Diffusive fractionation of noble gases in mantle with magma channels: Origin of low He/Ar in mantle-derived rocks

By crushing olivine and pyroxene phenocrysts in volcanic rocks from Kyushu Island, Japan, we determined He-3/He-4 of 3-7 Ra and Ar-40/Ar-36 of up to 1750. These values are lower than those of MORB. He-4/Ar-40* (down to 0.1) is much lower than the production ratio of He-4/Ar-40* (1-5), where an asterisk denotes correction for the atmospheric contribution. Such values are typical of mantle-derived samples from the island arcs and active continental margins. Although the origin of the low He-3/He-4 and Ar-40/Ar-36 of subcontinental mantle has been widely discussed, low He-4/Ar-40* has been given little attention. Actually, He-3/(4) He and He-4/Ar-40* of phenocrysts overlap with those of subcontinental mantle xenoliths. Although noble gas compositions of phenocrysts are affected considerably by diffusive fractionation in ascending magma, they have little effect on the noble gases in the mantle xenoliths because it takes 100 years for He/Ar fractionation of ca. 15% for a mantle xenolith with 5 cm diameter. Therefore, the low He-4/Ar-40* of the mantle xenoliths is inferred to result from another kinetic fractionation in the mantle. During generation and migration of magma in the mantle, lighter noble gases diffuse rapidly out into the magma. This diffusive fractionation can explain low He-4/Ar-40* and somewhat low He-3/He-4 in the residual mantle. Furthermore, the combination of the diffusive fractionation and subsequent radiogenic ingrowth explain the fact that data from subcontinental mantle xenoliths have extremely low He-3/He-4 and various He-4/Ar-40*. Consequently, He-4/Ar-40* and He-3/(4) He in mantle-derived materials are proposed as indicators of the degree of noble gas depletion of the source mantle. (C) 2009 Elsevier B.V. All rights reserved.