ALBERT

Description: ODP Leg 129 achieved one of its primary goals by recovering Jurassic-age ocean crust at Site 801 in the Pigafetta Basin, western Pacific Ocean. At Sites 800 and 802, although also located on presumed Jurassic-age crust, drilling terminated in the ubiquitous Cretaceous-age volcanic rocks that seem to blanket the Early Cretaceous and Jurassic basement of the western Pacific Ocean. Site 800 in the Pigafetta Basin penetrated 56 m of massive alkali dolerite sills. 40Ar/39Ar laser analyses of mineral separates revealed a crystallization age of 126.1 ± 0.6 Ma. The isotopic composition of these dolerite sills shows a strong HIMU component (high radiogenic Pb, low radiogenic Sr), nearly identical to similar age lavas recovered from nearby seamounts. Both the sills and seamounts of the Pigafetta Basin are Early Cretaceous products of the South Pacific Isotopic and Thermal Anomaly (SOPITA), which is responsible for the ocean islands and thermal (?) swell found in the South Pacific today. Site 801, drilled into the Jurassic Quiet Zone crust of the Pigafetta Basin, penetrated 131 m of basaltic flows divided into an upper sequence of alkalic basalts, an altered hydrothermal deposit, and a lower tholeiitic sequence. For the lower tholeiitic basalts, whole rock incremental- and laser-heating experiments revealed an age of 166.8 ± 4.5 Ma, almost exactly that predicted by simple linear extrapolation of the M-sequence magnetic lineation ages. The age and composition of the tholeiites offers important proof that the "Quiet Zone" crust of the western Pacific is indeed Jurassic mid-ocean ridge basalt (MORB), and not a product of mid-Cretaceous, intra-plate volcanic events. For the upper alkalic basalts at Site 801, 40Ar/39Ar mineral incremental-heating and laser-fusion experiments yielded an age of 157.4 ± 0.5 Ma. About 10 m.y. younger than that predicted by extrapolation of the M-sequence anomalies, the alkalic basalts were erupted in an off-ridge environment, which is consistent with both the geochemistry of the basalts and the Bathonian to Callovian radiolarian age of the overlying sediments. The 157.4 Ma age also serves as an important calibration point on the Geologic Time Scale (GTS), but further work may be needed to verify the correlation of the radiolarian zonations in the Pacific with the time scale stages based on European sections. Site 802 in the East Mariana Basin penetrated 51 m of basaltic pillow units and flows. Whole rock 40Ar/39Ar incrementalheating analysis of two samples revealed a crystallization age of 114.6 ± 3.2 Ma. Although the isotopic composition of these basalts indicates an ocean island basalt component, the major and trace element chemistry shows a strong MORB affinity. This is very similar to the contemporaneous basalts recovered at Site 462A in the Nauru Basin, and both seem more closely related to the formation of Ontong Java Plateau, ca. 120 Ma, rather than to products of the SOPITA.