Chemical and mineral compositions of recent and ancient sediments and sedimentary rocks from the Mariana Trench

On the bed and on the ocean slope of the southern latitudinal part of the Mariana Trench ancient sediments, as well as sedimentary and igneous rocks are exposed. In the lower part of the sampled part of the studied section Late Oligocene to Early Miocene chalk-like limestones and marls occur. Upward marly tuffites and tuffs (apparently alternating with carbonate rocks) occur. These rocks are overlain by Early Miocene tuffaceous clays and siliceous-clayey muds. In the upper part of the section there are Pleistocene pelagic clays and ethmodiscus oozes.

Chemistry and mineralogy of ODP Leg 124 sediment samples

From X-ray mineralogical studies and chemical analyses of the whole rocks and the fine fractions (<2 µm) of ten to fifteen samples at each site of ODP Leg 124, two major sources were identified in the sedimentary components of the Celebes and Sulu basins: (1) a terrestrial and continental contribution; (2) a volcanic influx that gives way to well-defined volcanic units or to a dilute contamination, consisting of coarse-grained minerals (Plagioclase, pyroxene, olivine, spinel) or a smectitic-rich fraction produced by the alteration of volcanic glasses and ashes. The continental signature increases the amount of quartz in the rocks and the phyllitic association is complex: micas, kaolinite, disordered interstratified clay-minerals. The chemical compositions of the bulk rocks and the fractions <2 µm are more potassic and aluminum-rich. The volcanic imprint depends on the grain-size and chemical properties of the components. Ca/Na contents highly variable compared to the K content of the bulk composition are due to the presence of coarse-grained volcanic Plagioclase. The fractions <2 µm are more magnesian than in the continental regime. The diagenesis is revealed by the crystallization of zeolites, the fixation of magnesium into the smectites that depletes the pore fluids in this element. Smectitization of the disordered interstratified clay minerals enriches the alkalinity of the pore fluids. Some deep formations of the Sulu Basin are affected by a thermal event, but no thermal event was recognized in the Celebes Basin.

Major- and trace-element chemistry of DSDP Leg 70 Holes basalts and their alteration products

Major and trace element compositions of basalts from the lower part of Hole 504B indicate their cogenetic nature. The cored sequence of interlayered pillow lavas and massive lava flows was produced by eruption of lavas, slightly variable in composition. Plagioclase and olivine crystallization in a shallow magma chamber, followed by small-scale fractionation at higher levels, is responsible for these variations. Except in highly fractured zones within the basement, there are systematic variations in the style and degree of rock alteration with depth. Trace element characteristics of altered rocks and secondary minerals indicate that progressive changes in sea water composition occurred as it reacted with basaltic crust.

Platinum-group, major, and trace element abundances in ODP Leg 183 basalts

Seventeen basalts from Ocean Drilling Program (ODP) Leg 183 to the Kerguelen Plateau (KP) were analyzed for the platinum-group elements (PGEs: Ir, Ru, Rh, Pt, and Pd), and 15 were analyzed for trace elements. Relative concentrations of the PGEs ranged from ~0.1 (Ir, Ru) to ~5 (Pt) times primitive mantle. These relatively high PGE abundances and fractionated patterns are not accounted for by the presence of sulfide minerals; there are only trace sulfides present in thin-section. Sulfur saturation models applied to the KP basalts suggest that the parental magmas may have never reached sulfide saturation, despite large degrees of partial melting (~30%) and fractional crystallization (~45%). First order approximations of the fractionation required to produce the KP basalts from an ~30% partial melt of a spinel peridotite were determined using the PELE program. The model was adapted to better fit the physical and chemical observations from the KP basalts, and requires an initial crystal fractionation stage of at least 30% olivine plus Cr-spinel (49:1), followed by magma replenishment and fractional crystallization (RFC) that included clinopyroxene, plagioclase, and titanomagnetite (15:9:1). The low Pd values ([Pd/Pt]_pm < 1.7) for these samples are not predicted by currently available Kd values. These Pd values are lowest in samples with relatively higher degrees of alteration as indicated by petrographic observations. Positive anomalies are a function of the behavior of the PGEs; they can be reproduced by Cr-spinel, and titanomagnetite crystallization, followed by titanomagnetite resorption during the final stages of crystallization. Our modeling shows that it is difficult to reproduce the PGE abundances by either depleted upper or even primitive mantle sources. Crustal contamination, while indicated at certain sites by the isotopic compositions of the basalts, appears to have had a minimal affect on the PGEs. The PGE abundances measured in the Kerguelen Plateau basalts are best modeled by melting a primitive mantle source to which was added up to 1% of outer core material, followed by fractional crystallization of the melt produced. This reproduces both the abundances and patterns of the PGEs in the Kerguelen Plateau basalts. An alternative model for outer core PGE abundances requires only 0.3% of outer core material to be mixed into the primitive mantle source. While our results are clearly model dependent, they indicate that an outer core component may be present in the Kerguelen plume source.

