Abstract
Abstract. 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 remove junk and huge volumes of accumulated drill cuttings. Hole-cleaning operations using junk baskets were successful, and they recovered large irregular samples that document a hitherto unseen sequence of evolving geological conditions and the intimate coupling between temporally and spatially intercalated intrusive, hydrothermal, contact-metamorphic, partial melting, and retrogressive processes. Hole 1256D is now clean of junk, and it has been thoroughly cleared 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. doi: 10.2204/iodp.sd.13.04.2011
Highlights
Hole 1256D is clean of junk, and it has been thoroughly cleared of the drill cuttings that hampered op
The high extent of metamorphic recrystallization exhibited by the granoblastic basalts, and operational factors, provide strong evidence that the granoblastic basalts, minor gabbros, and evolved plutonic rocks were sourced from the lowermost reaches of Hole 1256D (1494.9â1521.6 mbsf), most probably from below the lower-most recovered gabbroic interval (Fig. 4)
The extensive remedial operations on Expedition 335 precluded significant deepening of Hole 1256D, significant progress was made in improving the stability of the borehole
Summary
Insights on formation of intrusive crust at detachment-dominated, slow-spread lithosphere have been obtained (e.g., Holes 735B, U1309D, and other sites; ODP Legs 118, 153, 176, 209, and IODP Expeditions 304â305; Dick et al, 2000; Ildefonse et al, 2007; Kelemen et al, 2007), the thermal regime and the melt supply and delivery in these settings differ significantly from those of the axial zone in fast-spreading lithosphere. IODP Expedition 335 (13 April to 3 June 2011) was the fourth scientific drilling cruise of the Superfast campaign, and it returned to ODP Hole 1256D (6°44.163'N, 91°56.061'W) to deepen this ocean crust reference penetration a significant distance into cumulate gabbros.
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