Geochemistry of igneous rock samples of ODP Site 125-786

A morphologically complex igneous basement was penetrated at Leg 125 Site 786 beneath approximately 100 m of Eocene-Pleistocene sediments at 31°52.45 'N, 141°13.59'E in a 3082-m water depth. The site is located on the forearc basement high (FBH) of the Izu-Bonin (Ogasawara) Arc. In the broadest terms, the sequence in Hole 786B consists of a basal sheeted dike complex, heavily mineralized in places, with overlying pillow lavas giving way to a complex and repeated sequence of interlayered volcanic breccias and lava flows with some thin sedimentary intervals. The sequence has been further cut by dikes or sills, particularly of high-Ca and intermediate-Ca boninite, and is locally strongly sheared by faulting. The whole basement has been covered with middle Eocene-early Pleistocene sediments. A monomict breccia forms the shallowest portion of Hole 786B and a polymict breccia having Mn-oxide-rich clast coatings and matrix forms the deepest part of Hole 786A (-100-160 mbsf). The basement is tectonized in some places, and a mineralized stockwork is present in the deepest part of Hole 786B. A wide variety of rock types form this basement, ranging from mafic to silicic in character and including high-, intermediate-, and low-Ca boninites, intermediate- and low-Ca bronzite andesites, andesite, dacite, and rhyolite groups. Intragroup and intergroup relationships are complicated in detail, and several different upper mantle source(s) probably were involved. A significant role for orthopyroxene-clinopyroxene-plagioclase fractionation is indicated in the mafic-intermediate groups, and the most probable complementary cumulates should be noritic gabbros. Many overall similarities but some subtle differences are noted between the igneous basement at Site 786 and the subaerial outcrops of the FBH to the south in the type boninite locality of Chichijima. Both suites were derived by hydrous melting of a relatively shallow, refractory (harzburgitic) upper mantle source. These Bonin forearc basement rocks are similar in many respects to those of Eocene-Oligocene age now forming the forearc of the Marianas at Leg 60 Site 458 and on Guam. In sharp distinction, the geochemistry of the Eocene-Pleistocene ash sequences overlying the Bonin FBH must have been derived from a very different upper mantle source, implying considerable across-strike differences in sub-arc mantle composition.

REE and istotopic composition of ODP Site 127-797 clays

X-ray diffraction analyses have been carried out on 128 samples of Miocene to Quaternary sediments from ODP Sites 794, 795 and 797. Some clay fractions of samples from Site 797 have also been studied for rare earth elements and by Nd isotopic analyses. These three sites display similar lithological and clay assemblages (with dominant chlorite, illite and smectite) showing that the sedimentation was homogeneous throughout the whole Japan Sea Basin. Three mineralogical zones are recognized. The first zone (Lower Miocene sandy clay of Sites 794 and 797) is mainly composed of chlorite resulting from hydrothermal transformation of arc-derived smectite, due to sill injections during the initial oceanic spreading stage. The second zone (Lower Miocene to Lower Pliocene siliceous claystone and diatomaceous silty clay) is dominated by arc-derived smectite; the abundance of this mineral decreases upwards while illite and chlorite increase. This trend reflects a change of detrital source, from an eastern arc-derived source (epsilon -Nd**t>-3.3); variable LREE enrichment) to a western continental crust source (epsilon-Nd**t<-9.4; shale-like REE patterns); climatic modifications in the current dynamics are proposed as a cause for this change. The third zone (Upper Pliocene to Recent silty clay with minor diatom oozes) is characterized at Site 797 by increasing amounts of illite and chlorite. This reflects a more and more important western supply which is assumed to be related to tectonic rejuvenations of the Asian margin or climatic modifications affecting the alteration conditions or the current dynamics. At Sites 794 and 795, the more or less sharp supply of chlorite seems to be driven by the incipient subduction zone on the eastern margin of the Japan Sea.

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.

