Early differentiation and volatile accretion recorded in deep-mantle neon and xenon

Noble gas contents of the Iceland mantle plume show that neither the Moon-forming impact nor billions of years of mantle convection has erased the signature of Earth’s heterogeneous accretion and early differentiation. This analysis of noble gas content in Icelandic basaltic rocks — material produced by the melting of a deep upwelling of hot rock in Earth's mantle — indicates that their source is less degassed than that of mid-ocean-ridge basalts and that Earth's mantle has accreted volatiles from at least two separate sources. The author concludes that neither the Moon-forming impact nor billions of years of mantle convection has erased the signature of Earth's heterogeneous accretion and early differentiation. The isotopes 129Xe, produced from the radioactive decay of extinct 129I, and 136Xe, produced from extinct 244Pu and extant 238U, have provided important constraints on early mantle outgassing and volatile loss from Earth1,2. The low ratios of radiogenic to non-radiogenic xenon (129Xe/130Xe) in ocean island basalts (OIBs) compared with mid-ocean-ridge basalts (MORBs) have been used as evidence for the existence of a relatively undegassed primitive deep-mantle reservoir1. However, the low 129Xe/130Xe ratios in OIBs have also been attributed to mixing between subducted atmospheric Xe and MORB Xe, which obviates the need for a less degassed deep-mantle reservoir3,4. Here I present new noble gas (He, Ne, Ar, Xe) measurements from an Icelandic OIB that reveal differences in elemental abundances and 20Ne/22Ne ratios between the Iceland mantle plume and the MORB source. These observations show that the lower 129Xe/130Xe ratios in OIBs are due to a lower I/Xe ratio in the OIB mantle source and cannot be explained solely by mixing atmospheric Xe with MORB-type Xe. Because 129I became extinct about 100 million years after the formation of the Solar System, OIB and MORB mantle sources must have differentiated by 4.45 billion years ago and subsequent mixing must have been limited. The Iceland plume source also has a higher proportion of Pu- to 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 behaviour of the noble gases with respect to their radiogenic parents. Overall, these results show that Earth’s mantle accreted volatiles from at least two separate sources and that neither the Moon-forming impact nor 4.45 billion years of mantle convection has erased the signature of Earth’s heterogeneous accretion and early differentiation.