Geochemistry of oceanic crust of the Cocos Plate

New geochemical data from the Cocos Plate constrain the composition of the input into the Central American subduction zone and demonstrate the extent of influence of the Galápagos Hotspot on the Cocos Plate. Samples include sediments and basalts from Ocean Drilling Program (ODP) Site 1256 outboard of Nicaragua, gabbroic sills from ODP Sites 1039 and 1040, tholeiitic glasses from the Fisher Ridge off northwest Costa Rica, and basalts from the Galápagos Hotspot Track outboard of Central Costa Rica. Site 1256 basalts range from normal to enriched MORB in incompatible elements and have Pb and Nd isotopic compositions within the East Pacific Rise MORB field. The sediments have similar 206Pb/204Pb and only slightly more radiogenic 207Pb/204Pb and 208Pb/204Pb isotope ratios than the basalts. Altered samples from the subducting Galápagos Hotspot Track have similar Nd and Pb isotopic compositions to fresh Galápagos samples but have significantly higher Sr isotopic composition, indicating that the subduction input will have a distinct geochemical signature from Galápagos-type mantle material that may be present in the wedge beneath Costa Rica. Gabbroic sills from Sites 1039 and 1040 in East Pacific Rise (EPR) crust show evidence for influence of the Galápagos Hotspot ?100 km beyond the morphological hotspot track.

Geochemistry of DSDP Leg 65 basalts

Leg 65 of the Deep Sea Drilling Project successfully recovered basalts from four sites in the mouth of the Gulf of California, thus completing a transect begun during Leg 64 from the continental margin of Baja California to the east side of the East Pacific Rise (EPR). Sixty-three whole-rock samples from Sites 482, 483, and 485 have been analyzed by X-ray fluorescence techniques, and a further eleven samples by instrumental neutron-activation techniques, in order to assess magma variability within and between sites. Although the major element compositions and absolute hygromagmatophile (HYG) element abundances are quite variable, all of the basalts are subalkaline tholeiites exhibiting strong more-HYG element (e.g., Rb, La, Nb, Ta) depletion (LaN/YbN ~ 0.4; Nb/Zr ~ 0.02; Ba/Zr ~ 0.23; Th/Hf ~ 0.05). These ratios, together with La/Ta ratios of 20 and Th/Ta ratios of 1.25, demonstrate that the Leg 65 basalts resemble the depleted "N-type" ocean ridge basalts recovered from the Mid-Atlantic Ridge (MAR) at 22 °N and other sections of the EPR. Zr/Ti, Zr/Y, and La/Yb ratios increase with increasing fractionation. It is clear that the basalts recovered from Sites 482, 483, and 485 were all derived from a compositionally similar source and that the compositional differences observed between lithological units can be explained by varying degrees of open system fractional crystallization (magma mixing) in subridge magma chambers. The basaltic rocks recovered from Site 474 near the margin of Baja California, and Sites 477, 478, and 481 in the Guaymas Basin, all drilled during Leg 64, have consistently higher Th/Hf, La/Sm, Zr/Ti, and Zr/Y ratios and higher absolute Sr contents than the Leg 65 basalts. While some of these variations may be explained by different conditions of partial melting, it is considered more likely that the mantle source underlying the Guaymas Basin is chemically distinct from that feeding the EPR at the mouth of the Gulf. These source variations probably reflect the complex tectonic setting of the Gulf of California, the magmas formed at the inception of spreading and in the central part of the Gulf containing a minor but significant component of sub-continental (calc-alkaline) material.

Metamorphic processes in sediments at DSDP Leg 65 Holes

Sites 482, 483, and 485 were drilled during Leg 65 on young oceanic crust south of the Tamayo Transform Fault. The Leg 65 drilling program was part of a multinational effort to study the East Pacific Rise (EPR) and complements sea bottom surveys conducted both in this area (Lewis, 1979; Cyamex Scientific Team and Pastouret, 1981) and farther south at 21 °N (Larson, 1971; Normark, 1976; Cyamex Scientific Team; Rise Project Group, 1980). These studies, which included deep-tow, Angus and submersible surveys, were recently complemented by sea-beam surveys conducted by the Jean Charcot on the Tamayo Fracture Zone and farther south along the EPR. They have led to a better understanding of the magmatic, tectonic, and sedimentary processes occurring on the East Pacific Rise between the Tamayo and Rivera fracture zones. The purpose of this chapter is to describe the metamorphic processes affecting Pliocene through Quaternary sediments found in contact, or inter layered, with basaltic units drilled during Leg 65 at the mouth of the Gulf of California.

