Joel Etoubleau

Intense CH4 plumes generated by serpentinization of ultramafic rocks at the intersection of the 15°20′N fracture zone and the Mid-Atlantic Ridge

Abstract As part of the FARA French-US Program designed to study the Mid- Atlantic Ridge (MAR) between 15°N and the Azores, twenty-three dives with the submersible Nautile were conducted during the French-US Faranaut 15N cruise on the eastern and western parts of the 15°20′N Fracture Zone/Ridge axis intersection. South of the eastern ridge-transform fault intersection, nine Nautile dives were made within the rift valley and along the western rift valley wall. CH4 concentrations in the bottom waters reach 53.2 nmol/kg along faulted zones on top and on the east flank of the ultramafic inner corner high (15°05′N, 44°59′W) where serpentinized rocks outcrop. No 3He anomaly is associated with methane, ruling out any primary mantle component. Fourteen dives were also made in the rift valley to the north, close to the western intersection of the 15°20′N Transform. High CH4 anomalies (up to 22 nmol/kg) are also present in the bottom waters of the rift valley northern segment on both the western and eastern valley walls and on the inner high adjacent to the eastern wall where ultramafic rocks outcrop. Seven vertical hydrocasts carried out in the axial valley (4500 m deep) show an intense CH4 anomaly, with a maximum (35.8 nmol/kg) at 3200 m depth. This CH4 concentration is among the highest found along the Mid-Atlantic Ridge Charlou and Donval 1993 , Charlou et al 1997 . CH4 concentrations of 9.9–14.9 nmol/kg are also present on the western wall along the 3200 m isobath. The high CH4 concentrations correspond to only weak 3He anomalies. This CH4-rich plume is also associated with active fault zones that expose peridotite rocks. CH4 output from ultramafic outcrops on the western and eastern intersections of the 15°20′N Fracture Zone with the MAR is believed to reflect ongoing serpentinization. These results associated with many other CH4 anomalies measured in the water column above ultramafic outcrops found between 12°N and the Azores most likely reflect serpentinization processes extending along the whole slow spreading Mid-Atlantic ridge.

The age and distribution of mantle heterogeneity along the Mid-Atlantic Ridge (31–41°N)

New trace element and isotopic data for basalts from the mid-Atlantic ridge between 31 and 41oN allow a better description of the geochemical gradient south of the Azores triple junction, and the systematics of mantle source heterogeneity. There is a long wavelength enrichment in incompatible trace elements and isotopes associated with the Azores hot spot that extends from the Kurchatov fracture zone near 41oN to the Hayes fracture zone near 33oN. Superimposed on this gradient are local spikes of enrichment, the most prominent being the anomaly near the Oceanographer Fracture Zone (NOFZ). The Oceanographer anomaly spike is reflected modestly in the morphology of the ridge axis, but is not obviously related to a plume. The isotopic data alone are consistent with involvement of subcontinental material, but the samples do not contain the negative Nb‐Ta anomalies which are usually associated with the presence of continental material in the mantle source. Away from the prominent enrichment spikes associated with the Azores and Oceanographer fracture zone, there are systematic relationships in this region between parent=daughter element ratios and isotope ratios. The Pb, Sr and Nd isotope systems all give apparent ages in the range 100 Ma to 300 Ma, with the age increasing with likely parent=daughter fractionation during melting (U=Pb < Rb=Sr < Sm=Nd age). Monte Carlo simulations of an enrichment event in a depleted heterogeneous mantle at 250 Ma produce results that correspond well with the observations for all three isotopic systems. Since this age also corresponds to the pre-opening of the North Atlantic, it raises the possibility that some of the heterogeneity in this region is associated with shallow level mantle heterogeneity resulting from the rifting of Gondwanaland rather than from interaction with mantle plumes. The data may also reflect a mean mixing time for the heterogeneities in the upper mantle source. Sr isotope systematics reveal correlations in a 87 Sr= 86 Sr versus 87 Rb= 86 Sr plot, which are geographically controlled. Data points from 10‐24oN samples and data points from 31‐38oN samples (excluding NOFZ samples) plot on two offset trends of similar slope. Irrespective of the origin of the isotopic variations, these data require end member depleted mantle with distinct isotopic characteristics. Depleted sources with low 87 Rb= 86 Sr (0.005‐0.04) and low (La=Sm)N (<0.5), have 87 Sr= 86 Sr values that vary between 0.70215 and 0.7029. Therefore the depleted mantle source of N-MORB is not a homogeneous reservoir, but shows isotopic variations almost as large as the differences between generic depleted mantle (0.7025) and the enriched Atlantic plumes. Creation of a very heterogeneous depleted mantle in terms of isotopic composition needs to be included as a constraint on models of mantle mixing and convection.

