Geochemistry and minerals at DSDP Leg 65 Holes

Petrologic studies of mid-ocean ridge basalt (MORB) (e.g., Melson et al., 1975; Flower, et al., 1977; Byerly and Wright, 1978; Melson, 1979; Byerly and Sinton, 1980; Thompson, 1980) show that magmatic liquid-fraction trends are indicated by the compositions of fresh glass selvedges, usually, but not always, associated with pillow basalts. In contrast, whole-rock compositional variation will often reflect the complicating effects of syn- and post-eruptive phenocryst accumulation. Additional variation may be introduced by the reaction of basalts with seawater. While comparatively severe alteration of variable type was noted locally in the young basalts recovered across the mouth of the Gulf of California on Leg 65, most of the basalts were extremely fresh, making them ideal for studies of compositional variation.

Geochemistry and isotopic ratios of boninites and related rocks of the Izu-Bonin Forearc

Twenty-six samples representing the wide range of lithologies (low- and intermediate-Ca boninites and bronzite andesites, high-Ca boninites, basaltic andesites-rhyolites) drilled during Leg 125 at Sites 782 and 786 on the Izu-Bonin outer-arc high have been analyzed for Sr, Nd, and Pb isotopes. Nd-Sr isotope covariations show that most samples follow a trend parallel to a line from Pacific MORB mantle (PMM) to Pacific Volcanogenic sediment (PVS) but displaced slightly toward more radiogenic Sr. Pb isotope covariations show that all the Eocene-Oligocene samples plot along the Northern Hemisphere Reference Line, indicating little or no Pb derived from subducted pelagic sediment in their source. Two young basaltic andesite clasts within sediment do have a pelagic sediment signature but this may have been gained by alteration rather than subduction. In all isotopic projections, the samples form consistent groupings: the tholeiites from Site 782 and Hole 786A plot closest to PMM, the boninites and related rocks from Sites 786B plot closest to PVS, and the boninite lavas from Hole 786A and late boninitic dikes from Hole 786B occupy an intermediate position. Isotope-trace element covariations indicate that these isotopic variations can be explained by a three-component mixing model. One component (A) has the isotopic signature of PMM but is depleted in the more incompatible elements. It is interpreted as representing suboceanic mantle lithosphere. A second component (B) is relatively radiogenic (epsilon-Nd = ca 4-6; 206Pb/204Pb = ca 19.0-19.3; epsilon-Sr = ca -10 to -6)). Its trace element pattern has, among other characteristics, a high Zr/Sm ratio, which distinguishes it from the ìnormalî fluid components associated with subduction and hotspot activity. There are insufficient data at present to tie down its origin: probably it was either derived from subducted lithosphere or volcanogenic sediment fused in amphibolite facies; or it represents an asthenospheric melt component that has been fractionated by interaction with amphibole-bearing mantle. The third component (C) is characterized by high contents of Sr and high epsilon-Sr values and is interpreted as a subducted fluid component. The mixing line on a diagram of Zr/Sr against epsilon-Sr suggests that component C may have enriched the lithosphere (component A) before component B. These components may also be present on a regional basis but, if so, may not have had uniform compositions. Only the boninitic series from nearby Chichijima would require an additional, pelagic sediment component. In general, these results are consistent with models of subduction of ridges and young lithosphere during the change from a ridge-transform to subduction geometry at the initiation of subduction in the Western Pacific.

Chemical composition of ultrabasite and gabbro from the rift zones of the Arabian-Indian and Western Indian ridges

Results of petrographic studies of ultrabasite and gabbro from the rift zones of the Indian Ocean ridges are discussed using materials of R/V Vityaz Cruise 36. Rocks sampled from two sites 2700 km apart are close to each other in their composition. Petrographically ultrabasic rocks are divided into four subgroups: I - dunite; II - harzburgite, serpentinite; III - plagioclase lherzolite; and IV - metamorphically altered rocks. Petrographic description and chemical composition of basic rock varieties are presented as well as description of rock-forming minerals and their optical properties. Formation of pyroxene and plagioclase is shown to be related to autometasomatosis, which concludes the magmatic phase proper in rock mass formation accompanied by activity of residual intragranular liquid. Formation of ultrabasite in the rift zones is related to complicated processes.

