Chemistry and boron isotope ratios of Izu arc front volcanic rocks

The process of fluid release from the subducting slab beneath the Izu arc volcanic front (Izu VF) was examined by measuring B concentrations and B isotope ratios in the Neogene fallout tephra (ODP Site 782A). Both were measured by secondary ion mass spectrometry, in a subset of matrix glasses and glassy plagioclase-hosted melt inclusions selected from material previously analyzed for major and trace elements (glasses) and radiogenic isotopes (Sr, Nd, Pb; bulk tephra). These tephra glasses have high B abundances (~10-60 ppm) and heavy delta11B values (+4.5? to +12.0?), extending the previously reported range for Izu VF rocks (delta11B, +7.0? to +7.3?). The glasses show striking negative correlations of delta11B with large ion lithophile element (LILE)/Nb ratios. These correlations cannot be explained by mixing two separate slab fluids, originating from the subducting sediment and the subducting basaltic crust, respectively (model A). Two alternative models (models B and C) are proposed. Model B proposes that the inverse correlations are inherited from altered oceanic crust (AOC), which shows a systematic decrease of B and LILE with increasing depth (from basaltic layer 2A to layer 3), paralleled by an increase in delta11B (from ~ +1? to +10? to +24?). In this model, the contribution of sedimentary B is insignificant (<4% of B in the Izu VF rocks). Model C explains the correlation as a mixture of a low-delta11B (~ +1?) 'composite' slab fluid (a mixture of metasediment- and metabasalt-derived fluids) with a metasomatized mantle wedge containing elevated B (~1-2 ppm) and heavy delta11B (~ +14?). The mantle wedge was likely metasomatized by 11B-rich fluids beneath the outer forearc, and subsequently down dragged to arc front depths by the descending slab. Pb-B isotope systematics indicate that, at arc front depths, ~ 53% of the B in the Izu VF is derived from the wedge. This implies that the heavy delta11B values of Izu VF rocks are largely a result of fluid fractionation, and do not reflect variations in slab source provenance (i.e. subducting sediment vs. basaltic crust). Since the B content of the peridotite at the outer forearc (7-58 ppm B, mean 24 +/- 16 ppm) is much higher than beneath the arc front (~1-2 ppm B), the hydrated mantle wedge must have released a B-rich fluid on its downward path. This 'wedge flux' can explain (1) the across-arc decrease in B and delta11B (e.g. Izu, Kuriles), without requiring a progressive decrease in fluid flux from the subducting slab, and (2) the thermal structure of volcanic arcs, as reflected in the B and delta11B variations of volcanic arc rocks.

Lithologic, mineral, chemical and isotopic compositions of altered rocks with increasing distance across the eastern flank of the Juan de Fuca Ridge, ODP Leg 168 data

During ODP Leg 168, 10 sites were drilled across the eastern flank of the Juan de Fuca Ridge (JdFR), to examine the conditions of fluid-rock interaction in three distinct hydrothermal regimes (referred to as the Hydrothermal Transition (HT), Buried Basement (BB) and Rough Basement (RB) transects), extending over a ~120 km linear transect perpendicular to the spreading ridge. This was carried out in an attempt to constrain the conditions and processes that control the location, style and magnitude of low temperature (<150°C) fluid-rock interaction within this setting. This paper presents new data on the petrology, mineral chemistry and whole rock strontium and oxygen isotopic compositions of basalts from the eastern flank of the JdFR, in order to investigate the extent, style and sequence of low-temperature hydrothermal alteration and to establish how the hydrothermal regime evolved with time. Throughout the flank, a progressive sequence of low-temperature hydrothermal alteration has been identified, marked by changes in the dominant secondary mineral assemblage, changing from: chlorite+chlorite/smectite; to iron oyxhydroxide+celadonite; to saponite+/-pyrite; culminating at present with Ca- to CaMg(+/-Fe,Mn)-carbonate. The changes in secondary mineralogy have been used to infer a series of systematic shifts in the conditions of alteration that occurred as the basement moved off-axis and was progressively buried by sediment. In general, hydrothermal alteration of the uppermost oceanic crust commenced under open, oxidative conditions, with interaction between unmodified to slightly modified seawater and basaltic crust, to a regime in which circulation of a strongly modified seawater-derived fluid was more restricted, and alteration occurred under non-oxidative conditions. Across the flank, petrological observations and microprobe analyses indicate that the observed ranges in secondary mineral composition are directly related to changes in the geochemical and textural characteristics of the basement, as well as to interaction between fluids and phases from the four stages of alteration. This is suggestive of an increase in fluid-rock increased with time. Whole rock 87Sr/86Sr and d18O analyses of basalts from across the eastern flank of the JdFR reinforce petrological observations, with 87Sr/86Sr and d18O values slightly elevated above accepted pristine MORB values for this region. These results are consistent with an increase in the amount of fluid-rock interaction with time. Across the flank, enrichment in the 87Sr/86Sr and d18O relative to MORB, is influenced by a number of factors, including: local and regional variations in the crustal lithology and structure; the age of the crust; the extent of bulk rock alteration; and theoretically, the relative abundance of different isotopically-enriched secondary mineral phases in the crust.

