IODP Expedition 312 Preliminary Report

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Table T2. Key parameters for Hole 1256D and preliminary subdivisions of the upper oceanic crust at Site 1256 adopted for Expeditions 309 and 312.

Location: 6°44.163'N, 91°56.061'W
Water depth (m): 3634.7
Sediment thickness (m): 250
16-in casing string (m): 269.5
Interval in Hole 1256D drilled during
ODP Leg 206: 752 mbsf, 502 msb
IODP Expedition 309: 1255.1 mbsf, 1005.1 msb
IODP Expedition 312: 1507.1 mbsf, 1257.1 msb

Major lithologic zone

Sublithology

Top core, section,
interval (cm)

Top igneous unit

Depth (mbsf)

Criteria

Lavas
Lava Pond
206-1256D-2R-1, 0
1256D-1
(1256C-1 through 18)
~250
Massive ponded lava including overlying sheet flows.

Inflated Flows
206-1256D-13R-1, 0
1256D-2
350.3
Massive sheet and pillowed flows with rare inflation features.

Sheet and massive flows
206-1256D-43R-1, 0
1256D-16
533.9
Sheet flows with subordinate massive flows.

Transition zone

309-1256D-117R-1, 85
1256D-40
1004.2
Sheet flows with breccias, rare dikes, and alteration at (sub-) greenschist facies conditions.

Sheeted dike complex
Upper dikes
309-1256D-129R-1, 0
1256D-44a
1060.9
Massive basalts, common subvertical intrusive contacts. Elevated V_P and TC.

Granoblastic dikes
312-1256D-192R-1, 0
1256D-78
1348.3
Granoblastic texture; equant grains; secondary clinopyroxene, orthopyroxene, plagioclase, hornblende, magnetite, and illite.

Plutonic section
Gabbro 1
312-1256D-213R-1 (Piece 13, 52)
1256D-81
1406.62
Medium-grained gabbroic rocks intrude overlying dikes: gabbro, disseminated oxide gabbro, patchy gabbro, and quartz-rich oxide diorite.

Upper dike screen
312-1256D-225R-1 (Piece 1, 0)
1256D-90
1458.9
Fine-grained to cryptocrystalline basalt dike. Highly altered, granoblastic texture, secondary clinopyroxene, orthopyroxene, plagioclase, mt, illite, and hornblende.

Gabbro 2
312-1256D-230R-1 (Piece 5, 15)
1256D-91
1483.1
Plutonic rock intrudes upper and lower dike screens; medium-grained gabbro, oxide-bearing quartz gabbro, oxide gabbro, orthopyroxene-bearing gabbro, trondjhemite; leucocratic-gabbro; all highly altered; stoped basalt.
Gabbronorite of uncertain origin.
Late dike Unit 95 at 1502.6 mbsf.

Major lithologic zone	Sublithology	Top core, section, interval (cm)	Top igneous unit	Depth (mbsf)	Criteria

Lavas	Lava Pond	206-1256D-2R-1, 0	1256D-1 (1256C-1 through 18)	~250	Massive ponded lava including overlying sheet flows.
Inflated Flows	206-1256D-13R-1, 0	1256D-2	350.3	Massive sheet and pillowed flows with rare inflation features.
Sheet and massive flows	206-1256D-43R-1, 0	1256D-16	533.9	Sheet flows with subordinate massive flows.
Transition zone		309-1256D-117R-1, 85	1256D-40	1004.2	Sheet flows with breccias, rare dikes, and alteration at (sub-) greenschist facies conditions.
Sheeted dike complex	Upper dikes	309-1256D-129R-1, 0	1256D-44a	1060.9	Massive basalts, common subvertical intrusive contacts. Elevated V_P and TC.
Granoblastic dikes	312-1256D-192R-1, 0	1256D-78	1348.3	Granoblastic texture; equant grains; secondary clinopyroxene, orthopyroxene, plagioclase, hornblende, magnetite, and illite.
Plutonic section	Gabbro 1	312-1256D-213R-1 (Piece 13, 52)	1256D-81	1406.62	Medium-grained gabbroic rocks intrude overlying dikes: gabbro, disseminated oxide gabbro, patchy gabbro, and quartz-rich oxide diorite.
Upper dike screen	312-1256D-225R-1 (Piece 1, 0)	1256D-90	1458.9	Fine-grained to cryptocrystalline basalt dike. Highly altered, granoblastic texture, secondary clinopyroxene, orthopyroxene, plagioclase, mt, illite, and hornblende.
Gabbro 2	312-1256D-230R-1 (Piece 5, 15)	1256D-91	1483.1	Plutonic rock intrudes upper and lower dike screens; medium-grained gabbro, oxide-bearing quartz gabbro, oxide gabbro, orthopyroxene-bearing gabbro, trondjhemite; leucocratic-gabbro; all highly altered; stoped basalt. Gabbronorite of uncertain origin. Late dike Unit 95 at 1502.6 mbsf.

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