80 Million Years of Prolonged and Localized Fluid flow on Shatsky Rise

Shatsky Rise is a large igneous province in the northwest Pacific Ocean, which formed at an unstable ridge-ridge-ridge triple junction at the Jurassic/Cretaceous boundary, 147 mya. 40Ar/39Ar analyses of samples from TAMU and Ori Massif, two large scale volcanic features on Shatsky Rise, yield mixing ages between fresh plagioclase and sericite phases that range from several million years younger than the eruption age of 147 mya, to one analysis that was approximately 80 myr younger than the eruption age. This alteration sericite replacing plagioclase is caused by two things. Prolonged fluid flow, which can be found on IODP (Integrated Ocean Drilling Program) holes (1) U1347A, (2) U1349A, (3) U1350A, and (4) U1346A. Such fluid flow is also found on lesser extend on ODP (Ocean Drilling Program) hole 1213B. Localized fluid flow is evident in Ori Massif where particularly young ages of 65.8, 70.2 and 82.1 Mya are found in a unit of altered pillow basalt with interpillow sediment. Sericite 40Ar/39Ar alteration ages were determined by collecting grains of sericitic plagioclase by means of sieving, washing, magnetic separation, and mild acid leaching. Since the ages obtained are a mixture between fresh plagioclase and sericitic plagioclase, the true age of alteration cannot be obtained, the Verati and Jourdan model was employed with moderate success. The overall mechanism driving the additional heat required to form sericite is unclear, but our evidence points to extended residence of the fluids in a deep ocean crust aquifer, where it can be heated up over millions of years.