The magma-hydrothermal transition zone on the East Pacific Rise

Abstract

Seawater circulation in the subsurface of submarine hydrothermal systems has an important role in chemical exchange between seawater and ocean crust. The boundary layer between hydrothermally and magmatically dominated regions in these systems remains poorly understood, and in order to understand this better, the Integrated Ocean Drilling Program (IODP) drilled a hole in the eastern Pacific Ocean and recovered basement rocks that record the hydrothermal alteration history of this interval. This study investigates the mineralogy and geochemistry of granoblastic and gabbroic rocks recovered by IODP from this drill hole. Minerals were identified using microprobe analysis and temperatures at the time of rock formation were calculated from amphibole-plagioclase thermometer. These analyses indicate that hydrothermal fluids were still dominant at the deepest section of this well (> 1500 m below seafloor). The results from this study confirm the presence of hydrothermal activities at great depths and contributed to further understanding of this boundary layer in hydrothermal systems.