Geochemistry and petrology of basalts from Celebes Sea

Major-, trace-, and rare-earth element analyses are presented from a suite of basaltic rocks from the basement of the Celebes Sea. The major elements and trace-elements were determined by X-ray fluorescence techniques, and the rare-earth elements were analyzed by instrumental neutron activation analysis. Compositionally the Celebes Sea basalts are very similar to typical normal mid-ocean ridge basalts, such as those described from the Indian Ocean triple junction. Petrogenetic modeling shows that all of the basalts analyzed can be formed by 10% to 20% partial melting of a light rare-earth element-depleted spinel lherzolite followed by fractional crystallization of mixtures of olivine, Plagioclase, and iron oxide. The Celebes Sea is interpreted as a fragment of the basement of the Jurassic Argo abyssal plain trapped during the Eocene to the north of Australia.

Geochemistry of vein rocks of ODP Hole 118-735B

Rock samples from Hole 735B, Southwest Indian Ridge, were examined to determine the principal vein-related types of alteration that occurred, the nature of fluids that were present, and the temperatures and pressures of these fluids. Samples studied included veined metagabbro, veined mylonitic metagabbro, felsic trondhjemite, and late-stage leucocratic diopside-bearing veins. The methods used were standard petrographic analysis, mineral chemical analysis by electron microprobe, fluid inclusion petrography and analysis by heating/freezing techniques and laser Raman microspectroscopy, and oxygen isotopic analyses of mineral separates. Alteration in lithologic Units I and II (above the level of Core 118-735B-3OR; approximately 140 meters below the seafloor) is dominated by hydration by seawater-derived fluids at high temperature, up to about 700°C, and low water/rock ratio, during and immediately after pervasive ductile deformation. Below Core 118-735B-30R, pervasive deformation is less common, and brittle veining and brecciation are the major alteration styles. Leucocratic centimeter-scale veins, often containing diopside and plagioclase, were produced by interaction of hot (about 500°C) seawater-derived fluid and gabbro. The water/rock ratio was locally high at the veins and breccia zones, but the integrated water/rock ratio for the lower part of the hole is probably low. Accessory hydrous magmatic or deuteric phases formed from magmatic volatiles in some gabbro and in trondhjemite. Most subsequent alteration was affected by fluids that were seawater-derived, based on isotopic and chemical analyses of minerals and analyses of fluid inclusions. Many early-generation fluid inclusions, associated with high-temperature veining, contain appreciable methane as well as saline water. The source of methane is unclear, but it may have formed as seawater was reduced during low water/rock interaction with ultramafic upper mantle or ultramafic and mafic layer 3. Temperatures of alteration were calculated on the basis of coexisting mineral chemistry and isotopic values. Hydrothermal metamorphism commenced at about 720°C and continued to about 550°C. Leucocratic veining took place at about 500°C. Alteration within brecciated horizons was also at about 500° to less than 400°C, and the trondhjemite was altered at about 550° to below 490°C. Pressures calculated from a diopside-bearing vein, based on a combination of fluid inclusion and isotopic analysis, were 90 to 100 MPa. This pressure places the sample, from Core 118-735B-70R in Unit V, at about 2 km below the seafloor.

Geochemistry of peridotites from ODP Leg 125 diapiric serpentinites

Refractory spinel peridotites were drilled during Leg 125 from two diapiric serpentinite seamounts: Conical Seamount in the Mariana forearc (Sites 778-780) and Torishima Forearc Seamount (Sites 783-784) in the Izu-Ogasawara forearc. Harzburgite is the predominant rock type in the recovered samples, with subordinate dunite; no lherzolite was found. The harzburgite is diopside-free to sparsely diopside-bearing, with modal percentages of diopside that range from 0% to 2%. Spinels in the harzburgites are chrome-rich (Cr/[Cr + Al] = 0.38-0.83; Fe3+/[Fe3+ + Cr + Al] = 0.01-0.07). Olivine and orthopyroxene are magnesian (Mg# = 0.92). Discrete diopsides reveal extreme depletion of light rare earth elements. Primary hornblende is rare. The bulk major-element chemistry shows low average values of TiO2 (trace), Al2O3 (0.55%) and CaO (0.60%), but high Mg# (0.90). These rocks are more depleted than the abyssal peridotites from the mid-oceanic ridge. They are interpreted as residues of extensive partial melting (= 30%), of which the last episode was in the mantle wedge, probably associated with the generation of incipient island-arc magma, including boninite and/or arc-tholeiite. These depleted peridotites probably represent the residues of melting within mantle diapirs that developed within the mantle wedge.

