Carbonate-rich Sediments in the Hawaiian Plume: Evidence from Mahukona Volcano

Hawaiian shield lavas exhibit considerable heterogeneity in Sr-Nd-Hf-Pb isotope space, as well as in their major and trace element abundances. Some of these geochemical features are attributed to recycled oceanic crust, including sediments. Here we report major and trace element concentrations, and Sr-Nd-Hf-Pb isotopic compositions for shield lavas from Mahukona Seamount, Hawaii. Using these data, we identify a carbonate-rich sedimentary source component in the Mahukona lavas. Specifically, within Mahukona lavas, Nd-Hf-Pb isotopic ratios are highly correlated with each other, while 87Sr/86Sr is decoupled. Since the isotopic compositions were obtained on strongly acid-leached residues, the discordance of 87Sr/86Sr is not inferred to be alteration-related. Moreover, 87Sr/86Sr is positively correlated with Sr/Ce and negatively correlated with Rb/Sr and Ba/Sr. Because the Mahukona lavas are tholeiitic to transitional basalts, their Sr/Ce, Rb/Sr and Ba/Sr reflect their source values. We infer that one of the Mahukona source components has high Sr/Ce (>16) and 87Sr/86Sr (>0.7038), and low Rb/Sr (<0.015) and Ba/Sr (<0.15). Such a composition cannot be generated through mantle metasomatism, because high 87Sr/86Sr requires high Rb/Sr. We rather propose recycled ancient carbonate-rich sediment as a possible component in the Mahukona source. Carbonate-rich sediments, in general, have high Sr/Ce, and low Rb/Sr and Ba/Sr. Furthermore, ancient carbonate-rich sediments would have inherited their 87Sr/86Sr from seawater, which had 87Sr/86Sr greater than 0.7040 at 2 Ga. Since a carbonate-rich sedimentary component has very high Sr/X (X=Nd, Hf, Pb), sampling it would affect only 87Sr/86Sr and leave the Nd-Hf-Pb isotopic ratios unchanged.