Fluid Flow and Hydrothermal Alteration Patterns in Sheeted Dikes at Pito Deep

Upper crustal exposures of super fast spreading crust formed at the East Pacific Rise were investigated at Pito Deep using the submersible Alvin and Jason II ROV. Two major escarpments, with exposures of lavas, sheeted dikes and gabbros, were imaged and extensively sampled to investigate crustal accretion processes and, in particular, fluid flow pathways and hydrothermal alteration mechanisms. Our study focuses on the sheeted dike complex as this is where the fluid flux and fluid-rock interaction that relates to axial hydrothermal systems is concentrated. The sheeted dikes are steeply dipping and relatively intact with localized zones of brecciation and faulting. Sheeted dikes are variably altered to greenschist and amphibolite facies mineral assemblages. Samples with amphibole-dominated assemblages are predominantly concentrated in the lower dikes while chlorite-dominated assemblages are mostly concentrated in the upper dikes. The degree of alteration ranges from slight (2-10%) to pervasive (>80%) in the lower dikes and from fresh (<2%) to high (41-79%) in the upper dikes. Fault breccias and gouges show the most pervasive alteration. Veins are present throughout the sheeted dike complex and range from thin (<3 mm) to thick (<6 cm) composite veins with crack-seal textures. Thin veins, present at all depths in the sheeted dike complex, contain amphibole ± chlorite ± epidote ± quartz. Thicker veins, mostly restricted to the upper-sheeted dikes, contain quartz ± epidote ± chlorite ± sulfides. An important preliminary finding is that there is significant lateral variability in the degree of alteration and the conditions of fluid-rock interaction. Some dive transects record an increase in temperature with depth whereas others record high temperatures in the upper and lower dikes. Peak metamorphic temperatures, based on amphibole-plagioclase pairs, range from 559-870°C in the lower sheeted dikes to 505-681°C in the upper-sheeted dikes. Chlorite thermometry indicates peak metamorphic temperatures ranging from 204-312°C in the lower dikes and 214-287°C in the upper dikes. Heterogeneities in alteration range from large-scale regional patterns (10s-100s m) to small-scale patches (few mm). Regional scale lateral changes in the dominant alteration assemblages and extent of alteration appear to be gradual for the majority of samples, with more abrupt changes associated with faults.