Age model of ODP Site 162-983

At ODP Site 983, relative geomagnetic paleointensity and planktic and benthic delta18O records have been acquired for the last 350 kyr. The mean sedimentation rate in this interval is 11.3 cm/kyr. Magnetic properties and hysteresis ratios indicate that pseudo-single domain magnetite is the remanence carrier. Volume susceptibility (kappa), anhysteretic (ARM) and isothermal (IRM) remanence values vary by a factor of 3-4, well within the criteria usually cited for paleointensity studies. Natural remanent magnetization (NRM) is normalized by ARM and IRM to acquire the paleointensity proxy. Arithmetic means of NRM/ARM and NRM/IRM, calculated for five demagnetization steps in the 25-45 mT range, constitute the relative paleointensity estimates. Some paleointensity lows (particularly those at ~40, ~120 and ~188 ka) are associated with directional excursions of the field, especially the event at ~188 ka (referred to here as the Iceland Basin Event) that constitutes a short-lived polarity reversal. For the last 200 kyr, the records can be correlated with other high-resolution paleointensity records such as those from the Labrador Sea, Mediterranean/Somali Basin and Sulu Sea, implying that the millennial scale features are globally synchronous. A labeling system for paleointensity features is proposed that ties prominent highs and lows to oxygen isotope stages.

Chemistry of hydrothermal clasy and sulfides of ODP Site 106-649

Several meters of unconsolidated hydrothermal sediment were recovered from the Snake Pit hydrothermal field during ODP Leg 106. Polymetallic sulfides comprise most of the sediment with minor fragments of massive sulfide, organic debris, clay minerals, and fresh glass shards. Trace element and Sr-isotope contents of hydrothermal clays and sulfides from Holes 649B and 649G indicate that these minerals precipitated from a mixed hydrothermal fluid-seawater solution. Evaluation of the REE mineral data and the Snake Pit hydrothermal fluids shows that the REE distribution coefficients between the hydrothermal fluids and clay-sulfide mixes range from 100-500. This indicates that hydrothermal fluids originating in the root-zone of the Snake Pit hydrothermal system may be modified by the precipitation of hydrothermal minerals, either in the shallow subsurface or within chimney structures. Contrasting REE profiles of clay-sulfide aggregates and massive sulfides from Holes 649B and 649G may be accounted for by spatial and/or temporal variations in redox conditions in the plumbing system.

Stress tests of ODP Site 131-808 sediments

Sediments undergoing accretion in trench-forearc systems are subjected to conditions of large lateral thrusting. This stress regime controls the mechanism of faulting as well as the yield and strength properties of the sediment. Understanding them is therefore crucial for the construction of quantitative models of sediment dynamics in convergent margin settings. For this purpose triaxial and oedometer tests were performed on six whole-round core samples recovered from Site 808 from depths between 173 and 705 mbsf. Samples from five depth intervals were subjected to a triaxial test program that was primarily designed to define yield and strength behavior. Test specimens were cut parallel and normal to the core axis. Additional five oedometer tests with similarly prepared specimens were performed on samples from four depth intervals to evaluate the directional state and degree of sediment compaction. Test results show that the degree of sediment compaction is higher than expected from overburden. This overcompaction increases with depth. A well-developed mechanical anisotropy is evident in all samples tested, regardless of their depth and lithology. Values of yield limit, stiffness, and shear strength are up to 40% higher in the horizontal direction compared to the vertical direction. In addition the test data demonstrate that the axis of the volumetric yield loci have rotated into extensional stress field. This verifies that the mechanical state of sediment in the accretionary wedge is controlled by in-situ stress conditions of extensional nature. The coefficients of lateral stress inferred suggest that the extensional stress regime becomes increasingly effective with depth.

Marine barite in sediments of DSDP Site 54-424

The distribution of barite in sediments from D.S.D.P. sites 424 and 424A at the Galapagos hydrothermal mounds field is determined and the process of its formation is deduced. Barite in these deposits is associated with calcareous sediments and is completely absent from the hydrothermal material (manganese crusts and nontronite). Its concentrations tend to increase in the deeper sediments. Since manganese crusts contain significant amounts of Ba, a lack of barite in them is probably due to low concentrations of [SO4]2 in the sediment-seawater interface where they form. The formation of barite occurs within buried sediments, the interstitial waters of which are saturated with [SO4]2. The most probable source of [SO4]2- is the oxidation of H2S which is released from the hydrothermal fluids percolating upwards through the sediments. Although nontronite is formed within buried sediments the environmental conditions occurring during its formation (reducing) prevent barite formation. The association of barite with calcareous sediments is due to the release of Ba by calcareous microorganisms and/or to high concentrations of Ca in the pore waters which maintain a high pH and hence [SO4]2- is stable.

