On the ages and composition of lunar mare basalts

Abstract

Lunar volcanism lasted for about 1.5 b.y. and ceased at approximately2.5 b.y. ago. This implies that the temperature of the lunar mantle had then dropped below 1100 degrees C, necessary to produce basaltic melts on the Moon. From multispectral data sets it is also known that basalts from the lunar surface exhibit a broad variety in their composition, ranging from very low titanium (< 1 wt%) to high titanium (> 6 wt%). Investigation of the composition and the age of lunar mare basalts as well as estimates of their volumes allow to model the state and evolution of the Moon over time. In this context it is important to know, whether there exists any systematic relationbetween the age of a lunar basalt and its geochemical-mineralogical composition. Ages which are mainly based on stratigraphic superposition of units are often not precise enough or even wrong andtherefore misleading for this investigation. In order to quantisize the relationship between age and composition, new age determinations had to be obtained. We performed numerous crater counts on high resolution Lunar Orbiter IV images in combination with multispectral Galileo data from the Earth/Moon-Encounter 2. Our approach was to determine the ages for units which have been spectrally defined in a color ratio composite based on the Galileo data. From this data set we also calculated the mean titanium content of these units according to the empirical relation between the titanium content and the UV/VIS ratio. Results of this investigation will be presented for the Imbrium-, Serenitatis-, and Tranquillitatis basin, respectively.