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Table T1. Scientific objectives, Expedition 309/312.

Objectives (522-Full3) Achievements Outstanding goals
1. Drill an intact section of in situ upper ocean crust.
Current knowledge is based predominantly on exotic, tectonically exposed sections and suprasubduction zone ophiolites. These may not be representative of typical ocean crust, which comprises 60% of the Earth’s surface. Complete penetration of the lavas and, for the first time in the history of scientific ocean drilling, the sheeted dike complex. Significant penetration (>500 m) of the upper gabbros.
2. Test the relationship between ocean ridge spreading rate and depth to the low-velocity zone (LVZ) and investigate the geological nature of the LVZ.
Assuming that the LVZ is a melt lens that crystallizes to gabbro, the observed spreading rate depth to the axial LVZ relationship predicts that the dike–gabbro transition should be 1025 to 1300 msb at Site 1256, given a spreading rate of 220 mm/y. The dike/​gabbro boundary was drilled at 1157 msb at Site 1256, within the predicted LVZ depth range. This confirms: (1) the LVZ is a melt lens and (2) the spreading rate/LVZ depth relationship can be extrapolated to superfast spreading rates.The dike/​gabbro boundary drilled for the first time in in situ normal ocean crust. No outstanding goals.
3. Characterize the structure of superfast spreading rate crust, including
The lithology, thickness, structure, and geochemistry of the lavas and sheeted dikes and the nature of the lava–dike transition zone; and The textures, chemistry, and magmatic structure of the upper gabbros.And, hence, determine the processes of crustal accretion (gabbro glacier vs. sheeted sills models). Complete penetration of the lavas and dikes, allowing comparison with Hole 504B crust. Upper ~100 m of plutonics recovered (gabbro, oxide gabbro, quartz-rich oxide diorites, and trondjhemite dikelets, with dike screens and stoped dike clasts).The upper gabbros have chilled upper and lower contacts and are on average slightly more primitive than the dikes and lavas, but still within the EPR MORB field. They are not cumulates, nor are they representative of the entire lower crust. Evidence of upper gabbros with chilled upper and lower contacts favors the sheeted-sill model. However, given that the gabbros are fractionated (requiring a deeper magma chamber) and that 100 m represents only ~2% of the gabbro, further drilling is essential to determine the mode of accretion of lower ocean crust.
4. Correlate lithology and rock properties with remote geophysical measurements, in particular:
What do seismic Layers 2 and 3 correspond to? (Generally assumed to be the dike/​gabbro boundary, but it was intercepted within the sheeted dikes in Hole 504B, at an alteration boundary.)Which crustal layers contribute to the marine magnetic anomalies? dike/​gabbro boundary intercepted within L2 at Site 1256. Here the dike/​gabbro boundary is not the L2/L3 boundary or an alteration boundary within the dikes. Intercept the Layer 2/3 boundary at Site 1256 and determine its geological nature.Sample a longer, more representative gabbro section, determine the magnetic properties of the gabbros, and complete the crustal "magnetic budget."
5. Investigate the interactions between magmatic and alteration processes, including the nature and extent of water rock interaction, including
The alteration stratigraphy and chronology, fluid flow paths, and the nature of reaction/mixing zones;The variability of thermal, fluid, and chemical fluxes, the balance between low and high temperature alteration, and the implications for global geochemical budgets; andThe alteration of the dike–gabbro transition (the conductive boundary layer between the magma chamber and the hydrothermal system). Alteration of the lavas, dikes, upper gabbros, and transition zones described and compared to Hole 504B and ophiolites. A contact metamorphism zone was discovered in the lower dikes, due to intrusion of the upper gabbro, which was altered by high temperature fluids. Sample the entire hydrothermally altered portion of the crust to complete the crustal geochemical budgets.Determine the depth of hydrothermal fluid penetration and temperatures of interactions as this influences processes of crustal accretion and heat removal from the lower crust (significant to global heat output).

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