Geochemistry of Ontong Java Plateau basalts

Basement rocks from the Ontong Java Plateau are tholeiitic basalts that appear to record very high degrees of partial melting, much like those found today in the vicinity of Iceland. They display a limited range of incompatible element and isotopic variation, but small differences are apparent between sampled sites and between upper and lower groups of flows at Ocean Drilling Program Site 807.40Ar-39Ar ages of lavas from Site 807 and Deep Sea Drilling Project Site 289 are indistinguishable about an early Aptian mean of 122 Ma (as are preliminary data for the island of Malaita at the southern edge of the plateau), indicating that plateau-building eruptions ended more or less simultaneously at widely separated locations. Pb-Nd-Sr isotopes for lavas from Sites 289, 803, and 807, as well as southern Malaita, reflect a hotspot-like source with epsilon-Nd(T) = +4.0 to +6.3, (87Sr/86Sr)T = 0.70423-0.70339, and 206Pb/204Pb = 18.245-18.709 and possessing consistently greater 208Pb/204Pb for a given 206Pb/204Pb than Pacific MORB. The combination of hotspot-like mantle source, very high degrees of melting, and lack of a discernible age progression is best explained if the bulk of the plateau was constructed rapidly above a surfacing plume head, possibly that of the Louisville hotspot. Basalt and feldspar separates indicate a substantially younger age of ~90 Ma for basement at Site 803; in addition, volcaniclastic layers of mid-Cenomanian through Coniacian age occur at DSDP Site 288, and beds of late Aptian-Albian age are found at Site 289. Therefore, at least some volcanism continued on the plateau for 30 m.y. or more. The basalts at Site 803 are chemically and isotopically very similar to those at the ~122 Ma sites, suggesting that hot plume-type mantle was present beneath the plateau for an extended period or at two different times. Surviving seamounts of the Louisville Ridge formed between 70 and 0 Ma have much higher 206Pb/204Pb than any of the plateau basalts. Thus, assuming the Louisville hotspot was the source of the plateau lavas, a change in the hotspot's isotopic composition may have occurred between roughly 70 and 90 Ma; such a change may have accompanied the plume-head to plume-tail transition. Similar shifts from early, lower 206Pb/204Pb to subsequently higher 206Pb/204Pb values are found in several other oceanic plateau-hotspot and continental flood basalt-hotspot systems, and could reflect either a reduction in the supply of low 206Pb/204Pb mantle or an inability of some off-ridge plume-tails to melt refractory low 206Pb/204Pb material.

Mineralogy and geochemistry of a basalt from ODP Hole 119-738C

The Lower Cretaceous tholeiitic basalt cored at Site 738, on the southernmost part of the Kerguelen Plateau, shows anomalous Sr, Nd, and Pb isotopic compositions compared to other lavas from Kerguelen Island and the Kerguelen Plateau. The strongly negative value of eNd (- 8.5) and high 207Pb/204Pb ratio (15.71) reflect a long-term evolution in a source high in Nd/Sm and µ. These geochemical properties, not observed in the Indian Ocean mantle plumes (St. Paul, Kerguelen Islands), have been reported for alkali lavas erupted in East Antarctica, thus suggesting that they originate from the Gondwana subcontinental lithosphere.

Basal dolomitic sediments ofthe Tyrrhenian Sea

Basal dolomitic sediments were recovered at three drill sites in the Tyrrhenian Sea during Ocean Drilling Program (ODP) Leg 107 (Sites 650, 651, and 655). These sediments overlie the basaltic basement complex and are enriched in iron, and in some instances, also in manganese. The manganese enrichments, together with a very slight enrichment in trace transition elements, strongly suggest that the basal sediments have an affinity to deep-sea metalliferous deposits of hydrothermal origin. At Sites 651 and 655, the dolostones contain variable amounts of authigenic palygorskite, a Mgrich clay mineral. At Site 651, the basal sediments are 40 m thick and contain nonstoichiometric dolomite, sometimes Ca rich, but primarily Mg rich. The occurrence of Mg-rich dolomite with excess Mg up to 4% is unusual for the deep-sea environment; it may be associated with a hydrothermally driven flux of altered sea water through the directly underlying basement complex, which comprises basalt, dolerite, and serpentinized peridotite. Low-temperature alteration of the basement complex could produce solutions enriched in Mg. Oxygen-isotope equilibrium temperatures indicate that all of the studied dolomites formed under low-temperature conditions (i.e., < 70?C). The carbon-isotope compositions, together with the strong isotopic covariance, suggest that the Mg-rich dolomite precipitated more rapidly than the Carich dolomite. We suggest that the low-temperature, hydrothermal convection of Mg-rich solutions through the basal sediments in this back-arc basin environment (1) overcame kinetic problems related to the formation of massive dolostones, and (2) provided a mass-transport mechanism for dolomitization.

