moon

Rb-Sr constraints on the age of Moon formation

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Elsa Yobregat, Caroline Fitoussi, Bernard Bourdon
Icarus (in Press) Open Access
Link to Article [https://doi.org/10.1016/j.icarus.2024.116164]
Laboratoire de Géologie de Lyon, ENS Lyon, CNRS, UCBL, France

Determining the age of the Moon, which is commonly considered as the termination of Earth accretion has been a complex challenge for geochronology. A number of methods have been used to delineate the age of the Moon based either on absolute chronology of lunar rocks or have relied on more indirect methods using short-lived nuclides such as 182Hf that was present in the early history of the Solar System. Model ages usually require some assumptions that are sometimes controversial or harder to verify.

In this study, new high precision Sr isotope data (2.4 ppm, 2SD) were obtained for a well-dated lunar anorthosite (60025) in order to better constrain the initial 87Sr/86Sr of the bulk silicate Moon. This new data is then used to model the Sr isotope evolution of the Earth-Moon starting from the beginning of the Solar System. To comply with the Hfsingle bondW and stable isotope constraints, we then assume that the Earth and Moon were equilibrated at the time of Moon formation. By investigating systematically all the sources of uncertainties in our model, we show that compared with previous work on anorthosite, one can tighten the constraints on the youngest age of Moon formation to no >79 Ma after the beginning of the Solar System, i.e. the Moon cannot be younger than 4488 Ma.

Sound velocities in lunar mantle aggregates at simultaneous high pressures and temperatures: Implications for the presence of garnet in the deep lunar interior

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Marisa C. Wood1, Steeve Gréaux1, Yoshio Kono1, Sho Kakizaw1,2, Yuta Ishikawa1, Sayako Inoué1, Hideharu Kuwahara1, Yuji Higo2, Noriyoshi Tsujino2, Tetsuo Irifune1
Earth and Planetary Science Letters 641, 118792
Link to Article [https://doi.org/10.1016/j.epsl.2024.118792]
1Geodynamics Research Center, Ehime University, Matsuyama, Japan
2Japan Synchrotron Radiation Research Institute, SPring-8, Hyogo, Japan
Copyright Elsevier

Recent experimental and theoretical studies on lunar magma ocean crystallisation have suggested the presence of significant proportions of garnet in the deep lunar interior. While phase relation studies indicate a deep lunar mantle consisting of olivine, pyroxene, and garnet, the compatibility of such an assemblage with seismic models of the lunar interior is yet untested. In this study we report compressional and shear wave velocities in an iron-rich assemblage consisting of olivine, orthopyroxene, clinopyroxene, and garnet up to ∼8 GPa and 1300 K, by means of ultrasonic interferometry measurements combined with synchrotron techniques using the multi-anvil press apparatus. Sound velocity and density models of lunar mantle rocks along a selenotherm based on our experimental results find good agreement with the seismic and density profiles at lunar interior depths of 740–1260 km. Further models are constructed, allowing for the variation of chemical composition, phase proportion, and temperature; these suggest that a garnet-rich deep lunar mantle is compatible with present-day lower lunar mantle temperatures of between 1400–1800 K. Our results show that lunar mantle rocks with up to 33 wt.% garnet may provide an explanation for the observed high velocities of the lower lunar mantle. The presence of garnet in the lowermost part of the Moon’s mantle has significant implications for the depth and temperature of the Moon’s magma ocean as well as the composition, structure and internal dynamics of the solid Moon.