The helium paradoxes

The ratio He-3/He-4 (R) plays a central role in models of mantle evolution that propose an undegassed lower mantle, rich in the primordial isotope He-3. A large primordial volatile-rich reservoir, a feature of recent models, is inconsistent with high-temperature accretion and with estimates of crustal and bulk Earth chemistry, High R can alternatively reflect high integrated He-3/(U+Th) ratios or low He-4 abundances, as expected in refractory portions of the upper mantle, I show that high R materials are gas-pool-and are deficient in radiogenic He-4 compared with midocean ridge basalts, The seemingly primitive (i.e., high R) signatures in "hotspot" magmas may be secondary, derived from CO2-rich gases, or residual peridotite, a result of differential partitioning of U and He into magmas. A shallow and low He-3 source explains the spatial variability and the temporal trends of R in ocean islands and is consistent with a volatile-poor planet. A shallow origin for the "primitive" He signature in ocean island basalts, such as at Loihi, reconciles the paradoxical juxtaposition of crustal, seawater, and atmospheric signatures with inferred "primitive" characteristics. High U-238/Pb-204 components in ocean island basalts are generally attributed to recycled altered oceanic crust, The low U-238/He-3 component may be in the associated depleted refractory mantle, High He-3/He-4 ratios are due to low He-4, not excess He-3, and do not imply or require a deep or primordial or undegassed reservoir, Ar-40 in the atmosphere also argues against such models.