Probing the Pacific’s oldest MORB glass: mantle chemistry and melting conditions during the birth of the Pacific Plate

Major element chemistry of basalt from the southern East Pacific Rise (EPR) is different from that of the EPR at the time of the formation of the Pacific Plate at 170 Ma. Glass recovered from Jurassic age (170 Ma) Pacific ocean crust [Bartolini and Larson, Geology 29 (2001) 735-738] at Ocean Drilling Program Hole 801C records higher Fe sub(8) (10.77 wt%) and marginally lower Na sub(8) (2.21 wt%) compared to the modern EPR, suggesting deeper melting and a temperature of initial melting that was 60 degree C hotter than today. Trace element ratios such as La/Sm and Zr/Y, on the other hand, show remarkable similarities to the modern southern EPR, indicating that Site 801 was not generated on a hotspot-influenced ridge and that mantle composition has changed little in the Pacific over the past 170 Ma. Our results are consistent with the observation that mid-ocean ridge basalts (MORBs) older than 80 Ma were derived by higher temperature melting than are modern MORBs [Humler et al., Earth Planet. Sci. Lett. 173 (1999) 7-23], which may have been a consequence of the Cretaceous superplume event in the Pacific. Site 801 predates the formation of Pacific oceanic plateaus and 801C basalt chemistry indicates that higher temperatures of mantle melting beneath Pacific ridges preceded the initiation of the superplume.