The Behavior of Thorium in Lunar Picritic Magmas: Implications for the Bulk Thorium Content of the Lunar Mantle and Lunar Heat Flow

Introduction: Previous studies of heat flow from the lunar mantle have suggested that the amount of heat flow is strongly dependent on the abundance of radioactive isotopes in the mantle and is at least loosely correlated with Th content [1,2]. Measurements of heat flow from the Apollo missions had been used to imply that those measurements could be extended outside of KREEP terrains; however, the asymmetrical distribution of Th on the lunar surface and presumably in the deep lunar mantle challenges that notion. Fortunately, lunar pyroclastic glasses, which are thought to represent quenched products of near-primary melts from the deep lunar mantle [3], can provide information about the concentration of radiogenic elements in the lunar interior. High-pressure experiments on lunar picritic glasses indicate that the basalts they represent were in equilibrium with olivine + orthopyroxene residua [4]. Such a mineral assemblage is incapable of fractionating Sm from Th during melting, which means that the Sm/Th of the glass approximates the Sm/Th of the mantle sources for these magmas [4]. This abstract reports the Th and Sm results from in-situ analyses of pyroclastic glasses from the Apollo 11, 14, 15, and 17 landing sites as a means to evaluate Th and Sm systematics in the lunar mantle. Scientific Rationale and Analytical Approach: Prior to trace element analysis, individual phases were imaged, mapped and analyzed for major elements using a JEOL 733 superprobe. Trace elements in the glasses were measured using the Cameca ims 4f operated on the University of New Mexico campus by the Institute of Meteoritics. Analyses were made using primary O-ions accelerated through a nominal potential of 10 kV. A primary beam current of 15 nA was focused on the sample over a spot diameter of 10 to 15 µm. Sputtered secondary ions were energy filtered using a sample-offset voltage of 105 V and an energy window of ±25 V. Absolute concentrations of Th and Sm were calculated using empirical relationships of Trace Element/ 30 Si + ratios (normalized to known SiO 2 content in each of the glasses). Calibration curves were constructed using six basaltic glass standards. Major element analyses of these glasses have been well documented in other studies [e.g. 3 and 5].