Pristine helium from the Karoo mantle plume within the shallow asthenosphere beneath Patagonia.

Mantle xenoliths usually represent fragments derived from the depleted and degassed lithospheric mantle with 3 He/ 4 He isotope ratios (6 ± 1 R A ) lower than those of mid-ocean ridge basalts (8 ± 1 R A ). Otherwise, basalts from oceanic islands related to hotspots often have high 3 He/ 4 He ratios (>10 R A ), suggesting a deep and pristine undegassed mantle source. Here we present a striking high- 3 He/ 4 He component (up to 27.68 R A ) recorded by spinel-facies mantle xenoliths from Patagonia. Remarkably, the highest ratios were found in a long-lived trans-lithospheric suture zone related to the Carboniferous-Permian collision of two continental blocks: the Deseado and the North Patagonian massifs. The mantle xenoliths with notably high- 3 He/ 4 He ratios are inferred to be fragments of the shallow asthenosphere rising through the eroded and rejuvenated thin lithosphere. The pristine helium component is derived from the western margin of the Karoo mantle plume, related to the initial stages of the Gondwana fragmentation.