Hydrothermal Alteration of a Section of Upper Oceanic Crust in the Eastern Equatorial Pacific: A Synthesis of Results from Site 504 (DSDP Legs 69, 70, and 83, and ODP Legs 111, 137, 140, and 148)

Abstract

Mineralogical, chemical, and isotopic results from seven drilling legs that visited DSDP/ODP Hole 504B over 14 years are compiled here to present an integrated view of hydrothermal alteration of oceanic crust at Site 504. Hole 504B reaches to 2111 mbsf, through 274.5 m sediment, 571.5 m of volcanic rocks, a 209 m transition zone, and 1050 m into a sheeted dike complex. The volcanic section was altered through a series of processes involving interaction with seawater at low temperatures, with the effects of cold, oxidizing seawater decreasing downward. These processes and their effects on the volcanic section are generally similar to those in other oceanic upper crustal sections.The transition zone and upper dikes were altered in a subsurface mixing zone, where hydrothermal fluids upwelling through the dikes mixed with cooler seawater circulating in the overlying more permeable volcanic rocks. Alteration of the transition zone and upper dikes (down to 1500 mbsf) occurred in a series of stages, reflecting the thermal and chemical evolution of the hydrothermal system from (1) early chlorite, actinolite, albite-oligoclase, and titanite, to (2) quartz, epidote and sulfides, to (3) anhydrite, and finally to (4) zeolites and local calcite. The maximum temperature estimated for the first two stages is 350°-380°C, and the inferred mineral assemblages for these early stages are typical of the greenschist facies.The lower dikes (1500-2111 mbsf) underwent an early, high-temperature (>400°C) alteration stage, resulting in the formation of hornblende and calcic secondary plagioclase, consistent with reactions inferred to occur in deep subsurface reaction zones, where hydrothermal vent fluids acquire their final compositions. Much of the subsequent reactions produced greenschist assemblages at ~300°-400°C. The lower dikes have lost metals and sulfur and are a source of these elements to hydrothermal vent fluids and seafloor sulfide deposits. The lower dikes underwent subsequent alteration stages similar to the upper dikes, with rare epidote + quartz veins recording the presence of upwelling hydrothermal fluids, and limited late off-axis effects (zeolites and prehnite). Anhydrites in the lower dikes indicate more reacted fluid compositions than in the upper dikes.Alteration of the sheeted dikes from Hole 504B is heterogeneous, with recrystallization controlled by fracturing and access of fluids. Defining the position of the seismic Layer 2/3 transition depends upon the scale of observation, but the change at Site 504 occurs within the sheeted dikes and is correlated with progressive changes in porosity and hydrothermal alteration. However, we still do not know the nature of the transition from sheeted dikes to gabbros in in situ ocean crust, or the nature of the inferred fault at the base of Hole 504B and its role in fluid flow and alteration.

Crustal faults exposed in the Pito Deep Rift: Conduits for hydrothermal fluids on the southeast Pacific Rise

“…Other elements, and mobile and immobile trace elements in particular, vary in more complicated ways depending upon the abundance and compositions of other silicates and accessory phases. The distribution and composition of hydrothermal mineral phases result from the reaction progress as seawater-derived hydrothermal fluids pass through the ocean crust at variable pressure, temperature, pH, and redox state of the fluids [Mottl, 1983;Seyfried et al, 1991;Alt et al, 1996]. Such reaction progress is also tracked by stable isotope compositions of altered rocks, which are reported elsewhere for PDR samples [Barker et al, 2007[Barker et al, , 2008.…”

“…One of the most influential models for axial hydrothermal systems holds that as seawater recharges the hydrothermal system, it loses its magnesium in a reaction zone at the base of the SDC and returns up through a discharge zone where quartz and sulfide mineralization dominates [e.g., Alt et al, 1996]. Most workers now recognize that there is no ''reaction zone'' per se but rather that seawater-rock chemical exchange occurs along the entire flow path [e.g., Bickle and Teagle, 1992].…”

“…Another apparently universal property of the ocean crust is a metamorphic gradient with depth wherein low-temperature clay minerals dominate the alteration mineralogy in the lavas, with an abrupt jump to ''greenschist'' through ''amphibolite'' metamorphism in the sheeted dikes [Alt et al, 1996;Gillis et al, 2001]. [53] We describe a conceptual model that may help reconcile these various perspectives on the hydrothermal system by focusing on fluid flow through faults and fractures (Figure 12).…”

“…[3] There are two methods of accessing subsurface ocean crust: deep crustal drilling [Alt et al, 1996;Wilson et al, 2006] and submersible investigations of tectonic escarpments, also known as ''tectonic windows'' [Karson, 2002;Karson et al, 2002aKarson et al, , 2002bPockalny et al, 2004]. The most recently investigated Pacific-crust tectonic window is the Pito Deep Rift (PDR), where in situ ocean crust generated at the SEPR is exposed along escarpments with kilometer-scale relief (Figure 1).…”

Fluid flow patterns in fast spreading East Pacific Rise crust exposed at Hess Deep

“…In Hole 504B, chlorite only replaces clinopyroxene in the shallow sheeted dikes [Ishizuka, 1989], whereas this relationship is prevalent throughout the Hess Deep sheeted dike complex. Similarly, metal (Cu, Zn) and O isotope (<4.5%o) depletion at Hole 504B is restricted to the lower dikes [Alt et al, 1996], whereas these elements are locally depleted at different depths and localities along the scarp. The depth-related trends in the Hole 504B dikes are most similar to Hess Deep dikes from the eastern half of area 2 (Figures 2 and 5).…”