Intensifying Weathering and Land Use in Iron Age Central Africa

A Price of Civilization Large expanses of rainforests in parts of Central Africa were abruptly replaced by savannas around 3000 years ago, presumably because of climate change. However, that succession occurred at a time of expansion by Bantu tribes, from near the border of present-day Cameroon and Nigeria to the south and east, in a migration that brought with it agriculture and iron-smelting technologies. Bayon et al. (p. 1219, published online 9 February; see the Perspective by Dupont) analyzed the nearby marine sedimentary record and found that chemical weathering in Central Africa also increased markedly at this time. This increase in weathering could have been caused by forest clearing by the Bantu to create arable land and to fuel their smelters, rather than climate change alone. Savannas abruptly replaced rainforests around 3000 years ago on account of both climate and human land-use changes. About 3000 years ago, a major vegetation change occurred in Central Africa, when rainforest trees were abruptly replaced by savannas. Up to this point, the consensus of the scientific community has been that the forest disturbance was caused by climate change. We show here that chemical weathering in Central Africa, reconstructed from geochemical analyses of a marine sediment core, intensified abruptly at the same period, departing substantially from the long-term weathering fluctuations related to the Late Quaternary climate. Evidence that this weathering event was also contemporaneous with the migration of Bantu-speaking farmers across Central Africa suggests that human land-use intensification at that time had already made a major impact on the rainforest.

Geological setting and mineralogical and geochemical investigations on sulfide deposits near 13°N on the East Pacific Rise

Geological, mineralogical and geochemical investigations of hydrothermal deposits on one of the most studied segments of the East Pacific Rise, near 13°N, show that sulfides occur in four main tectonic settings. These are: (1) in the middle part of the central graben where active zones are located, (2) at the top of the central graben where a second line of deposits is observed, (3) on the marginal high, which has sulfides both on its western tectonic side and at its top where a large deposit is located, and (4) on the flank of the Southeastern Seamount which is the site of the most important deposit of the area. The nature of the distribution of the samples through the area studied shows that hydrothermal activity has existed for more than 20,000 years. Mineral assemblages are dependent on the sample location. The sulfide samples exhibit a wide range of mineralogical assemblages varying from high-temperature hydrothermal Cu-Fe-rich chimneys (black smokers), which are characteristic structures of young edifices, to mature Cu-Fe-rich massive sulfides similar to those found in the core of massive sulfide lenses mined on shore. With respect to these observations a classification into nine types of sulfide assemblages is proposed. Hydrothermal activity at the axis is now increasing in intensity (increase in temperature, decrease in f(S2) and f(O2)). The relative paucity of Zn-rich massive sulfides on the off-axis structures is likely to be a consequence of the depletion of Zn in the hydrothermal fluids. R-mode factor analysis applied to a population of nineteen representative bulk chemical analyses shows fractionation of the reduced sulfur fluid during the discharge into three main groups (Cu-Fe-Co-Se-Ca-Sr, Zn-Cd-Ag-As-Pb and Si-Ba) displaying the principal types of sulfide samples collected in the 13°N area. Microprobe investigations conducted on sulfide minerals define the distribution of trace-metal elements such as Ag, Co, Se, Cd and As during the process of hydrothermal maturation of the deposits.

