The magmatic plumbing systems of Kilauea and Mauna Loa volcanoes are generally thought to be physically isolated from one another. Two primary pieces of evidence support this idea: (1) extensive 3-D geophysical modeling of magma-induced earthquakes beneath Kilauea delineates a “closed” magmatic plumbing system with no obvious connection to Mauna Loa, and (2) distinct differences in Pb, Sr and Nd isotopic ratios and major- and trace-element chemistry between Kilauea and Mauna Loa have been consistent for most of the past 230 kyr. However, twenty-five sequential prehistoric (~1.0-0.5 ka) lava flows from the northwest wall of Kilauea at Uwekahuna Bluff analyzed for Pb, Sr, and Nd isotopic ratios and major- and trace- element abundances in this study have unusual compositions that span the geochemical spectrum between Mauna Loa and Kilauea. Rhodes et al. (1989) used major- and trace- element abundances to conclude that the unique chemistry of Uwekahuna Bluff lavas could be explained by “magmatic intercourse”, where Mauna Loa magmas periodically invade Kilauea’s magma reservoir. However, the new data presented in this study suggest Uwekahuna Bluff lavas originated from partial melting of an unusual mantle source component within the Hawaiian plume. Like historical (1790-present) Kilauea summit lavas, Uwekahuna Bluff lavas display systematic fluctuations in Pb, Sr, and Nd isotopic ratios and ratios of highly over moderately incompatible trace elements that are thought to reflect partial melting of small-scale mantle heterogeneities intrinsic to the Hawaiian plume. The distinctive 206Pb/204Pb and 87Sr/86Sr ratios of Uwekahuna Bluff lavas have only been previously observed in prehistoric Mauna Loa lavas from both the volcano’s summit (1.7-1.0 ka) and its submarine southwest rift zone (300-100 ka). The similarity in eruption age and the unusual isotopic signature of Uwekahuna Bluff and young prehistoric Mauna Loa lavas could be explained if both volcanoes tapped a laterally extensive (at least 35 km long) heterogeneity in the mantle source region. From the base to the middle portion of the Uwekahuna Bluff section studied here, Pb isotopic and highly over moderately incompatible trace-element ratios record a gradual transition towards historical Mauna Loa-like values followed by a reversal towards historical Kilauea-like compositions. This probably represents the melting of the large-scale heterogeneity beneath these volcanoes and its subsequent removal from the melting region due to the active upwelling of the Hawaiian plume. Assuming a cylindrical heterogeneity was tapped for 500 years with a rate of mantle upwelling of 10 m/yr, the minimum size of the heterogeneity is 35 km wide x 7 km thick. The degree of partial melting (9.6-15.2 %) modeled for Uwekahuna Bluff lavas is more robust than Kilauea’s historical record (4.7-10.3 %) suggesting the unusual large-scale heterogeneity is more fertile than Kilauea’s typical mantle source.