Concentration of the rare-earth elements in metalliferous sediments from DSDP Leg 92 holes

DSDP Leg 92 drilled at four sites along an east-west transect at 19°S on the western flank of the East Pacific Rise (EPR), in an area where sediments are essentially a mixture of hydrothermal and biogenic components, with only a minimal contribution of clastic material. Rare-earth element (REE) data on the metalliferous (non-carbonate) fraction of samples ranging in age from ~2 to ~27 Ma indicate the existence of two distinct groups of patterns corresponding to two broad age groups, one <=8 Ma, the other >=10 Ma. Within each group, REE patterns have characteristics which are near-uniform, despite large variations in total REE abundances. Sediments of the younger group are enriched in light REE (LREE) relative to deep bottom waters influenced by the hydrothermal plume extending west from the EPR at 19°S. Sediments of the older groups show further relative LREE enrichment and/or heavy REE (HREE) depletion. Surficial sediments deposited beneath the lysocline have high Sum REE concentrations resulting from slow accumulation rates, and patterns resembling older sediments due to early diagenetic effects. A correlation between the mass accumulation rates (MAR) of Sum REE and Fe + Mn suggests that ferromanganese particulate matter supplied by the hydrothermal plume scavenges REE; during this process the LREE are preferentially removed from plume seawater. The MAR of Fe + Mn shows a general decrease with age above basement, whereas Sum REE concentrations in the metalliferous component increase with age above basement. This supports the Ruhlin and Owen model wherein limited scavenging of REE, due to rapid burial of sediment near the palaeo-axis, leads to low concentrations (but high MAR-values) for the REE. Following deposition and burial of the hydrothermal component, further relative flattening of the REE pattern takes place, probably the result of diagenetic reactions over several million years. Phase partitioning data indicate that the proportion of REE residing in more poorly crystalline phases tends to increase with age (from ~45% to 90% of Sum REE). This suggests that as initial ferromanganese precipitates undergo diagenetic recrystallization, REE are transferred to the poorly crystalline phases, and/or are scavenged from pore waters by these phases. Because of the various modifications to REE patterns apparently produced both in the water column and post-depositional settings, the REE patterns of metalliferous sediments will not reflect fine-scale REE variations in associated oceanic water masses.

(Table 4) Mean diameter of Orbulina universa d'Orbigny at DSDP Leg 54 Holes

On Deep Sea Drilling Project Leg 54, we recovered upper Pliocene (Globigerinoides obliquus: PL6 zone) to Pleistocene sediments from the equatorial East Pacific Rise (EPR) and Galapagos spreading center (GSC). Progressively older sediments were drilled at increasing distances from the crest, with the exception of the sediment drilled in the deepest trough known in the Siqueiros fracture zone. The anomalous age obtained at the latter site suggests that the basalt which was drilled may represent fracture zone volcanism. Paleoenvironmental analysis using the planktonic foraminifers at the EPR sites indicated the presence of environmental cycles of shorter wave length during the interval from 0 to 0.24 Ma, whereas cycles of longer wave length occurred from 0.43 to 2.17 Ma. The planktonic foraminiferal taphocoenoses at the EPR sites were strongly affected by selective dissolution which indicated that these EPR sites have been near either the lysocline or carbonate compensation surface since the upper Pliocene. The planktonic foraminiferal thanatocoenoses at the GSC sites were preserved better than those at the EPR sites. The number of planktonic foraminiferal species generally was greatly reduced in the green mud associated with the GSC hydrothermal mounds. More species were found in older than in younger green mud; this suggests that there probably was an increase in the rate of production of green mud sometime after the initiation of the hydrothermal system.

(Table 1) Downcore age and bulk geochemistry of sediment core GS7202-35

The distribution of redox-sensitive metals in sediments is potentially a proxy for past ocean ventilation and productivity, but deconvolving these two major controls has proved difficult to date. Here we present a 740 kyr long record of trace element concentrations from an archived sediment core collected at ~15°S on the western flank of the East Pacific Rise (EPR) on 1.1 Myr old crust and underlying the largest known hydrothermal plume in the world ocean. The downcore trace element distribution is controlled by a variable diagenetic overprint of the inferred primary hydrothermal plume input. Two main diagenetic processes are operating at this site: redox cycling of transition metals and ferrihydrite to goethite transition during aging. The depth of oxidation in these sediments is controlled by fluctuations in the relative balance of bottom water oxygen and electron donor input (organic matter and hydrothermal sulfides). These fluctuations induce apparent variations in the accumulation of redox-sensitive species with time. Subsurface U and P peaks in glacial age sediments, in this and other published data sets along the southern EPR, indicate that basin-wide changes in deep ocean ventilation, in particular at glacial-interglacial terminations II, III, IV, and V, alter the depth of the oxidation front in the sediments. These basin-wide changes in the deep Pacific have significant implications for carbon partitioning in the ocean-atmosphere system, and the distribution of redox-sensitive metals in ridge crest sediment can be used to reconstruct past ocean conditions at abyssal depths in the absence of alternative proxy records.

