Leg 137

Hole 504B

Hole 504, perhaps the most important in-situ reference section for the structure and composition of the oceanic crust, was reopened for future drilling and downhole measurements by remedial operations during Leg 137 which cleared a large diamond bit and assorted hardware left in the bottom of the hole at the end of Leg 111 in 1986.

Hole 504B was reopened after less than a week of fishing and milling the junk and was then deepened by 59.2 m to a total depth of 1621.5 mbsf or 1347 m into basement. Engineering tests were conducted to compare the efficiency of the standard ODP wireline rotary core barrel (RCB) and a conventional oilfield diamond core barrel. While the RCB system advanced the hole at an acceptable rate with recovery of the order of 14%, the diamond core barrel yielded excellent recovery but with very slow progress. Of the 59.2 m drilled during Leg 137, 48.6 m was cored with a recovery of 8.77 m. The recovered rocks are all massive basalts interpreted as a continuation of the sheeted-dike complex, although no intrusive dike margins were actually recovered, and the trend of increasing proportion of actinolite in the secondary mineral assemblage, recognized on Leg 111, continued with depth.

Temperatures in the deeper kilometer of the hole were consistent with values logged during Leg 111 and record a linear gradient of 61 deg C/km, inferring a temperature of 165 deg C at the bottom of the hole (1562 mbsf). In the upper 500 m of the hole, corresponding to the permeable layer 2A, temperatures were considerably depressed, suggesting a renewal of the downhole flow of ocean-bottom water into the upper levels of basement. Since Leg 111, this downhole flow has increased to a rate of approximately 15 m/hr.

During the second test of the diamond core barrel, the 18-m-long outer barrel, with the bit at the bottom, broke off in the hole and further fishing operations were scheduled for Leg 140.