Geochemistry of DSDP Leg 91 basalts

Basalts recovered from Sites 595 and 596 on Mesozoic crust in the southwest Pacific range from olivine-bearing tholeiites to ferrobasalts. Despite having undergone extensive low-grade alteration, which has raised K and Rb abundances, the basalts have consistent interelement ratios of Ti, Zr, Hf, rare-earth elements, Y, Th, Ik, and Nb. La/Ta (-18), Lan/Ybn (0.6), Ti/Zr (115), Zr/Nb (20), and Th/Hf (0.08) ratios all fall within the range of N-type mid-ocean-ridge basalt. The basalts from Sites 595 and 596 indicate that the Mesozoic Pacific crust was derived from a mantle source by processes similar to those operating at the present-day East Pacific Rise.

Geochemistry and ages of rock samples from Kirwanveggen, Antarctica

The basement of southern Kirwanveggen (western Dronning Maud Land) is formed by a SSW-dipping section consisting of (from SW to NE): migmatic gneisses; granitoid; low-grade/prograde meta-pelites, meta-psammites and meta-basalts (= "Polaris Formation"); ortho-gneiss; quartzite mylonite; Polaris Formation; quartzite mylonite; meta-turbidites. These units are (partly) separated by at least four SSW-dipping, NE to N directed major thrusts. Most probably, this thrust system is of Pan-African age. Towards north, the section is followed by the molasse-like Urfjell Group, deposited later than approx. 550 Ma and earlier than 450 Ma. Similarities with the Pan-African of the Shackleton Range (thrusting, molasse) led to the assumption, that the East/West Gondwana suture runs from the Shackleton Range towards Sor Rondane (eastern Dronning Maud Land) passing southern Kirwanveggen at its south-east.

Analysis of Manganese nodules from the Challenger Expedition

Manganese nodules from Stations 252 and 281 of the Challenger Expedition, collected in 1875, have recently been discovered in the Redpath Museum. The nodules have been found to be quite typical specimens of two areas in the Central Pacific Ocean except for dehydration and other changes that have taken place during storage. The principal resolvable manganese mineral in nodules from Station 252 proved to be 10 A manganite; there was a very thin surface coating of birnessite. Delta manganite was the only manganese mineral found in nodules from Station 281. Through electron microprobe studies, findings from chemical, optical and x-ray crystallographic work were correlated with the detailed picture of the occurrence and quantities of the different elements within the nodules. In all cases it was found that the iron and manganese had an antithetical relationship, and that nickel and copper were associated with the manganese. Special study was given to a 300-micron-square area in a nodule from Station 252 which included a segregation of 49.39% Mn, 5.31 % Ni, and 1.64% Cu. Crystallization of the manganese phases is thought to have provided a mech¨anism for formation of segregations which were further enriched through chemical scavenger action as long as ocean floor conditions permitted.

Composition of marine phosphorites and phosphate-bearing sediments

The book deals with behavior of phosphorus and its concentration in oceanic phosphorites. The major stages of marine geochemical cycle of phosphorus including its supply to sedimentary basins, precipitation from sea water, distribution and speciation in bottom sediments, diagenetic redistribution, and relation to other elements are under consideration. Formation of recent phosphorites as a culmination of phosphate accumulation in marine and oceanic sediments is examined. Distribution, structure, mineral and chemical compositions of major phosphorite deposits of various age on continental margins, as well as on submarine plateaus, uplifts and seamounts and some islands are described. A summary of trace element abundances in oceanic phosphorites is presented. Problems of phosphorite origin are discussed.

Grain size composition and geochemistry of volcaniclastic sediments in the Lau Basin

