He–Ne–Ar–N 2 –CO 2 Systematics of Fernando de Noronha Mantle Xenoliths: Confirmation of Mantle Plume Origin

The Fernando de Noronha archipelago (southwest Atlantic, 345 km from the coast of Brazil) is considered as the result of mantle plume activity. However, data on the isotopic composition of helium and neon, which are, perhaps, the only unambiguous geochemical criterion for deep mantle plumes have not been published yet for the region. In this paper, we present the first data on the isotopic composition of helium, neon, argon, and nitrogen, obtained by stepwise crushing of mantle xenoliths from the basanites of the San José Formation. The results obtained may indicate that fluid inclusions contain the very first portions of the exsolved gases—they are ultra-depleted in helium in relation to neon and especially argon. This conclusion is also supported by He–Ar–CO 2 systematics. The isotopic composition of helium ( 4 He/ 3 He = 31 879 ± 6796) and neon ( 21 Ne/ 22 Ne (mantle) = 0.0453 ± 0.0012) indicates that it was indeed a mantle plume, identical in noble gas composition to the Kerguelen plume. According to the Ar–Ne isotope systematics 40 Ar/ 36 Ar (mantle) = 7455 ± 2290. Nitrogen is characterized by a heavy isotopic composition (δ 15 N = +5.4 ± 0.2‰), which corresponds to the hypothesis of the subduction nature of nitrogen in deep mantle plumes.