Coring and drilling strategy

General operations plan

The Expedition 324 operations plan (Table T1) is based on previous experience in coring LIPs, such as Kerguelen Plateau (Leg 183), Ontong Java Plateau (Leg 192), and the Emperor Seamounts (ODP Leg 197). Such operations are reasonably well understood, having been accomplished numerous times in the past, and relatively simple in that they do not require special equipment or advanced coring techniques.

At each of the drill sites, the JOIDES Resolution will drop a beacon and establish position over the site, lower the drill string to the seafloor, and proceed with drilling and coring until unable to continue or allotted time runs out. At all sites, only rotary core barrel (RCB) drilling is anticipated. By not coring with the advanced piston corer (APC)/extended core barrel (XCB) bottom-hole assembly (BHA), several days at each site are saved by not having to run the APC/XCB BHA to the bottom and core the upper sedimentary section. The consequence of coring only with the RCB is that soft sediments will be deformed by the coring process. This is deemed an acceptable loss because the sedimentary section has been cored on each of the Shatsky Rise massifs (and many times at Tamu Massif), coring sediments is not an objective of the expedition, and the drill sites have been chosen in places where the sediment section is thin. In addition, as another time-saving method, we are seeking Environmental Protection and Safety Panel approval not to core the sediment section until reaching a penetration depth of ~50 m above basement. The rationale is that the upper sedimentary section has been cored at other nearby locations, so there is not much to be learned by additional sediment coring.

Coring igneous rocks is often time consuming and wearing on the drill bit. In coring time calculations, an igneous rate of penetration (ROP) of 2–3 m/h has been assumed, based on similar results from Leg 192 (Ontong Java Plateau) (Mahoney, Fitton, Wallace, et al., 2001). Lesser or greater penetration at Shatsky Rise sites will depend on the average ROP at each site. In general, during igneous coring an RCB drill bit will last ~40–60 h (rotating time). At a rate of 3 m/h, for example, this implies a penetration of 120–180 m before the bit is too worn to continue. This will place a limit on the total igneous penetration at sites where only a single bit is planned. Changing the bit requires raising the entire drill string, changing the bit, sending the drill string back to the seafloor, and reentering the hole. This typically takes on the order of 1 day of operations time with a free-fall funnel (FFF). During Expedition 324, the use of FFFs is planned for multibit hole reentry at proposed Site SRSH-3B and possibly at proposed Site SRCH-5. Lesser penetration may actually occur owing to unfavorable hole conditions or unavoidable operations problems, such as not being able to find and reenter the FFF.

The general coring plan is to core at three sites on Tamu Massif, so age and geochemical trends across the putative plume head eruption can be examined, and one site each on the Ori and Shirshov massifs to study the age and geochemical evolution of the younger part of the plateau. Because Expedition 324 departs from Yokohama and must end in Townsville it is necessary to work from north to south, thus beginning at proposed Site SRNH-2 and ending at proposed Site SRSH-8. This is not necessarily the best approach (one could argue that Tamu Massif sites have higher priority), but because the cruise must end to the south, working from north to south saves ~1.5 days of transit time.