Geochemistry of lavas from the Lau-Tonga arc and back-arc system

Detailed comparison of mineralogy, and major and trace geochemistry are presented for the modern Lau Basin spreading centers, the Sites 834-839 lavas, the modern Tonga-Kermadec arc volcanics, the northern Tongan boninites, and the Lau Ridge volcanics. The data clearly confirm the variations from near normal mid-ocean-ridge basalt (N-MORB) chemistries (e.g., Site 834, Central Lau Spreading Center) to strongly arc-like (e.g., Site 839, Valu Fa), the latter closely comparable to the modern arc volcanoes. Sites 835 and 836 and the East Lau Spreading Center represent transitional chemistries. Bulk compositions range from andesitic to basaltic, but lavas from Sites 834 and 836 and the Central Lau Spreading Center extend toward more silica-undersaturated compositions. The Valu Fa and modern Tonga-Kermadec arc lavas, in contrast, are dominated by basaltic andesites. The phenocryst and groundmass mineralogies show the strong arc-like affinities of the Site 839 lavas, which are also characterized by the existence of very magnesian olivines (up to Fo90-92) and Cr-rich spinels in Units 3 and 6, and highly anorthitic plagioclases in Units 2 and 9. The regional patterns of mineralogical and geochemical variations are interpreted in terms of two competing processes affecting the inferred magma sources: (1) mantle depletion processes, caused by previous melt extractions linked to backarc magmatism, and (2) enrichment in large-ion-lithophile elements, caused by a subduction contribution. A general trend of increasing depletion is inferred both eastward across the Lau Basin toward the modern arc, and northward along the Tongan (and Kermadec) Arc. Numerical modeling suggests that multistage magma extraction can explain the low abundances (relative to N-MORB) of elements such as Nb, Ta, and Ti, known to be characteristic of island arc magmas. It is further suggested that a subduction jump following prolonged slab rollback could account for the initiation of the Lau Basin opening, plausibly allowing a later influx of new mantle, as required by the recognition of a two-stage opening of the Lau Basin.

Mineralogy and geochemistry of Holocene sediments from the North Atlantic

Bulk mineralogy, Sm, Nd and Pb elemental and isotopic compositions of the clay-size fraction of Holocene sediments were analyzed in three deep North Atlantic cores to trace the particle provenance. The aims of the present paper are to identify the origin of the particles driven by deep currents and to reconstruct deep circulation changes over the Holocene in the North Atlantic. The three cores are retrieved in fracture zones; two of them are located in the Island Basin along the gyre of North Atlantic Deep Water, and the third core is located off the present deep circulation gyre in the Labrador Sea. Whereas sedimentary supplies in the Labrador Sea were constantly derived from proximal sources, the geochemical mixing trends in the Iceland Basin samples indicate pronounced changes in the relative contribution of continental margin inputs over the past 6 kyr. Supplies from western European margin that sharply increased at 6 kyr were progressively diluted by a larger contribution of Scandinavian margins over the last 3 kyr. Changes in composition of the particles imply significant reorganization of paleocirculation of the deep North Atlantic components in the eastern basins: mainly reorganizations for both Iceland-Scotland Overflow Water and Norwegian Sea Overflow Water. Moreover the unusual Pb isotopic composition of the oldest sediments from the southern Iceland Basin indicates that distal supplies from Greenland margin were driven into the Iceland Basin, supporting a deep connection between Labrador Sea and Iceland Basin through the Charlie Gibbs Fracture Zone prior the Holocene Transition period.

Whole rock geochemistry and stable isotopes of rocks, fragments and foraminifers from two ODP Leg 166 holes

Total carbon and carbonate contents, quantitative carbonate mineralogy, trace metal concentrations, and stable isotope compositions were determined on a suite of samples from the Miocene sections at Sites 1006 and 1007. The Miocene section at Site 1007, located at the toe-of-slope, contains a relatively high proportion of bank-derived components and becomes fully lithified at a depth of ~300 meters below seafloor (mbsf). By contrast, Miocene sediments at Site 1006, situated in Neogene drift deposits in the Straits of Florida and composed primarily of pelagic carbonates, do not become fully lithified until a depth of ~675 mbsf. Diagenetic and compositional contrasts between Sites 1006 and 1007 are reflected in geochemical data derived from sediment samples from each site.

