The source mantle of the basaltic ocean crust on the western half of the Pacific Plate was examined using Pb-Nd-Hf isotopes. The results showed that the subducted Izanagi-Pacific Ridge (IPR) formed from both Pacific (180-∼80 Ma) and Indian (∼80-70 Ma) mantles. The western Pacific Plate becomes younger westward and is thought to have formed from the IPR. The ridge was subducted along the Kurile-Japan-Nankai-Ryukyu (KJNR) Trench at 60-55 Ma and leading edge of the Pacific Plate is currently stagnated in the mantle transition zone. Conversely, the entire eastern half of the Pacific Plate, formed from isotopically distinct Pacific mantle along the East Pacific Rise and the Juan de Fuca Ridge, largely remains on the seafloor. The subducted IPR is inaccessible; therefore, questions regarding which mantle might be responsible for the formation of the western half of the Pacific Plate remain controversial. Knowing the source of the IPR basalts provides insight into the Indian-Pacific mantle boundary before the Cenozoic. Isotopic compositions of the basalts from borehole cores (165-130 Ma) in the western Pacific show that the surface oceanic crust is of Pacific mantle origin. However, the accreted ocean floor basalts (∼80-70 Ma) in the accretionary prism along the KJNR Trench have Indian mantle signatures. This indicates the younger western Pacific Plate of IPR origin formed partly from Indian mantle and that the Indian-Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous. Key Points: The isotope geochemistry of the western Pacific Plate slab is reported The Pacific Plate originated from both Indian and Pacific mantle The Indian-Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous.