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doi:10.2204/iodp.proc.309312.205.2009 IntroductionDuring Integrated Ocean Drilling Program (IODP) Expedition 312, the structural geology team was impressed by the microstructure of chilled margins recovered from the sheeted dike complex (SDC) of the Pacific crust drilled in Hole 1256D. Microstructures and crosscutting relationships within and around the chilled margins led the structure team to postulate that the fracturing and hydrothermal alteration of the chilled margins occurred more or less during dike intrusion. If so, the microstructures are evidence for weakening of the crust by elevated fluid pressure (Harper and Tartarroti, 1996; Umino et al., 2008) and provide a mechanism for the generation of hydrothermal plumes at the seafloor by diking (Delaney et al., 1998). The purpose of this data report is to summarize shipboard observations of the SDC chilled margins and to present new, higher resolution data on the microstructures, mineralogy, and compositions of the dike margins. We review shipboard hand sample and thin section studies and present new backscatter electron images (BSEI) of chilled margins. We report on qualitative identification of phases from electron dispersive spectroscopy (EDS). Also presented are some wavelength dispersive spectroscopy (WDS) analyses of feldspars in the chilled margins; the feldspars preserve alteration microstructures, including K-feldspar zoning and variable barium concentrations. Other hydrothermal phases include lenses of anhydrite, sphene, and more widely reported hydrothermal minerals such as albite, chlorite, actinolite, calcite, and quartz. We close with a brief discussion of the probable origin and significance of the microstructure, mineralogy, and composition of the chilled margins. |