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Lunar and Planetary Science XXIX 1243.pdf

ON THE RELATION OF AGE AND TITANIUM CONTENT OF BASALTS. Harald Hiesinger1,2, R. Jaumann1, G. Neukum1, and J.W. Head III2. 1DLR-Institute of Planetary Exploration, Berlin/Germany; 2Department of Geologi- cal Sciences, Brown University, Providence, RI 02906 USA. [email protected] Introduction: It has long been known that lunar mare classification by [4] which allows to distinguish four different

basalts exhibit a broad variety of geochemical compositions as classes of TiO2-concentrations. Low-titanium basalts exhibit

well as ages [1]. From the investigation of returned lunar sam- TiO2-contents of less than 2 wt%, medium-titanium basalts ples it is also known that titanium rich basalts are generally show less than 4 wt%, medium-to-high titanium basalts are older than titanium-poor basalts. On the other hand we learned characterized by 3-7 wt% titanium and high-titanium basalts from remote sensing data that there exist young basalts with contain more than 6 wt% titanium. As this classification al-

high TiO2-concentrations. Therefore the questions are: How lows variations in titanium content of up to several wt%, it

did TiO 2-concentrations vary with time? Is there a correlation was necessary to define the titanium concentration more pre-

between ages and TiO2-concentrations and if so what kind of cisely in order to calculate the mean TiO 2-content of each unit. correlation is this. Here we present results of our investigation Therefore we used the mean value of each class (1 wt%, 2 on the relationship between ages and titanium content of lunar wt%, 5 wt%, and 9 wt%) for the calculation of the mean tita- basalts for four impact basins on the nearside of the Moon nium concentration in each unit. As a result we obtained a data

(Tranquillitatis, Humorum, Serenitatis, and Imbrium). set which consists of the age and the mean TiO 2-content for 98 Approach and description of the data base: As de- different units in the four basins. scribed earlier [2] we obtained ages of spectrally defined ba- : For the basalts exposed in Mare salt units in Mare Tranquillitatis, Mare Humorum, Mare Ser- Tranquillitatis we obtained the highest values of titanium con- enitatis, and . To get the titanium concentrations centrations of all investigated maria. Large areas of Mare of each unit we made use of the Galileo data set obtained dur- Tranquillitatis are characterized by mean titanium contents

ing the second Earth-Moon encounter in 1992 and our earth- higher than 6 wt%, other areas show TiO 2-concentrations of 3- based telescopic data. The images were corrected for radio- 7 wt%. Recent investigations [5] which are based on high- metric and photometric effects, were map-projected, resolution multispectral Clementine and Galileo data were mosaiced, and normalized to MS2, a spectral calibration area made to analyse the stratigraphy of basaltic units in this maria. in . The UV/VIS ratio is an appropriate We found that the maximum volcanic activity was in the upper method to derive titanium concentrations of unsampled mature Imbrian system at 3.6-3.7 b.y. The ages of the mare basalts mare regions from multispectral remote sensing data [3]. We vary between 3.39 b.y. and 3.80 b.y. and there is obviously no used this empirical relationship in order to determine the mean trend showing younger basalts systematically poorer in tita-

TiO2-concentrations of our units. To do so, we made use of the nium than the older basalts (Fig. 1).

Fig. 1: Ages and titanium content in the investigated mare regions. Each bar represents a single basalt unit. Basins are aligned by age. Ne=Nectarian, Im1=Early Imbrian, Im2=Late Imbrian, Er=Eratosthenian Lunar and Planetary Science XXIX 1243.pdf

