Isotopic systematics of the early Mauna Kea shield phase and insight into the deep mantle beneath the Pacific Ocean

The 3500 m deep Hawai'i Scientific Drilling Project core provides a ~680 kyr record of the magmatic history and source components of Mauna Kea volcano. We report high‐precision Pb‐Sr‐Nd isotopic compositions of 40 basalts from the last 408 m of the final drilling phase (HSDP2‐B and HSDP2‐C) and show that these lowermost basalts represent the early shield stage of Mauna Kea's growth history. Two sample groups are distinguished based on their isotopic variability compared to the rest of the core. Over a depth interval of 210 m (3098.2–3308.2 mbsl), the basalts show very restricted isotopic variation and represent sampling of a relatively homogeneous source. Samples from the bottom 192 m record the largest range of 206 Pb/ 204 Pb and 208 Pb/ 204 Pb in the core, reflecting the greater isotopic variability of the earlier stages of volcanism compared to subsequent stages. The heterogeneity of Mauna Kea lavas is explained by mixing variable proportions of four distinct components intrinsic to the Hawaiian mantle plume. One of these components, Kea, is a prevalent and long‐lived composition within the Hawaiian plume, whereas the other three components are involved at different stages of the volcano's history and contribute to the short‐term isotopic variability of Mauna Kea. The compositional similarity of the Kea component to “C” and to the super‐chondritic bulk‐silicate Earth suggests that Kea may be part of the primitive mantle of a non‐chondritic Earth. Other Pacific oceanic island basalts share Kea‐like compositions, indicating that the Kea component is a common, widespread composition within the Pacific deep mantle.