Evolution of helium isotopes in the Earth's mantle
Abstract
Degassing of the Earth's mantle through magmatism results in the irreversible loss of helium to space, and high 3He/4He ratios observed in oceanic basalts have been considered the main evidence for a `primordial' undegassed deep mantle reservoir. Here we present a new global data compilation of ocean island basalts, representing upwelling `plumes' from the deep mantle, and show that island groups with the highest primordial signal (high 3He/4He ratios) have striking chemical and isotopic similarities to mid-ocean-ridge basalts. We interpret this as indicating a common history of mantle trace element depletion through magmatism. The high 3He/4He in plumes may thus reflect incomplete degassing of the deep Earth during continent and ocean crust formation. We infer that differences between plumes and the upper-mantle source of ocean-ridge basalts reflect isolation of plume sources from the convecting mantle for ~1-2Gyr. An undegassed, primordial reservoir in the mantle would therefore not be required, thus reconciling a long-standing contradiction in mantle dynamics.
- Publication:
-
Nature
- Pub Date:
- August 2005
- DOI:
- 10.1038/nature03930
- Bibcode:
- 2005Natur.436.1107C