Early Pliocene to Pleistocene volcaniclastic sediments recovered during Ocean Drilling Program Leg 135 from Sites 834 to 839 in the Lau Basin show a wide range of chemical and mineralogical compositions extending the spectrum previously known from the Lau Basin, Lau Ridge and Tofua Arc. The following major types of volcaniclastics have been distinguished: (1) primary fallout ashes originating from eruptions on land, (2) epiclastic deposits that resulted from subaerial and submarine eruptions, (3) subaqueous fallout and pyroclastic flow deposits resulting from explosive submarine eruptions, and (4) hyaloclastites resulting from mechanical fragmentation and spalling of chilled margins of submarine pillow tubes and sheet-lava flows. Vitric shards are mostly basaltic andesitic to rhyolitic and broadly follow two major trends in terms of K2O enrichment: a low-K series (LKS) with about 1 wt% K2O at 70 wt% SiO2, and a very low-K series (VLKS) with only about 0.5 wt% K2O at 70 wt% SiO2. Sites 834 and 835 on "old" backarc basin crust, >4.2 and 3.4 m.y. old, comprise LKS rhyolites >3.3 m.y. old. Calc-alkaline basaltic turbidites originating from the Lau Ridge flowed in at 3.3 Ma. In the period from 3.3 to 2.4 Ma basaltic andesitic to rhyolitic, fine-grained LKS and VLKS volcaniclastics were deposited by turbidity currents and subaerial fallout. Three thin, discrete fallout layers (2.4-3.2 m.y. old) with high-K calc-alkaline compositions probably erupted in New Zealand. Volcaniclastics from Site 836, all <0.6 m.y. old, make up 24% of the sediments and comprise local basaltic andesitic to andesitic hyaloclastites with low Ba/Zr ratios of 0.9 to 1.4 and polymict andesitic sediments with Ba/Zr ratios of up to 5.5, containing clasts altered to lower greenschist facies. In Sites 837-839, drilled on young crust (1.8-2.1 m.y. old), volcaniclastics make up 45%-64% of the total sediment. Glass compositions are often bimodal with a mafic and a rhyolitic population. Large-volume rhyolitic, silt- to lapilli-sized volcaniclastics are interpreted as pyroclastic flows from explosive eruptions on a seamount 25-50 km away from the sites. Ba/Zr ratios are 2 to 4, partially overlapping with some Lau Basin basement lavas that show an "arc" signature, and they can reach values >5 in thin volcaniclastic layers <0.6 m.y. old.

Sedimentology and geochemistry of ODP Leg 129 siliceous deposits

Siliceous deposits drilled on Ocean Drilling Program Leg 129 accumulated within a few degrees of the equator during the Jurassic through early Tertiary, as constrained by paleomagnetic data. During the Jurassic and Early Cretaceous, radiolarian ooze, mixed with a minor amount of pelagic clay, was deposited near the equator, and overall accumulation rates were moderate to low. At a smaller scale, in more detail, periods of relatively higher accumulation rates alternated with periods of very low accumulation rates. Higher rates are represented by radiolarite and limestone; lower rates are represented by radiolarian claystone. Our limited data from Leg 129 suggests that accumulation of biogenic deposits was not symmetrical about the equator or consistent over time. In the Jurassic, sedimentation was siliceous; in the Cretaceous there was significant calcareous deposition; in the Tertiary claystone indicates significantly lower accumulation rates at least the northern part of the equatorial zone. Accumulation rates for Leg 129 deposits in the Cretaceous were higher in the southern part of the equatorial zone than in the northern part, and the southern side of this high productivity zone extended to approximately 15°S, while the northern side extended only to about 5°N. Accumulation rates are influenced by relative contributions from various sediment sources. Several elements and element ratios are useful for discriminating sedimentary sources for the equatorial depositional environments. Silica partitioning calculations indicate that silica is dominantly of biogenic origin, with a detrital component in the volcaniclastic turbidite units, and a small hydrothermal component in the basal sediments on spreading ridge basement of Jurassic age at Site 801. Iron in Leg 129 sediments is dominantly of detrital origin, highest in the volcaniclastic units, with a minor hydrothermal component in the basal sediments at Site 801. Manganese concentrations are highest in the units with the lowest accumulation rates. Fe/Mn ratios are >3 in all units, indicating negligible hydrothermal influence. Magnesium and aluminum concentrations are highest in the volcaniclastic units and in the basal sediments at Site 801. Phosphorous is very low in abundance and may be detrital, derived from fish parts. Boron is virtually absent, as is typical of deep-water deposits. Rare earth element concentrations are slightly higher in the volcaniclastic deposits, suggesting a detrital source, and lower in the rest of the lithologic units. Rare earth element abundances are also low relative to "average shale." Rare earth element patterns indicate all samples are light rare earth element enriched. Siliceous deposits in the volcaniclastic units have patterns which lack a cerium anomaly, suggesting some input of rare earth elements from a detrital source; most other units have a distinct negative Ce anomaly similar to seawater, suggesting a seawater source, through adsorption either onto biogenic tests or incorporation into authigenic minerals for Ce in these units. The Al/(Al + Fe + Mn) ratio indicates that there is some detrital component in all the units sampled. This ratio plotted against Fe/Ti shows that all samples plot near the detrital and basalt end-members, except for the basal samples from Site 801, which show a clear trend toward the hydrothermal end-member. The results of these plots and the association of high Fe with high Mg and Al indicate the detrital component is dominantly volcaniclastic, but the presence of potassium in some samples suggests some terrigenous material may also be present, most likely in the form of eolian clay. On Al-Fe-Mn ternary plots, samples from all three sites show a trend from biogenic ooze at the top of the section downhole to oceanic basalt. On Si-Fe-Mn ternary plots, the samples from all three sites fall on a trend between equatorial mid-ocean spreading ridges and north Pacific red clay. Copper-barium ratios show units that have low accumulation rates plot in the authigenic field, and radiolarite and limestone samples that have high accumulation rates fall in the biogenic field.