AGES AND TITANIUM CONTENT OF LUNAR MARE BASALTS: H. Hiesinger et al

Mare Humorum: Basalts in Mare Humorum were previ- rich than younger basalts. We found no such correlation be-

ously investigated by [6], [7], and [8]. As the Galileo EM2 tween the age and the corresponding TiO2-content in any of

data do not cover the Humorum area, we were restricted to our the investigated basins. In each basin the TiO2-concentrations telescopic data, obtained at the Mauna Kea Observatory in seem to vary independently from the ages of the units. How- August 1989. For the basalt units in the Humorum basin we ever, our data suggest that there is a trend which shows the

calculated mean TiO2-concentrations of 3-6 wt%. We found basalts of the older basins to be more titanium rich than the the highest values in the passage to and basalts of the younger basins. It is also obvious that with de- the lowest values in the Southwest of the basin. Our crater creasing basin age the variety of ages and compositions in- counts reveal ages of 3.33-3.75 b.y. for the basalts, with a creases. From the stratigraphic columns we learn that the maximum at 3.4-3.5 b.y. and a weaker maximum at 3.6-3.7 basalts exhibiting the relativly highest titanium contents often b.y. Fig. 1 shows that the titanium concentration remained appear simultanuously in all investigated basins. From the more or less constant over the whole period of basalt volcan- narrow distribution of the ages in Mare Tranquillitatis and ism in the Humorum basin. Mare Humorum we conclude that these basins may have been Mare Serenitatis: The basalts in Mare Serenitatis are filled in relatively short periods of time and therefore showing

characterized by a very broad variety in ages and TiO2- a very similar geochemical composition. For the other two concentrations. Our calculations show mean titanium contents basins differentiation processes in the magma reservoirs or the of 1-7 wt% and ages of 2.44-3.81 b.y. We observe two lunar mantle during the longer active volcanic period may maxima in the volcanic activity, one at 3.4-3.5 b.y. and a sec- have lead to the different compositions. ond at 3.6-3.8 b.y. In Mare Serenitatis low and high titanium References: [1] Taylor, S.R., (1982). Planetary science: a lunar per- basalts erupted at about the same time, so there is no strong spective. Lunar and Planetary Inst., Houston. [2] Hiesinger, H., Jau- correlation between TiO2-content and their age. We were able mann, R., Neukum, G., Head, J.W.III, (1998). Ages of lunar mare to identify 5 high titanium units, four of them during the late basalts. This volume. [3] Charette, M.P., McCord, T.B., Pieters, Imbrian. C.M., Adams, J.B., (1974). Application of remote spectral reflectance measurements to lunar geology: classification and determination of Mare Imbrium: In Mare Imbrium we identified 30 dif- titanium content of lunar soils. J. Geophys. Res. 79, 1605-1613. [4] ferent basalt units with ages of 2.01-3.57 b.y. 14 of these units Pieters, C.M., (1993). Compositional diversity and stratigraphy of the exhibit ages of 3.3-3.6 b.y., the other units all are younger than lunar crust derived from reflectance spectroscopy. Topics in remote 3.3 b.y. The majority of units were emplaced between 3.3-3.6 sensing 4, Cambridge University Press, 309-339. [5] Staid, M.I., b.y. and we observe a steady, but lower frequency, of units Pieters, C.M., Head, J.W.III, (1996). Mare Tranquillitatis: basalt em- placement history and relation to lunar samples. Journal Geophys. emplaced until 2.6 b.y. Like the basalts im Mare Serenitatis, Res., Vol. 101, No. E10, 23213-23228. [6] Pieters, C.M., Head, the basalts of Mare Imbrium show a broad variety in ages and J.W.III, McCord, T.B., Adams, J.B., Zisk, S., (1975). geochemical and

composition. We calculated the TiO2-content to be in the range geological units of Mare Humorum: definition using remote sensing of 1-4 wt%. In Mare Imbrium the TiO -content tends to de- and lunar sample information. Proc., Lunar Planet. Sci. Conf. 6th, 2 2689-2710. [7] Johnson, T.V., Pieters, C.M., McCord, T.B., (1973). crease until about 3.0-3.2 b.y. and was higher again in the Mare Humorum: an integrated study of spectral reflectivity. Icarus 19, Eratosthenian period. 224-229. [8] Lucey, P.G., Bruno, B.C., Hawke, B.R., (1991). Pre- Conclusions: Geochemical analyses of returned lunar liminary results of imaging spectroscopy of the Humorum basin re- samples generally suggest that older basalts are more titanium gion of the Moon. Proc. Lunar Planet. Sci. Conf. 21st, 391-403.