Further efforts to limit lunar internal temperatures from electrical conductivity determinations

Abstract

A Monte Carlo procedure is used to generate a representative set of eighteen electrical conductivity profiles that are consistent with previously published lunar transfer function data in the frequency range 10−5 to 10−3 Hz. The electrical conductivity is most strongly constrained by these data in the approximate depth range 450 to 1350 km. Published laboratory electrical conductivity versus temperature data (extrapolated to higher and lower temperatures) are applied to convert the eighteen electrical conductivity profiles into temperature profiles for a series of radially homogeneous olivine‐pyroxene mixtures ranging from 100% olivine [(Mg/(Mg + Fe) = 0.91] to 100% aluminous orthopyroxene (6.8 wt% Al2O3). The 100% olivine composition yields selenotherms that approach the Ringwood‐Essene solidus at depths near 500 km while the addition of aluminous orthopyroxene (either 1.9 or 6.8 wt% Al2O3) in concentrations exceeding 15–30 vol% leads to cooler selenotherms that approach the solidus only at depths greater than about 1000 km. The shape of the temperature‐depth profiles is not found to be strongly sensitive to the olivine/pyroxene ratio if the assumption of radial homogeneity is valid at depths greater than 450–500 km.On the basis of independent geophysical constraints (low seismic shear wave attenuation in the upper mantle, maintenance of mascon anisostasy over 3–4 b.y., locations of the deep moonquake foci), the profile envelopes for compositions containing more than 15–30 vol% aluminous orthopyroxene are considered to be most probable. A comparison of the latter with present day selenotherms calculated according to thermal history models indicates the greatest consistency with the model of Toksöz et al. [1978] which assumed initial partial melting and differentiation to a depth of 500 km and significant radial heat transport by subsolidus convection only during the first few b.y. of lunar history.