(Table T1) Geochemical composition of xenoliths and host rocks from ODP Hole 193-1188A and dredge haul MD-138, PACMANUS field

A number of intensely altered, dark xenoliths with palimpsest quench textures were recorded within altered dacitic host rocks at Site 1189 (Roman Ruins, PACMANUS) during Ocean Drilling Program (ODP) Leg 193. Several of these displayed puzzling marginal fringes, apparently of altered plagioclase with variolitic texture, protruding into adjacent host rocks. Despite their alteration, the xenoliths were interpreted as fragments of rapidly chilled, possibly olivine-bearing basalts incorporated into the dacitic magmas either within the crustal plumbing system or during eruption at the seafloor (figures F15, F16, F17, F42, and F43 in Shipboard Scientific Party, 2002, doi:10.2973/odp.proc.ir.193.104.2002). An additional example of formerly spinifex-textured xenolith, the first from Site 1188 (Snowcap) and the first from the upper cristobalite-bearing zone of alteration, has been revealed by postcruise studies. Furthermore, a pristine sample of the parent lithology has been found within a dredge haul (MD-138, Binatang-2000 Cruise of Franklin; 3°43.60'S, 151°40.35'E, 1688 meters below sea level) from the Satanic Mills hydrothermal field at PACMANUS, near ODP Site 1191. The purpose of this report is to document these discoveries and thereby to confirm and build on shipboard interpretations. To my knowledge, similar xenoliths have never before been found in modern island arc or backarc volcanic sequences. Spinifex textures are most common in Archean komatiites, some of which are bimodally associated with calc-alkaline felsic volcanic rocks.

Pliocene sediment and silicon isotope record of ODP Site 145-882

Increases in the low-field mass-specific magnetic susceptibility (chi), dropstones and the terrigenous sediment component from Ocean Drilling Program (ODP) Site 882 (~45°N) have been interpreted to indicate a major onset of ice-rafting to the sub-Arctic northwest Pacific Ocean during marine isotope stage (MIS) G6 (from ~2.75 Ma). In contrast, studies of the terrigenous content of sediments cored downwind of ODP Site 882 indicate that dust and disseminated volcanic ash deposition in the sub-Arctic Pacific increased markedly during MIS G6. To investigate the relative contribution of dust, volcanic ash and ice rafting to the Pliocene chi increase, we present new high-resolution environmental magnetic and ice-rafted debris records from ODP Sites 882 and 885. Our results demonstrate that the chi increase at both sites across MIS G6 is predominantly controlled by a previously overlooked mixture of aeolian dust and volcanic ash. Our findings call into question the reliability of chi as a proxy for ice-rafting to the North Pacific. They also highlight a previously undocumented link between iron fertilisation and biogeochemical cycling in the North Pacific at a key stage during intensification of late Pliocene northern hemisphere glaciation.

Whole-rock geochemical analyses of serpentinized harzburgites from ODP Site 209-1270

Small-scale shear zones are present in drillcore samples of abyssal peridotites from the Mid-Atlantic ridge at 15°20'N (Ocean Drilling Program Leg 209). The shear zones act as pathways for both evolved melts and hydrothermal fluids. We examined serpentinites directly adjacent to such zones to evaluate chemical changes resulting from melt-rock and fluid-rock interaction and their influence on the mineralogy. Compared to fresh harzburgite and melt-unaffected serpentinites, serpentinites adjacent to melt-bearing veins show a marked enrichment in rare earth elements (REE), strontium and high field strength elements (HFSE) zirconium and niobium. From comparison with published chemical data of variably serpentinized and melt-unaffected harzburgites, one possible interpretation is that interaction with the adjacent melt veins caused the enrichment in HFSE, whereas the REE contents might also be enriched due to hydrothermal processes. Enrichment in alumina during serpentinization is corroborated by reaction path models for interaction of seawater with harzburgite-plagiogranite mixtures. These models explain both increased amounts of alumina in the serpentinizing fluid for increasing amounts of plagiogranitic material mixed with harzburgite, and the absence of brucite from the secondary mineralogy due to elevated silica activity. By destabilizing brucite, nearby melt veins might fundamentally influence the low-temperature alteration behaviour of serpentinites. Although observations and model results are in general agreement, due to absence of any unaltered protolith a quantification of element transport during serpentinization is not straightforward.

Rock magentic investigations of ODP Site 162-983

We report natural remanent magnetization (NRM) directions and geomagnetic paleointensity proxies for part of the Matuyama Chron (0.9-2.2 Ma interval) from two sites located on sediment drifts in the Iceland Basin. At Ocean Drilling Program Sites 983 and 984, mean sedimentation rates in the late Matuyama Chron are 15.9 and 11.5 cm/kyr, respectively. For the older part of the record (>1.2 Ma), oxygen isotope data are too sparse to provide the sole basis for age model construction. The resemblance of the volume susceptibility record and a reference d18O record led us to match the two records to derive the age models. This match, based on Site 983/984 susceptibility, is consistent with available Site 983/984 benthic d18O data. Paleointensity proxies were derived from the slope of the NRM versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30-60 mT peak field range. Paleointensity lows correspond to polarity reversals at the limits of the Jaramillo, Olduvai, Cobb Mountain, and Réunion Subchrons and to seven excursions in NRM component directions. Magnetic excursions (defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator) are observed at 932, 1048, 1115, 1190-1215 (Cobb Mountain Subchron), 1255, 1472-1480, 1567-1575 (Gilsa Subchron), and 1977 ka. The results indicate that geomagnetic directional excursions, associated with paleointensity minima, are a characteristic of the Matuyama Chron and probably of polarity chrons in general.