We will drill as many single-bit holes as possible in the allotted time, perhaps as many as 10-12 holes. Each hole should penetrate ~50 m into basaltic basement, recovering sufficient mid-ocean ridge basalt to enable a satisfactory analytical program. Much of the region is devoid of measurable sediment cover and all sites are located on localized sediment pockets. At each site, we intend to locate specific drilling targets by running a short single-channel seismic survey that crosses the precruise survey line. These data will ensure sufficient sediment thickness for borehole initiation. Because these sediments are expected to be reworked and possibly winnowed, they will be recovered only by rotary core barrel (RCB) drilling. Because basement penetration at as many sites as possible is the primary objective of this leg, we may choose, on a site-by-site basis, to drill through the overlying sediment without expending time on wireline core recovery. A review of recent deep-water legs suggests that at least 10 such short basement penetration holes can reasonably be achieved during a single leg. During Leg 144, for example, 20 holes were drilled at 10 sites on guyots in the northwest Pacific. Basaltic basement was recovered in at least one hole at nine of the sites. For Leg 187, the minimum number of holes required for an acceptable definition of the off-axis isotopic boundary is six, but much higher resolution can be obtained with eight or more holes, especially if the program is able to respond to the results of onboard geochemical analyses of the recovered basalts. For example, Figure 5 shows the along-axis distribution of Zr/Ba and Rb/Ba ratios across the isotopic boundary, strongly indicating that these and other ratios can be used off axis to reliably distinguish Pacific from Indian mantle sources.
The best use of the drillship will result from a reactive drilling strategy, predicated on our ability to distinguish "Indian" from "Pacific" mantle using trace element ratios measured on board by inductively coupled plasma (ICP) or direct-current plasma (DCP) spectrometry. In the event that adequate onboard analysis is unsuccessful, a worst-case plan will allow for acceptable definition of the boundary by onshore isotopic analysis.
The following discussion describes one example of how such a strategy might proceed, but there are numerous other possibilities and final decisions have not yet been made. Site numbers are shown in Figure 6.
An initial series of three holes is drilled at Sites 36, 8c, and 21. These sites straddle likely positions of the boundary, other than the most rapid long-term migration. Each of these sites will prove to have basalts that are either derived from Indian (I) or Pacific (P) mantle.
• Scenario 1. Basalts at all three sites (I I I) are derived from Indian mantle. This implies rapid migration of the boundary from the east. Sites 14, 13b, and 1b are drilled to establish the location of the I/P boundary, followed by one or more of Sites 4c, 2b, and 29 to locate the boundary farther west.
• Scenario 2. Indian-type basalt is at Sites 36 and 8c. Pacific-type basalt is at Site 21 giving an I I P pattern. This implies slower migration, most likely tied to the depth anomaly. Sites 23 and 16 are drilled to better locate the boundary, followed by one or more of Sites 28, 29, and 2b to locate the boundary close to the eastern AAD fracture zone. Finally, one or more of Sites 3b, 33, 34, 35, and 27 are drilled to locate the boundary within the AAD.
• Scenario 3. Indian-type basalt is at Site 36. Pacific-type basalt is at Sites 8c and 21 (I P P pattern). This implies a long-term assocation of the boundary with the eastern AAD. Working from north to south, the following sites will better define its geometry: Sites 27, 35, 34, 33, 28, 29, and 3b.