Mineralogical and geochemical analyses of serpentines from ODP Hole 125-778A

Thirty-five samples from Hole 778A were prepared for X-ray diffraction (XRD) mineralogical analyses and for chemical analyses of major and trace elements. Most of the selected samples were silt- and sand-sized sedimentary serpentinites or microbreccias except for a soft clast of mafic rock, a hard clast of massive serpentinized peridotite, and a pebble of consolidated, undeformed serpentine microbreccia that contained planktonic foraminifers. Both mineralogical and geochemical analyses allow discrimination of three groups among the analyzed samples. These groups correspond to three stratigraphic intervals present along the drilled section. Group A contains the upper samples (lithologic Unit I). These consist of poorly consolidated serpentine muds carrying hard-rock clasts (serpentinized peridotites, metabasalts). They are characterized by the following mineralogical assemblage: serpentine, Fe-oxides and hydroxides, aragonite, and halite. They exhibit variable SiO2, MgO contents, but are characterized by a SiO2/MgO ratio near 1. CaO content is high in relation to development of aragonite. Al2O3 content is low. Relatively high K2O, Na2O, and Sr contents are present, presumably in relation to interactions with seawater. Group B (30-77 mbsf) contains samples exhibiting very homogeneous chemical and mineralogical compositions. They consist of serpentinite microbreccias exhibiting frequent shear structures. Hard-rock clasts are also present (serpentinized peridotites, metabasalts, one possible chert fragment). The mineralogy of the Group B samples is characterized by the presence of serpentine and authigenic minerals: hydroxycarbonates and hydrogrossular. Calcite and chlorite are also present, but all the samples lack aragonite. Their chemical compositions are remarkably similar to compositions of their parent rocks. Group C contains silt- and sand-sized serpentine and serpentine microbreccias, which are locally rich in red clasts, probably strongly altered (oxidized?) mafic fragments. Intervals having clasts of more diverse origin than those higher in the section were recovered. Clast lithology includes serpentinized peridotites, metabasalts, metavolcaniclastite, meta-olivine gabbro, and amphibolite sandstone. Mineralogy and geochemistry reflect these compositions. Serpentine content of the samples is less than in previous groups. Correlatively, sepiolite, palygorskite, and chlorite-smectite are mineral phases present in the analyzed samples. Accessory igneous minerals (amphiboles, pyroxenes, hematite) also were found. The chemical compositions of most of Group C samples differ from that of massive serpentinized peridotites. The main differences are (1) higher SiO2, CaO, TiO2 and Al2O3 contents, (2) a SiO2/MgO ratio greater than 1, and (3) a negative correlation between Al2O3, and MgO, Cr, and Ni. These characteristics suggest new constraints relative to the flow structure of the flank of Conical Seamount.

Mineral composition of mid-ocean-ridge basalts from ODP Leg 187 sites

The chemical compositions of olivine, plagioclase, pyroxene, and spinel in lavas collected during Ocean Drilling Program Leg 187 in the Australian Antarctic Discordance, Southeast Indian Ridge (41°-46°S, 126°-135°E) were analyzed, and modeling of the theoretical equilibrium petrogenetic conditions between olivine and melt was conducted. The cores of larger olivine phenocrysts, particularly in the isotopic Indian-type mid-ocean-ridge basalt (MORB), are not equilibrated with melt compositions and are considered to be xenocrystic. Larger plagioclase phenocrysts with compositionally reversed zonation are also xenocrystic. The compositions of primary magma were calculated using a "maximum olivine fractionation" model for primitive MORB that should fractionate only olivine. Olivine compositions equilibrated with calculated primary magma and compositions of calculated primary magma suggest that (1) isotopic Pacific-type MORB is more fractionated than Indian-type MORB, (2) Pacific-type MORB was produced by higher degrees of partial melting than Indian-type MORB, and (3) primary magma for Indian-type MORB was segregated from mantle at 10 kbar (~30 km depth), whereas that for Pacific-type MORB was segregated at 15 kbar (~45 km depth).

