"Seismic Reflection Imaging of the Heat Source of an Ultramafic-Hosted Hydrothermal System: (Rainbow, Mid-Atlantic Ridge 36º 10-17’N)" by Juan Pablo Canales, Woodshole Oceanographic Institution, will be held at room Sala d'Actes of Institut de Ciencies del Mar (ICM - www.icm.csic.es).
Most of our understanding of hydrothermal systems and the nature of their heat sources comes from models and observations at fast and intermediate spreading ridges. In these settings, hydrothermal systems are mainly located within the axial zone of a spreading segment, hosted in basaltic rock, and primarily driven by heat extracted from crystallization of crustal melt sills. In contrast, hydrothermal systems at slow-spreading ridges like the Mid-Atlantic Ridge (MAR) show a great variety of venting styles and host-rock lithology, and are located in diverse tectonic settings like axial volcanic ridges, non-transform discontinuities (NTDs), the foot of ridge valley walls, and off-axis inside corner highs. Among MAR systems, the Rainbow hydrothermal field (RHF) stands out as an end-member of this diversity: an ultramafic-hosted system emitting H2 and CH4-rich fluids at high temperatures and high flow rates, which suggests a magmatic heat source despite the lack of evidence for recent volcanism and its location within an NTD with presumably low magma budget.
We present 2D multichannel seismic reflection images across the Rainbow massif from the NSF-funded MARINER multidisciplinary geophysical study that reveal, for the first time, the magmatic system driving hydrothermal circulation in an ultramafic setting. Data were acquired in 2013 on board the RV M. Langseth with an 8-km-long hydrophone streamer. The images have been obtained from pre-stack depth migrations using a regional 3D P-wave velocity model from a coincident controlled-source seismic tomography experiment using ocean bottom seismometers. Our images show a complex system of partially molten and solidified plutonic sills intruding fresh or partially (<15%) serpentinized peridotite centered beneath the RHF. The sill complex occupies an areal extent of ~4.6x8 km², with most of the sills at depths between 3-6 km below the seafloor, but some as deep as 10 km below seafloor. Our data also image high-amplitude 35°-45° dipping reflections within the massif coincident with strong lateral velocity gradients that may arise from detachment fault planes, lithological contacts, and/or alteration boundaries. Our results are an important step towards understanding the interactions of detachment faulting, magmatic intrusion, and hydrothermal circulation in sections of the global mid-ocean ridge system dominated by mantle exposures.