DRILLING STRATEGY
The mass balance equation and continental growth from the study of
the input and output fluxes of chemical elements cycled through the
subduction factory can be determined by: (1) the study of the various parts
of the mass balance equation at multiple margins and (2) the study of the
inputs and outputs across a selected margin. The latter approach is the
strategy chosen for drilling in the Mariana and the Izu-Bonin arc systems
during Leg 185. The Mariana and Izu margins are nonaccretionary margins
where old, cold slabs of oceanic crust (sediments and basement) are
completely subducted, and backarc spreading is present. Although the
Marina and Izu-Bonin arcs share the same subducting plate, they have
distinct geochemical differences. Because, as stated in previous sections,
significant progress had been made on many aspects of the chemical flux
equation through previous DSDP and ODP drilling (i.e., ODP Legs 125, 126,
and 129) on both sides of the trench, Leg 185 was designed to drill two
sites to fill in the missing gaps of the recycling equation. These were
deep water sites: (1) an existing ODP hole (Hole 801C) located seaward of
the Mariana Trench and (2) a new site, Bon-10A (Site 1149), east of the
Izu-Bonin Trench (Table 2).
The drilling strategy for Hole 801C, an ODP legacy hole drilled during
Leg 129 (Lancelot, Larson, et al., 1990), was to re-enter and to deepen the
hole by an additional 250 m (to a maximum of 400 m total basement
depth) past the upper oxidative alteration zone of the basaltic crust (Fig.
9). During Leg 129 only 63 m of the normal tholeiites was cored. Based on
data from Hole 504B and other basement sites with sufficient
penetration, the upper oxidative zone of alteration, which contains most
important element budgets (e.g., K, B), lies in the upper 200-300 m of the
basaltic crust (Alt et al., 1986; Staudigel et al., 1995). The transition
from volcanics to sheeted dikes may not lie much deeper: 500-600 m in
Hole 504B (Detrick et al., 1994); 450 m to Layer 2b (Carbotte et al., 1997);
and only a few hundred meters at Hess Deep (Francheteau et al., 1992).
During Leg 185 Hole 801C was deepened by an additional 332 m into
basaltic crust with the RCB system, and by an additional 7 m with the
diamond core barrel (DCB) system to a new total depth of 935.7 mbsf,
placing this site as the DSDP and ODP drill hole with the sixth greatest
penetration into normal oceanic crust. Average core recovery was 47%,
and a complete suite of ODP downhole logs was run to 850 mbsf. After six
reentries the hole conditions remained good for coring operations,
although greater difficulty was experienced by cuttings filling the hole in
increasing amounts between reentries. With that exception, the hole is in
good condition for future reoccupation and deepening.
The drilling strategy at Site 1149 was to core the entire sedimentary
section, inferred to be ~470 m thick, and as far into the upper oxidative
alteration zone of the basaltic basement as possible, to a maximum of 430
m (Fig. 9). Previous drilling had failed to penetrate successfully through
resistant cherts, so most of the sediment column was still unsampled.
During Leg 185 a total thickness of ~410 m was cored in the four holes
that were drilled. To a depth of 180 mbsf, recovery of sediments with the
APC and the XCB systems was good (91%). However, low recovery (32%)
and numerous hole problems were encountered when drilling the chert and
porcellanite units. Although poorly recovered, the units were logged,
which will allow for a continuous record to calculate chemical fluxes to
the trench. The sediment/basement contact was recovered in two holes
(Holes 1149B and 1149C), and successful penetration into the basaltic
crust was achieved at Hole 1149C where a total of 133 m was cored, with
an average recovery of 21%.
To 185 Site 801 Principal Results
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