Chemical and mineral compositions of Northwest Pacific bottom sediments and their grain size fractions

A detailed study of chemical composition of bottom sediments along a profile through the Northwest Pacific Basin has allowed to identify and describe four lithofacies types of bottom sediments. Distinguished types of sediments form a genetic series reflecting changing conditions of sedimentation from near-shore to central regions of the ocean. Along the strike of pelagic clays a gradual transition from ash containing clays to zeolite containing clays is established. Ash particles and zeolites have similar forms of occurrence. Together with other data it suggests that zeolites have been formed by diagenetic transformation of rhyolitic glass. Regular changes of CaCO3, amorphous SiO2, Fe and Mn contents in bottom sediments from the coast to the pelagic zone are shown.

Composition and structural formulas of ODP Site 200-1224 Eocene ferrobasalt minerals

The ~46-m.y.-old igneous basement cored during Leg 200 in the North Pacific represents one of the few cross sections of Pacific oceanic crust with a total penetration into basalt of >100 m. The rocks, emplaced during the Eocene at a fast-spreading rate (~14 cm/yr; full rate) are strongly differentiated tholeiitic basalts (ferrobasalts) with 7-4.5 wt% MgO, relatively high TiO2 (2-3.5 wt%), and total iron as Fe2O3 (9.1-16.8 wt%). The differentiated character of these lavas is related to unusually large amounts of crystallization differentiation of plagioclase, clinopyroxene, and olivine. The lithostratigraphy of the basement (cored to ~170 meters below seafloor) is divided into three units. The deepest unit (lithologic Unit 3), is a succession of lava flows of no more that a few meters thickness each. The intermediate unit (lithologic Unit 2) is represented by intermixed thin flows and pillows, whereas the shallowest unit (lithologic Unit 1), comprises two massive flows. The rocks range from aphyric to sparsely clinopyroxene-plagioclase-phyric (phenocryst content = <3 vol%) and from holocrystalline to hypohyaline. Chilled margins of pillow fragments show holohyaline to sparsely vitrophyric textures. Site 1224 oxide minerals present a type of alteration not previously seen, where titanomagnetite is only partially destroyed and the pure magnetite component is partially removed from the mineral, leaving, in the most extreme case, a nearly pure ulvöspinel residuum. As a result of this dissolution, iron, mainly in the oxidized state, is added to the circulating solvent fluids. This means that a considerable metal source can result from low-temperature reactions throughout the upper ocean crust. The coarsest-grained lithologic Unit 1 rocks have interstitial myrmekitic intergrowths of quartz and sodic plagioclase (~An12), roughly similar in mineralogy and bulk composition to tonalite/trondhjemite veinlets in abyssal gabbros from the southwest Indian Ocean and Hess Deep, eastern equatorial Pacific. Based on idiomorphic relationships and projections into the simplified Q-Ab-Or-H2O granite ternary system, the myrmekitic intergrowths formed at the same time as, or just after, the oxide minerals coprecipitated and at low water vapor pressure (~0.5 kbar). Their compositions correspond to SiO2-oligoclase intergrowths that are considerably less potassic than dacitic glasses that erupt, although rarely, along the East Pacific Rise or that have been produced experimentally by partial melting of gabbro. Based on the crystallization history and comparison to experimental data, the original interstitial siliceous liquids resulted from late-stage immiscible separation of siliceous and iron-rich liquids. The rare andesitic lavas found along the East Pacific Rise may be hybrid rocks formed by mixing of these immiscible siliceous melts with basaltic magma.