Lunar and Planetary Science XXXII (2001) 1815.pdf NEW AGE DETERMINATIONS OF LUNAR MARE BASALTS IN MARE COGNITUM, MARE NUBIUM, OCEANUS PROCELLARUM, AND OTHER NEARSIDE MARE H. Hiesinger1, J. W. Head III1, U. Wolf2, G. Neukum2 1 Department of Geological Sciences, Brown University, Providence, RI 02912, [email protected] 2 DLR-Inst. of Planetary Exploration, Rutherfordstr. 2, 12489 Berlin/Germany Introduction see a second small peak in volcanic activity at ~2-2.2 Lunar mare basalts cover about 17% of the lunar b.y. surface [1]. A significant portion of lunar mare basalts are exposed within Oceanus Procellarum for which Oceanus Procellarum, Mare Cognitum, Mare Nubium absolute radiometric age data are still lacking. Here we (Binned Ages of Mare Basalts) present age data that are based on remote sensing 20 techniques, that is, crater counts. We performed new crater size-frequency distribution measurements for spectrally homogeneous basalt units in Mare Cogni- 15 tum, Mare Nubium, and Oceanus Procellarum. The investigated area was previously mapped by Whitford- Stark and Head [2] who, based on morphology and 10 spectral characteristics, defined 21 distinctive basalt Frequency types in this part of the lunar nearside. Based on a high-resolution Clementine color ratio composite (e.g., 5 750-400/750+400 ratio as red, 750/990 ratio as green, and 400/750 ratio as blue), we remapped the distribu- 0 tion of distinctive basalts and found that their map well 1.1 1.5 2 2.5 3 3.5 4 discriminates the major basalt types. However, based Age [b.y.; bins of 100 m.y.] on the new high-resolution color data several of their units can be further subdivided into spectrally different Fig. 1:Distribution of basalt ages in Oceanus Procellarum, Mare basalt sub-types. The purpose of remapping the basalts Cognitum, and Mare Nubium in Oceanus Procellarum was to define spectrally ho- mogeneous units for which we assume that they were All Investigated Basins formed within a short period of time with to a first (Binned Ages of Mare Basalts) order homogeneous mineralogy, such as a single erup- 50 tive phase. Definition of homogeneous units is one of the crucial prerequisites for reliable age determina- 40 tions with crater size-frequency distribution measure- ments. Having defined such units with Clementine 30 images, we transferred the unit boundaries to high- resolution Lunar Orbiter IV images in order to meas- ure the crater size-frequency distribution. Clementine Frequency 20 images are not well suited for crater counts because of their high sun angle. The technique of crater size- 10 frequency distribution measurements on spectrally homogeneous regions has been previously applied to 0 basalts in Mare Imbrium, Serenitatis, Tranquillitatis, 1.1 1.5 2 2.5 3 3.5 4 Humorum, Humboldtianum, Australe and is described Age [b.y.; bins of 100 m.y.] in detail by [3]. Fig. 2:Distribution of basalt ages in all investigated basins (Im- Results brium, Serenitatis, Tranquillitatis, Oceanus Procellarum, Based on the new age data, Figure 1 shows the Cognitum, Nubium, Humorum, Humboldtianum, Australe) distribution of basalt ages in the investigated mare regions in Oceanus Procellarum, Mare Cognitum, and Mare Nubium. The largest number of basalt units per time bin was Our new crater size frequency distribution data of formed in the late Imbrian Period at ~3.3-3.5 b.y. We the remapped basalt units indicate that the ages of Lunar and Planetary Science XXXII (2001) 1815.pdf NEW AGE DETERMINATIONS OF LUNAR MARE BASALTS HIESINGER, H. ET AL. basalts in Mare Cognitum vary from 3.32 b.y. to 3.65 cate thicknesses of up to tens of meters. Our volume b.y. We dated ~7 count areas in Mare Cognitum but estimates are based on the assumption that a single only one unit showed evidence for a resurfacing basalt flow unit is 10 m thick. This thickness is a con- event. In