as 60 m and for data point in probably unrealistic, we feel justified in identifying craters 500 m in diameter. retaining it as it fits the data points, although a curve is Each unit was displayed in Adobe Photoshop and the possible within the bounds of the errors bars listed in Table 1. number pixels in random sample areas were obtained from the Basaltic volcanism on the terrestrial planets (Table “histogram” tool. The sizes of the sample areas are given in 8.8.1) gives Apollo site crater densities relative to the average Table 1. The “paintbrush tool” was used as circle with the maria crater density. It shows the Apollo 15 relative crater correct number of pixels in diameter to represent 500 m. It density as 0.5. Schultz and Spudis (1983) give the Apollo 15 was placed over each crater and the crater counted if its landing site as 5.0 × 10−2 km−2 (for craters >0.5 km in diameter was larger than 500 m. Craters sitting on borders of diameter). Therefore, the average maria crater density (for sampling areas were included in an area if more that half of craters >0.5 km in diameter) equals 10.0 × 10−2 km−2. The Basalts in Mare Humorum and SE Procellarum 481

Table 1. Results. Craters in 2 2 Unit sample area Sample area/km Craters/km Age (Ga) FeO wt% ±1.3% TiO2 wt% ±1% 17 99 3744 0.026 ± 0.0027 1.76 ± 0.19 18.4 8.7 9 61 2214 0.028 ± 0.0035 1.90 ± 0.19 19.2 11.8 31 35 912 0.038 ± 0.0035 2.57 ± 0.49 17.8 6.5 1 52 1304 0.040 ± 0.0055 2.71 ± 0.37 18.7 8.1 119 173 4196 0.041 ± 0.0031 2.79 ± 0.23 18.8 10.0 57 35 774 0.045 ± 0.0076 3.06 ± 0.23 18.6 8.7 19 40 846 0.047 ± 0.0075 3.16 ± 0.14 18.8 10.2 85 132 2685 0.049 ± 0.0043 3.25 ± 0.04 16.2 3.5 75 79 1481 0.053 ± 0.0060 3.28 ± 0.06 17.7 6.3 143 52 977 0.053 ± 0.0074 3.28 ± 0.06 16.9 4.2 29 24 423 0.057 ± 0.0116 3.32 ± 0.06 17.4 5.4 21 46 789 0.058 ± 0.0086 3.33 ± 0.04 17.6 6.5 2 41 700 0.059 ± 0.0092 3.34 ± 0.04 18.5 8.6 34 67 1120 0.060 ± 0.0073 3.35 ± 0.03 17.5 4.2 5 87 1384 0.063 ± 0.0067 3.36 ± 0.03 18.0 6.6 10 71 1118 0.064 ± 0.0075 3.36 ± 0.04 18.6 7.7 101 55 883 0.062 ± 0.0084 3.36 ± 0.04 17.7 6.4 98 183 2799 0.065 ± 0.0048 3.37 ± 0.03 13.3 2.0 117 38 587 0.065 ± 0.0105 3.37 ± 0.06 16.7 5.8 59 38 580 0.066 ± 0.0106 3.38 ± 0.06 17.4 4.4 111 193 2819 0.068 ± 0.0049 3.38 ± 0.03 16.2 3.4 106 67 976 0.069 ± 0.0084 3.39 ± 0.04 15.4 3.1 35 112 1591 0.070 ± 0.0067 3.40 ± 0.03 18.6 7.3 84 106 1470 0.072 ± 0.0070 3.41 ± 0.04 18.0 5.7 49 159 2193 0.073 ± 0.0057 3.42 ± 0.03 17.8 5.6 99 37 491 0.075 ± 0.0124 3.42 ± 0.09 14.2 2.4 12 65 870 0.075 ± 0.0093 3.42 ± 0.07 18.5 8.7 24 80 1063 0.075 ± 0.0084 3.42 ± 0.07 17.8 6.3 89 76 941 0.081 ± 0.0093 3.45 ± 0.05 13.4 2.3 90 53 611 0.087 ± 0.0119 3.50 ± 0.05 12.5 2.1 27 78 908 0.086 ± 0.0097 3.50 ± 0.04 16.6 3.7 136 315 3492 0.090 ± 0.0051 3.51 ± 0.03 18.4 7.5 3 130 1426 0.091 ± 0.0080 3.52 ± 0.04 18.5 8.7 crater densities relative to the average lunar maria for other The crater density of our sample area in each unit is given landing sites can be obtained from BVTP. Crater densities for in Table 1. This was used to determine the age of the unit from the Apollo 11 and Apollo 17 landing sites are 1.3 and 1.2× the Fig. 1. The ages quoted in Fig. 1 are given to high precision lunar maria average, respectively. (0.01 Ga) to suggest the order of eruption, although we The Apollo 12 figure quoted in BVTP is incorrect recognize that many of the error bars overlap. (personal communication with Paul Spudis). Spudis has recounted the area and has found the actual density, relative to RESULTS the average lunar maria, to be 4.8 × 10−2 km−2. The crater densities at the Apollo mare sites are The data for the 33 units, for which an area >500 km2 therefore: could be crater counted, are presented in Table 1 (in age order). Apollo 11: 13 × 10−2 km−2Apollo 15: 5.0 × 10−2 km−2 Units 3 and 19 were originally thought to be a single unit, Apollo 12: 4.8 × 10−2 km−2Apollo 17: 12 × 10−2 km−2 as were units 17 and 21, 1, and 142, and also 49 and 143 because there is no significant difference between their FeO There appears to be a correlation between absolute dates content and TiO2 content. However, it was later decided that and crater densities in Fig. 1 because of the straight lines that they should be split as they have significantly different crater can be drawn almost perfectly through them. We have used count densities indicating flows of different ages but having these to infer the ages of units directly from crater counts. The approximately the same FeO and TiO2 content. The TiO2 steep rise in crater densities pre-Apollo 15 is caused by the values for units 1 and 2, and also 12 and 17 are not one late heavy bombardment events. standard deviation apart, but have been regarded as separate 482 T. Hackwill et al.

