Drilling Strategy | Table of Contents
During Leg 205, we will return to Leg 170 drill sites; the Leg 205 sites are 1039R, 1040R, and
alternate Site 1043R. Science objectives for Leg 205 have two primary foci. The first is the igneous
and alteration history of the basement at reference Site 1039R on the incoming plate. The second is
on the three hydrological systems: in basement at Site 1039R; in the uppermost section of the
subducting sediment section at Site 1040R; and along the décollement and upper conduit at Site
1040R. An alternate site, Site 1043R, has the same objectives as those at Site 1040R. These goals
will be accomplished as described in detail below by limited coring of selected intervals, downhole
temperature measurements, logging at Site 1039R, the installation of long-term observatories to
monitor temperature and pressure, and sampling fluids and gases at key hydrological intervals.
Site 1039R Science Objectives
During Leg 205, coring and sampling will begin at Site 1039R 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 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
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 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.
Site 1040R Science Objectives
Limited coring through the décollement and into the uppermost underthrust section at Site 1040 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
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 Site 1040R is the fluid conduit (perhaps a thrust) encountered at 180-220
mbsf during Leg 170 coring. 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.
Alternate Site 1043R Science Objectives
Site 1043R has been approved as an alternate site. Installation of a modified CORK with a
sampling interval in the décollement here would accomplish all the décollement science objectives at
Site 1040. Penetrations at both Sites 1040 and 1043 would permit close comparison of
hydrological and compositional characteristics along a section of the décollement 1 km apart.
Drilling Strategy | Table of Contents