Geodiversity of Hydrothermal Processes Along the Mid‐Atlantic Ridge and Ultramafic‐Hosted Mineralization: a New Type Of Oceanic Cu‐Zn‐Co‐Au Volcanogenic Massive Sulfide Deposit

OS21C-08. Eberhart, G. L., P. A. Rona, and J. Honnorez (1989), Geologic controls of hydrothermal activity in the Mid-Atlantic Ridge rift valley; tectonics and volcanics, Mar. Geophys. Res., 10(3-4), 233–259. Edmond, J. M., A. C. Campbell, M. R. Palmer, G. P. Klinkhammer, C. R. German, H. N. Edmonds, H. Elderfield, G. Thompson, and P. Rona (1995), Time series studies of vent fluids from the TAG and MARK sites (1986, 1990) Mid-Atlantic Ridge: a new solution chemistry model and a mechanism for Cu/Zn zonation in massive sulphide orebodies, in Hydrothemal Vents and Processes, edited by L. M. Parson et al., pp. 77–86, Geol. Soc., London. Edmonds, H. N., P. J. Michael, E. T. Baker, D. P. Connelly, J. E. Snow, C. H. Langmuir, H. J. B. Dick, R. Muhe, C. R. German, and D. W. Graham (2003), Discovery of abundant hydrothermal venting on the ultraslow-spreading Gakkel ridge in the Arctic, Nature, 421(6920), 252–256. Elderfield, H., et al. (1993), Preliminary geochemical results from the Broken Spur hydrothermal field, 29° N, Mid-Atlantic Ridge, Eos Trans. AGU, 74(43), Fall Meet. Suppl., 99. Escartín, J., and M. Cannat (1999), Ultramafic exposures and the gravity signature of the lithosphere near the Fifteen-Twenty fracture zone (Mid-Atlantic Ridge, 14°–16.5° N), Earth Planet. Sci. Lett., 171(3), 411–424. Escartín, J., and J. Lin (1998), Tectonic modification of axial crustal structure; evidence from spectral analyses of residual gravity and bathymetry of the Mid-Atlantic Ridge flanks, Earth Planet. Sci. Lett., 154(1-4), 279–293. Escartín, J., D. K. Smith, J. Cann, H. Schouten, C. H. Langmuir, and S. Escrig (2008), Central role of detachment faults in accretion of slow-spreading oceanic lithosphere, Nature, 455(7214), 790–794. Fouquet, y. (1997), Where are the large hydrothermal sulphide deposits in the oceans?, Philos. Trans. R. Soc. London, Ser. A, 355(1723), 427–440. Fouquet, y., et al. (1993a), Sulfide mineralizations associated with ultramafic rocks on the MAR near 15° 20′N, Terra Nova Abstr., 5, suppl. 1, 444–445. Fouquet, y., U. von Stackelberg, J. L. Charlou, J. Erzinger, P. M. Herzig, R. Muehe, and M. Wiedicke (1993b), Metallogenesis in back-arc environments; the Lau Basin example, Econ. Geol., 88(8), 2150–2177. Fouquet, y., A. Wafik, P. Cambon, C. Mevel, G. Meyer, and P. Gente (1993c), Tectonic setting and mineralogical and geochemical zonation in the Snake Pit sulfide deposit (Mid-Atlantic Ridge at 23° N), Econ. Geol., 88(8), 2014–2032. Fouquet, Y., J. L. Charlou, I. Costa, J. P. Donval, J. Radford-Knoery, H. Pellé, H. Ondréas, N. Lourenço, M. Ségonsac, and M. KingstonTivey (1994), A detailed study of the Lucky Strike hydrothermal site and discovery of a new hydrothermal site: Menez Gwen. Preliminary results of the DIVA1 cruise (5–29 May, 1994), InterRidge News, 3(2), 14–18. Fouquet, Y., H. Ondréas, J. L. Charlou, J. P. Donval, J. RadfordKnoery, I. Costa, N. Lourenço, and M. K. Tivey (1995), Atlantic lava lakes and hot vents, Nature, 377, 201. Fouquet, Y., R. Knott, P. Cambon, A. Fallick, D. Rickard, and D. Desbruyeres (1996), Formation of large sulfide mineral deposits along fast spreading ridges; example from off-axial deposits at 12° 43′N on the East Pacific