Cerium-Nd isotope evidence for an incompatible element depleted Moon

1E. Hasenstab-Dübeler,1C. Münker,1J. Tusch,1,2M.M. Thiemens,3 D. Garbe-Schönberg,4E. Strub,5P. Sprung
Earth and Planetary Science Letters 606, 118018 Link to Article [https://doi.org/10.1016/j.epsl.2023.118018]
1Institut für Geologie und Mineralogie, Universität zu Köln, Köln, Germany
2Laboratoire G-Time, Département Géosciences, Environnement et Société, Université Libre de Bruxelles, Brussels, Belgium
3Institut für Geowissenschaften, Christian-Albrechts- Universität zu Kiel, 24118 Kiel, Germany
4Division of Nuclear Chemisty, University of Cologne, 50674 Cologne, Germany
5Hot Laboratory Division (AHL), Paul Scherrer Institut, Villigen, Switzerland
Copyright Elsevier

The Moon is a key example of a planetary body that originated from a giant impact collisional event. By better understanding its bulk composition, we gain critical constraints on the building blocks of the Earth-Moon system. Combined measurements of long-lived 147Sm-143Nd and short-lived 146Sm-142Nd isotope compositions of Earth and Moon have lead to controversial interpretations in the past and it remains ambiguous, whether or not the Moon is similar to primitive chondrites in its refractory lithophile element composition. We investigated coupled 138La-138Ce and 147Sm-143Nd isotope and trace element compositions across a wide range of lunar rock types to provide an independent assessment of the bulk Moon composition. All measured lunar rocks define a tight array in 138Ce-143Nd space, intersecting initial
at an initial εCe
, significantly lower than the currently accepted chondritic 138Ce reference value. The results of combined modeling of 138Ce-143Nd-176Hf isotope and trace element behavior during lunar magma ocean (LMO) crystallization are in good agreement with the bulk silicate Moon having a slight depletion in its highly incompatible trace element inventory. Our calculated composition of the silicate Moon evolves towards
and εNd =+1.4 at 3.30 Ga, the approximate age of most lunar samples investigated here. This proposed lunar isotope composition at 3.30 Ga agrees well with the intersection of the
Ga lunar array and the terrestrial array defined by rocks from the Archean Pilbara and the Kaapvaal Cratons. We take this as evidence that accessible silicate Earth and the Moon may share a common reservoir slightly depleted in highly incompatible trace elements, named here Slightly Depleted Earth-Moon reservoir (SDEM). The SDEM reservoir proposed here is generally in line with previous models claiming a depleted composition of the accessible silicate Earth, but the degree of depletion is significantly smaller than previously proposed.


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