Chemical composition of ODP Leg 138 sediments

A major oceanographic event preserved in the Cocos plate sedimentary column survived subduction and is recorded in the changing composition of Nicaraguan magmas. A uranium increase in these magmas since the latest Miocene (after 7 Ma) resulted from the 'carbonate crash' at 10 Ma and the ensuing high organic carbon burial in the sediments. The response of the arc to this paleoceanographic event requires near steady-state sediment recycling at this margin since 20 Ma. This relative stability in sediment subduction invites one of the first attempts to balance sedimentary input and arc output across a subduction zone. Calculations based on Th indicate that as much as 75% of the sedimentary column was subducted beneath the arc. The Nicaraguan margin is one of the few places to observe such strong links between the oceans and the solid earth.

Age and chemical composition of dredged igneous rocks from the Vityaz Ridge, Pacific slope of the Kuril Island Arc

Original results of igneous rock studies are presented. The rocks were dredged during a marine expedition (cruise 37 of R/V Akademik M.A. Lavrent'ev in August-September, 2005) in the region of the submarine Vityaz Ridge and the Kuril Arc outer slope. Several age complexes (Late Cretaceous, Eocene, Late Oligocene, Miocene, and Pliocene-Pleistocene) are recognizable on the Vityaz Ridge. These complexes are characterized by a number of common geochemical features since all of them represent formations of island arc calc-alkali series. At the same time, they also have individual features reflecting different geodynamic settings. The outer slope of the Kuril Arc demonstrates submarine volcanism. Pliocene-Pleistocene volcanic rocks dredged here are similar to volcanites of the Kuril-Kamchatka Arc frontal zone.

Geochemistry at DSDP Sites 69-505, 69-504 and 70-504

Deep basement penetration during Legs 69 and 70 at Hole 504B in the Panama Basin allowed the recovery of a 561.5-meter sequence of basaltic pillows, thin flows, and breccias interspersed with thick massive flows. The lavas, which are aphyric to moderately plagioclase-olivine-clinopyroxene phyric, are petrologically indistinguishable from typical mid-ocean-ridge basalts (MORB). Some units are distinctive in that they carry accessory chrome-spinel microphenocrysts or emerald green clinopyroxene phenocrysts. Major and trace element analyses were carried out on 67 samples using X-ray fluorescence techniques. The basalts resemble normal MORB in terms of major elements. However, the trace element analyses show that most of the basalts are characterized by very strong depletion in the more incompatible elements compared with, for instance, normal (N type) MORB from the Atlantic at 22°N. Interdigitated with these units are one or two units that have distinctly higher incompatible element concentrations similar to those in basalts of the transitional (T) type from the Reykjanes Ridge (63°N in the Mid-Atlantic Ridge). All the basalts appear to have undergone some high-level crystal fractionation, although this has not proceeded to the extent of yielding ferrobasalts as it has at the adjacent Galapagos Spreading Center or along the East Pacific Rise. The magnetic anomalies are of lower amplitude than in the latter two regions, which suggests that the absence of ferrobasalts may be a general feature of the ocean crust generated at the Costa Rica Rift. The presence of two distinct magma types, one strongly depleted and the other moderately enriched in incompatible elements, suggests that magma chambers at the spreading center are discontinuous rather than continuous and that there is some chemical heterogeneity in the underlying mantle source. Observed variations in incompatible element ratios of basalts from the more depleted group could, however, reflect mixing between these two magma types. In general it would appear that the mantle feeding the Costa Rica Rift is significantly more depleted in incompatible trace elements than that feeding the Mid-Atlantic Ridge.

