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Basaltic rocks recovered from three drill sites in the western Pacific during Ocean Drilling Program Leg 129 have fairly distinct Sr, Nd, and Pb isotopic compositions. The Cretaceous alkali olivine dolerites from Site 800 in the northern part of Pigafetta Basin have fairly low Sr/Sr: (0.70292-0.70320) and Nd/Nd; (0.51277-0.51281) and high present-day Pb isotopic ratios (206pb/204pb _ 20.53-21.45; Pb/Pb = 15.70-15.77; Pb/Pb = 40.02-40.68). The Middle Jurassic tholeiites from Site 801 in the southern part of the basin have low Sr/Sη (0.70237-0.70248), high l 4 Nd/ 4 Ndi (0.51298-0.51322), and moderate present-day Pb isotopic ratios (Pb/Pb = 18.20-19.12; 2 0 Pb/ 0 Pb = 15.47-15.60; 2 0 Pb/ 0 Pb = 37.56-38.18); isotopic compositions of the alkali olivine basalts overlying the tholeiites fall between those of the tholeiites and Site 800 dolerites. The Cretaceous tholeiites from Site 802 in the East Mariana Basin have high Sr/Sri (0.70360-0.70372), fairly low Nd/Ndj (0.51277-0.51280), and fairly low and homogeneous present-day Pb isotopic ratios (Pb/Pb = 18.37-18.39; 2 0 Pb/ 0 Pb = 15.49-15.51; 2 0 Pb/ 0 Pb 38.34-38.39). Isotopic compositions of Site 801 tholeiites are indistinguishable from those of modern mid-ocean ridge basalts, consistent with the proposal that these tholeiites are a part of the oldest Pacific crust. The diverse isotopic compositions of the younger basalts appear to be the result of Jurassic Pacific plate migration over the geologically anomalous south-central Pacific region, wherein they acquired their distinct isotopic compositions. The anomalous region was volcanically more active during the Cretaceous than at present.

Isotope Geochemistry of Leg 129 Basalts: Implications for the Origin of the Widespread Cretaceous Volcanic Event in the Pacific