overview of thesis research in the MIT/WHOI joint program
Why do I spend all these hours in the laboratory?
The focus of my thesis research is hydrothermal activity associated
with seafloor spreading in back-arc basin environments. More
specifically, my academic pursuits are to understand the conditions and
processes responsible for the formation of metal-rich (e.g., gold,
silver, copper, zinc) hydrothermal vent deposits at the seafloor.
Hydrothermal Systems in the Eastern Manus Basin: Fluid Chemistry and Magnetic Structure as guides to Subseafloor Processes
Seafloor hydrothermal activity is associated with transfer of energy (heat) and mass from the interior of the earth to the oceans, and is associated with the alteration of oceanic crust and the formation of metal-rich sulfide mineral deposits. Circulation of seawater-borne hydrothermal fluids through hot oceanic crust modifies the composition of the host rock and of the circulating fluids. The generation of heated, reduced, acidic and metal-bearing hydrothermal fluids provides the necessary chemical and energy requirements to drive deep-sea biological activity and the conditions requisite for the formation of metal-rich sulfide mineral deposits on the seafloor.
Numerous investigators have studied hydrothermal processes occurring along basalt-hosted mid-ocean ridge (MOR) spreading centers over the last 30 years and have advanced our understanding of mass and energy transfer, the controls over hydrothermal fluid chemistry, the conditions and processes responsible for the formation of vent mineral deposits and the requirements for biological habitation of these environments. Substrate composition, permeability of the oceanic crust and geometry of the underlying heat source are recognized as important variables influencing fluid-rock interactions within seafloor hydrothermal systems.
Back-arc basin (BAB) spreading centers are known to be an important host for seafloor hydrothermal systems; however, studies of hydrothermal systems in these environments , for the most part, less detailed and understood than their MOR counterparts. Because the tectonic-magmatic relations and the composition and permeability of oceanic crust in BAB settings are substantially different [as compared to MOR environments], it is to be expected that fundamental processes of fluid-rock interaction, generation of evolved hydrothermal fluid and formation of metal-rich mineral deposits should also differ.
Hydrothermal activity in the Manus Basin (Bismarck Sea, Papua New Guinea) is now recognized at several discrete and geochemically unique areas, including the Vienna Woods, PACMANUS, DESMOS and SuSu Knolls hydrothermal fields. The recent NSF-ODP funded mgln06mv cruise (M. Tivey, W. Bach, M.K. Tivey, J. Seewald, Prinicpal Investigators, Woods Hole Oceanographic Institution) to hydrothermal systems in the Manus Basin sampled and recovered the most comprehensive suite of hydrothermal fluid and sulfide mineral samples from a back-arc environment to date. The over-arching scientific questions that we wish to address are 1) what is the influence of substrate composition (andesite-dacite) on the composition of evolved hydrothermal fluids, 2) how does magmatic volatile degassing in this area contribute to the source and transport of trace metals in the systems to the seafloor, 3) what physiochemical conditions are required to deposits concentrated metal sulfide hydrothermal deposits on the seafloor and 4) can we use geochemical proxies within sulfide deposits and fluids as guides to subseafloor hydrothermal processes? Detailed geochemical (elemental and isotopic) examination of recovered vent deposit samples from the Manus Basin forms the basis of my thesis research.
Collaborative Research: Sampling and Initial Characterization of Hydrothermal Fluids, Deposits, Microfauna, and Megafauna at Vent Fields along the Eastern Lau Spreading Center
The NSF-Ridge2000 Program is focused on examining the transfer of mass and energy from within the Earth's mantle to the oceanic crust. Since the discovery of active seafloor hydrothermal systems, significant progress has been made in understanding the role that hydrothermal activity plays in this energy and mass transfer. Convective circulation of seawater through hot oceanic crust alters the geochemical composition and physical properties of this crust, and produces hot, reduced hydrothermal fluids that are an important source of energy and nutrients for large and diverse biological communities at depth in the ocean basins where photosynthetic processes are excluded. These heated, reduced hydrothermal fluids are also an important source of metals, which under optimal conditions can form poly-metallic massive sulfide deposits at or close to the seafloor at active vent sites. These deposits are indicated as contemporary analogs for some terrestrial volcanic-associated massive sulfide ore deposits that humans exploit today.
Most studies of these systems have been conducted along mid-ocean ridge (MOR) sites; reflecting an emphasis on the global significance of crustal emplacement and energy and mass transfer within this tectonic environment. These studies have documented a range of fluid compositions, crustal alteration styles, deposit compositions and structure, and biological community dynamics. This variety may be attributed to (but not limited to) differences, along the global MOR, in 1) substrate composition, 2) subsurface hydrology (styles of seawater and hydrothermal fluid circulation and interaction), 3) presence or absence (and depth to) of an intrusive magmatic heat source, 4) transient periods of volcanic/magmatic vs. tectonically driven hydrothermal activity and 5) presence of volatile (gaseous) species within hydrothermal fluids.
Back-arc basin (BAB) hosted hydrothermal systems are increasingly recognized as being important for models that describe global energy and mass transfer in the oceanic crust. The dynamics of crustal emplacement along sreading axes in BAB environments is significantly different to the global MOR, and produces oceanic crust of differing chemical composition, yield strength and viscosity. Such differences will affect subsurface hydrology, magmatic vs tectonic influences on hydrothermal activity and inputs of mantle-derived chemical species (gases and metals) into hydrothermal systems. The Eastern Lau Spreading Center (ELSC) in the Lau Basin has been identified as a key geographic region in which differences in, and effects of, crustal accretion and composition, subsurface hydrology, water depth and depth of subseafloor seawater circulation on hydrothermal activity can be examined. Collaborative research studies will document the range in fluid chemistry and sulfide mineral composition and geochemistry at major fields along the ELSC. In addition, these studies will identify the key processes and parameters that influence fluid chemistry, sulfide deposit formation and the range of habitats available for microbial and megafaunal populations. These studies reflect the interdependence between geology, geochemistry, physics and biology at seafloor hydrothermal vent systems.
Principal Investigators in this research proposal are Meg Tivey (Woods Hole Oceanographic Institution), Jeff Seewald (Woods Hole Oceanographic Institution), Stacy Kim (Moss Landing Marine Laboratories), Mike Mottl (Univ. Hawai'i), Anna-Louise Reysenbach (Portland State Univ.) and Geoff Wheat (Univ. Alaska).