Rogue mantle helium and neon.

Area of Science:

• Geochemistry
• Geophysics
• Planetary Science

Background:

• The canonical model posits an undegassed lower mantle, but this is challenged by evidence of recycled materials in ocean island basalts.
• Mantle helium, particularly isotopes like helium-3 (3He) and helium-4 (4He), is readily extracted by magmatic processes, making its long-term preservation in fertile mantle rocks unlikely.

Purpose of the Study:

• To propose a new model for the distribution of primordial helium and solar neon in Earth's mantle.
• To explain the observed variations in helium isotope ratios (3He/4He) in different types of basalts.

Main Methods:

• This study is primarily theoretical, proposing a geochemical model based on diffusion and rock properties.
• It involves interpreting existing geochemical data on helium and neon isotopes in basalts.

Main Results:

• High 3He/4He ratios and solar neon components are attributed to early-stage diffusion from primordial materials into refractory mantle rocks (e.g., dunites).
• These 'reservoir' rocks, acting as sinks for primordial volatiles, explain the geochemical signatures observed in ocean island basalts and mid-ocean ridge basalts.
• The differential tapping and stretching of these reservoirs during melting account for the distinct isotopic ratios.

Conclusions:

• The lower mantle may not be uniformly undegassed; instead, it contains reservoirs of primordial volatiles.
• This model reconciles the apparent conflict between the canonical mantle model and geochemical observations of basalts.
• Early Earth processes, including volatile diffusion into refractory rocks, played a crucial role in shaping mantle geochemistry.