Investigations on volcanic rocks in sediment core CRP-3 from the Ross Sea, Antarctica

The petrography, mineralogy and geochemistry of volcanic and subvolcanic rocks in CRP-3 core have been examined in detail in order to characterise and to compare them with volcanic and subvolcanic rocks cropping out in the Victoria Land area, and to define the clast provenance or to establish possible volcanic activity coeval with deposition. Clasts with sizes ranging from granule to boulder show geochemical and mineralogical features comparable with those of Ferrar Supergroup rocks. They display a subalkaline affinity and compositions ranging from basalts to dacite. Three different petrographic groups with distinct textural and grain size features (subophitic, intergranular-intersertal, and glassy-hyalopilitic) are recognised and are related to the emplacement/cooling mechanism. In the sand to silt fraction, the few glass shards that have been recognised are strongly altered: however chemical analyses show they have subalkalic magmatic affinity. Mineral compositions of the abundant free clinopyroxene grains found in the core, are less affected by alteration processes, and indicate an origin from subalkaline magmas. This excludes the presence, during the deposition of CRP-3 rocks of alkaline volcanic activity comparable with the McMurdo Volcanic Group. Strong alteration of the magmatic body intruded the Beacon sandstones obliterates the original mineral assemblage. Geochemical investigations confirm that intrusion is part of the Ferar Large Igneous Province.

Chemical composition of bulk sediments from ODP Hole 199-1215A

Elemental concentrations were determined on 21 samples from Hole 1215A to evaluate the influence of hydrothermal sources on bulk sediment composition. Rare earth element (REE) concentrations were also determined on 10 of these samples. Concentration profiles and REE patterns are consistent with a strong hydrothermal influence on sediment composition at the base of Hole 1215A.

Petrographic description and geochemistry of basalts and altered volcanic rocks from ODP Leg 183 sites

The basalts recovered during Legs 183 and 120 from the southern, central, and northernmost parts of the Kerguelen Plateau (Holes 1136A, 1138A, 1140A, and 747C, respectively), as well as those recovered from the eastern part of the crest of Elan Bank (Hole 1137A), represent derivates from tholeiitic melts. In the northern part of the Kerguelen Plateau (Hole 1140A), basalts may have formed from two sources located at different depths. This is reflected in the presence of both low- and high-titanium basalts. The basalts are variably altered by low-temperature hydrothermal processes (at temperatures up to 120°C), and some are affected by subaerial weathering. The hydrothermal alteration led mainly to the formation of smectites, chlorite minerals, mixed-layer hydromica-smectite and smectite-chlorite minerals, hydromica, serpentine(?), clinoptilolite, heulandite, stilbite, analcime, mordenite, thomsonite, natrolite(?), calcite, quartz, and dickite(?). Alteration of extrusive basalts is mainly related to horizontal fluid flow within permeable contact zones between lava flows. Under a nonoxidizing environment of alteration, the tendency to lose most of elements, including rare earth elements, from basalts dominates. Under on oxidizing environment, basalts accumulate many elements.

Igneous geochemistry of OP Leg 126 samples

Major element, trace element, and radiogenic isotope compositions of samples collected from Ocean Drilling Program Leg 126 in the Izu-Bonin forearc basin are presented. Lavas from the center of the basin (Site 793) are high-MgO, low-Ti, two-pyroxene basaltic andesites, and represent the products of synrift volcanism in the forearc region. These synrift lavas share many of the geochemical and petrographic characteristics of boninites. In terms of their element abundances, ratios, and isotope systematics they are intermediate between low-Ti arc tholeiites from the active arc and boninites of the outer-arc high. These features suggest a systematic geochemical gradation between volcanics related to trench distance and a variably depleted source. A basement high drilled on the western flank of the basin (Site 792) comprises a series of plagioclase-rich two-pyroxene andesites with calc-alkaline affinities. These lavas are similar to calc-alkaline volcanics from Japan, but have lower contents of Ti, Zr, and low-field-strength elements (LFSE). Lavas from Site 793 show inter-element variations between Zr, Ti, Sr, Ni, and Cr that are consistent with those predicted during crystallization and melting processes. In comparison, concentrations of P, Y, LFSE, and the rare-earth elements (REE) are anomalous. These elements have been redistributed within the lava pile, concentrating particularly in sections of massive and pillowed flows. Relative movement of these two-element groupings can be related to the alteration of interstitial basaltic andesite glass to a clay mineral assemblage by a post-eruptive process. Fluid-rock interaction has produced similar effects in the basement lavas of Site 792. In this sequence, andesites and dacites have undergone a volume change related to silica mobility. As a result of this process, some lithologies have the major element characteristics of basaltic andesite and rhyolite, but can be related to andesitic or dacitic precursors by silica removal or addition.

