Keywords:He isotopes, Ar isotopes, Mantle convection, Geodynamic model, Geochemistry, Volatiles
The dichotomy in 3He/4He isotopes ratios between mid ocean ridge and ocean island basalt samples (MORB and OIB) has provided a fundamental cornerstone in defining the need for a long-lived, volatile rich, and possibly primordial, geochemical reservoir within the deep mantle. However, this is somewhat at odds with evidence from other radiogenic isotope systems (e.g., U-Th-Pb, Lu-Hf, Sm-Nd, Rb-Sr), as well as geophysical evidence, for recycled surficial/crustal material in the deep mantle. Geodynamic models of mantle convection have been developed to incorporate earth-like phase and viscosity changes and can reproduce earth-like surface heat flow and plate motion. When such models take into account the higher density of the subducting oceanic crust, compared to the surrounding mantle, it provides a mechanism of retarding convection in the deepest mantle and reproduces observed geochemical distributions in multiple isotope systems (U-Th-Pb, Rb-Sr, Sm-Nd, Lu-Hf and Re-Os) that define the DMM, HIMU and EMI mantle endmembers (Brandenburg et al., 2008, Jones et al., 2017). Here we use a geodynamical model of mantle convection (Brandenburg et al., 2008), and combine it with geochemical input parameters, to investigate whether the subduction of dense eclogite can retard lower mantle convection sufficiently to explain the observed differences in 3He/4He obtained for MORB and OIB. We go on to analyse the effect of 3He outgassing from the Earth’s core and early recycling of extra-terrestrial material containing high 3He (e.g., interplanetary dust particles (IDPs)) into the mantle. The K-Ar system is also incorporated into the model and earth-like 40Ar planetary degassing from the mantle is reproduced. We explore the impact of 36Ar recycling on the 40Ar/36Ar compositions of MORB and OIB. The results suggest recycling of atmospheric Ar can produce the observed 40Ar/36Ar compositions of the upper and lower mantle.
Brandenburg, J.P., Hauri, E.H., van Keken, P.E., Ballentine, C.J., 2008. Earth and Planetary Science Letters 276, 1-13.
Jones, R., van Keken, P. E., Hauri, E., Vervoort, J. D., & Ballentine, C. J., 2017. AGU Fall Meeting Abstracts.