The helium paradoxes

Abstract

The ratio ^3He/^4He (R) plays a central role in models of mantle evolution that propose an undegassed lower mantle, rich in the primordial isotope ^3He. 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 ^3He/(U+Th) ratios or low ^4He abundances, as expected in refractory portions of the upper mantle. I show that high R materials are gas-poor and are deficient in radiogenic ^4He compared with midocean ridge basalts. The seemingly primitive (i.e., high R) signatures in “hotspot” magmas may be secondary, derived from CO_2-rich gases, or residual peridotite, a result of differential partitioning of U and He into magmas. A shallow and low ^3He 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 ^(238)U/^(204)Pb components in ocean island basalts are generally attributed to recycled altered oceanic crust. The low ^(238)U/^3He component may be in the associated depleted refractory mantle. High ^3He/^4He ratios are due to low ^4He, not excess ^3He, and do not imply or require a deep or primordial or undegassed reservoir. ^(40)Ar in the atmosphere also argues against such models.