Secondary mineral assemblages in a volcanic sequence of ODP Hole 104-642E

Mineralogical and geochemical analyses of alteration products from upper and lower volcanic series recovered during ODP Leg 104 reveal variations both in composition and order of crystallization of clay minerals vesicles and voids filling and replacing glass. These results provide information about successive alteration stages of rocks and interlayered volcaniclastic sediments. The first stage, related to initial basalt-seawater interaction, is characterized by development of Fe-smectites, especially Fe-rich saponite. A second stage of intermittently superimposed subaerial weathering is marked by iron-oxides-halloysite-kaolinite formation. The third episode, interpreted as hydrothermal on the basis of O-isotopic data, is defined by postburial coprecipitation of Fe-poor, Mg-rich saponite and celadonite. A distinct final and pervasive hydrothermal stage, occurring mainly in the lower series and dominated by Al-smectites-zeolites assemblage, indicates changes toward a more reducing alteration environment.

Geochemistry of the sheeted dike complex at Pito Deep

Alteration of sheeted dikes exposed along submarine escarpments at the Pito Deep Rift (NE edge of the Easter microplate) provides constraints on the crustal component of axial hydrothermal systems at fast spreading mid-ocean ridges. Samples from vertical transects through the upper crust constrain the temporal and spatial scales of hydrothermal fluid flow and fluid-rock reaction. The dikes are relatively fresh (average extent of alteration is 27%), with the extent of alteration ranging from 0 to >80%. Alteration is heterogeneous on scales of tens to hundreds of meters and displays few systematic spatial trends. Background alteration is amphibole-dominated, with chlorite-rich dikes sporadically distributed throughout the dike complex, indicating that peak temperatures ranged from <300°C to >450°C and did not vary systematically with depth. Dikes locally show substantial metal mobility, with Zn and Cu depletion and Mn enrichment. Amphibole and chlorite fill fractures throughout the dike complex, whereas quartz-filled fractures and faults are only locally present. Regional variability in alteration characteristics is found on a scale of <1-2 km, illustrating the diversity of fluid-rock interaction that can be expected in fast spreading crust. We propose that much of the alteration in sheeted dike complexes develops within broad, hot upwelling zones, as the inferred conditions of alteration cannot be achieved in downwelling zones, particularly in the shallow dikes. Migration of circulating cells along rides axes and local evolution of fluid compositions produce sections of the upper crust with a distinctive character of alteration, on a scale of <1-2 km and <5-20 ka.

(Table 2) Oxygen, carbon, and strontium isotopic composition of carbonate-brucite material from the Lost City hydrothermal field

The isotopic (dD, d18O, d13C, and 87Sr/86Sr) and geochemical characteristics of hydrothermal solutions from the Mid-Atlantic Ridge and the material of brucite-carbonate chimneys at the Lost City hydrothermal field at 30°N, MAR, were examined to assay the role of the major factors controlling the genesis of the fluid and hydrothermal chimneys of the Lost City field. The values of dD and d18O in fluid samples indicates that solutions at the Lost City field were produced during the serpentinization of basement ultramafic rocks at temperatures higher than 200°C and at relatively low fluid/rock ratios (<1). The active role of serpentinization processes in the genesis of the Lost City fluid also follows from the results of the electron-microscopic studying of the material of hydrothermal chimneys at this field. The isotopic (d18O, d13C, and 87Sr/86Sr) and geochemical (Sr/Ca and REE) signatures indicate that, before its submarine discharging at the Lost City field, the fluid filtered through already cold altered outer zones of the Atlantis Massif and cooled via conductive heat loss. During this stage, the fluid could partly dissolve previously deposited carbonates in veins cutting serpentinite at the upper levels of the Atlantis Massif and the carbonate cement of sedimentary breccias underlying the hydrothermal chimneys. Because of this, the age of modern hydrothermal activity at the Lost City field can be much younger than 25 ka.

Geochemistry of minerals at DSDP Hole 70-504B

Hole 504B, drilled into the 5.9 Ma crust of the southern flank of the Costa Rica Rift, tapped a hydrothermal system in its conductive stage. Three alteration zones were encountered along the 561.5 meters of basement drilled. The upper alteration zone, 274.5 to 584.5 meters below the seafloor (BSF), is characterized by the presence of color zonation in which red halos are located between dark gray inner rock portions and dark gray outer bands. The red halos are characterized by an abundance of iddingsite, and they have higher K2O contents and Fe3+/FeT ratios, but lower SiO2 contents, than the adjacent dark gray inner zones. The dark gray outer bands are characterized by the presence of celadonite-nontronite. Saponite is omnipresent in these three alteration bands. Phillipsite is the only zeolite that occurs in the upper alteration zone. The upper alteration zone is interpreted as being the result of low-temperature alteration, with large amounts of cold oxygenated seawater percolating through the upper ocean crust. In the upper alteration zone, the formation of red halos was both preceded and followed by formation of dark gray outer bands. Then followed formation of dark gray cores. The lower alteration zone (584.5-835.5 m BSF) is characterized by the absence of color zonation, the downward-increasing abundance of pyrite and saponite, and the presence of quartz, talc, and calcite. The chemical changes (downhole MgO enrichment and concomitant CaO depletion) observed in the basalts of the lower alteration zone are thought to result from reactions of oceanic basalts with evolved seawater (i.e., solutions derived from seawater that has already reacted with ocean crust), which is thus depleted in oxygen, potassium, and radiogenic strontium. This alteration process, which was responsible for saponite formation in both the upper and lower alteration zones, was rock dominated, and it took place under suboxic to anoxic conditions during a second stage of alteration. Reaction temperatures could have progressively increased with depth. There is also a zeolitic zone that essentially coincides with the lower part of the upper alteration zone (between 528.5 and 563 m BSF). The host rock adjacent to veins of zeolite exhibits a greenish discoloration due to the intensive replacement of the igneous minerals. The replacement minerals result in significant increases in the bulk rock K2O, MgO, CaO, CO2, and H2O+ contents. The solutions circulating along the newly opened fissures had high Ca activity, and minerals probably precipitated in these fissures at 60°C or 110°C. These hydrothermal solutions circulated later than those responsible for the formation of the minerals that characterize the upper and lower alteration zones.