Effect of shallow-level contamination on the helium isotope systematics of ocean-island lavas
The bimodal distribution of helium isotope ratios in ocean-island lavas has provided powerful constraints on the composition and evolution of the Earth's mantle. 'High-3He hotspot' ratios (3He/4He greater than in mid-ocean-ridge basalt, MORB) found on Hawaii and Iceland are thought to...
| Main Authors: | , , |
|---|---|
| Format: | Journal article |
| Language: | English |
| Published: |
1995
|
| Summary: | The bimodal distribution of helium isotope ratios in ocean-island lavas has provided powerful constraints on the composition and evolution of the Earth's mantle. 'High-3He hotspot' ratios (3He/4He greater than in mid-ocean-ridge basalt, MORB) found on Hawaii and Iceland are thought to trace pristine plumes from the deep(er) mantle, whereas 'low-3He hotspot' values (3He/4He ≤ MORB value) at Tristan da Cunha and St Helena are considered to characterize plumes composed (in part) of recycled oceanic or continental crust. Here we report the observation of both 'high-3He' and 'low-3He' characteristics in lavas from a single ocean island - Heard Island, in the Indian Ocean. Whereas the high-3He lavas provide unambiguous evidence for the involvement of a deep-seated plume in their genesis, we argue that the low 3He/4He ratios in other lavas result from shallow-level contamination by radiogenic helium before eruption. These observations call into question the presumed association between low-3He ratios (at Heard Island and elsewhere) and ancient crustal material recycled back into the mantle. |
|---|