Ancient carbonate sedimentary signature in the Hawaiian plume: Evidence from Mahukona volcano, Hawaii

Lavas from Mahukona, a small Hawaiian volcano on the Loa trend, exhibit major and trace element abundance variations exceeding those in lavas from large Hawaiian shields, such as Mauna Loa and Mauna Kea. Mahukona lavas define three geochemically distinct groups of tholeiitic shield basalt and a transitional group of postsshield basalt. At 10% MgO the tholeiitic groups range from 9 to 12% CaO; such differences in CaO can reflect partial melts derived from garnet pyroxenite (low CaO) and peridotite (high CaO), but the negative CaO-Yb (both at 10% MgO) trend formed by Mahukona lavas is inconsistent with this explanation. Within Mahukona lavas, radiogenic Nd-Hf-Pb isotopic ratios are highly correlated with each other; however, Sr-87/Sr-86 is decoupled from these radiogenic isotopic ratios. Rather, Sr-87/Sr-86 is correlated with trace element abundance ratios involving Sr, and importantly, Mahukona lavas define a negative Rb/Sr-Sr-87/Sr-86 trend, implying that a Sr-rich source component characterized by high Sr-87/Sr-86 is important in the petrogenesis of Mahukona lavas. We infer that this Sr-rich source component is recycled ancient carbonate-rich sediments. Intershield heterogeneity among Hawaiian shields also shows a negative Rb/Sr-Sr-87/Sr-86 trend. For example, Makapuu-stage Koolau lavas have higher Sr-87/Sr-86 but lower Rb/Sr than Mauna Kea lavas. Consequently, we infer that a recycled ancient carbonate-rich sedimentary source component is important in the Hawaiian plume. Although most lavas from Loa and Kea trend volcanoes define distinct fields in isotopic ratios of Sr, Nd, Hf, and Pb, the majority of Mahukona lavas have isotopic ratios at the boundary between the fields defined by Loa and Kea trend lavas. However, a subgroup of Mahukona shield lavas have Kea-like isotopic and trace element signatures, an observation that can be explained by vertical heterogeneity in a bilaterally asymmetrical plume.