Rise, Earth Planet. Sci. Lett., 144(1-2), 147–162. Fouquet, y., et al. (1997), Discovery and first submersible investigations on the Rainbow Hydrothermal Field on the MAR (36°14N), Eos Trans. AGU, 78(46), Fall Meet. Suppl., F832. Fouquet, y., et al. (1998a), FLORES diving cruise with the Nautile near the Azores. First dives on the Rainbow field: Hydrothermal seawater/mantle interaction, InterRidge News, 7(1), 24–28. Fouquet, y., K. Henry, R. Knott, and P. Cambon (1998b), Geochemical section of the TAG hydrothermal mound, in TAG: Drilling an Active Hydrothermal System on a Sediment-Free 362 ULTRAMAFIC-HOSTED SULFIDE MINERALIZATION ALONG THE MAR Slow-Spreading Ridge, edited by P. M. Herzig et al., Proc. Ocean Drill. Program Sci. Results, 158, 363–388. Fouquet, y., et al. (2000), Hydrothermal processes in oceanic ultramafic environments; the Rainbow hydrothermal sulfide deposit, paper presented at 31st International Geological Congress, Int. Union of Geol. Sci., Rio de Janeiro, Brazil. Fouquet, Y., G. Cherkashov, J. L. Charlou, H. Ondréas, M. Cannat, N. Bortnikov, S. Silantyev, J. Etoubleau, and P. Serpentine (2007), Diversity of ultramafic hosted hydrothermal deposits on the Mid Atlantic Ridge; first submersible studies on Ashadze, Logatchev 2 and Krasnov vent fields during the Serpentine cruise, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract T51F-03. Fouquet, y., et al. (2008), Serpentine cruise–ultramafic hosted hydrothermal deposits on the Mid Atlantic Ridge: First submersible studies on Ashadze 1 and 2, Logatchev 2 and Krasnov vent fields, InterRidge News, 18, 15–19. Fournier, R. O., R. J. Rosenbauer, and J. L. Bischoff (1982), The Solubility of quartz in aqueous sodium chloride solution, Geochim. Cosmochim. Acta, 46, 1975–1978. Francheteau, J., et al. (1979), Massive deep-sea sulfide ore deposits discovered on the East Pacific Rise, Nature, 277, 523–528. Fruh Green, G. L., D. S. Kelley, S. M. Bernasconi, J. A. Karson, K. A. Ludwig, D. A. Butterfield, C. Boschi, and G. Proskurowski (2003), 30,000 years of hydrothermal activity at the Lost City vent field, Science, 301(5632), 495–498. Gaal, G., and J. Parkkinen (1993), Early Proterozoic ophiolitehosted copper-zinc-cobalt deposits of the Outokumpu type, in Mineral Deposit Modeling, edited by R. V. Kirkham et al., pp. 335–341, Geol. Assoc. of Canada, Toronto, Ont., Canada. Gablina, I. F., N. N. Mozgova, y. S. Borodaev, T. V. Stepanova, G. A. Cherkashev, and M. I. Il’in (2000), Copper sulfide associations in recent oceanic ores of the Logatchev hydrothermal field (Mid-Atlantic Ridge, 14° 45′ N), Geol. Ore Deposits, 42(4), 296–316. Gallant, R. M., and K. L. Von Damm (2006), Geochemcial controls on hydrothermal fluids from the Kairei and Edmond vent fields, 23°–25° S, Central Indian Ridge, Geochem., Geophys., Geosyst., 7, Q06018, doi:10.1029/2005GC001067. German, C. R., and J. Lin (2004), The thermal structure of the oceanic crust, ridge-spreading and hydrothermal circulation: How well do we understand their inter-connections?, in Mid-Ocean Ridges: Hydrothermal Interactions Between the Lithosphere and Oceans, Geophys. Monogr. Ser., vol. 148, edited by C. R. German, J. Lin, and L. M. Parson, pp. 1–18, AGU, Washington, D. C. German, C. R., and L. M. Parson (1998), Distributions of hydrothermal activity along the Mid-Atlantic Ridge; interplay of magmatic and tectonic controls, Earth