Episodic entrainment of deep primordial mantle material into ocean island basalts

Abstract

Chemical differences between mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) provide critical evidence that the Earth's mantle is compositionally heterogeneous. MORBs generally exhibit a relatively low and narrow range of 3He/4 He ratios on a global scale, whereas OIBs display larger variability in both time and space. The primordial origin of 3He in OIBs has motivated hypotheses that high 3He/4 He ratios are the product of mantle plumes sampling chemically distinct material, but do not account for lower MORB-like 3He/4 He ratios in OIBs, nor their observed spatial and temporal variability. Here we perform thermochemical convection calculations which show the variable 3He/4 He signature of OIBs can be reproduced by deep isolated mantle reservoirs of primordial material that are viscously entrained by thermal plumes. Entrainment is highly time-dependent, producing a wide range of 3He/4 He ratios similar to that observed in OIBs worldwide and indicate MORB-like 3He/4 He ratios in OIBs cannot be used to preclude deep mantle-sourced hotspots.