Deep mantle neon and xenon preserve a record of early planetary differentiation and heterogeneous volatile accretion

Xe, produced from the radioactive decay of extinct I, and Xe, produced from extinct Pu and extant U, have provided important constraints on early mantle outgassing and volatile loss from Earth. The low ratios of radiogenic to non-radiogenic xenon (Xe/Xe) in ocean island basalts (OIBs) compared to mid-ocean ridge basalts (MORBs) have been used as evidence for the existence of a relatively undegassed primitive deep mantle reservoir. However, the low Xe/Xe ratios in OIBs have also been attributed to mixing between subducted atmospheric Xe with MORB Xe, which obviates the need for a less degassed deep mantle reservoir. Here I present new noble gas measurements (He, Ne, Ar, Xe) from the Iceland plume that show differences in elemental abundances and Ne/Ne ratios between MORB and OIBs. The new observations demonstrate for the first time that the lower Xe/Xe ratios in OIBs are derived from a lower I/Xe ratio in the OIB mantle source and cannot be explained by mixing atmospheric Xe with MORB-type Xe. As I became extinct ~100 Myrs after the start of the Solar System, OIB and MORB mantle sources must have differentiated by 4.45 Ga and subsequent mixing must have been limited. The Iceland plume source also has a higher proportion of Puto U-derived fission Xe, requiring the plume source to be less degassed than MORBs, a conclusion that is independent of noble gas concentrations and the partitioning behavior of the noble gases with respect to their radiogenic parents. Overall, these results demonstrate that the Earth’s mantle accreted volatiles from at least two separate sources and 4.45 Gyrs of mantle convection has not erased the signature of Earth’s early differentiation.