Planet. Sci. Lett., 160(3-4), 327–341. German, C. R., et al. (1994), Hydrothermal activity on the Reykjanes Ridge: The Steinahóll vent-field at 63°06′N, Earth Planet. Sci. Lett., 121, 647–654. German, C. R., et al. (1999), A segment scale study of fluxes through the Rainbow hydrothermal plume, 36°N Mid-Atlantic Ridge, Eos Trans. AGU, 80(46), Fall Meet. Suppl., F957– F958. Gibson, H. L., R. L. Morton, and G. J. Hudak (1999), Submarine volcanic processes, deposits, and environments favorable for the location of volcanic-associated massive sulfide deposits, in Volcanic-Associated Massive Sulfide Deposits; Processes and Examples in Modern and Ancient Settings, edited by C. T. Barrie and M. D. Hannington, Rev. Econ. Geol., 8, 13–51. Goodfellow, W. D., and J. M. Franklin (1993), Geology, mineralogy, and chemistry of sediment-hosted clastic massive sulfides in shallow cores, Middle Valley, northern Juan de Fuca Ridge, Econ. Geol., 88(8), 2037–2068. Gracia, E., D. Bideau, R. Hekinian, Y. Lagabrielle, and L. M. Parson (1997), Along-axis magmatic oscillations and exposure of ultramafic rocks in a second-order segment of the Mid-Atlantic Ridge (33° 43′N to 34° 07′N), Geology, 25(12), 1059–1062. Gracia, E., J. L. Charlou, J. Radford Knoery, and L. M. Parson (2000), Non-transform offsets along the Mid-Atlantic Ridge south of the Azores (38° N–34° N): Ultramafic exposures and hosting of hydrothermal vents, Earth Planet. Sci. Lett., 177(1-2), 89–103. Haase, K. M., et al. (2007), young volcanism and related hydrothermal activity at 5° S on the slow-spreading southern Mid-Atlantic Ridge, Geochem., Geophys., Geosyst., 8, Q11002, doi:10.1029/ 2006GC001509. Halbach, P., et al. (1989), Probable modern analogue of Kurokotype massive sulphide deposits in the Okinawa Trough back-arc basin, Nature, 338(6215), 496–499. Halbach, P., B. Pracejus, and A. Maerten (1993), Geology and mineralogy of massive sulfide ores from the central Okinawa Trough, Japan, Econ. Geol., 88(8), 2210–2225. Halls, C., and R. Zhao (1995), Listvenite and related rocks: Perspectives on terminology and mineralogy with reference to an occurrence at Cregganbaun, Co. Mayo, Republic of Ireland, Mineral. Dep., 30, 303–313. Hannington, M., P. Herzig, S. Scott, G. Thompson, and P. Rona (1991), Comparative mineralogy and geochemistry of goldbearing sulfide deposits on the mid-ocean ridges, Mar. Geol., 101(1-4), 217–248. Hannington, M., et al. (2001), First observations of high-temperature submarine hydrothermal vents and massive anhydrite deposits off the north coast of Iceland, Mar. Geol., 177(3-4), 199–220. Hannington, M. D., and S. D. Scott (1988), Gold mineralisation in volcanogenic massive sulphides; modern and ancient, in Bicentennial Gold; 1988; Extended Abstracts; Oral Programme, edited by A. D. T. Goode and L. I. Bosma, pp. 353–358, Geol. Soc. of Aust., Sydney, N.S.W., Australia. Hannington, M. D., and S. D. Scott (1989), Sulfidation equilibria as guides to gold mineralization in volcanogenic massive sulfides; evidence from sulfide mineralogy and the composition of sphalerite, Econ. Geol., 84(7), 1978–1995. Hannington, M. D., G. Thompson, P. A. Rona, and S. D. Scott (1988), Gold and native copper in supergene sulphides from the Mid-Atlantic Ridge, Nature, 333(6168), 64–66. Hannington, M. D., I. R. Jonasson, P. M. Herzig, and S. Petersen (1995), Physical and chemical processes of se

