IODP Expedition 335: Deep Sampling in ODP Hole 1256D

Observations of the gabbroic layers of untectonized ocean crust are essential to test theoretical models of the accretion of new crust at mid-ocean ridges. Integrated Ocean Drilling Program (IODP) Expedition 335 (“Superfast Spreading Rate Crust 4”) returned to Ocean Drilling Program (ODP) Hole 1256D with the intention of deepening this reference penetration of intact ocean crust a significant distance (~350 m)into cumulate gabbros. Three earlier cruises to Hole 1256D(ODP 206, IODP 309/312) have drilled through the sediments,lavas, and dikes and 100 m into a complex dike-gabbro transition zone.Operations on IODP Expedition 335 proved challenging throughout, with almost three weeks spent re-opening and securing unstable sections of the hole. When coring commenced, the comprehensive destruction of the coring bit required further remedial operations to removejunk and huge volumes of accumulated drill cuttings. Hole-cleaning operations using junk baskets were successful, and they recoveredlarge irregular samples that document a hitherto unseen sequenceof evolving geological conditions and the intimate coupling between temporally and spatially intercalated intrusive, hydrothermal,contact-metamorphic, partial melting, and retrogressiveprocesses. Hole 1256D is now clean of junk, and it has been thoroughlycleared of the drill cuttings that hampered operations during this and previous expeditions. At the end of Expedition 335, we briefly resumed coring before undertaking cementing operations to secure problematic intervals. To ensure the greatest scientific return from the huge efforts to stabilize this primary ocean lithosphere reference site, it would be prudent to resume the deepening of Hole 1256D in the nearest possible future while it is open to full depth.