Subducted oceanic lithosphere and the origin of the 'high mu' basalt helium isotopic signature

The isotope geochemistry of ocean island basalts has been used to infer the presence of ancient recycled oceanic crust in the mantle. The helium isoto pic ratios of basalts from HIMU sources, having high U/Pb (mu) and low Rb/S r, are thought to be derived from recycled ocean crust, but are only slight ly more radiogenic than average mid-ocean ridge basalt (MORB) values (i.e. He-3/He-4 of 6-7 R/Ra, compared to similar to 8 R/Ra). These values appear to be inconsistent with ancient recycled oceanic crustal sources because th e helium should be significantly more radiogenic. We propose a simple model to explain this helium isotopic composition. The basic hypothesis is that the entire oceanic lithosphere is subducted, of which the oceanic crust is only a small fraction. Based on measurements from xenoliths, the residual l ithosphere contains similar to5 x 10(-7) ccSTP/g of helium. The relatively low uranium content in the mantle lithosphere ( < 0.5 ppb) leads to He-3/He -4 ratios slightly more radiogenic than the upper mantle ratio after 2 Ga, and could be the source of the HIMU helium isotopic composition by mixing w ith the subducted oceanic crust. Although the helium content of the subduct ed lithosphere is highly uncertain, existing data suggest that a simple clo sed system evolution model can explain the helium isotopic composition of t he HIMU mantle sources. (C) 2001 Elsevier Science B.V. All rights reserved.