Unusually large NbTa depletions in North Chile ridge basalts at 36°50′ to 38°56′S: major element, trace element, and isotopic data

Abstract We present new major and trace element data for 28 basalts and 10 basaltic glasses recovered from 16 locations from the North Chile Ridge (NCR) at 36°50′ to 38°56′S. This part of the Chile Ridge consists of three short ridge segments, which are characterized by deep (3200–4100 m) axial valleys. Chemical compositions of the basaltic glasses vary from primitive to moderately fractionated basalts (MgO = 9.54−7.28 wt%). All rocks are incompatible element depleted, mid-ocean ridge basalts (MORB) with average chondrite-normalized (La/Sm) N ratios of 0.46 ± 0.08. The radiogenic isotopic ratios of 10 representative samples display a narrow range in 87 Sr 86 Sr ratios from 0.70241 to 0.70249. 143 Nd 144 Nd ratios also vary within a small range from 0.51312 to 0.51318, and the 206 Pb 204 Pb ratios range from 18.2 to 18.6, with one exception which has a 206 Pb 204 Pb of 19.1. Overall, isotopic compositions are similar to average depleted MORB from the EPR (East Pacific Rise) and MAR (Mid-Atlantic Ridge), but 207 Pb 204 Pb ratios are significantly higher. Pb isotopic systematics of East Pacific MORB reflect large-scale heterogeneities, which are probably the result of long-lived differences in Th/U ratios in the mantle. Significant differences exist in the inferred primary melt compositions between the NCR basalts and depleted MORB from the South EPR and Galapagos Spreading Centre (GSC). For a given MgO content, basalt glasses from the NCR have systematically higher Na and Ti and lower Ca concentrations than those from the South EPR and the GSC. This has been interpreted as indicating relatively low average degrees of melting, which is possibly the result of cooling the shallow asthenosphere near transform offsets. NCR basalts are, on average, more primitive than basalts from the EPR and GSC, implying the lack of a robust magmatic system in this part of the Chile Ridge. This, together with the characteristic ridge topography and short average segment length, suggests that the magmatic system is short-lived, similar to slow-spreading ridges. This can be attributed to lower upwelling rates and, consequently, low magma supply rates near the transform offsets. The majority of the basalts from the NCR are unusually depleted in Nb and Ta relative to average depleted MORB. Trace element modelling shows that the distinct trace element characteristics of the NCR lavas could be the result of melting a mantle which has experienced a previous melting episode. This episode was possibly related to upwelling and melting of the mantle beneath the Pacific-Farallon ridge more than a million years ago.