The Southeast Indian Ridge between 88°E and 118°E: Gravity anomalies and crustal accretion at intermediate spreading rates

Although slow spreading ridges characterized by a deep axial valley and fast spreading ridges characterized by an axial bathymetric high have been extensively studied, the transition between these two modes of axial morphology is not well understood. We conducted a geophysical-survey of the intermediate spreading rate Southeast Indian Ridge between 88 degrees E and 118 degrees E, a 2300-km-long section of the ridge located between the Amsterdam hot spot and the Australian-Antarctic Discordance where satellite gravity data suggest that the Southeast Indian Ridge (SEIR) undergoes a change from an axial high in the west to an axial valley in the east. A basic change in axial morphology is found near 103 degrees 30'E in the shipboard data; the axis to the west is marked by an axial high, while a valley is found to the east. Although a well-developed axial high, characteristic of the East Pacific Rise (EPR), is occasionally present, the more common observation is a rifted high that is lower and pervasively faulted, sometimes with significant (> 50 m throw) faults within a kilometer of the axis. A shallow axial valley (< 700 m deep) is observed from 104 degrees E to 114 degrees E with a sudden change to a deep (>1200 m deep) valley across a transform at 114 degrees E. The changes in axial morphology along the SEIR are accompanied by a 500 m increase in near-axis ridge flank depth from 2800 m near 88 degrees E to 3300 m near 114 degrees E and by a 50 mGal increase in the regional level of mantle Bouguer gravity anomalies over the same distance, The regional changes in depth and mantle Bouguer anomaly (MBA) gravity can be both explained by a 1.7-2.4 km change in crustal thickness or by a mantle temperature change of 50 degrees C-90 degrees C. In reality, melt supply (crustal thickness) and mantle temperature are linked, so that changes in both may occur simultaneously and these estimates serve as upper bounds. The along-axis MBA gradient is not uniform. Pronounced steps in the regional level of the MBA gravity occur at 103 degrees 30'E-104 degrees E and at 114 degrees E-116 degrees E and correspond to the changes in the nature of the axial morphology and in the amplitude of abyssal hill morphology suggesting that the different forms of morphology do not grade into each other but rather represent distinctly different forms of axial (s)tructure and tectonics with a sharp transition between them. The change from an axial high to an axial valley requires a threshold effect in which the strength of the lithosphere changes quickly. The presence or absence of a quasi-steady state magma chamber may provide such a mechanism. The different forms of axial morphology are also associated with different intrasegment MBA gravity patterns. Segments with an axial high have an MBA low located at a depth minimum near the center of the segment, At EPR-like segments, the MBA low is about 10 mGal with along-axis gradients of 0.15-0.25 mGal/km, similar to those observed at the EPR, Rifted highs have a shallower low and lower gradients suggesting an attenuated composite magma chamber and a reduced and perhaps episodic melt supply. Segments with a shallow axial valley have very flat along-axis MBA profiles with little correspondence between axial depth and axial MBA gravity.

(Table 1) Chemical composition of Fe-rich layered silicates from the East Korean Rise, Japan Sea

Mineral and chemical compositions of highly ferruginous layered silicates (HLS) of glauconite sands occurred on the East Korean Rise outside volcanic structures and on an unnamed volcano and the Chentsov Volcano have been studied. The use of cluster and discriminant analyses has resulted to more objectively distinguished groups among HLS, and the use of factor analysis - to illustrate correlations between chemical elements in different groups. It has been found that green mineral assemblages of the East Korean Rise are heterogeneous in terms of morphology, composition and origin, and their formation is a complex multistage process including both neoformation and degradation.

Chemical composition and fluxes of deep sea suspended matter within and over the 9°50'N hydrothermal field, EPR

Results of geochemical studies of suspended matter from the water mass over the hydrothermal field at 9°50'N on the East Pacific Rise are reported. The suspended matter was sampled in background waters, in the buoyant plume, and in the near-bottom waters. Contents of Si, Al, P, Corg, Fe, Mn, Cu, Zn, Ni, Co, As, Cr, Cd, Pb, Ag, and Hg were determined. No definite correlations were found between the elements in the background waters. Many of the chemical elements correlated with Fe and associated with its oxyhydroxides in the buoyant plume. In the near-bottom waters trace elements are associated with Fe, Zn, and Cu (probably, with their sulfides formed during mixing of hydrothermal fluids with seawater). Chemical composition of sediment matter precipitated in a sediment trap was similar to the near-bottom suspended matter.