Fig. 1. Ages of selected areas based on densities of craters with diameters >500 m. Redrawn from Schultz and Spudis (1983) with Apollo 11 and 17 added. AB: area B in Young (1977), LIF: late Imbrium flows, MS: Mare Smythii, SWM: Southwest of Maestlin, LM: Letronne mare, GM: Gruithuisen mare, FRM: Flamsteed Ring mare, SEL: Southeast of Lichtenberg, Copernicus (ejecta), Tycho (ejecta). Error bars are given in Table 1. units as the color difference on the false color map is obvious. in Fig. 4 are the latest eruptions; in each case there is almost The false color image with the boundaries of the units is certainly basalt from earlier eruptions underneath. shown as Fig. 2. Each unit is annotated with its identifying Furthermore, small impacts will have caused a degree of number from Table 1. Areas of dark mantling deposit are mixing between the topmost layer and those underneath shown as “dmd.” (Melosh 1989). If the lower layers are of different composition or maturity, any vertical mixing will affect the DISCUSSION composition and apparent maturity of the surface unit.

Eruption Sequence in Mare Humorum and SE Sequence of Eruption within Mare Humorum Procellarum The eruption sequence of units within Mare Humorum Figure 3 shows the 33 main units on a time line. There and their FeO and TiO2 content from Table 1 are summarized are three distinct clusters indicating periods of increased in Table 2. volcanic activity. These are separated by quiet periods. The Table 2 shows the FeO wt% and TiO2 wt% content of the vertical lines separate the periods that are described in this main units within Mare Humorum in age order, with the discussion as “A,” “B,” and “C.” youngest ones towards the top. Some of the age error bars for Figure 3 clearly shows that the period of greatest units 136, 84, 111, and 117 overlap so the exact sequence of volcanic activity in the area occurred 3.6 to 3.1 Ga ago. This them may not be entirely correct. There appears to be no trend is well inside the period of maximum lunar activity, between in either FeO or TiO2 wt% with time as demonstrated in 3.8 and 3.0 Ga ago (Spudis 1996). Fig. 5. Unfortunately, it was not possible to age units 120 and Figure 4 was constructed to see if there is any broad 121 accurately because they contain too few primary craters spatial direction in three periods of volcanic activity in the with diameters >500 m. region. The later activity tends to be in northern latitudes, but The oldest unit (136) is in the middle of the basin, with with only five “B” and “C” eruptions, this could hardly be younger ones surrounding it. It being the oldest can be stated described as a trend. It is difficult to look for a trend within the with some confidence because its error bars do not overlap “A” eruptions because they are so close together in time that those of the other units. It is possible that 136, being the oldest many of the error bars (in Table 1) overlap earlier and later and in the middle, slowly sank because of its own weight. eruptions and so there is some doubt over the exact sequence. Solomon and Head (1979) show that this may cause a However, the broad trend of the sequence is probably correct compressive stress underneath the load and radial extensional because in most cases the error bars only overlap the ages of stresses at its edges. This extensional stress may have created a few earlier or later units. It should be noted that units shown the many grabens that occur around the perimeter of Mare Basalts in Mare Humorum and SE Procellarum 483