Formation of carbonate chimneys in the Mediterranean Sea linked to deep-water oxygen depletion

Marine sediments at ocean margins vent substantial amounts of methane1, 2. Microbial oxidation of the methane released can trigger the precipitation of carbonate within sediments and support a broad diversity of seafloor ecosystems3, 4. The factors controlling microbial activity and carbonate precipitation associated with the seepage of submarine fluid over geological time remain poorly constrained. Here, we characterize the petrology and geochemistry of rocks sampled from metre-size build-ups of methane-derived carbonate chimneys located at the Amon mud volcano on the Nile deep-sea fan. We find that these carbonates comprise porous structures composed of aggregated spherules of aragonite, and closely resemble microbial carbonate reefs forming at present in the anoxic bottom waters of the Black Sea5. Using U-series dating, we show that the Amon carbonate build-ups formed between 12 and 7 thousand years ago, contemporaneous with the deposition of organic-rich sediments in the eastern Mediterranean, the so-called sapropel layer S1. We propose that the onset of deep-water suboxic or anoxic conditions associated with sapropel formation resulted in the development of intense anaerobic microbial activity at the sea floor, and thus the formation of carbonate chimneys.

Water column anomalies associated with hydrothermal activity between 11°40′ and 13°N on the East Pacific rise: discrepancies between tracers

Abstract During the HYDROFAST cruise (R.V. Jean Charcot , December 1986) on the East Pacific Rise (EPR), entrainment and vertical transport of deep ocean water by buoyant hydrothermal plumes were clearly seen between the two (12°53′ and 12°37′N) Overlapping Spreading Centers (OSC). Geochemical anomalies (CH 4 , Mn, 3 He), coupled with deviations from ambient T/S linearity, confirmed the existence of a hydrothermal field around 12°50′. New intense hydrothermal venting around 12°43′ was also discovered. These two submarine hydrothermal fields produce effluent plumes which coalesce and form a 250 m thick layer. T/S diagrams show the complex structure of temperature anomalies in the water masses affected by hydrothermal inputs. The hydrothermal clouds are depleted in dissolved oxygen relative to deep-sea water. Simple relationships do not exist between temperature and chemical tracers. The discrepancies result from a combination of various reasons: variable vent fluid characteristics, residence time of tracers in seawater, scavenging and oxidation kinetics of elements and biological activity. However, CH 3 / 3 He and Mn/CH 4 ratios are comparable to those observed in other hydrothermal fields at spreading axes. These elements and compounds can be used at zero age to describe the variation in hydrothermal activity.

Geochemistry of the Hollister Ridge: relation with the Louisville hotspot and the Pacific–Antarctic Ridge

The Hollister Ridge is located on the western flank of the Pacific–Antarctic Ridge (PAR), between the Udintsev fracture zone (FZ) and the Eltanin fault system. It is a linear aseismic structure, 450 km long, oblique with respect to the PAR. Data show that the most recent activity is located in the central part of the chain, which can be considered as being still volcanically active. Both major/trace element and isotopic data suggest that some interaction occurred between the Pacific–Antarctic Ridge and the Hollister Ridge. The source of the Hollister Ridge samples has its own geochemical characteristics. The geochemical variations observed along the ridge can be explained by mixing between two major end-member components: (1) a PAR depleted source, and (2) a Hollister enriched source. A small contribution (20% maximum) of Louisville plume material is likely to exist in the middle of Hollister Ridge. These data unequivocally reject the possibility that the Hollister Ridge could be the present location of the Louisville hotspot. Ages and geochemistry data support the idea of an influence of intraplate deformation as a probable cause of the origin of the Hollister Ridge.

Potential Contamination from Glassware in the Determination of Phosphorus in Natural Waters

Abstract Phosphorus was determined in samples of glass from laboratory glassware, using a hydrofluoric digestion followed by colorimetry. Glass from such sources is shown to contain up to 0.002% of phosphorus. Experiments on storage of samples in glass bottles show that glassware may contribute significant phosphorus contamination to natural water samples, particularly seawater, as a result of partial dissolution of glass into the sample during storage. The release of phosphate from glassware into the sample is greater than the concentration expected from the P/Si ratio in glass and the silicate dissolution in the sample. This is attributed to the formation of a leached layer at the surface of the glass in contact with the water. Under the alkaline digestion conditions applied in this work for the total phosphorus determination, the dissolution of glass does not appear to be a source of measurable phosphate contamination. Nevertheless each analyst must satisfy himself that no phosphate contamination occur...