Conglomerate and sandstone petrography at DSDP Hole 58-445

Conglomerates and sandstones in lithologic unit V at DSDP Site 445 comprise lithic clasts, detrital minerals, bioclasts, and authigenic minerals. The lithic clasts are dominantly plagioclase-phyric basalt and microdolerite, followed by plagioclase-clinopyroxene-phyric basalt, aphyric basalt, chert, and limestone. A small amount of hornblende schist occurs. Detrital minerals are dominantly plagioclase, augite, titaniferous augite, olivine, green to pale-brown hornblende, and dark-brown hornblende, with subordinate chromian spinel, epidote, ilmenite, and magnetite, and minor amounts of diopside, enstatite, actinolite, and aegirine-augite. Bioclasts are Nummulites boninensis, Asterocyclina sp. cf. A. penuria, and some other larger foraminifers. Correlation of cored and dredged samples indicates that the Daito Ridge is mainly composed of igneous, metamorphic, ultramafic, and sedimentary rocks. The igneous rocks are mafic (probably tholeiitic) and alkalic. The metamorphic rocks are hornblende schist, tremolite schist, and diopside-chlorite schist. The ultramafic rocks are alpinetype peridotites. Mineralogical data suggest that there were two metamorphic events in the Daito Ridge. The older one was intermediate- to high-pressure metamorphism. The younger one was contact metamorphism caused by a Paleocene volcanic event, possibly related to the beginning of spreading of the west Philippine Basin. The ultramafic rocks suffered from the same contact metamorphism. During the Eocene, exposed volcanic and metamorphic rocks on the uplifted Daito Ridge may have supplied pebble clasts to the surrounding coast and shallow sea bottom. The steep slope offshore may have caused frequent slumping and transportation of the pebble clasts and shallow-water benthic organisms into deeper water, forming the conglomerates and sandstones treated here.

Composition and age of bottom sediments from the Southern Ocean

Geomorphology, geology, stratigraphy, lithology and geochemistry of bottom sediments in the South Ocean are under consideration. Regularities of distribution of iron-manganese nodules, features of occurrence of ore components in the nodules, nodule abundance in bottom sediments have been studied.

Chemistry of serpentinized peridotites from ODP Hole 109-670A

In this paper we describe textural relationships in hydrated upper mantle peridotites emplaced at a nonconstructive ridge segment. Development of serpentinites and partially serpentinized peridotites takes place in four main stages: (1) pervasive serpentinization forming mainly lizardite, (2) a tensional stage forming chrysotile + talc + chlorite, (3) a deformational stage forming antigorite + tremolite, and (4) a late local tensional stage forming another generation of chrysotile veinlets. Mineral chemistry of serpentine pseudomorphs reflects in part primary mineral compositions. Olivine pseudomorphs are typically nickeliferous and depleted in aluminum and chromium. Orthopyroxene pseudomorphs have lower nickel contents and relatively high iron, aluminum, and chromium contents. Clinopyroxene pseudomorphs have very low nickel contents and relatively high aluminum and chromium contents. These chemical patterns in the serpentinites can be used to help discriminate between harzburgitic and lherzolitic protoliths. Oxygen isotopes and mineral parageneses suggest serpentine is derived from circulation of hydrothermal (200?C) fluids through the peridotite body. Crystallization of tremolite, talc, and chlorite may have occurred at temperatures up to 525?C if C02/H20 ratios were less than 0.25. Open fissures developing in aging upper mantle provide paths for important seawater circulation through a thin basaltic carapace down to shallow mantle rocks.