Fig. 2. A false color image of Mare Humorum and SE Procellarum, 30°W–50°W, 0°–40°S. The units are numbered. The light gray area represents highland. 484 T. Hackwill et al.

overall increase in TiO2 wt% with a much wider range of values in the “A” period. Later eruptions display, on average, higher values of TiO2 wt%. Figure 7 plots the FeO wt% against TiO2 wt% values for all units within the area. It shows a slow increase in TiO2 wt% as FeO rises from about 9% to about 17% and then increases rapidly for FeO values above 17%. This creates two gradients with trends that appear to be (almost) straight lines with a few points in between (circled) which we could not assign to either trend with any confidence. They may be part of the either trend or could be a separate group. The units are shown in Fig. 8 with the low FeO/TiO2 wt% (light gray), high FeO/ Fig. 3. Timeline showing the eruption times for the 33 main units in TiO2 wt% (dark gray) and the intermediate ones (mid-gray). Mare Humorum and SE Procellarum. Vertical lines separate clusters Large craters, their ejecta and dark mantling deposits are of activity. For discussion purposes, the periods are described in the shown in black. Highland material is left blank. text as “A,” “B,” and “C.” The Imbrian-Eratosthenian boundary is at 3.2 Ga and the Eratosthenian-Copernican boundary is at 1.1 Ga. The low FeO/TiO2 wt% units are nearly all isolated areas (Wilhelms 1987). of basalt. These appear to be thin, as high-resolution Lunar Orbiter IV images of these units show plenty of examples of Humorum. Petrycki and Wilson (1999) indicate that there is a highland peaks protruding through the basalt. The Clementine relationship between graben and maria as most graben occur FeO image shows many craters that have pierced through the near maria. The stresses may have cracked the lithosphere basalt and ejected highland rock to the surface. Figure 9 under what is now filled with basalt. Chenet et al. (2004 and shows northern Lacus Excellentiae as an example of highland personal communication) estimate the thickness of the crust peaks protruding through the basalts. ∼ under Mare Humorum to be 10–25 km. The stresses may In contrast, high FeO/TiO2 wt% units are all located in have created dykes to a depth that allowed an intrusion of lava the deep basins of Mare Humorum and SE Procellarum. to flow into the basin above. It seems likely that the Many of the intermediate FeO/TiO2 wt% units are on the extensional stresses caused by the sinking of unit 136 would edges on Mare Humorum or SE Procellarum and all are near have occurred similarly in all directions around it causing the to highland. cracks in the lithosphere around it and thus providing a path There is more chance of highland material being for the subsequent eruptions. Lucchitta and Watkins (1978) transported vertically to the surface in thin basalts than thick show that graben appear to have formed prior to 3.6 ± 0.2 Ga. ones. This is because there are a greater number of smaller This can fit with our dating of unit 136 appearing at 3.51 Ga impacts than larger ones (Neukum 1975) and smaller impacts and the first outer eruption, unit 84, at 3.41 Ga. do not penetrate as deeply as larger ones. There is also more We suggest a plausible sequence of events for chance of highland material reaching a unit by lateral emplacement of basalts in Mare Humorum. Unit 136 erupted transport if the unit is adjacent to the highland rather than into the middle of basin and was the first major unit to be being surrounded by other large units of basalt. However, we emplaced at 3.51 Ga. It began to sink. This sinking cracked see little evidence of lateral transport in our study area. While the lithosphere beneath and caused graben in the highlands we accept that lateral transport of ejecta occurs, (Li and around it. Subsequent eruptions around unit 136 (units 84, Mustard 2000), it is unlikely to have a significant effect on the 111, 117, 85, and 119) occurred between 3.41 and 2.79 Ga. average FeO and TiO2 wt% values of our units. We have inspected the Clementine FeO and TiO2 images of our study EVOLUTION OF FeO AND TiO2 IN MAGMAS area and found some evidence of a fringe of highland material petering out on the mare side of the mare/highland contact of Figure 6a demonstrates the variation of FeO wt% of some of our units, while others show no evidence of it at all. flows in Mare Humorum and SE Procellarum. It clearly Where the fringe occurs, it only extends to about 5 km on to shows a much wider range of FeO wt% in the “A” period than the mare, and this is very small in comparison the size of the in the “B,” and “C.” Additionally, the average FeO wt% vast majority of our units. Deep basaltic units that are content increases during the period. This result supports the adjacent to highland are unlikely to display much laterally findings of Hiesinger et al. (2001) who carried out a similar transported highland material on their surfaces because they investigation but over ∼220 units in 9 mare regions. It is probably contain many layers of basalt and any laterally recognized that FeO wt% and TiO2 wt% are a mixture of transported highland will have been sandwiched between basalts from earlier eruptions because of mixing as a result of them. Gault et al. (1974) show that it requires 107 years to be vertical mixing (Melosh 1989) with other material being 99% sure that the top 1 cm has been turned over once and so imported laterally (Li and Mustard 2000). Figure 6b shows an little of this sandwiched material would reach the present Basalts in Mare Humorum and SE Procellarum 485

