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%.

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