Zirconium, hafnium, and rare earth element partition coefficients for ilmenite and other minerals in high‐Ti lunar mare basalts: An experimental study

Abstract

Partition coefficients were determined for Gd, Lu, Hf, and Zr among ilmenite, armalcolite, and synthetic high‐Ti mare basaltic melts at temperatures from 1122° to 1150°C, and at oxygen fugacities of IW × 100.5, by in situ analysis with the electron microprobe, using samples doped to percent concentration levels. Coefficients for Zr were also measured for samples containing 600–1600 ppm Zr, using a special high‐sensitivity electron microprobe technique. In addition, coefficients were determined for Hf and Zr between chromian ulvospinel and melt, for Hf between pigeonite and melt, and for Lu between olivine and melt by microprobe analysis of samples doped to percent levels. Recommended values for ilmenite are D(Gd) = 0.006, D(Lu) = 0.09, D(Hf) = 0.42, and D(Zr) = 0.33. Recommended values for armalcolite are D(Gd) = 0.006, D(Lu) = 0.046, D(Hf) = 1.5, and D(Zr) = 0.9. Values measured using the microprobe are in general agreement with values measured by analyzing mineral separates from the sanle run products by isotope dilution at the USGS Isotope Lab in Denver. Agreement between microprobe and ID is within 3% for Hf, and within 10% for Zr. For Lu, the ID results were 15–35% lower than the microprobe values. The source of this disagreement has not been identified. Parition coefficients measured in this study and the collaborative ID study confirm the general results of earlier measurements. D values for ilmenite are less than 0.01 for the LREE (light rare earth element), are around 0.1 for the HREE (heavy rare earth elements), and are several times greater than this for Zr and Hf. These results constitute the first simultaneous experimental measurement of ilmenite distribution coefficients for the three important elements Lu, Hr, arid Zr in mare basaltic systems.