Mineralogy of ODP Site 105-645

Cores from the upper 70 meters below seafloor (mbsf) (upper Pleistocene) at Ocean Drilling Program (ODP) Site 645 in Baffin Bay show dramatic meter-scale changes in color and mineralogy. Below this interval, mineralogical changes are more gradual to the top of the Miocene at about 550 mbsf. The Pliocene-Pleistocene section can be divided into five facies: Facies 1 - massive, poorly sorted, gravel-bearing muds; Facies 2 - gray silty clays and silty muds; Facies 3 - laminated detricarbonate silty muds; Facies 4 - silty sand and sandy silt; and Facies 5 - poorly sorted muddy sands and silty muds. Facies 4 and 5 are restricted to the Pliocene section below depths of about 275 mbsf. The mineralogical/color cycles in the upper 70 mbsf are the result of alternations between Facies 2 and three lithotypes of Facies 1: lithotype A - tan-colored, carbonate-rich, gravel-bearing mud; lithotype B - weak, red-colored, gravel-bearing mud rich in sedimentary rock fragments; and lithotype C - gray, gravel-bearing mud. A fourth lithotype, D, is restricted to depths of 168-275 mbsf and is dark gray, carbonate-poor, gravel-bearing mud. We believe that all lithotypes of Facies 1 and the sand and gravel fractions of Facies 2 and 3 were deposited by ice rafting. Depositional processes for Facies 4 and 5 probably include ice rafting and bottom- and turbidity-current transport. Data from petrographic analyses of light and heavy sand-sized grains and X-ray analyses of silt- and clay-size fractions suggest that tan-colored sediments (lithotype A of Facies 1; Facies 3) were derived mainly from Paleozoic carbonates of Ellesmere, Devon, and northern Baffin islands. Weak red sediments (lithotype B) contain significant red sedimentary clasts, reworked quartzarenite grains and clasts, and rounded colorless garnets, all derived from Proterozoic sequences of the Borden and Thule basins, and from minor Mesozoic red beds. Other sediments in the upper 335 mbsf at Site 645 contain detritus from a heterogeneous mixture of sources, including Precambrian shield terranes around Baffin Bay. Sediments from 335 to 550 mbsf (Facies 5) are rich in friable sedimentary clasts and detrital micas and contain glauconite and, in a few samples, reworked diatoms. These components suggest derivation from poorly consolidated Mesozoic-Tertiary sediments in coastal outcrops and beneath the modern shelves of northeastern Baffin Island and western Greenland. For the upper Pleistocene section (about 0-100 mbsf), marked mineralogical cyclicity is attributed to fluctuating glacial margins, calving rates, and iceberg melting rates, particularly around the northern end of Baffin Bay. Tan-colored, carbonate-rich units were derived at times of maximum advance of glaciers on Ellesmere and Devon islands, during relatively warm intervals induced by incursion of warm Atlantic surface water into the bay. At the beginning of these warmer episodes, most icebergs were contributed by glaciers near sea level around the Arctic channels, which resulted in deposition of weak red, ice-rafted units rich in Proterozoic sedimentary clasts.

Geochemical compositions of minerals from chilled dike margins of ODP Hole 140-504B

Mineral and whole-rock geochemical data are presented for chilled dike margins from the lower sheeted dike complex of Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) Hole 504B. Compositions of phenocrystic plagioclase (An80-89); olivine (Fo82-86); clinopyroxene (Wo52En40Fs8, with Cr2O3 up to 1.2%); and rare chromian spinel (Cr# 43) are consistent with those from the lavas and the upper dike complex recovered previously (DSDP Legs 69, 70, 83, and ODP Leg 111). Major and trace element compositions fall in group D of Autio and Rhodes (1983) and have high CaO/Na2O, and low TiO2, K2O, and (La/Sm)N values consistent with previous analyses from this site.

Mineralogy and textures of gabbros from ODP Hole 118-735B

Abundant iron-titanium (Fe-Ti) oxide gabbro, olivine gabbro, and troctolite were drilled at Hole 735B adjacent to the Atlantis II Fracture Zone of the Southwest Indian Ridge during Leg 118. The Fe-Ti oxide gabbro occurs as intrusive bodies into olivine gabbro with very sharp intrusive contacts. The size of the intrusive bodies varies from a millimeter to a few tens of meters. Mineralogical parameters, such as anorthite content of plagioclase and Mg/(Mg+Fe) ratios of mafic minerals exhibit bimodal distributions corresponding to olivine and Fe-Ti oxide gabbros, respectively. When the two major gabbro types are looked at separately, several downhole mineralogical cycles are recognized. The Fe-Ti oxide gabbros exhibit two such cycles with plagioclase becoming more sodic and mafic minerals becoming more iron-rich downward in the drill core. The olivine gabbros and troctolites, however, exhibit two cycles showing an upward increase in sodium in plagioclase and iron in mafic minerals. The mineralogical variations of these gabbros and the intrusive contact relationships probably resulted from downward intrusion of evolved magma into underlying solid or almost solidified olivine gabbros and troctolite. The dense evolved melt at the top of the cumulus pile probably formed from the crystallization of olivine gabbro cumulates followed by extreme fractional crystallization of residual melt in an isolated, ephemeral magma chamber. The interlayered occurrence of evolved and primitive gabbros from Hole 735B represents a typical section of lower ocean crust formed at a very slow spreading ridge.