There are three general objectives of this proposed work. First,
to understand the structural controls of hydrothermal venting at superfast
spreading rates. Do boundaries of morphotectonic/structural 4th order
segments correspond to the boundaries of hydrothermal activity on
superfast- spreading ridge segments as proposed for the fast-spreading
northern EPR and intermediate-spreading Juan de Fuca ridge? Does the degree
of hydrothermal activity along individual tectonic segments of ridge crest
correspond more closely to high-frequency variations in the rate of magma
supply (e.g., cross-sectional inflation, axial depth) than to low-frequency
variations (e.g., spreading rate)? Large-scale surveys of hydrothermal
activity on intermediate- to superfast-spreading ridges indicate that the
relative spatial frequency of hydrothermal plumes increases linearly with
spreading rate. This correlation implies that variations in hydrothermal
activity are a function of large-scale, and thus low-frequency, variations
in the magma supply rate. However, plotting plume incidence against either
axial depth or cross-sectional area also yields linear correlations. This
is because in the three areas in the Pacific surveyed to date, the mean
values of axial depth, inflation, and spreading rate are nearly perfectly
correlated. To identify which parameter is dominant we need a large survey
area with morphological trends much different than the previously surveyed
areas. Our proposed study area offers an ideal laboratory for examining the
effect of these three parameters on the distribution and composition of
hydrothermal discharge. Is the degree of hydrothermal activity greater
along a plate boundary undergoing rapid reorganization than along segments
of similar morphology and spreading rate along a stable boundary as
proposed based on DSDP results near 19ūS? Our proposed study area is unique
because of the large-scale reorganization of spreading center geometry
presently occurring by duelling rift propagation that may be evolving
toward microplate tectonics, and thus offers a unique opportunity to
evaluate the effect of such structures on the development of hydrothermal
circulation. Second, to understand the temporal controls of hydrothermal
venting. Do high ratios of volatiles/heat and volatiles/metals in
hydrothermal fluids and in the overlying water-column plumes indicate that
the ridgecrest volcanic/hydrothermal system has been recently perturbed by
input of magma, as proposed for the Juan de Fuca ridge and northern EPR?
Third, to understand the relative importance of hydrothermal venting and
deep ocean currents in forming far-field plumes. Is the absence of a
far-field helium plume to the west of the EPR at ~30ūS due to the pattern
of deep ocean currents which carry the hydrothermal effluent eastward at
this latitude, or to the absence of hydrothermal sources on the EPR axis
south of the Easter Microplate?
In order to test these hypotheses we propose an integrated
geophysical/hydrothermal survey. We first propose to collect
high-resolution deep-towed sidescan and bathymetry using the WHOI DSL-120
system to map the detailed patterns of faults, fissures, and recent
volcanic eruptive sites. CTD/nephelometers mounted on the vehicle and wire
will provide precise plume distributions in conjunction with the deep-tow
geophysical measurements. We then propose continuous mapping of
hydrothermal anomalies using the PMEL SUAVE system in the tow-yo technique,
continual raisings and lowerings of the instrumentation through the plume
interval while the ship slowly steams ahead, to determine two-dimensional
anomalies of temperature, particle concentration (light
scattering/attenuation), and the dissolved fraction of certain chemical
species (e.g., Fe and Mn). Tow-yo surveys are powerful tools for both
thorough reconnaissance mapping and high-resolution discharge location.
Comparison of plume surveys with vent location by camera or submersible has
shown close agreement. We then propose to collect discrete samples from
tows and vertical casts to determine the first-order composition of the
discharging hydrothermal fluids.
The combination of high-resolution bathymetric, acoustic, and
hydrothermal plume data we plan to acquire will allow us to make
quantitative measurements of the distribution and composition of
hydrothermal venting and its relation to specific geologic characteristics
of the ridge at three spatial scales of progressively increasing size. This
unique data set will be used to test a series of hypotheses that address
fundamental questions about the relation of hydrothermal processes to the
morphotectonic/structural environment in which they exist. This proposed
work will result in significant advances in understanding the pattern of
hydrothermal venting at the fastest present-day spreading rates.