[S-IT16] Planetary cores: Structure, formation, and evolution

[SIT16-01] Helium in the Earth’s core

*William F McDonough¹ (1.Department of Earth Science, Sendai, Miyagi 980-8578, Japan)

Keywords:Earth's core, helium, volcanic gases, core--mantle exchange

To explain the often observed occurrence of ³He-enriched isotopic compositions in some ocean island basalts (OIB) the Earth's core is often invoked as a source of primitive helium gas with a solar, or nearly so, isotopic composition (i.e., R_core/R_A > 120, where R_A = (³He/⁴He) of the atmosphere (i.e., 1.340 ± 0.006)). Here we examine this assumption and consider a number of likely scenarios where the core might or might not be a supplier of isotopically primitive helium gas. Given experimentally determined solubilities for He in a core forming metallic melt and compositional models for the core, we show here that alpha emitting radioisotopes of platinum group elements alone can reduce by about half the original primitive solar isotopic composition of helium. Moreover, adding about 1 picogram/gram of U into the core further reduces the core's ³He/⁴He value (<50). These observations should motivate further studies on the nature of helium in the core.