Early Moon formation inferred from Hafnium-Tungsten systematics

Area of Science:

• Geochemistry
• Planetary Science
• Cosmochemistry

Background:

• Lunar age estimates vary widely, impacting our understanding of Earth's origin.
• A key debate centers on the higher abundance of tungsten-182 (¹⁸²W) in the silicate Moon compared to Earth.
• This difference was previously attributed to varying amounts of late accretion after core formation.

Purpose of the Study:

• To investigate the origin of the ¹⁸²W anomaly in lunar samples.
• To refine the timeline of the Moon-forming impact.
• To re-evaluate the role of late accretion in lunar and terrestrial evolution.

Main Methods:

• High-precision trace element analysis of lunar samples using inductively coupled plasma mass spectrometry.
• Measurement of Hafnium/Tungsten (Hf/W) ratios in lunar silicates.
• Integration of experimental data on partition coefficients.

Main Results:

• The Hf/W ratio in the silicate Moon is higher than in the bulk silicate Earth.
• The observed ¹⁸²W excess is consistent with the decay of the short-lived isotope Hafnium-182 (¹⁸²Hf).
• ¹⁸²Hf was present only during the first 60 million years of solar system formation.

Conclusions:

• The Moon formed early, around 50 million years after the solar system's formation.
• The ¹⁸²W excess in the lunar silicate is explained by radiogenic decay, not late accretion.
• This finding provides a more precise constraint on the timing of the Moon-forming impact and early Earth history.