[摘要] Observations of the gabbroic layers of untectonized oceancrust are essential to test theoretical models of the accretionof new crust at mid-ocean ridges. Integrated Ocean DrillingProgram (IODP) Expedition 335 ("Superfast Spreading RateCrust 4") returned to Ocean Drilling Program (ODP) Hole1256D with the intention of deepening this reference penetrationof 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-gabbrotransition zone.
Operations on IODPExpedition 335 proved challengingthroughout, withalmost three weeks spentre-opening and securingunstable sections of thehole. When coring commenced,the comprehensivedestruction of the coringbit required further remedialoperations to removejunk and huge volumes ofaccumulated drill cuttings.Hole-cleaning operationsusing junk baskets weresuccessful, and they recoveredlarge irregularsamples that document ahitherto unseen sequenceof evolving geological conditionsand the intimatecoupling between temporallyand spatially intercalatedintrusive, hydrothermal,contact-metamorphic,partial melting, and retrogressiveprocesses.
Hole 1256D is now cleanof junk, and it has been thoroughlycleared of the drillcuttings that hampered operationsduring this and previous expeditions. At the end ofExpedition 335, we briefly resumed coring before undertakingcementing operations to secure problematic intervals.To ensure the greatest scientific return from the huge effortsto stabilize this primary ocean lithosphere reference site, itwould be prudent to resume the deepening of Hole 1256D inthe nearest possible future while it is open to full depth.
doi:
10.2204/iodp.sd.13.04.2011