Geochemistry of low-temperatuer altered basalts of ODP Hole 136-843B

The mineralogy and chemistry of altered basalts and the stable isotopic compositions of secondary vein carbonates were studied in cores from Ocean Drilling Program Hole 843B, located in 95-Ma crust of the Hawaiian Arch. Millimeter- to centimeter-sized dark alteration halos around veins are 5%-15% altered to celadonite and Fe-oxyhydroxides, plus minor saponite and calcite. Adjacent gray host rocks are about 15% altered to saponite and calcite. The dark halos are enriched in H2O+, CO2, FeT, K2O, MnO, and Fe3+/FeT and depleted in SiO2, Al2O3, MgO, and TiO2 relative to gray host rocks. Brown alteration halos occur around veins where veins are more abundant, and are similar to dark halos, but contain more Fe-oxyhydroxides and exhibit greater Fe2O3T contents and higher Fe3+/FeT. Stable isotopic compositions of vein carbonates are consistent with their precipitation from seawater at temperatures of 5°-40°C. Crosscutting relationships of veins and zoned vein and vesicle fillings reveal a sequence of secondary mineral formation and alteration conditions. Celadonite and Fe-oxyhydroxides formed and dark alteration halos developed relatively early, under oxidizing conditions at low temperatures (<50°C). Saponite formed later at lower seawater/rock ratios and under more reducing conditions. Calcite and pyrite formed last in veins and vesicles from more evolved, seawaterderived fluids at temperatures of 5°-40°C. A second stage of celadonite, with compositions distinct from the early celadonite, also occurred relatively late (within the "calcite stage"), and may be related to refracturing of the crust and introduction of less-evolved seawater solutions into the rocks. Trends to higher K2O contents are attributed to alteration, but high K/Ti, Ba, and Zr contents indicate the presence of enriched or transitional MORB. CO2 contents of Pacific ODP cores exhibit a general increase with age suggesting progressive fixation of CO2 as calcite in the crust, but this could be complicated by local heterogeneities in fracturing and calcite formation in the crust.

Geochemistry and age determination of sediments from teh Rainbow hydrothermal plume

A geochemical investigation has been conducted of a suite of four sediment cores collected from directly beneath the hydrothermal plume at distances of 2 to 25 km from the Rainbow hydrothermal field. As well as a large biogenic component (>80% CaCO3) these sediments record clear enrichments of the elements Fe, Cu, Mn, V, P, and As from hydrothermal plume fallout but only minor detrital background material. Systematic variations in the abundances of "hydrothermal" elements are observed at increasing distance from the vent site, consistent with chemical evolution of the dispersing plume. Further, pronounced Ni and Cr enrichments at specific levels within each of the two cores collected from closest to the vent site are indicative of discrete episodes of additional input of ultrabasic material at these two near-field locations. Radiocarbon dating reveals mean Holocene accumulation rates for all four cores of 2.7 to 3.7 cm.kyr?1, with surface mixed layers 7 to 10+ cm thick, from which a history of deposition from the Rainbow hydrothermal plume can be deduced. Deposition from the plume supplies elements to the underlying sediments that are either directly hydrothermally sourced (e.g., Fe, Mn, Cu) or scavenged from seawater via the hydrothermal plume (e.g., V, P, As). Holocene fluxes into to the cores' surface mixed layers are presented which, typically, are an order of magnitude greater than "background" authigenic fluxes from the open North Atlantic. One core, collected closest to the vent site, indicates that both the concentration and flux of hydrothermally derived material increased significantly at some point between 8 and 12 14C kyr ago; the preferred explanation is that this variation reflects the initiation/intensification of hydrothermal venting at the Rainbow hydrothermal field at this time - perhaps linked to some specific tectonic event in this fault-controlled hydrothermal setting.

Geochemistry of interstitial waters of DSDP Leg 96 sites

Interstitial water studies were done at 9 of the 11 sites visited in the Mississippi Fan and Orca and Pigmy Basins during DSDP Leg 96. High concentrations of sulfate were observed at Mississippi Fan Sites 616, 617, 620, and 623. The maximum sulfate value of 38.8 mM, recorded at Site 617, is the highest ever found in DSDP sediments. Hypersaline interstitial water was observed at Site 618 in Orca Basin. Concentration ratios of salinity to chlorinity and to sodium in interstitial waters are similar to those of Orca Basin bottom water, suggesting that the chemistry of interstitial water is affected by the dissolution of buried salt.

