Dados da Submissão

Título

VESTIGES OF THE KAROO MANTLE PLUME WITHIN THE SHALLOW ASTHENOSPHERE BENEATH PATAGONIA REVEALED BY PRIMORDIAL HELIUM FROM SPINEL-BEARING MANTLE XENOLITHS

Texto do resumo

Mantle xenoliths usually represent fragments derived from the depleted and degassed lithospheric mantle with 3He/4He isotope ratios lower (6 ± 1 RA) than those of mid-ocean ridge basalts (8 ± 1 RA). Otherwise, basalts from oceanic islands related to hotspots often have high 3He/4He ratios (>10 RA), suggesting a deep and pristine undegassed mantle source. Helium isotope ratios discussed here are assumed as mantle-derived because they were extracted by both stepwise and single crushing methods under high vacuum, which avoid the cosmogenic or radiogenic contributions from the mineral matrix. The 3He/4He isotopic ratios reveal the existence of intrinsic lithospheric mantle beneath Central-North Patagonia (5.33–6.99 RA), MORB-like (7.26–7.60 RA), and primordial (21.65–27.68 RA) components. Notably, mantle xenoliths with high-3He/4He isotopic ratios are found in a region characterized by low values of elastic thickness estimations (Te <20 km) and low-velocity seismic anomaly, suggesting the presence of thin and weak continental lithosphere. Comparatively, mantle xenoliths from Patagonia show compositional affinity with those from Ferrar Large Igneous Province (Antarctica), which mainly preserved the SCLM reservoir (3He/4He = 5.86–6.98 RA) superimposed by MORB-type (3He/4He = 7.00–9.05 RA) and OIB-like (3He/4He = 10.60–26.80 RA) components. In addition, South Atlantic hotspots (Shona, Bouvet, and Discovery) show high-3He/4He isotopic ratios (12-15 RA) and are assumed as the expression of the Jurassic Karoo-Ferrar mantle plume. Regarding the lower isotopic ratios recorded by these samples compared to our results, it can be explained by the less frequent occurrence of hotspots with high 3He/4He ratios in the Atlantic large low-shear velocity province (LLSVP). Alternatively, the lower 3He/4He ratios observed in these hotspot lavas can be a consequence of the plume-ridge interaction in which a deeper and undegassed component (OIB) interacted with a shallower and degassed (MORB) mantle source. Therefore, mantle xenoliths with high-3He/4He ratios from Patagonia 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.

Palavras Chave

Mantle xenoliths; Primordial helium component; Karoo mantle plume; lithospheric thinning; Patagonia.