Reconstructing the Interaction between Magma Emplacement and Hydrothermal Cooling Underneath Oceanic Spreading Ridges

Nobumichi Shimizu, Geology & Geophysics
Johan Lissenberg, Geology & Geophysics

DOEI Project Funded: 2006

Abstract

The fundamental process that drives the Earth’s geological evolution is cooling. This process is manifested principally at oceanic spreading ridges, where tectonic plates drift apart, creating oceanic crust. As such, oceanic spreading ridges are responsible for formation of more than two-thirds of the Earth’s crust, and they are major sites of thermal and chemical exchange between the mantle, crust and oceans. The mechanisms of growth of the oceanic crust, as well as the intensity and distribution of hydrothermal circulation, are controlled primarily by the thermal structure of spreading ridges. How the thermal structure of a spreading ridge evolves is determined by the interplay between magma emplacement, which introduces heat, and hydrothermal circulation, which removes heat from the crust and transports it into the oceans. Constraints on the thermal structure of ocean ridges come primarily from geophysical models. These suggest that at fast-spreading ridges, where magma supply is abundant, the crust is hot, whereas low melt supply at slow-spreading ridges leads to enhanced hydrothermal circulation and cool crust and upper mantle.

This proposal aims to place direct geological constraints on the interaction between magma emplacement and hydrothermal cooling underneath ocean ridges with different spreading rates. We will perform U/Pb age dating of zircons from abyssal gabbros at the Northeast National Ion Microprobe Facility at WHOI. The age data will establish whether long time scales of crust formation, which may result from the crystallization of melts in the mantle at (very) slow spreading ridges, is common, and whether it correlates with spreading rate. Zircon thermometry will establish crystallization temperatures of the gabbros, which, combined with crystallization depths derived from the ages, allows for a reconstruction of isotherms underneath the spreading ridge. Chlorine contents of magmatic amphibole coexisting with zircon will track interaction of the gabbros with hydrothermally altered rocks, constraining the extent and depth of hydrothermal circulation. Combined, the data allow us to examine the relationships between melt emplacement, hydrothermal cooling and the thermal structure at various spreading rates, placing important constraints on how oceanic crust forms. An additional goal is to develop a routine for U/Pb dating of extremely young zircons, laying the foundation for a new area of inquiry at WHOI.