Major, trace and rare earth elements and minerals at DSDP Legs 69-70 Holes

We obtained major and trace element data on 113 samples from basalts drilled during DSDP Legs 69 and 70 in the Costa Rica Rift area. The majority have major and trace element characteristics typical of ocean-ridge tholeiities. Most of the basalts are relatively MgO rich (MgO > 8 wt.%) and have Mg values (MgO/MgO + 0.85FeO x 100) of about 53, characteristics that clearly indicate that the various magmas underwent only a small amount of crystal fractionation before being erupted onto the seafloor. According to their normative mineralogies, the rocks are olivine tholeiites. A few samples plot close to the diopside-hypersthene join of the projected basalt tetrahedron. Except for basalts from two thin intervals in Hole 504B, which differ significantly from all the other basalts of the hole, practically no chemical downhole variation could be established. In the two exceptional intervals, both TiO2 and P2O5 contents are markedly enriched among the major oxides. The trace elements in these intervals are distinguished by relatively high contents of magmatophile elements and have flat to enriched chondrite-normalized distribution patterns of light rare earth elements (LREE). Most of the rocks outside these intervals are strongly depleted in large-ionlithophile (LIL) elements and LREE. We offer no satisfactory hypothesis for the origin of these basalts at this time. They might have originated within pockets of mantle materials that were more primitive than the LIL-element-depleted magmas that were the source of the other basalts. A significant change with depth in the type of alteration occurs in the 561 meters of basalt cored in Hole 504B. According to the behavior of such alteration-sensitive species as K2O, H2O-, CO2, S, Tl, and the iron oxidation ratio, the alteration is oxidative in the upper part and nonoxidative or even reducing in the lower part. The oxidative alteration may have resulted from low temperature basalt/seawater interaction, whereas hydrothermal solutions may be responsible for the nonoxidative alteration.

Geochemistry of dikes and lavas from the north wall of the Hess Deep Rift

An investigation of ~1-m.y.-old dikes and lavas from the north wall of the Hess Deep Rift (2°15'N, 101°30'W) collected during Alvin expeditions provides a detailed view of the evolution of fast spreading oceanic crust. The study area encompasses 25 km of an east-west flow line, representing ~370,000 years of crustal accretion at the East Pacific Rise. Samples analyzed exhibit depleted incompatible trace element abundances and ratios [(La/Sm)N < 1]. Indices of fractionation (MgO), and incompatible element ratios (La/Sm, Nb/Ti) show no systematic trends along flow line. Rather, over short (<4 m) and long (~25 km) distances, significant variations are observed in major and trace element concentrations and ratios. Modeling of these variations attests to the juxtaposition of dikes of distinct parental magma compositions. These findings, combined with studies of segmentation of the subaxial magma chamber and lateral magma transport in dikes along rift-dominated systems, suggest a more realistic model of the magmatic system underlying the East Pacific Rise relative to the commonly assumed twodimensional model. In this model, melts from a heterogeneous mantle feed distinct portions of a segmented axial magma reservoir. Dikes emanating from these distinct reservoirs transport magma along axis, resulting in interleaved dikes and host lavas with different evolutionary histories. This model suggests the use of axial or flow line lava compositions to infer the evolution of axial magma chambers should be approached with caution because dikes may never erupt lava or may transport magma significant distances along axis and erupt lavas far from their axial magma chamber of origin.

Pb isotopes in Cenozoic sediments of ODP Site 177-1090

Nd and Pb isotopic compositions extracted from bulk deep sea sediments have been shown to be robust proxies for deep water circulation as well as weathering provenance and intensity over geologically young time scales. In this study we evaluated ten deep sea samples from Ocean Drilling Program (ODP) site 1090 ranging in age from mid Eocene to early-Miocene to test whether Pb isotopic compositions extracted from geologically older sediments record reliable seawater isotopic ratios and to evaluate the source of the extracted Pb. The sequential extraction protocol used in this study is similar to protocols reported for previous studies and produces acetic acid, hydroxylamine hydrochloride (HH) and residue fractions. Each extracted fraction was analyzed for Pb isotopes, rare earth elements (REEs), and a suite of major elements. Similar 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios are recorded from the acetic acid and HH fractions for ~70-80% of the samples, suggesting that either the acetic acid dissolves Fe-Mn oxides or multiple phases are recording the same seawater isotopic value. Several indirect tests, such as Al mass balance, comparison of Sr isotopes in HH extracts to contemporaneous seawater Sr isotopes, and comparison of Nd isotopic compositions in HH extracts to published fossil fish teeth values, provide evidence that Pb isotopic compositions measured in our bulk HH extracts record bottom water values. The relationship between Pb, Mn and Ca concentrations in HH fractions indicates that Fe-Mn oxides and a Mn-bearing carbonate are the dominant phases contributing seawater Pb. Comparison of REE patterns derived from the HH fraction and total digestions of Fe-Mn nodule standards reveals that the trivalent REEs exhibit patterns consistent with the parent archive, but Ce can be fractionated during extraction. Ratios of REEs also produce unique fields for each fraction and can be used to test the purity of the seawater signal of the extraction protocol. Finally, an initial evaluation of Pb isotopic compositions in fossil fish indicates that this archive is not suitable for bottom water Pb isotope studies.