Geochemistry at DSDP Leg 75 Holes

Samples of sediments and rocks collected at DSDP Sites 530 and 532 were analyzed for 44 major, minor, and trace elements for the following purposes: (1) to document the downhole variability in geochemistry within and between lithologic units; (2) to document trace-element enrichment, if any, in Cretaceous organic-carbon-rich black shales at Site 530; (3) to document trace-element enrichment, if any, in Neogene organic-carbon-rich sediments at Site 532; (4) to document trace-element enrichment, if any, in red claystone above basalt basement at Site 530 that might be attributed to hydrothermal activity or weathering of basalt. Results of the geochemical analyses showed that there are no significant enrichments of elements in the organic-carbon-rich sediments at Site 532, but a number of elements, notably Cd, Co, Cr, Cu, Mo, Ni, Pb, V, and Zn, are enriched in the Cretaceous black shales. These elements have different concentration gradients within the black-shale section, however, which suggests that there was differential mobility of trace elements during diagenesis of interbedded more-oxidized and less-oxidized sediments. There is little or no enrichment of elements from hydrothermal activity in the red claystone immediately overlying basalt basement at Site 530, but slight enrichments of several elements in the lowest meter of sediment may be related to subsea weathering of basalt

Geochemistry at DSDP Leg 74 Holes

Clay mineralogical and inorganic geochemical data from the Campanian to the Pleistocene provide information bearing on the evolution of both continental and marine paleoenvironments in the Walvis Ridge area. (1) Alteration processes of basalts occurred under subaerial conditions during the Campanian and Maestrichtian and were virtually absent in deeper marine environments. (2) Strong tectonic effects were present during the Campanian and persisted until the early Eocene. (3) Subsidence of this part of the Walvis Ridge became important during the late Maestrichtian and continued into the Paleocene and Eocene. (4) The influence of global climatic cooling was evident from the late Eocene on. (5) Modification of oceanic circulation and the increasing influence of surface and deep water masses on the sedimentation characterized the Cenozoic.

Geochemistry, isotopic ratios and mineral composition of samples from ODP Leg 163 sites

Thirty-five samples from the drill core of the three Leg 163 sites (Sites 988, 989, and 990) off the southeast coast of Greenland were analyzed for 27 major, minor, and trace elements by X-ray fluorescence (XRF) and for 25 trace elements, including 14 rare-earth elements (REEs), by an inductively coupled plasma source mass spectrometer (ICP/MS). Sr- and Nd-isotope data are reported for seven samples and oxygen-isotope data are reported for 19 plagioclase separates. In addition, a reconnaissance survey of the composition of the main mineral phases, plagioclase, pyroxene, and oxides was determined on an electron microprobe to provide the basic information required for petrogenetic modeling. Olivine pseudomorphs are present in many of the samples, but in no case was an olivine grain found that was fresh enough to give a reliable analysis. The chemical and isotopic data recorded here were determined to provide a comparison with the larger data sets acquired by the Edinburgh, Copenhagen, and Leicester laboratories from both Legs 152 and 163 drill cores. This will permit a detailed comparison of the North Atlantic flood basalt province as a whole with the better known Columbia River, Deccan, and Karoo continental flood basalt provinces, for which substantial chemical data sets are already available at Washington State University.

Interstitial water d13C and sediment geochemistry of ODP Leg 166 sites

A review of interstitial water samples collected from Sites 1003-1007 of the Bahamas Transect along with a shore-based analysis of oxygen and carbon isotopes, minor and trace elements, and sediment chemistry are presented. Results indicate that the pore-fluid profiles in the upper 100 meters below seafloor (mbsf) are marked by shifts between 20 and 40 mbsf that are thought to be caused by changes in sediment reactivity, sedimentation rates, and the influence of strong bottom currents that have been active since the late Pliocene. Pore-fluid profiles in the lower Pliocene-Miocene sequences are dominated by diffusion and do not show significant evidence of subsurface advective flow. Deeper interstitial waters are believed to be the in situ fluids that have evolved through interaction with sediments and diffusion. Pore-fluid chemistry is strongly influenced by carbonate recrystallization processes. Increases in pore-fluid Cl- and Na+ with depth are interpreted to result mainly from carbonate remineralization reactions that are most active near the platform margin. A lateral gradient in detrital clay content observed along the transect, leads to an overall lower carbonate reactivity, and enhances preservation of metastable aragonite further away from the platform margin. Later stage burial diagenesis occurs at slow rates and is limited by the supply of reactive elements through diffusion.

