Open system behavior of helium in case of the HIMU source area

Abstract

Lower 3He/4He than that of mid-ocean ridge basalts (MORBs) observed in HIMU-ocean island basalts (OIBs) has been considered to be the evidence that the HIMU source is related to recycled materials. However, typical 3He/4He value of Polynesian HIMU-OIBs (∼6.8 RA) is too high to be explained by addition of in situ radiogenic 4He in the recycled oceanic crust. Mixing of the upper mantle material with the recycled oceanic crust cannot explain 3He/4He of HIMU-OIBs as revealed from mixing calculation in the He-Pb isotope system. As another possibility, if we assume that large mobility of heliumaffects 3He/4He of recycled materials, the model calculation indicates that the balance between radiogenic 4He production in the recycled oceanic crust and diffusive loss Of 4He to the surrounding mantle accounts for 3He/4He of HIMU. The model suggests the thickness of HIMU source body to be in the order of 1 km.