Hydrothermal alteration of a 1 km section through the upper oceanic crust, Deep Sea Drilling Project Hole 504B: Mineralogy, chemistry and evolution of seawater‐basalt interactions

Abstract

Deep Sea Drilling Project hole 504B penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes. Alteration of the crust at site 504 occurred in four stages, related to the movement of the crust away from the spreading axis: (1) Dikes reacted with seawater (200–>300°C) in the upwelling zone of an axial convection cell at the spreading axis, resulting in the formation of greenschist facies parageneses in veins and host rocks. Mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone, causing a steep temperature gradient at the base of the pillow section. Secondary minerals formed in the lower pillow section from the resultant reducing “mixed” fluids at temperatures of around 100°C. At the same time, the initial effects of “seafloor weathering” began in the upper 320 m of the pillow section at low temperatures (<50°C) and under conditions of open circulation of oxidizing seawater. (2) Following refracturing of the dikes, a second stage of axial upwelling occurred; hydrothermal fluids (200°–380°C) were probably similar to those presently sampled from spreading ridges on the seafloor. Mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz, epidote, and sulfides in veins and formation of a sulfide‐rich stockworklike zone within this transition zone. Alteration of the pillow section proceeded under the prior conditions, with the effects of seafloor weathering extending progressively downward into the crust. (3) Seawater recharge penetrated to depths of at least 1075.5 m subbasement and deposited anhydrite locally in veins. (4) Off‐axis alteration of the dikes was characterized by formation of zeolites, calcite, and prehnite in veins and host rocks from more highly evolved and lower temperature (100°–250°C) fluids. Alteration in the upper pillow section evolved from “seafloor weathering” conditions to more reducing and rock‐dominated, as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment. Zeolites and calcite were the last phases to form throughout the pillow section. Sealing of the crust to convective cooling also allowed conductive reheating of the crust from below. Calculations of seawater‐crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt‐seawater interactions are a sink for seawater Mg and K and a source for Si and Ca.

Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on mid-ocean ridges: The role of assimilation at spreading centers

“…If alteration is caused by circulation of hydrothermal fluids, it is likely that this is largely restricted by permeability to the base of the sheeted dikes and uppermost gabbros of Layer 3. This is also consistent with observations from drill cores from lower sheeted dikes and upper gabbros that have experienced low-to medium-grade metamorphism (up to greenschist or amphibolite facies; Alt et al, 1986;Coogan et al, 2003). Although hydrothermal circulation has little effect on the major element chemistry, the lower sheeted dikes and upper gabbros commonly show lower K 2 O, Na 2 O and δ 18 O values, variable CO 2 , higher H 2 O and Cl concentrations (e.g.…”

“…δ 18 O values are higher than unaltered MORB compositions (~5.6 ± 0.2‰; Eiler, 2001) in the upper volcanic section of the crust but decrease to lower than mantle values in the lower sheeted dikes and gabbros (for profiles see Alt and Teagle, 2000). The δ 18 O of the sheeted dike and gabbro layers is quite variable but the sheeted dikes at Hess Deep have an average value of 5.1‰ (Gillis et al, 2001) and are as low as 2.9‰ (Agrinier et al, 1995), significantly different than mantle values of 5.6‰ (e.g., Alt et al, 1986;Eiler, 2001). Bulk melting of surrounding wall rock will result in δ 18 O values similar to the altered whole rock values, however, partial melts in equilibrium with the altered basalts will have oxygen isotope ratios generally less than mantle-equilibrated values, but higher (by~1‰; e.g., Muehlenbach and Byerly, 1982) than the altered basalt itself.…”

“…The bulk composition and mineralogy of altered crustal material may vary, depending on the temperature and pressure of alteration and the fluid/rock ratios (Alt et al, 1986). Constraints on the depth of assimilation come from studies of volatile concentrations in glasses (Michael and Cornell, 1998;le Roux et al, 2006) and textural observations in ophiolites (e.g.…”

Relationships between Strengths and Physical Properties of Samples Recovered during Legs 137, 140, and 148 from Hole 504B

“…Furthermore, alteration continues to affect rock strength even at sites with ages greater than the 5.9 Ma at Site 504, in agreement with its observed effect on other physical properties. However, differences in alteration with depth in Hole 504B (see, e.g., Alt et al, 1986, and related papers in this volume) do not appear to affect the relationship between strength and other physical properties of the samples we have tested.…”

Determination of lattice parameter, oxidation state, and composition of individual titanomagnetite/titanomaghemite grains by transmission electron microscopy

“…Our observations thus far suggest that the degree of oxidation of samples in the same drill hole and therefore of the same age can vary significantly, according to factors such as rock porosity which in turn relates to water/rock ratio [Alt et al, 1986;Honnorez et al, 1996]. We selected samples in this study primarily to determine the relation between z and a, and not with respect to factors affecting the relation between z and age.…”