JA Daily Science Report for Expedition 312, 22 November 2005


LOCATION: Site 1256


SCIENCE UPDATE: Cores 174R to 177R were cored from 1265.4 to 1285.7 mbsf with an average recovery of 22.8%. Penetration has slowed to approximately 1 m/hr. Cores 175R through 177R recovered six individual basalt units (68-72)  distinguished by subtle textural variation and interpreted as dikes. All are variable in grain size from cryptocrystalline to fine-grained, in some cases grading inward from chilled margins. A steeply-dipping, sharp, cryptocrystalline chilled margin is preserved where Unit 69 intrudes Unit 68 and grades over ~40 cm to a microcrystalline to fine-grained dike core. This is the best evidence so far that we are in the sheeted dike complex.


The basalts are slightly to highly altered with the most intense alteration developed in complex cm-scale patches in which primary minerals are replaced by combinations of chlorite, quartz, pyrite, epidote, and actinolite(?). This is the first observation of epidote-rich patches in Hole 1256D. The basalts are cross-cut by numerous mm-scale quartz, chlorite, pyrite, anhydrite, prehnite or laumontite veins commonly with mm-wide chloritic halos. Subvertical veins are cut in most places by sub-horizontal veins. Dike chilled margins are commonly brecciated, generally highly altered, pyrite-mineralized, and disrupted by margin parallel and perpendicular quartz, chlorite, pyrite veins.