Major oxides, trace elements and rare earth elements of selected basalt samples at DSDP Hole 83-504B

DSDP Hole 504B is the deepest section drilled into oceanic basement, penetrating through a 571.5-m lava pile and a 209-m transition zone of lavas and dikes into 295 m of a sheeted dike complex. To define the basement composition 194 samples of least altered basalts, representing all lithologic units, were analyzed for their major and 26 trace elements. As is evident from the alteration-sensitive indicators H2O+, CO2, S, K, Mn, Zn, Cu, and the iron oxidation ratio, all rocks recovered are chemically altered to some extent. Downhole variation in these parameters enables us to distinguish five depth-related alteration zones that closely correlate with changes in alteration mineralogy. Alteration in the uppermost basement portion is characterized by pronounced K-uptake, sulfur loss, and iron oxidation and clearly demonstrates low-temperature seawater interaction. A very spectacular type of alteration is confined to the depth range from 910 to 1059 m below seafloor (BSF). Rocks from this basement portion exhibit the lowest iron oxidation, the highest H2O+ contents, and a considerable enrichment in Mn, S, Zn, and Cu. At the top of this zone a stockwork-like sulfide mineralization occurs. The chemical data suggest that this basement portion was at one time within a hydrothermal upflow zone. The steep gradient in alteration chemistry above this zone and the ore precipitation are interpreted as the result of mixing of the upflowing hydrothermal fluids with lower-temperature solutions circulating in the lava pile. Despite the chemical alteration the primary composition and variation of the rocks can be reliably established. All data demonstrate that the pillow lavas and the dikes are remarkably uniform and display almost the same range of variation. A general characteristic of the rocks that classify as olivine tholeiites is their high MgO contents (up to 10.5 wt.%) and their low K abundances (-200 ppm). According to their mg-values, which range from 0.60 to 0.74, most basalts appear to have undergone some high-level crystal fractionation. Despite the overall similarity in composition, there are two major basalt groups that have significantly different abundances and ratios of incompatible elements at similar mg-values. The majority of the basalts from the pillow lava and dike sections are chemically closely related, and most probably represent differentiation products of a common parental magma. They are low in Na2O, TiO2, and P2O5, and very low in the more hygromagmaphile elements. Interdigitated with this basalt group is a very rarely occurring basalt that is higher in Na2O, TiO2, P2O5, much less depleted in hygromagmaphile elements, and similar to normal mid-ocean ridge basalt (MORB). The latter is restricted to Lithologic Units 5 and 36 of the pillow lava section and Lithologic Unit 83 of the dike section. The two basalt groups cannot be related by differentiation processes but have to be regarded as products of two different parental magmas. The compositional uniformity of the majority of the basalts suggests that the magma chamber beneath the Costa Rica Rift reached nearly steady-state conditions. However, the presence of lavas and dikes that crystallized from a different parental magma requires the existence of a separate conduit-magma chamber system for these melts. Occasionally mixing between the two magma types appears to have occurred. The chemical characteristics of the two magma types imply some heterogeneity in the mantle source underlying the Costa Rica Rift. The predominant magma type represents an extremely depleted source, whereas the rare magma type presumably originated from regions of less depleted mantle material (relict or affected by metasomatism).

Rare earth elements of modern seeps from the Gulf of Mexico, the Black Sea, and the Congo Fan

At marine seeps, methane is microbially oxidized resulting in the precipitation of carbonates close to the seafloor. Methane oxidation leads to sulfate depletion in sediment pore water, which induces a change in redox conditions. Rare earth element (REE) patterns of authigenic carbonate phases collected from modern seeps of the Gulf of Mexico, the Black Sea, and the Congo Fan were analyzed. Different carbonate minerals including aragonite and calcite with different crystal habits have been selected for analysis. Total REE content (SumREE) of seep carbonates varies widely, from 0.1 ppm to 42.5 ppm, but a common trend is that the SumREE in microcrystalline phases is higher than that of the associated later phases including micospar, sparite and blocky cement, suggesting that SumREE may be a function of diagenesis. The shale-normalized REE patterns of the seep carbonates often show different Ce anomalies even in samples from a specific site, suggesting that the formation conditions of seep carbonates are variable and complex. Overall, our results show that apart from anoxic, oxic conditions are at least temporarily common in seep environments.

Rare earth elements in bottom sediments from the deeps of the Red Sea rift zone

Analysis of rare earth element (REE) distribution and behavior in ore-bearing hydrothermal-sedimentary deposits from the Red Sea is carried out. Geochemical patterns and mechanisms of REE accumulation in metalliferous sediments of the open ocean and in deposits adjoined to areas of hydrothermal discharge are shown. Main factors, which determine composition of REE and the level of their accumulation in hydrothermal occurrences of the Red Sea, are considered.

Contents and accumulation rates of the rare earth and some other elements in metalliferous sediments from DSDP Hole 92-598

The sediments recovered during DSDP Leg 92 (Site 598) include a complete 16 m.y. record of hydrothermal sedimentation along the western flank of the East Pacific Rise at 19°S. Fifty samples from this sediment column were analyzed to test the hypothesis that the REE composition of the hydrothermal component is primarily acquired via scavenging from seawater. Site 598 provides an ideal sample suite for this purpose: the sediments are lithologically "simple," primarily consisting of a mixture of hydrothermal materials and biogenous carbonates; the composition of the hydrothermal component is essentially constant through space and time; and the sediments have undergone minimal diagenetic alteration. The following observations suggest the above-stated hypothesis is true. The Ce anomaly as well as key indices of light and heavy REE behavior all show that the REE pattern of hydrothermal sediments approaches that of seawater with increasing paleodistance from the rise crest. Moreover, shale-normalized REE patterns are similar to that of seawater, varying only in absolute REE content: the REE content increases with distance from the paleo-rise crest and exhibits a pronounced increase in sediments deposited below the paleolysocline. Based on significant correlative relationships between paleodistance from the rise crest and both the concentration and mass accumulation rates (MARs) of REEs and Fe, we conclude the REEs in the hydrothermal component are derived from the interaction of seawater and Fe in the hydrothermal plume.