Geochemistry of felsic rocks and minerals of ODP Hole 135-841B

A felsic volcanic series (605-825 mbsf) overlain by upper Eocene shallow-water sediments (500-605 mbsf) and basalticandesitic sills that intruded into sediments of Holocene to Miocene age (0-500 mbsf) was drilled in the forearc region of the Lau Basin at a water depth of 4810 m. The volcanic sequence at Site 841 includes altered and mineralized calc-alkaline rhyolites and dacites, dacitic tuffs, lapilli tuffs, flow breccias, and welded tuffs. These rocks formed subaerially or in a very shallow-water environment suffering a subsidence of >5000 m since Eocene times. Calculations of gains and losses of the major components during alteration show most pronounced changes in the uppermost 70 m of the volcanic sequence. Here, Al, Fe, Mg, and K are enriched, whereas Si and Na are strongly depleted. Illite, vermiculite, chlorite, and hematite predominate in this part of the hole. Throughout the section, quartz, plagioclase, kaolinite, and calcite are present. Sulfide mineralization (up to 10 vol%) consisting mainly of disseminated pyrite (with minor pyrrhotite inclusions) and marcasite together with minor amounts of chalcopyrite is pervasive throughout. Locally, a few sulfide-bearing quartz-carbonate veins as well as Ti-amphibole replacement by rutile and then by pyrite were observed. Strong variations in the As content of sulfides (from 0 to 0.69 wt%) from the same depth interval and local enrichments of Co, Ni, and Cu in pyrite are interpreted to result from fluctuations in fluid composition. Calculations of oxygen and sulfur fugacities indicate that fO2 and fS2 were high at the top and lower at the bottom of the sequence. Sulfur isotope determinations on separated pyrite grains from two samples give d34S values of +6.4ë and +8.4ë, which are close to those reported from Kuroko and Okinawa Trough massive sulfide deposits and calc-alkaline volcanic rocks of the Japanese Ryukyu Island Arc. Calculated chlorite formation temperatures of 265°-290°C at the top of the sequence are consistent with minimum formation temperatures of fluid inclusions in secondary quartz, revealing a narrow range of 270°-297°C. Chlorite formation temperatures are constant downhole and do not exceed 300°C. The presence of marcasite and 4C-type pyrrhotite indicates a formation temperature of <= 250°C. At a later stage, illite was formed at the top of the volcanic series at temperatures well below 200°C.

Geochemistry of the volcanic basement of ODP Hole 134-831B

The basement of Bougainville Guyot drilled at Site 831 consists of andesitic hyalobreccias derived from a submarine arc volcano. The volcanic sequence has been dated by K/Ar at approximately 37 Ma. The 121 m of andesitic hyalobreccias drilled in Hole 831B have been divided into five subunits of two types: one appears to be primary, and the other contains evidence of reworking and a subaerial clastic input. Variations are attributed to fluctuations in water depth. The distinctive hyalobreccias consist of andesitic blebs with chilled margins and peripheral fractures set in a chaotic greenish matrix that is mainly altered glass, with crystals similar to those in the blebs or clasts. Their formation is attributed to violent reaction of andesitic magma discharged into seawater, in perhaps the submarine equivalent of fire-fountaining. There was limited reworking by currents and debris flows on the flanks of the submarine volcano. The andesite shows no significant compositional variation in phenocryst phases throughout the drilled sequence and contains phenocrysts of plagioclase (An88-43), clinopyroxene (Ca44Mg46Fe10-Ca41Mg40Fe19), orthopyroxene (Ca4Mg79Fe17-Ca3Mg58Fe39), and titanomagnetite. There is a systematic change in volcanic composition with height in the section, from more mafic andesites at the base, to overlying more acid andesites, and strong evidence exists that magma mixing may have played a significant role in the genesis of these lavas. The andesites have affinities with the low-K arc tholeiite series. Trace element and isotopic systematics for these rocks indicate very minor involvement of a LILE- and 87Sr-enriched slab-derived fluid in their petrogenesis. This accords with the previous suggestion that Bougainville Guyot forms part of an Eocene proto-island arc developed along the southern side of the d'Entrecasteaux Zone, above a southward-dipping subduction zone.

Geochemistry from DSDP Holes 67-497 and 67-496

Mineralogical identification, glass chemistry, and instrumental neutron activation analyses of Quaternary volcanic ash layers from Leg 67 Holes 496, 497, and 499 are used to correlate the drill holes and on-land sources. We have identified two units at Hole 496 that correspond to the 23,000-yr.-old Pinos Altos ash (Samples 496-3-4, 55-57 cm and 496-3-5, 74-76 cm); the 84,000-yr.-old Los Chocoyos ash corresponds with Sample 496-5-4, 134-146 cm, but this latter correlation is less certain.