Alteration of the Upper Oceanic Crust: Mineralogy and Processes in Deep Sea Drilling Project Hole 504B, Leg 83

Abstract

Leg 83 of the Deep Sea Drilling Project has deepened Hole 504B to over 1 km into basement, 1350 m below the seafloor (BSF). The hole previously extended through 274.5 m of sediment and 561.5 m of pillow basalts altered at low temperature (< 100°C), to 836 m BSF. Leg 83 drilling penetrated an additional 10 m of pillows, a 209-m transition zone, and 295 m into a sheeted dike complex. Leg 83 basalts (836-1350 m BSF) generally contain superimposed greenschist and zeolite-facies mineral parageneses. Alteration of pillows and dikes from 836 to 898 m BSF occurred under reducing conditions at low water/rock ratios, and at temperatures probably greater than 100°C. Evolution of fluid composition resulted in the formation of (1) clay minerals, followed by (2) zeolites, anhydrite, and calcite. Alteration of basalts in the transition zone and dike sections (898-1350 m BSF) occurred in three basic stages, defined by the opening of fractures and the formation of characteristic secondary minerals. (1) Chlorite, actinolite, pyrite, albite, sphene, and minor quartz formed in veins and host basalts from partially reacted seawater (Mg-bearing, locally metal-and Si-enriched) at temperatures of at least 200-250°C. (2) Quartz, epidote, and sulfides formed in veins at temperatures of up to 380°C, from more evolved (Mg-depleted, metal-, Si-, and 18 O-enriched) fluids. (3) The last stage is characterized by zeolite formation: (a) analcite and stilbite formed locally, possibly at temperatures less than 200°C followed by (b) formation of laumontite, heulàndite, scolecite, calcite, and prehnite from solutions depleted in Mg and enriched in Ca and 18 O, at temperatures of up to 250°C. The presence of small amounts of anhydrite locally may be due to ingress of relatively unaltered seawater into the system during Stage 3. Alteration was controlled by the permeability of the crust and is characterized by generally incomplete recrystallization and replacement reactions among secondary minerals. Secondary mineralogy in the host basalts is strongly controlled by primary mineralogy. The alteration of Leg 83 basalts can be interpreted in terms of an evolving hydrothermal system, with (a) changes in solution composition because of reaction of seawater fluids with basalts at high temperatures; (b) variations in permeability caused by several stages of sealing and reopening of cracks; and (c) a general cooling of the system, caused either by the cooling of a magma chamber beneath the spreading center and/or the movement of the crust away from the heat source. The relationship of the high-temperature alteration in the transition zone and dike sections to the low-temperature alteration in the overlying pillow section remains uncertain.

Stable and Strontium Isotopic Profiles through Hydrothermally Altered Upper Oceanic Crust, Hole 504B

“…Some of these patches (e.g., Sample 148-504B-252R-1, 15-20 cm) clearly formed around primary pore space, where 1-to 5-mm-sized former vugs are now filled with needle-like amphibole. Similar features are observed throughout the dike section, and the patches probably represent more extensive hydrothermal reaction around locally enhanced primary pore spaces (Alt et al, 1985(Alt et al, , 1989bZuleger et al, 1995;Bach et al, this volume).…”

“…Hblde = hornblende. Data from this work and Alt et al (1985Alt et al ( , 1986aAlt et al ( , 1986bAlt et al ( , 1995.…”

Permeability versus depth in the upper oceanic crust: In situ measurements in DSDP hole 504B, eastern equatorial Pacific

“…We have little evidence with which to add age dependence to the hole 504B results, let alone to a general relationship. The alteration of the minerals found in cores from layer 2C shows clear indications of a previous, higher-temperature history, followed by much colder conditions than those currently found in the hole [Alt and Laverne, 1984]. Both colder and hotter circulation systems imply that the permeability in layer 2C was higher, and the circulation was more vigorous in the past than at present.…”

Mineral Chemistry of Primary and Secondary Phases in Basaltic Rocks, Leg 129

“…The brownish olive green variety is a smectite with significant Fe-Mg occupancy and is a member of the saponite-ferrosaponite series of trioctahedral smectites. True chlorites, together with di-and trioctahedral smectites, have been recorded in deeper oceanic basement sequences such as at Sites 501, 504, and 505 (Adamson, 1981;Alt et al, 1985).…”