Noble gas systematics of the Reunion mantle plume source and the origin of primordial noble gases in Earth's mantle

摘要

New noble gas data of ultramafic xenoliths from Reunion Island, Indian Ocean, further constrain the characteristics of primordial and radiogenic noble gases in Earth's mantle plume reservoirs. The mantle source excess of nucleogenic ~(21)Ne is significantly higher than for the Hawaiian and Icelandic plume reservoirs, similar to excess of radiogenic ~4He. ~(40)Ar/~(36)Ar of the Reunion mantle source can be constrained to range between 8000 and 12 000, significant ~(128)Xe and fission Xe excess are present. Regarding the relative contribution of primordial and radiogenic rare gas nuclides, the Reunion mantle source is intermediate between Loihi- and MORB-type reservoirs. This confirms the compositional diversity of plume sources recognized in other radioisotope systematics. Another major result of this study is the identification of the same basic primordial component previously found for the Hawaiian and Icelandic mantle plumes and the MORB reservoir. It is a hybrid of solar-type He and Ne, and 'atmosphere-like' or 'planetary' Ar, Kr, Xe (Science 288 (2000) 1036). ~(20)Ne/~(22)Ne ratios entend to maximum values close to 12.5 (Ne-B), which is the typical signature of solar neon implanted as solar corpuscular radiation. This suggests that Earth's solar-type noble gas inventory was acquired by small (less than km-sized) precursor planetesimals that were irradiated by an active early sun in the accretion disk after nebular gas dissipation, or, alternatively, that planetesimals incorporated constituents irradiated in transparent regions of the solar nebula. Previously, such an early irradiation scenario was suggested for carbonaceous chondrites which follow common volatile depletion trends in the sequence CI-Cm-CV-Earth. In turn, CV chondrites closely match Earth's mantle composition in ~(20)Ne/~(22)Ne, ~(36)Ar/~(22)Ne and ~(36)Ar/~(38)Ar. This indicates that mantle Ar could well be a planetary component inherited from precursor planetesimals. However, a corresponding conclusion for mantle Kr and Xe is less convincing yet, but this may be just due to the lack of appropriate 'meteoritic' building blocks matching terrestrial composition. Alternatively, heavy noble gases in Earth's mantle could be due to admixing of severely fractionated air, but this effect must have affected all mantle sources to a very similar extent, e.g. by global subduction before the last homogenization of the mantle reservoirs.