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.