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 He/He isotope ratios (6 ± 1 R) lower than those of mid-ocean ridge basalts (8 ± 1 R). Otherwise, basalts from oceanic islands related to hotspots often have high He/He ratios (>10 R), suggesting a deep and pristine undegassed mantle source. Here we present a striking high-He/He component (up to 27.

Slab melting boosts the mantle wedge contribution to Li-rich magmas.

The lithium cycling in the supra-subduction mantle wedge is crucial for understanding the generation of Li-rich magmas that may potentially source ore deposition in continental arcs. Here, we look from the mantle source perspective at the geological processes controlling the Li mobility in convergent margins, by characterizing a set of sub-arc mantle xenoliths from the southern Andes (Coyhaique, western Patagonia). The mineral trace element signatures and oxygen fugacity estimates (FMQ >  + 3) in some of these peridotite xenoliths record the interaction with arc magmas enriched in fluid-mobile elements originally scavenged by slab dehydration.

Plume-subduction interaction forms large auriferous provinces.

Gold enrichment at the crustal or mantle source has been proposed as a key ingredient in the production of giant gold deposits and districts. However, the lithospheric-scale processes controlling gold endowment in a given metallogenic province remain unclear. Here we provide the first direct evidence of native gold in the mantle beneath the Deseado Massif in Patagonia that links an enriched mantle source to the occurrence of a large auriferous province in the overlying crust.