Stable isotope ratios of interstitial fluids from ODP Leg 110 sites

Delta18O values of pore waters from the northern Barbados accretionary prism range from -0.3 to -3.6? and reflect pervasive reaction of volcanic ash to form smectite within the sedimentary sequence and continued low temperature alteration of basalt in the underlying ocean crust with the overprint of diffusive exchange between water in the sediment pores and the open ocean. Delta D values of pore waters in sediments sampled seaward of the deformation front drop from +5? at the sediment surface to -6? at the deepest levels sampled. These changes may also be related to alteration processes but remain largely enigmatic. Sediment deformation caused by impingement of the Caribbean plate on the Atlantic plate has instigated migration of chemically and isotopically distinct fluid along faults and coarse-grained sedimentary beds; delta18O values of pore waters are also locally affected by thrust stacking which increases diffusive pathlengths and possibly modifies diagenetic reaction rates in Pleistocene sediments. Migrating fluids are distinguished by anomalous delta18O values that are as much as 1? higher than those of surrounding fluids. Uncertainties in hydrogen isotope fractionation resulting from processes occurring under these conditions hinder identification of the hydrogen isotope composition of expelled fluid. Stable isotope analyses of pore waters help constrain the fluid migration history of the accretionary prism by limiting the source of fluids, the paths along which fluid flows, and the timing of faulting and subsequent fluid flow.

Fluid compositions and mineralogy of precipitates from Mid Atlantic Ridge hydrothermal vents at 4°48'S

The effect of volcanic activity on submarine hydrothermal systems has been well documented along fast- and intermediate-spreading centers but not from slow-spreading ridges. Indeed, volcanic eruptions are expected to be rare on slow-spreading axes. Here we report the presence of hydrothermal venting associated with extremely fresh lava flows at an elevated, apparently magmatically robust segment center on the slow-spreading southern Mid-Atlantic Ridge near 5°S. Three high-temperature vent fields have been recognized so far over a strike length of less than 2 km with two fields venting phase-separated, vapor-type fluids. Exit temperatures at one of the fields reach up to 407°C, at conditions of the critical point of seawater, the highest temperatures ever recorded from the seafloor. Fluid and vent field characteristics show a large variability between the vent fields, a variation that is not expected within such a limited area. We conclude from mineralogical investigations of hydrothermal precipitates that vent-fluid compositions have evolved recently from relatively oxidizing to more reducing conditions, a shift that could also be related to renewed magmatic activity in the area. Current high exit temperatures, reducing conditions, low silica contents, and high hydrogen contents in the fluids of two vent sites are consistent with a shallow magmatic source, probably related to a young volcanic eruption event nearby, in which basaltic magma is actively crystallizing. This is the first reported evidence for direct magmatic-hydrothermal interaction on a slow-spreading mid-ocean ridge.

Shell size, body mass and average pO2 in mantle cavity water of different marine mollusc species

Marine invertebrates with open circulatory system establish low and constant oxygen partial pressure (Po2) around their tissues. We hypothesized that as a first step towards maintenance of low haemolymph and tissue oxygenation, the Po2 in molluscan mantle cavity water should be lowered against normoxic (21 kPa) seawater Po2, but balanced high enough to meet the energetic requirements in a given species. We recorded Po2 in mantle cavity water of five molluscan species with different lifestyles, two pectinids (Aequipecten opercularis, Pecten maximus), two mud clams (Arctica islandica, Mya arenaria), and a limpet (Patella vulgata). All species maintain mantle cavity water oxygenation below normoxic Po2. Average mantle cavity water Po2 correlates positively with standard metabolic rate (SMR): highest in scallops and lowest in mud clams. Scallops show typical Po2 frequency distribution, with peaks between 3 and 10 kPa, whereas mud clams and limpets maintain mantle water Po2 mostly <5 kPa. Only A. islandica and P. vulgata display distinguishable temporal patterns in Po2 time series. Adjustment of mantle cavity Po2 to lower than ambient levels through controlled pumping prevents high oxygen gradients between bivalve tissues and surrounding fluid, limiting oxygen flux across the body surface. The patterns of Po2 in mantle cavity water correspond to molluscan ecotypes.

Isotopic geochemistry of lavas at DSDP Holes 81-553 and 81-555

It is demonstrated by K-Ar analyses that the age of reversely magnetized basalts, which immediately predate magnetic Anomaly 24B, is 53.5 ± 1.9 m.y. Samples from deep levels appear to be grossly contaminated by an extraneous argon component with a uniform argon-40/argon-36 ratio 440. This component is thought to have been derived from fluids circulating in the lava pile during burial. The age result corroborates the assignment previously made to Anomaly 24B by Hailwood et al. (1979) and Lowrie and Alvarez (1981). It additionally suggests that lava extrusion formed part of a much larger magmatic event, which affected wide areas of the North Atlantic margins around the Paleocene/Eocene boundary, and can therefore probably be considered a good estimate of the age of this boundary. Initial 143Nd/144Nd ratios lie in the very restricted range 0.512920 ± 19 to 0.513026 ± 24 and initial 8 7Sr/86Sr ratios from ca. 0.703 to ca. 0.705. Acid leaching reduces the latter range to 0.70264 ± 4 to 0.70384 ± 4, suggesting that the higher 87Sr/86Sr ratios resulted from interaction with seawater. The array of data for treated samples is closely conformable on a 143Nd/144Nd-87Sr/86Sr diagram with the main oceanic mantle array and with previously published fields for Atlantic Ocean basalts. No evidence for any continental crustal contamination has been found. This suggests, but does not prove, that continental crust played no part in the genesis of these rocks.

(Table T1) Permeability, electrical resistivity and density of ODP Hole 193-1188A and Site 193-1189 minicores, PACMANUS field

Magmatic fluids, heat fluxes, and fluid/rock interactions associated with hydrothermal systems along spreading centers and convergent margins have a significant impact on the genesis of major sulfide deposits and biological communities. Circulation of hydrothermal fluids is one of the most fundamental processes associated with localized mineralization and is controlled by inherent porous and permeable properties of the ocean crust. Heat from magmatic intrusions drives circulation of seawater through permeable portions of the oceanic crust and upper mantle, discharging at the seafloor as both focused high-temperature (250°-400°C) fluids and diffuse lower-temperature (<250°C) fluids. This complex interaction between the circulating hydrothermal fluids and the oceanic basement greatly influences the physical properties and the composition of the crust (Thompson, 1983; Jacobson, 1992, doi:10.1029/91RG02811; Johnson and Semyan, 1994, doi:10.1029/93JB00717). During Ocean Drilling Program (ODP) Leg 193, 13 holes were drilled in the PACMANUS hydrothermal system (Binns, Barriga, Miller, et al., 2002, doi:10.2973/odp.proc.ir.193.2002). The hydrothermal system consists of isolated hydrothermal deposits lined along the main crest of the Pual Ridge, a 500- to 700-m-high felsic neovolcanic ridge in the eastern Manus Basin. The principal drilling targets were the Snowcap (Site 1188) and Roman Ruins (Site 1189) active hydrothermal fields. Samples from these two sites were used for a series of permeability, electrical resistivity, and X-ray computed tomography measurements.

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.