MARINER

Seismic Investigation of the Rainbow Hydrothermal Field and its Tectono/Magmatic Setting, Mid-Atlantic Ridge 36deg 14'N

Heat extraction from the Earth via hydrothermal systems along mid-ocean ridges (MORs) is a fundamental process affecting the Earth: hydrothermal systems extract approximately one third of the global yearly heat loss through ridges and are a primary means of chemical exchange between the solid Earth and the oceans. It is generally believed that sections of MORs with greater magma supply host a greater abundance of hydrothermal systems. While this simple conceptual model provides a framework within which to understand hydrothermal heat generation, the relative roles of magmatic heat input, tectonic heat advection, and faulting in controlling ridge thermal structure and hydrothermal circulation are still poorly understood. This is particularly important for hydrothermal circulation at slow- and ultra-slow spreading ridges, where venting occurs in a variety of host-rock lithology and tectonic setting. The Rainbow hydrothermal field (RHF) is a methane-, hydrogen- and iron-rich system located on an ultramafic massif within a tectonized non-transform discontinuity (NTD) of the Mid-Atlantic Ridge, where current models predict that long-term magma supply should be very low. Yet Rainbow vents high-temperature fluids at high flow rates, which is difficult to explain without a magmatic heat source. This conundrum stands in the way of our ability to develop general models for the roles of magmatic heat input and tectonic faulting on controlling ridge thermal structure and hydrothermal circulation, particularly for hydrothermal systems located in regions dominated by ultramafic lithologies, which are common at slow and ultra-slow MORs.

This project consists of a three-part seismic study of the physical architecture of the crustal and upper mantle at Rainbow to address the following fundamental question: What is the relationship between magmatism, faulting, substrate lithology, and hydrothermal circulation at the Rainbow hydrothermal field? A combination of 3D and 2D high-resolution active-source seismic tomography, 2D multichannel seismic reflection, and passive microearthquake monitoring will allows to determine the tectonic and thermal structure, melt content, and microseismicity around and beneath the RHF to determine:

- The nature and location of the heat source driving hydrothermal circulation at RHF.

- The nature and origin of the Rainbow massif.

- The role of the local stress field on focusing hydrothermal discharge.