Hf-W AND THE ISOTOPIC CRISIS FOR THE GIANT IMPACT ORIGIN OF THE MOON

Introduction: It has become clear over the last few years that the widely accepted model for the origin of the Earth-Moon system as the result of a Mars-sized giant impactor colliding with proto-Earth is inconsistent with a variety of new isotopic data. Generally, it is thought that the Earth and the Mars-sized Moonforming impactor were isotopically different (for mass independent isotopic variations). Therefore, according to the canonical Moon forming SPH simulations, the Earth and the Moon should end up isotopically different from each other and that is inconsistent with observations. In contrast, isotopic studies have demonstrated that the Earth and Moon are remarkably similar in their isotopic compositions for many elements (notably mass independent isotopic compositions of O, Ti, Cr and W, and mass dependent isotope compositions of Mg and Si). This similarity between the Earth and Moon is unique in our Solar System when compared to other planetary bodies. This unexpected similarity has been called an isotopic crisis for the giant impact origin of the Moon [1]. It provides a fundamental constraint on the theory of the origin of the Moon; any successful model must properly explain such similarity between the Earth and Moon. Thus, a model for the Earth-Moon isotopic equilibration subsequent to the giant impact was developed [2], but such a model require rather special conditions to be valid. Therefore, recently [3] and [4] discussed new and different SPH simulations that predicted ways of making the Earth and Moon into almost isotopic twins. In the new giantimpact models [5,6] lunar material is derived either from a range of depths in the proto-Earth’s mantle or equally from the entire mantles of two colliding halfEarths. These scenarios are more likely to produce a Moon with the same or very similar isotopic fingerprint as the Earth. Here we explore the case of the Hf-W evolution as many previous studies have argued that the W/W isotopic compositions of the Earth and Moon are essentially the same. We show that this radiogenic isotope system is not very likely to lead to identical W isotope compositions of the Earth and Moon. The W isotope composition of lunar samples: Measurements of the W isotope compositions of lunar samples potentially provide constraints on both (i) the age of the Moon and its earliest differentiation and (ii) the bulk W isotope composition of the Moon and whether or not it is identical to the Earth’s mantle. The measured W isotopic composition of metals from lunar High-Ti, Low-Ti mare basalts and KREEP-rich samples [7], which contain only trace amounts of Ta and consequently require negligible correction for cosmogenic W from Ta, reveal that W of these metals (0.09 ±0.10) are indistinguishable from each other and from the terrestrial samples. A measurement on ferroan anorthosite 60025 by [8] yields W = -0.03 ±0.30 and also supports identical W isotope signatures of the Earth’s mantle and the Moon. Recent very high precision measurements argue that the W/W of the Moon is only 10 to 15 ppm higher than the bulk silicate Earth [9].