Lithology, mineralogy and geochemistry of ODP Hole 118-735B and 176-735B respectively

Ocean Drilling Program (ODP) Hole 735B, located on Atlantis Bank on the Southwest Indian Ridge, penetrated 1508 meters below seafloor with an average recovery of 87%, providing a nearly continuous sample of a significant part of oceanic Layer 3. Based on variations in texture and mineralogy, 12 major lithologic units are recognized in the section, ranging from 39.5 to 354 m thick. The principal lithologies include troctolite, troctolitic gabbro, olivine gabbro and microgabbro, gabbro, gabbronorite and Fe-Ti oxide gabbro, gabbronorite, and microgabbro. Highly deformed mylonites, cataclasites, and amphibole gneisses are locally present, as are small quantities of pyroxenite, anorthositic gabbro, and trondhjemite. Downhole variations in mineral composition, particularly for olivine and clinopyroxene, show a number of cyclic variations. Plagioclase compositions show the widest variations and correspond to different degrees of deformation and alteration as well as primary processes. Downhole chemical variations correspond reasonably well with variations in mineral compositions. Iron and titanium mainly reflect the presence of Fe-Ti oxide gabbros but show some cyclical variations in the lower part of the core where oxide gabbros are sparse. CaO is highly variable but shows a small but consistent increase downhole. MgO is more uniform than CaO and shows a very small downward increase. Sulfur and CO2 contents are generally low, but S shows significant enrichment in lithologic Unit IV, which consists of Fe-Ti oxide gabbro, reflecting the presence of sulfide minerals in the sequence. The lithologic, mineralogical, and geochemical data provided here will allow detailed comparisons with ophiolite sections as well as sections of in situ ocean crust drilled in the future.

Iron isotope and chemical compositions of sediments and basalts drilled seaward of the Mariana Trench

The purpose of this work is to study the mobility and budget of Fe isotopes in the oceanic crust and in particular during low-temperature interaction of seawater with oceanic basalt. We carried out this investigation using samples from Ocean Drilling Program (ODP) Site 801C drilled during Leg 129 and Leg 185 in Jurassic Pacific oceanic crust seaward of the Mariana Trench. The site comprises approximately 450 m of sediment overlying a section of 500 m of basalt, which includes intercalated pelagic and chemical sediments in the upper basaltic units and two low-temperature (10-30°C) ocherous Si-Fe hydrothermal deposits. Fe was chemically separated from 70 selected samples, and 57Fe/54Fe ratios were measured by MC-ICP-MS Isoprobe. The isotopic ratios were measured relative to an internal standard solution and are reported relative to the international Fe-standard IRMM-14. Based on duplicate measurements of natural samples, an external precision of 0.2? (2 sigma) has been obtained. The results indicate that the deep-sea sediment section has a restricted range of d57Fe, which is close to the igneous rock value. In contrast, large variations are observed in the basaltic section with positive d57Fe values (up to 2.05?) for highly altered basalts and negative values (down to ?2.49?) for the associated alteration products and hydrothermal deposits. Secondary Fe-minerals, such as Fe-oxyhydroxides or Fe-bearing clays (celadonite and saponite), have highly variable d57Fe values that have been interpreted as resulting from the partial oxidation of Fe(2+) leached during basalt alteration and precipitated as Fe(3+)-rich minerals. In contrast, altered basalts at Site 801C, which are depleted in Fe (up to 80%), display an increase in d57Fe values relative to fresh values, which suggest a preferential leaching of light iron during alteration. The apparent fractionation factor between dissolved Fe(2+) and Fe remaining in the mineral is from 0.5? to 1.3? and may be consistent with a kinetic isotope fractionation where light Fe is stripped from the minerals. Alternatively, the formation of secondary clays minerals, such as celadonite during basalt alteration may incorporate preferentially the heavy Fe isotopes, resulting in the loss of light Fe isotopes in the fluids. Because microbial processes within the oceanic crust are of potential importance in controlling rates of chemical reactions, Fe redox state and Fe-isotope fractionation, we evaluated the possible effect of this deep biosphere on Fe-isotope signatures. The Fe-isotope systematics presented in this study suggest that, even though iron behavior during seafloor weathering may be mediated by microbes, such as iron-oxidizers, d57Fe variations of more than 4? may also be explained by abiotic processes. Further laboratory experiments are now required to distinguish between various processes of Fe-isotope fractionation during seafloor weathering.