Mare Nubium we dated ~20 count areas. servative estimate because it is at the lower end of Ages in Mare Nubium are generally similar to ages thickness estimates found in the literature [e.g., 4, 5]. obtained for basalts in Mare Cognitum but show a Once the areal extent of a basalt unit is measured, wider range of ages of 2.77-3.67 b.y. At least two the volume can be calculated and plotted in cumula- basalt units in Mare Nubium show two clearly distin- tive form versus the age. Figure 3 shows the flux of guishable ages, indicating that a resurfacing event basalts in all investigated areas, that is Imbrium, affected these units. Combined with our previously Serenitatis, Tranquillitatis, Humorum, Humboldtia- presented ages, the new crater counts indicate that num, Australe, Oceanus Procellarum, Cognitum, and active mare volcanism in Oceanus Procellarum ranges Nubium. Our data indicate that the largest basalt vol- over a long period of time from about 1.14 b.y. to umes erupted 3.3-3.7 b.y. ago. However, it has to be about 3.93 b.y., a total of ~2.8 b.y. kept in mind that older units, that are partially cov- Figure 2 shows the distribution of ages of ~220 ered with younger units are systematically underesti- basalt units that are exposed in all investigated basins. mated in their volume. The implication is that the flux For this plot we combined results from our previous curve may be steeper at older ages. Flattening of the age determinations for basalts in Mare Imbrium, curve at ages >3.7 b.y. is very likely an effect of cov- Serenitatis, Tranquillitatis, Humorum, Humboldtia- ering older units and not caused by a small flux. num, and Australe with new results for Oceanus Pro- Making use of the new data, we are now able to in- cellarum, Cognitum, and Nubium [3]. The data indi- vestigate the stratigraphy of nearside basalts and to cate that lunar volcanism in the large nearside mare study variations in basalt mineralogy with time [6] started at ~4 b.y. ago and ended at ~1.1 b.y. Most of the investigated basalts on the lunar nearside erupted Conclusions during the late Imbrian Period. Based on our new age determinations for basalts Combining age data with volume calculations of that are exposed in Oceanus Procellarum we conclude distinctive basalt units, we can make contributions to that (1) volcanism was active over a long period of estimates of the flux of lunar volcanism. time, starting at ~4 b.y. and ending at ~1.1 b.y.; (2) the largest number of basalt units were formed in the Cumulative Volumes late Imbrian Period at ~3.3-3.5 b.y.; (3) there is (Based on Estimated Thickness of 10m) All Investigated Basins probably a second peak in volcanic activity at ~2-2.2 3.5 104 b.y.; (4) the flux of lunar basalts was largest during 3 104 the late Imbrian Period, especially between ~3.3 and ~3.7 b.y., (5) the flux of basalts is significantly 4 ] 2.5 10 3 smaller during the Eratosthenian Period. 2 104 References 1.5 104 [1] Head, 1976, Rev. of Geophys. Space Phys. 14, 265-300; [2] 4 Whitford-Stark, J.L., Head III, J.W., [1980]. J. Geophys. Res. 85, 1 10 No. B11, 6579-6609; [3] Hiesinger et al., 2000, J. Geophys. Rev., in press; [4] Schaber, 1973, Proc. Lunar Planet. Sci. Conf., 4, 73- Cumulative Volume [km 5000 92; [5] Gifford and El-Baz, 1981, Moon and Planets 24, 391-398; 0 [6] Hiesinger et al., 2001, LPSC XXXII, this issue; -5000 1 1.5 2 2.5 3 3.5 4 Age [b.y.] Fig. 3: Cumulative volumes of basalts in all investigated basins (Imbrium, Serenitatis, Tranquillitatis, Oceanus Procellarum, Cog- nitum, Nubium, Humorum, Humboldtianum, Australe) For the calculation of the volume of a basalt unit, we have to rely on assumptions on the thickness of the flow unit. We do not know the thickness of each individual basalt unit, but measurements of flow units in Mare Imbrium and at the Apollo landing sites indi-.