Heterogeneous mantle-derived helium isotopes in the Canary Islands and other ocean islands

Consistent He-3/He-4 ratios have been measured for >25 years in geothermal fluids and gases from Cumbre Vieja, La Palma (9.4 +/- 0.1R(A), where R-A is the He-3/He-4 of air), and Teide, Tenerife (6.8 +/- 0.3R(A)), Canary Islands. Both locations are characterized by similar CO2/He-3 (similar to 2 to 4 x 10(9)), mantle-like delta C-13 (-3.3 parts per thousand to -4.4 parts per thousand) and CO2 output (0.1-0.2 x 10(10) mol yr(-1)). Helium isotopic differences between the islands cannot be explained by differential aging and He-4 ingrowth in their mantle sources. Instead, distinct He reservoirs exist, with a high-mu (HIMU)-type mantle source for La Palma and a more enriched mantle, with possible litho spheric mantle influence, for Tenerife. Geothermal samples from the Canary Islands record a present-day He distribution distinct from higher He-3/He-4 in olivine from older eastern Canary Island lavas, indicating temporal variability in sources. Comparison of geothermal sample data versus olivine, pyroxene, and glass He isotope data for the Canary Islands, Azores, Cape Verde, Hawaiian islands, and Iceland reveals generally good correspondence, even across >1 m.y. of stratigraphy. However, in addition to the Canary Islands, there are examples of inter-island heterogeneity for He isotopes at Hawaii, the Azores, and within Iceland, preserved in hydrothermal samples, minerals, and glasses. In particular, in northwest Iceland, olivine separates from older lavas preserve higher He-3/He-4 than present-day geothermal samples from the same region. This difference likely reflects a reduced mantle-derived He-3 input to Icelandic magmatism since the Miocene. Temporal variability in He-3/He-4, assessed using geothermal and geological materials in conjunction, offers a powerful tool for examining heterogeneity and temporal evolution of mantle sources at intraplate volcanoes.

Automated summary

Consistent He-3/He-4 ratios have been measured in geothermal fluids and gases from Cumbre Vieja and Teide in the Canary Islands for over 25 years. The differences in helium isotopes between the two islands can be attributed to distinct mantle sources, with La Palma having a high-mu (HIMU)-type mantle source and Tenerife having a more enriched mantle. Temporal variability in He-3/He-4 distribution in geothermal samples compared to older lavas indicates heterogeneity and evolution of mantle sources at intraplate volcanoes.