Chemical composition and K-Ar age of rocks from the northern Sea of Japan

The Middle Paleozoic complex consists of terrigenous and volcanogenic materials metamorphized in greenschist facies. Clastic rocks have arkosic composition and are formed by alteration of basalts and metamorphic rocks. Metaeffusives were formed from basaltoid products of oceanic tholeiite magma indicating that underwater rise structures of the northern Sea of Japan were emplaced on the oceanic crust.

Chemical composition of celadonite-glauconite minerals and cristobalite from altered oceanic basalts of DSDP Hole 49-407

Features of associations of newly formed minerals in Oligocene basalts from the Reykjanes Ridge (DSDP Hole 407), which consist in wide development of celadonite-glauconite group minerals, as well as saponite, cristobalite and specific Fe-Si formations is shown. Detailed mineralogical and geochemical characteristics of these newly formed minerals are reported. It is firstly shown that at underwater alteration of basalts not only celadonite can be generated, but also glauconite. Dependence of secondary mineral composition from petrochemical features of original basalts and from their permeability is revealed. It is concluded that complexes of secondary minerals formed during alteration of basalts with similar petrochemical characteristics in the ocean and on the continent are different.

Geochemistry of ODP Hole 128-798B

The capability of determining elemental concentrations through geochemical logging has recently been established. However, the quality of these data obtained in some environments has yet to be quantified. We assess the quality of geochemical logs compared with XRF results from a suite of core samples from Hole 798B. The resulting core/log correlations are only fair, because the tool has been adversely affected by the very high porosity of the formation. The results, however, do fall within the statistical uncertainties predicted by the processing. The recent application of a modified boron sleeve to the Ocean Drilling Program's geochemical logging tool is shown to reduce interference of borehole chlorine on the resultant chemistry.

Petrology, geochemistry and K-Ar Age determination of De Gerlache Seamount basalts

The De Gerlache Seamounts are two topographic highs in the Bellingshausen Sea, southeastern Pacific. Petrological and geochemical studies together with K-Ar age determinations were carried out on four dredged basalt samples collected during a RV Polarstern expedition (ANT-XII/4) in 1995. Minor and trace element composition suggest alkaline basalt compositions. Compared to alkaline basalts of adjacent West Antarctica (the Jones Mountains) and of Peter I Island, the samples have lower mg-numbers, lower Ni and Cr contents and lower high field-strength elements (HFSE)/Nb and large-ion lithophile elements (LILE)/HFSE ratios. Three of the four samples have low K, Rb, and Cs concentrations relative to alkaline basalts. The K-depletion and other elemental concentrations may be explained by 1.1% melting of amphibole bearing mantle material. Additionally, low Rb and Ba values suggest low concentrations of these elements in the mantle source. K-Ar age determinations yield Miocene ages (20-23 Ma) that are similar in age to other alkaline basalts of West Antarctica (Thurston Island, the Jones Mountains, Antarctic Peninsula) and the suggested timing of onset of Peter I Island volcanism (~10-20 Ma). The occurrence of the DGS and Peter I Island volcanism along an older but reactivated tectonic lineation suggests that the extrusions exploited a zone of pre-existing lithospheric weakness. The alkaline nature and age of the DGS basalts support the assumption of plume activity in the Bellingshausen Sea.

Chemistry and fluid inclusions of the sheeted dike comples of ODP Holes 137-504B and 140-504B

