Water and the Electrical Conductivity of the Lunar Mantle

Introduction. The electrical conductivity of minerals increases with temperature and impurity content [e.g., 1]. Aluminum in orthopyroxene is one such impurity that has been invoked to fit the electrical conductivity of the deep interior of the Moon while maintaining subsolidus temperatures [2-4]. However, the required bulk alumina abundance in pyroxene is comparable to that inferred for the entire pristine lunar mantle. Water in trace quantities—tens of ppm—can account for the observed lunar conductivity and is suggested by recent analytic measurements [5]. Background. Electrical conductivity constrains the temperature and composition of planetary interiors. A large body of work has used electromagnetic induction to derive the lunar conductivity-depth profile and thence infer mantle temperatures from assumed compositions [2-4,6-9]. Effect of Al 2 O 3. Early work relating lunar conductivity to laboratory temperature-conductivity measurements of olivine and low-alumina pyroxenes derived excessively high temperatures for the Moon's interior. Subsequent laboratory studies revealed that orthopyroxene (opx) conductivity increases with the contents of aluminum and chromium [2]. Substitution of a trivalent cation for Si 4+ causes a charge-compensating increase in ferric iron content, Fe