Rogue Mantle Helium and Neon
Francis Albarède
The canonical model of helium isotope geochemistry describes
the lower mantle as undegassed, but this view conflicts with
evidence of recycled material in the source of ocean island
basalts. Because mantle helium is efficiently extracted by magmatic
activity, it cannot remain in fertile mantle rocks for long
periods of time. Here, I suggest that helium with high
3He/
4He
ratios, as well as neon rich in the solar component, diffused
early in Earth's history from low-melting-point primordial material
into residual refractory "reservoir" rocks, such as dunites.
The difference in
3He/
4He ratios of ocean-island and mid-ocean
ridge basalts and the preservation of solar neon are ascribed
to the reservoir rocks being stretched and tapped to different
extents during melting.