Fluid inclusions in variably altered diabase recovered from Ocean Drilling Program Legs 137 and 140 at Hole 504B, Costa Rica Rift, exhibit fluid salinities up to 3.7 times that of seawater values (11.7 wt% NaCl equivalent) and exhibit uncorrected homogenization temperatures of 125°C to 202°C. The liquid-dominated inclusions commonly are entrapped in zones of secondary plagioclase and may be primary in origin. Fluid salinities are similar to compositions of fluids venting on the seafloor (0.4-7.0 wt% NaCl) and overlap with those measured in metabasalt samples recovered from near the Kane Fracture Zone on the Mid-Atlantic Ridge and from the Troodos ophiolite, Cyprus. The salinity variations may reflect hydration reactions involving formation of secondary mineral assemblages under rock-dominated conditions, which modify the ionic strength of hydrothermal fluids by consuming or liberating water and chloride ion. Rare CO2-CH4-bearing inclusions, subjacent to zones where talc after olivine becomes an important secondary mineral phase (1700 mbsf), may have formed due to local interaction of seawater and olivine at low water to rock ratios. Corrected average fluid inclusion homogenization temperatures exhibit a gradient from 159°C at a depth of 1370 mbsf to 183°C at a depth of 1992 mbsf and are in apparent equilibrium with the present conductive downhole temperatures. These data indicate that fluid inclusions may be used to estimate downhole temperatures if logging data are unavailable. The compositional and thermal evolution of the diabase-hosted fluids may reflect late-stage, off-axis circulation and conductive heating of compositionally modified seawater in the sheeted dike complex at Hole 504B.

Sedimentology, geochemistry and paleomagnetic of ODP Hole 151-913B

The Eocene and Oligocene epochs (55 to 23 million years ago) comprise a critical phase in Earth history. An array of geological records (Zachos et al., 2001, doi:10.1126/science.1059412; Lear et al., 2000, doi:10.1126/science.287.5451.269; Coxall et al., 2005, doi:10.1038/nature03135; Pekar et al., 2005; doi:10.1130/B25486.1; Strand et al., 2003, doi:10.1016/S0031-0182(03)00396-1) supported by climate modelling (DeConto and Pollard, 2003, doi:10.1038/nature01290) indicates a profound shift in global climate during this interval, from a state that was largely free of polar ice caps to one in which ice sheets on Antarctica approached their modern size. However, the early glaciation history of the Northern Hemisphere is a subject of controversy (Coxall et al., 2005, doi:10.1038/nature03135; Tripati et al., 2005, doi:10.1038/nature03874; Wolf-Welling et al., 1996, doi:10.2973/odp.proc.sr.151.139.1996; Moran et al., 2006, doi:10.1038/nature04800). Here we report stratigraphically extensive ice-rafted debris, including macroscopic dropstones, in late Eocene to early Oligocene sediments from the Norwegian-Greenland Sea that were deposited between about 38 and 30 million years ago. Our data indicate sediment rafting by glacial ice, rather than sea ice, and point to East Greenland as the likely source. Records of this type from one site alone cannot be used to determine the extent of ice involved. However, our data suggest the existence of (at least) isolated glaciers on Greenland about 20 million years earlier than previously documented (Winkler et al., 2002, doi:10.1007/s005310100199), at a time when temperatures and atmospheric carbon dioxide concentrations were substantially higher.

Results of analyses of unmelted meteoritic debris collected from Eltanin ejecta in three sediment core from POLARSTERN cruise ANT-XII/4

A total of 1.7 g of unmelted meteorite particles have been recovered from FS Polarstern piston cores collected on expedition ANT XII/4 that contain ejecta from the Eltanin impact event. Most of the mass (1.2 g) is a large, single specimen that is a polymict breccia, similar in mineralogy and chemistry to howardites or the silicate fraction of mesosiderites. Most of the remaining mass is in several large individual pieces (20-75mg each) that are polymict breccias, fragments dominated by pyroxene, and an igneous rock fragment. The latter has highly fractionated REE, similar to those reported in mafic clasts from mesosiderites. Other types of specimens identified include fragments dominated by maskelynite or olivine. These pieces of the projectile probably survived impact by being blown off the back surface of the Eltanin asteroid during its impact into the Bellingshausen Sea.

Geochemistry and Fe/(Ti + Ti), x, Curie temperature, and oxidation parameter z at DSDP Hole 597C

Electron microprobe and thermomagnetic analyses of selected basalt samples from Hole 597C were performed. The main purpose of this work was to investigate and estimate the degree of oxidation of the samples using the ratios of Fe to Ti and the Curie temperatures obtained from thermomagnetic curves. The results show that the magnetic properties of samples from Hole 597C change at a sub-bottom depth of 100 m, and that low-temperature and high-temperature oxidation processes prevailed above and below 100 m, respectively.