Contents of rare earth and other chemical elements in Fe-Mn micronodules, nodules, and bottom sediments from the Clarion-Clipperton ore province in the Pacific Ocean

Processes governing the formation of rare earth element (REE) composition are under consideration for ferromanganese deposits (nodules, separate parts of nodules, and micronodules of different size fractions) within the Clarion-Clipperton ore province in the Pacific Ocean. It is shown that ferromanganese oxyhydroxide deposits with different chemical compositions can be produced in sediments under similar sedimentation conditions. In areas with high bioproductivity size of micronodules has positive correlation with Mn content and Mn/Fe and P/Fe ratios and negative correlation with Fe, P, REE, and Ce anomaly. Behavior of REE in micronodules from sediments within bioproductive zones is related to increase of influence of diagenetic processes in sediments as a response to the growth of size of micronodules. Distinctions in chemical composition of micronodules and nodules are related to their interaction with associated sediments. Micronodules grow in sediments using hydrogenous ferromanganese oxyhydroxides. As they grow, micronodules are enriched in labile fraction of sediments reworked during diagenesis. Sources of material of ferromanganese nodules are governed by their formation at the water bottom interface. Their upper part is formed by direct settling of iron oxyhydroxides from bottom water, whereas the lower part is accumulated due to diagenetic processes in sediments. Differences of REE compositions in ferromanganese deposits are caused by the reduction of manganese during diagenesis and its separation from iron. Iron oxyhydroxides form a sorption complex due to sorption of phosphate-ion from bottom and pore waters. Sorption of phosphate-ion results in additional sorption of REE.

Composition of ferromanganese crusts from the Atlantic, Pacific and Indian Oceans

A collection of layered ferromanganese ores (27 samples) from the Atlantic and Pacific oceans was studied. Trace element and PGE contents were determined layer-by-layer (up to 10 microlayers) in 13 of these samples. The trace, rare earth, and platinum group element distributions, including their layer-to-layer variations, were compared in hydrogenic and hydrothermal crusts from different regions. It was found that the main PGE variations (by a factor of 10-50) are related to their layer-to-layer variations within a given ore field. The distributions of PGE and trace elements are strongly heterogeneous, which is related, first, to different contents of the elements in the layers of different age in ferromanganese crusts (FMC) and, second, to the observed regional heterogeneity and influence of hydrothermal fluids. Geochemical data indicate that CFC formation was mainly caused by the hydrochemical precipitation of material from seawater. This process was accompanied by diagenetic phenomena, water-rock interaction, and influence of volcanic and hydrothermal sources.

Rocks and minerals from the Strakhov Fracture Zone, 4°N Mid-Atlantic Ridge

Geological-geophysical data obtained during Cruises 7, 11, and 12 of R/V Akademic Nikolay Strakhov (1989-1991) within the international project EQUARIDGE in the Strakhov Fracture Zone region (4°N) are presented. The trough of the fracture is interpreted as an open extension joint, a graben produced by stretching along the axis of the Mid-Atlantic Ridge. Bedrock studies showed that typical mid-ocean tholeiitic basalts occur within the narrow (60 nm wide) axial rift zone, whereas igneous rocks not typical for the ocean were found on the eastern and western flank plateaus. This allows to suppose that a reworked relict continental-type basement of pre-Upper Jurassic age possibly exists beneath the flank plateaus, within the segment under discussion. The above data correspond to the hypothesis of E. Bonatti about nonspreading nature of the basement of Mid-Atlantic Ridge within the equatorial segment and the Strakhov Fracture Zone.