Koolau Scientific Drilling Project: A transition from typical Koolau to Mauna Loa-like compositions at ∼325 mbsl

The well established geochemical differences between adjacent Hawaiian shields provide important constraints on the sources and processes involved in Hawaiian hotspot volcanism. Recent studies of lavas forming the Kilauea, Mauna Loa and Mauna Kea shields show that there are important geochemical variations during the growth of individual Hawaiian shields. Koolau volcano is significant because the surficially exposed shield lavas define endmember geochemical characteristics for major and trace elements, radiogenic and oxygen isotopic ratios in Hawaiian shield lavas. There is strong evidence that these extreme geochemical characteristics reflect a source component derived from recycled oceanic crust including abyssal sediment. Is this endmember composition characteristic of the entire Koolau shield? Studies of lavas and glasses exposed in the Nuuanu landslide block suggest that older Koolau lavas were unlike the subaerially exposed Koolau lavas; however, stratigraphy of blocks within the landslide is complex. With the objective of directly defining temporal geochemical variations during growth of the Koolau shield, the Koolau Scientific Drilling Project (KSDP) penetrated the shield to a depth of 679m. Only chips are available from the upper part of the hole that was rotary drilled, but coring of the lower 328m recovered 103 lava flows. Based on petrography and major element compositions, Haskins and Garcia (submitted) argue that the distinctive Koolau lavas defining endmember geochemical characteristics "form only a veneer with maximum thickness of ∼400m in the volcano's summit region." We have determined trace element abundances in cored KSDP lavas from 304m to 625m and find that a gradual transition from Koolau-like to Mauna Loa-like lavas occurs at ~325m in the core. For example, Frey et al. (1994) showed that among Hawaiian shield lavas, subaerially exposed Koolau lavas have the highest La/Nb and Sr/Nb. In the KSDP core with decreasing age these ratios show a long-term increase with significant scatter. Most of these KSDP lavas have La/Nb and Sr/Nb within the field of Mauna Loa lavas and ratios typical of uppermost Koolau lavas occur only in the depth interval of ∼304 to 336m; although a few samples deeper in the core, ∼525m, also have some geochemical characteristics similar to the uppermost Koolau shield lavas. We concur with Haskins and Garcia that the endmember Koolau composition forms a thin, several hundred meter, veneer of the Koolau shield. Consequently, the source component with a recycled slab geochemical signature was not a long-lived characteristic of hotspot volcanism.