RESULTS

Bulk mineralogical analyses were compiled for two sites drilled during Leg 193. These results are shown for the Snowcap diffuse venting area (Site 1188) and the Roman Ruins high-temperature area (Site 1189) in Figures F3 and F4, respectively. In addition, data for both sites are shown in Tables T1, T2, T3, and T4.

With the exception of fresh glassy rhyodacite and dacite near the seafloor, hydrothermal alteration is ubiquitous in the subsurface at Snowcap and Roman Ruins (Binns, Barriga, Miller, et al., 2002). Hydrothermal alteration is complex and multistage and includes pervasive replacement of igneous material and alteration halos along anhydrite pyrite quartz veins. Mineral assemblages arising from hydrothermal alteration vary with depth and are complicated by overprinting relationships. At both sites, rocks are highly to completely altered to cristobalite and/or quartz, clay minerals (e.g., illite, chlorite, smectite, and mixed layers), anhydrite, and disseminated pyrite. In Hole 1188A a zone from ~50 to 120 mbsf is characterized by a chlorite illite-christobalite-plagioclase assemblage intercalated with a pyrophyllite assemblage (Fig. F3). A lower sequence below 120 mbsf in Hole 1189B comprises hydrothermal alteration features (Fig. F4B) that are distinctly different from those encountered at Snowcap (Site 1188). K-feldspar is abundant, and magnetite is rare. K-feldspar appears to be associated with quartz-chlorite alteration, which alternates with less strongly altered cristobalite-bearing rocks in the lower portion of Hole 1189B. A detailed description on clay mineral assemblages, clay and whole rock chemistry, and clay mineral isotopic compositions of the altered rocks from both sites is given in Lackschewitz et al. (2004).

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