ALBERT

Description: Alteration patterns in the lavas and dykes of the Troodos Ophiolite, Cyprus, record a complex history of axial hydrothermal alteration, crustal aging, and subsequent uplift and emplacement of the ophiolite. Field mapping shows that distribution of five alteration zones, each with distinct mineralogical, geochemical, and hydrologie characteristics, is influenced by igneous stratigraphy, structure, and the nature and thickness of the overlying sediments. Paragenetic sequences of secondary minerals indicate that alteration conditions changed progressively as the crust cooled and moved off-axis. Along spreading axes, low temperatures (≤50°C) were maintained by the rapid flow of seawater in and out of the lavas, and only minimal alteration took place. In contrast, lower water/rock ratios and higher temperatures (〉200°C) in the dykes promoted extensive seawater-rock interaction. Although the sharp rise in temperature between the two regimes generally coincides with the lava-dyke transition, late-stage intrusions or hydrothermal upwelling zones locally cause high-temperature alteration to extend upward into the lavas. As a segment of crust moved off-axis, temperatures remained low in the lavas and progressively decreased, from 〉250° to 〈80°C, in the dykes. High permeability in the uppermost lavas led to the downward migration of an oxidative alteration front whose thickness and spatial distribution was dependent upon the rate and nature of sedimentation and, thus, the original seafloor morphology. Although field relations show that alteration has a consistent vertical pattern in Troodos, the alteration zones are not laterally continuous, and the stratigraphie depth of their boundaries varies considerably.

Description: Many studies have focused on low-temperature alteration of the oceanic crust, nevertheless little is known about the initial processes of low-temperature alteration and their influence on the mineralogical and chemical composition of the oceanic crust once it leaves the spreading axis. The lack of such studies is primarily related to the difficulty of recovering samples representative of this initial alteration style. Such information is nevertheless important not only for crust–ocean mass-balances but also to characterize an important input to the subduction zones. We have studied dredged basalts from the eastern flank of the East Pacific Rise at 14°15′S, concentrating on the products of a single spreading segment in a corridor perpendicular to the spreading axis and covering a range of crustal ages from 0 to 9 Ma. Electron microprobe, X-ray fluorescence, X-ray diffraction and ICP-MS analyses have been carried out to examine the mineralogical and chemical changes in the basalts which make up the surface of the upper crust caused by low-temperature alteration. Fresh rocks were sampled at the ridge axis; off-axis basalts show features of progressive alteration. Celadonite is the main alteration component in 0.12–4.6 Ma old rocks, whereas phillipsite is more abundant in rocks older than 4.6 Ma. Changing compositions of secondary minerals, progressive sealing of fractures and the occurrence of more alteration rinds on older rocks show evidence for a slight change in redox conditions, from an oxidizing, water-dominated to a more reducing, rock-dominated environment with time. Iron oxyhydroxide and celadonite are the first alteration products, partly replaced or covered by saponite under more reducing conditions. The Fe necessary for the formation of these minerals is furnished by the dissolution of glass and the breakdown of olivine. Phillipsite is present in fractures and veins in rocks older than 1 Ma. Analyses also indicate an illite–smectite mixed layered mineral which is believed to be an intermediate between saponite and celadonite and small amounts of a chlorite/smectite mixed layered mineral. All samples are characterized by the lack of minerals formed by hydrothermal processes. We conclude that the alteration took place under seawater-dominated conditions at low temperature. A comparison of trace element analyses from altered whole rock samples and their appendant fresh glass chips provides a record of element mobility during alteration. Off-axis basalts show significant uptakes of Rb, Cs and Ba which are supplied by seawater and incorporated in or on secondary minerals. An enrichment of U is also apparent and appears to be especially strong when alteration conditions are oxidative. K2O is also gained in all altered off-axis basalts, believed to be linked to the formation of celadonite. However, the volume of rock being altered is so small relative to the volume of the oceans that this scavenging has no noticeable effect on the composition of the oceans. The composition of the oceanic crust, on the other hand, is affected significantly.