Isotopically depleted, alkalic lavas from Bowie Seamount, northeast Pacific Ocean

Abstract

Bowie Seamount, located 200 km west of the Queen Charlotte Islands, British Columbia, is one of the southernmost of the Pratt–Welker Seamounts in the Gulf of Alaska. Eight dredge hauls recovered hawaiite and mugearite, with major-and trace-element compositions similar to those of Hawaiian alkali basalts. All samples are enriched in light incompatible elements but are isotopically MORB-like: 87Sr/86Sr = 0.70269–0.70285; 143Nd/144Nd = 0.51307–0.51315; and 206Pb/204Pb = 18.26–19.18. Sr- and Pb-isotope ratios correlate positively, whereas Sr and Nd correlate negatively, and lie on a mixing trend between DMM and HIMU mantle components. Juan de Fuca MORB have similar ratios and define similar mixing trends.The combined geochemical data require that the mantle source of Bowie (and Pratt–Welker) magmas has only recently been enriched in incompatible elements. The enrichment process could involve (i) metasomatism of the underlying lithosphere by CO2-rich fluids from a depleted source or (ii) mixing of silicate melts from a HIMU source with small-degree partial melts from depleted mantle. Alternatively, the magmas may be partial melts of subducted, 1.2 Ga oceanic crust that was variably enriched in U and Sr(?) during sea-floor alteration.