Er and Yb isotope fractionation in planetary materials

Abstract

Terrestrial planets are depleted in volatile elements relative to solar abundances. Little is known, however, about volatility at the high temperatures relevant to asteroidal collisions and to the giant lunar impact. Although refractory rare-earth elements have overall similar crystallochemical properties, some differ in their temperatures of condensation from the nebular gas. This is the case for Yb, which condenses at ∼1490K and in the vapor is mostly in elemental form. By contrast, Er, largely present as ErO, condenses at ∼1660K. We analyzed the Er and Yb isotopic compositions in 33 terrestrial basalts, garnets, different classes of chondrites and achondrites, and lunar samples by MC-ICP-MS. The range of mass-dependent isotope fractionation is larger for Yb (0.43‰ per amu) than Er (0.23‰) isotopes. For terrestrial rocks, a positive correlation between δYb and La/Yb suggests that the isotopic differences between Er and Yb can be accounted for by the presence of small fractions of Yb2+. Yb is isotopically heavy in kimberlite and light in garnets. Ytterbium behaves similarly to Fe, with Yb3+ being more incompatible than the much less abundant Yb2+. In addition, the coexistence of divalent and trivalent sites in the garnet structure and the preference of heavy isotopes for stable bonds makes Yb in garnet isotopically light.The deficit of heavy Yb isotopes in lunar basaltic samples relative to the Earth, chondrites, and eucrites provides new evidence that the Moon formed by the condensation of silicate vapor in the aftermath of the giant lunar impact. Separation of vapor from melt and of heavy from light isotopes is first expected during the adiabatic expansion of the initial vapor plume. Subsequently, friction between melt and gas tends to further enrich the Moon feeding zone in silicate vapor to compensate the inward migration of melt out of the pre-lunar disk. A major consequence of interpreting the present lunar data by vapor/melt segregation is that the relative abundances of refractory elements in the Moon are unlikely to be chondrite-like or even Earth-like.Erbium isotope ratios in lunar samples reflect the capture of neutrons produced by galactic cosmic rays. The first resonance of 167Er for neutron capture will help cover an energy range poorly covered by other nuclides.