Melting in the Deep Lunar Mantle

Introduction: Whereas previous experimental studies [1,2,3] and interpretation of Apollo seismic experiments [4,5] are permissive for garnet in the deep lunar mantle, interpretations of Lu-Hf isotopic systematics [6] and trace element data [7] seem to require garnet in the source of selected mare basalts. Understanding the mineralogy of the deep lunar mantle is critical to our further interpretation of the structure of the Moon and modeling early lunar differentiation. Also, as it has been suggested that some mare basalts may be the product of melting that was initiated at depths to 1000 km, the stability of phases such as garnet may affect basalt composition and magma transport. To better understand melting in the deep lunar mantle we have initiated a study that focuses upon both the trace element characteristics of mare basalts and pyroclastic glasses and the high pressure (> 2.5 GPa) phase equilibria of pyroclastic glass compositions. Here we report the initial high pressure results on picritic glass compositions and ion probe trace element study of the pyroclastic glasses with an emphasis on elements that could potentially be fractionated by garnet. In addition to continuing this line of experiments, an additional facet of this study will focus upon a comparison of trace elements between mare basalts and pyroclastic glasses using comparable ion probeICP-MS data sets. Analytical Approach: For this study, we initially started with the Apollo 15 and 17 pyroclastic glasses. Prior to trace element analysis, individual glasses were imaged and analyzed using a JEOL 733 superprobe. Selected trace elements (Sc, Y, Zr, Ce, Yb, Hf, Hf) that would be useful in evaluating the presence of garnet in the source were measured using the Cameca ims 4f operated on the University of New Mexico campus by IOM. Analyses were made using primary O ions accelerated through a nominal potential of 10.0 kV. A primary beam current of 20 nA was focused on the sample over a spot diameter of 20 μm. Sputtered secondary ions were energy filtered using a sample offset voltage of 105 V and an energy window of ± 25 V. Analyses involved repeated cycles of peak counting. The analytical procedure included counting on a background position to monitor detection noise. Absolute concentrations of each element were calculated using empirical relationships of Trace Element/ Si ratios (normalized to known SiO2 content) to element concentrations as derived from daily calibration. Calibration curve was constructed using multiple glass standards (> 3) for each element. Calibration curves for each element have correlation coefficients of greater than 0.97. Average values for Hf for the green and orange glasses as determined by ion probe are 0.63 and 7.3 ppm, respectively. The average values for Hf for bulk Apollo 15 green and Apollo 17 orange glasses are 0.6 and 5.8 ppm, respectively All analyses are referenced to a single basalt glass standard that will then be used as a comparative reference to future ICPMS analysis of crystalline mare basalts. Experimental Approach: High pressure experiments are being carried out in the IOM high pressure experimental lab at pressures between 2.0 and 4.5 GPa using synthetic Apollo 15 green C glass and Apollo 14 black glass compositions. These compositions span nearly the entire range of lunar pristine glasses, and these pressures range to the deepest parts of the lunar mantle. Results: High Pressure Experiments. Apollo 15 green glass C is saturated with garnet and clinopyroxene at 4.5 GPa and 1775°C (Fig. 1). Additional experiments will determine which phase is on the liquidus.