Fig. 4. The “A,” “B,” and “C” periods of volcanic activity suggested in Fig. 3 displayed as a gray scale in (b), (c), and (d) to see if any spatial trend exists with time; (a) displays the small, un-aged units. surface. This would also be the case for units nearer the Nearly all of the intermediate FeO/TiO2 wt% basalts middle of the basin. From this, we conclude that there should occur near the edges of the major basins in Fig. 8. Here, we be more highland material available for mixing on the surface suggest that the highland is deeper than in the thin island-type of thin basalts than thick ones. units because it is on the slope leading down to the center of 486 T. Hackwill et al.

Fig. 5. Evolution of (a) FeO wt% and (b) TiO2 wt% of units 84, 111, 117, 85, 119 and 136 in Mare Humorum.

Fig. 6. a) Age against FeO content for each of the 33 main units. b) Age against TiO2 wt% for each of the 33 main units. Error bars are given in Table 1.

Table 2. Mare Humorum units. the deep basin. Fewer impacts will have penetrated through to Unit Age (Ga) FeO wt% ±1.3% TiO2 wt% ±1% the highland than in the isolated low FeO/TiO2 wt% units, but 119 2.79 ± 0.23 18.8 10.0 more than have reached the base of the very thick high FeO/ 85 3.25 ± 0.04 16.2 3.5 TiO2 wt% basalts nearer the middle of the basin. We suggest 117 3.37 ± 0.06 16.7 5.8 that here there will be less highland contamination than in the 111 3.38 ± 0.03 16.2 3.4 thin isolated outcrops of basalt, but more than in those that 84 3.41 ± 0.04 18.0 5.7 occur in the deep basins. This will result in intermediate-type 136 3.51 ± 0.03 18.4 7.5 FeO/TiO2 wt% basalts. Ages of units within Mare Humorum (in chronological order) and their FeO wt% and TiO2 wt% content. CONCLUSIONS

We have used Clementine FeO wt%, TiO2 wt%, “true” and false color images to identify 109 basaltic units in Mare Humorum and SE Procellarum (30°W–50°W, 0°–40°S). We have derived model ages for 33 units from crater densities. We have found evidence for three phases of eruption during the Imbrian and Eratosthenian, although no geographical trend could be detected. A possible sequence of the eruptions that formed the major units in Mare Humorum is suggested with the middle one appearing first and the surrounding ones erupting later. We found no trend in FeO or TiO2 wt% with time for the major units within Mare Humorum. We have Fig. 7. A plot of FeO wt% against TiO2 wt% for all units within the shown that within the study area, there is a wide range of FeO study area. Error bars given in Table 1. We suggest that there are two and TiO2 wt% in the oldest units while in later units the FeO trends: high FeO/TiO2 and low FeO/TiO2; straight lines indicate and TiO wt% became concentrated into a much narrower these. Those that could not confidently be assigned to either trend 2 band of values. Finally, we plotted FeO wt% against TiO were circled and regarded as intermediate FeO/TiO2. 2 Basalts in Mare Humorum and SE Procellarum 487

Fig. 9. Highland peaks protruding through basalt suggesting that the basalt is thin. This example is northern Lacus Excellentiae. Lunar Orbiter IV Frame 148-H3.

Acknowledgments–We thank Brian Fessler of the Lunar and Planetary Institute, Houston, for preparing the Clementine maps for us. We are also very grateful to the anonymous reviewer for commenting on the original manuscript.

Editorial Handling—Dr. Beth Ellen Clark

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