SCIENTIFIC OBJECTIVES
During Leg 205, we will return to near the Leg 170 drill sites. The planned Leg 205 sites are 1039R-A, 1040R-A, 1040R-B, and 1040R-C. Science objectives for Leg 205 have two primary foci. The first is the igneous and alteration history of the basement at reference Site 1039R-A on the incoming plate. The second is on the three hydrological systems: in basement at Site 1039R-A; in the uppermost section of the subducting sediment section at Site 1040R-A; and along the décollement and upper conduit at Sites 1040R-B and 1040R-C. These goals will be accomplished as described in detail below by limited coring of selected intervals, downhole temperature measurements, logging at Site 1039R-A, the installation of long-term observatories to monitor temperature and pressure, and sampling fluids and gases at key hydrological intervals.
Site 1039R-A Science Objectives
During Leg 205, coring and sampling will begin at Site 1039R-A within the carbonates above the
sill encountered during Leg 170, will continue through the sill and the previously undrilled
sediments beneath the sill, and ~100 m into basement. The scientific objectives to be addressed
through coring, sample analysis, and logging at Site 1039R-A are as follows.
1.Quantify the amount of carbonate in the subducting sediment and uppermost altered basaltic
crust for comparison with CO2 fluxes out of the volcanoes to evaluate carbon recycling
through the arc.
2.Determine the distribution of metalliferous carbonates above the sill and above basement
and determine the concentrations of elements such as Cu, Cr, Ni, V, and Pb to construct
element fluxes into the trench and to constrain their flux out of the basement.
3.Determine the extent of sill emplacement and their contribution to the bulk composition of
the subducting igneous crust.
4.Determine the igneous and alteration mineralogy, petrology, and geochemistry in the
uppermost 100 m of the oxidative alteration zone and characterize the original volcanic
structure within the basement. Use the geochemical data to calculate subduction fluxes.
Attention will be paid to low-temperature alteration features that may result from near trench
fluid flow as well as that deriving from ridge-crest and near off-axis hydrothermal
circulation.
5.Determine physical properties in the core and borehole that may affect estimations of
basement composition and lithologic variation or that relate to fluid flow and deformation
such as porosity, density, permeability, fracture distribution and orientation, and strength.
A long-term borehole observatory (i.e., a modified CORK) will also be emplaced at Site 1039R-A to sample fluids and to monitor temperature and pressure within the uppermost permeable basement. The science objectives for this are to
1.Use pressure, temperature, fluid, and gas compositions and fluid flow rates together with
downhole measurements to characterize the fluid and heat fluxes responsible for the
abnormally low heat flow in the vicinity of this site because of seawater incursion to
basement.
2.Evaluate the thermal, hydrological, and chemical implications of this extensive fluid
circulation for the thermal structure of the uppermost part of the subducting plate, the
hydrological pathways available in the shallow subduction zone and overlying prism, and
global element fluxes.
Sites 1040R-A, 1040R-B, and 1040R-C Science Objectives
Limited coring of the décollement and installation of modified CORKs to monitor and sample
within the area of maximum flow of deeply sourced fluids in the décollement and in the underthrust
sediment section address the following scientific objectives.
1.Determine physical properties of the décollement horizon from further structural
experiments on whole-round samples to constrain hydrological modeling and permit
integration of fluid flow and deformation models.
2.Determine chemistry of pore fluid profiles from décollement whole rounds to compare with
profiles measured during Leg 170 and to evaluate possible heterogeneity.
3.Determine pressure, temperature, and composition of fluids and gases along the décollement
and evaluate any possible changes through time for hydrologic modeling. This same data
set will constrain the flux of elements out of the downgoing sediment section along the
décollement to evaluate the role of fluid egress on element fluxes to the ocean and its
corollary, changing composition of the residual slab because of fluid loss.
4.Use selected elements, element ratios, and isotopic compositions in the fluids from the
décollement in an attempt to constrain dehydration reactions at the updip and, perhaps,
downdip limits of the seismogenic zone.
5.Determine pressure, temperature, and fluid composition in the zone of compaction
dewatering beneath the décollement to constrain pathways of fluid return to the surface and
to evaluate the effects of this flow system on element fluxes.
6.Collect whole-round samples from the décollement under appropriate conditions for
postcruise microbiological investigations to determine the resident microbial ecology of the
zone for comparison with eventual microbial experiments on fluids collected from the
décollement.
A lower-priority target at these sites is the fluid conduit (perhaps a thrust) encountered at 180-220 mbsf during Leg 170 coring at Site 1040. Pore fluid chemistry profiles across this horizon show advective flow of deeply sourced fluids, similar to those along the décollement. If necessary, the objectives above for the décollement could also be addressed at this horizon.